Universidade de Aveiro Departamento de Engenharia Me âni a2011Bruno Pinto Lopes Desenvolvimento de pre ursores para o fabri o deespumas metáli asDevelopment of pre ursors to produ e metalli foams

Universidade de Aveiro Departamento de Engenharia Me âni a2011Bruno Pinto Lopes Desenvolvimento de pre ursores para o fabri o deespumas metáli asDevelopment of pre ursors to produ e metalli foamsDissertação apresentada à Universidade de Aveiro para umprimento dos req-uisitos ne essários à obtenção do grau de Mestre em Engenharia Me âni a,realizada sob orientação ientí a de Doutora Isabel Maria AlexandrinoDuarte, Investigadora Auxiliar do Departamento de Engenharia Me âni ada Universidade de Aveiro e de Prof. Doutora Móni a Sandra AbrantesDe Oliveira Correia, Professora Auxiliar do Departamento de EngenhariaMe âni a da Universidade de Aveiro.

Dedi o este trabalho a todos aqueles que me são queridos...

O júri / The juryPresidente / President Prof. Doutor Rui Pedro Ramos CardosoProfessor Auxiliar da Universidade de AveiroVogais / Committee Prof. Doutor José Maria da Fonte FerreiraProfessor Asso iado / agreg. da Universidade de AveiroDoutora Isabel Maria Alexandrino DuarteInvestigadora Auxiliar da Universidade de Aveiro (orientador)Prof. Doutora Móni a Sandra Abrantes De Oliveira CorreiaProfessora Auxiliar da Departamento de Engenharia Me âni a ( o-orientador)

Agrade imentos /A knowledgements The main obje tive of this thesis was the development of pre ursor materialfor the manufa ture of metal foams. It was a very pra ti al nature of workthat had the help of several people and businesses. Thus I want to pay aspe ial thanks to my advisors: Dr. Isabel Duarte and Dr. Móni a Oliveirafor suggestions, help and tireless support provided during the work.An a knowledgement to ompanies Chemetall (Frankfurt, Germany) andAlpo o (Nottingham, UK) for supplying the powders of titanium hydrideand aluminium alloys respe tively. To MJ Amaral ompany in the person ofMr. Manuel Vide to the support on manufa turing the mould to produ ethe pre ursor material.I also want to thank Professor Dr. José Maria Ferreira from Department ofCerami s and Glass of the University of Aveiro for the help and availability.Also, I would like to thank the laboratory te hni ians Célia Miranda andAna Ribeiro also from Department of Cerami s and Glass of University ofAveiro for support in the use of equipment for arrying out the work.Finally I would like to thank Inês for all her support and help providedduring this dissertation.Thank you to everyone...

Keywords Aluminium foams; Powder metallurgy; pre ursor material; Titanium HydrideAbstra t The metal foams produ ed by powder metallurgy have been in reasinglyused in various appli ations. Currently, these materials are being used inlight weight stru tures, energy absorption and sound muing in ars, trains,trams and air rafts. The main obje tive of this thesis was the developmentof quality pre ursor material for the manufa ture of Aluminium alloy foams.For that, ommer ial powders of three types of Aluminium alloys (Al-alloy)and two types of Titanium Hydride (TiH2) of dierent hemi al omposi-tions and sizes were tested. To attain the latter, the work was divided intofour phases: (1) Evaluation of the powder properties, in luding the study ofoxidation and thermal de omposition rea tion of TiH2 powders by thermalanalysis, omplemented by X-ray dira tion (XRD), s anning ele tron mi- ros opy (SEM) and energy dispersive spe tros opy X-ray (EDX), and alsothe thermal behaviour of Al alloys by hot mi ros opy te hnique. (2) Op-timization of the parameters in the manufa turing pro ess of the pre ursormaterial. (3) Evaluation of the ability of the pre ursor materials developedin the previous step to produ e Al alloy foams with good quality. (4) Eval-uation of the properties of the obtained foams in this work, su h as density, ompressive strength and energy absorption apa ity. As a result of thiswork, only two of the pre ursor materials developed during the exe utionof this work have demonstrated to be able to manufa ture aluminium alloyfoams with quality. These pre ursors materials were obtained using powdersof the same type of Al-alloy (Al 88.45% and 11.19% Sili on) with an averagediameter of 16 µm, only diering in the type of used TiH2 powders (6.89µm and 15.39 µm). Both TiH2 powders were previously thermally treatedat 480C during 180 minutes. From these studies, it an on luded thatit is possible to obtain pre ursors with densities above 80% of the theoreti- al density by applying a ombination of old and a hot pressing at 400Cusing 200bar. The pre- ompa ted material at old temperature must beheated to 400C and kept at this temperature during 40 minutes, before hotpressing. The Al-alloy foams obtained using TiH2 powder with an averagediameter of 6.89 µm presented densities between 652.28 and 527.44 kg/m3.The obtained foams using TiH2 powder with an average diameter of 15.39µm have shown lower density values, ie. between 487.8 and 421.29 kg/m3.From the evaluation of the me hani al properties, it was found that higherdensity foams have better me hani al properties, namely Young's modulus,stress level and energy absorption apa ity. For example, Young's modulusfor the higher density foams lies between 106.30 and 147.54 MPa, whereasfor the lower density foams the values lie between 27.44 and 63.88 MPa.

Palavras- have Espumas de ligas de Alumínio; metalurgia de pós; material pre ursor; Hidretode TitânioResumo As espumas metáli as produzidas por metalurgia de pós têm vindo a ser adavez mais utilizadas nas mais diversas apli ações. A tualmente estes mate-riais estão a ser utilizados em estruturas ultraleves, de absorção de energiae de amorte imento sonoro em veí ulos automóveis, omboios, elé tri os eaeronaves. O prin ipal obje tivo desta dissertação prendeu-se om o desen-volvimento de material pre ursor de qualidade para o fabri o de espumas deligas de Alumínio. Para o efeito foram testados pós omer iais de três tiposde ligas de Alumínio (liga de Al) e dois tipos de Hidreto de Titânio (TiH2) dediferentes omposições quími as e granulometrias. Para atingir este obje -tivo, o trabalho dividiu-se em quatro fases: (1) Avaliação das propriedadesdos pós, in luindo o estudo da oxidação e da rea ção de de omposição tér-mi a dos pós de TiH2 através de análises térmi as, omplementadas omdifra ção de raio-X (DRX), mi ros opia ele tróni a de varrimento (SEM) eespe tros opia de energia dispersiva de raio-X (EDX), e ainda o estudo do omportamento térmi o das ligas de Al através da té ni a de mi ros opia aquente. (2) Optimização dos parâmetros no pro esso de fabri o do materialpre ursor. (3) Avaliação da apa idade dos materiais pre ursores desenvolvi-dos na etapa anterior para produzir espumas de ligas de Al de qualidade. (4)Avaliação das propriedades das espumas obtidas neste trabalho, nomeada-mente a densidade, a resistên ia à ompressão e a apa idade de absorçãode energia. Como resultado deste trabalho, apenas dois dos materiais pre- ursores desenvolvidos durante a exe ução deste trabalho demonstraram ter apa idade de fabri ar espumas de ligas de Alumínio de qualidade. Estesmateriais pre ursores foram obtidos usando pós do mesmo tipo de liga de Al(88.45% de Al e 11.19% de Silí io) de diâmetro médio de 16 µm, diferindoapenas no tipo de pós de TiH2 usados (6.89 µm e 15.39 µm). Ambos ospós de TiH2 foram previamente tratados termi amente a 480C durante 180minutos. Dos estudos realizados, on lui-se que é possível obter pre ursores om densidades superiores a 80% da densidade teóri a através da ombi-nação de uma prensagem a frio e de uma prensagem a quente a 400C de200bar. O pré- ompa tado a frio deve ser aque ido até 400C e estar aesta temperatura durante 40 minutos, antes da prensagem a quente. Asespumas de ligas de Al obtidas usando pós de TiH2 de diâmetro médio de6.89 µm apresentaram densidades entre 652.28 e 527.44 kg/m3. As obtidasusando pós de TiH2 de diâmetro médio de 15.39 µm apresentaram valoresde densidade inferiores, ou seja entre 487.8 e 421.29 kg/m3. Da avaliaçãodas propriedades me âni as, veri ou-se que as espumas de maior densidadeapresentam valores de propriedades me âni as superiores, nomeadamente omódulo de Young, tensão de patamar e apa idade de absorção de energia.Por exemplo, o módulo de Young das espumas de maior densidade apresentavalores entre 106.30 e 147.54 MPa, enquanto as espumas de densidades in-ferior tem valores entre 27.44 e 63.88 MPa.

ContentsList of Tables iiiList of Figures ivNomen lature ix1 Introdu tion 12 State of the Art 32.1 Introdu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Classes of metalli foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Manufa turing Pro esses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3.1 Produ tion of foams from metalli melts . . . . . . . . . . . . . . . 52.3.1.1 Dire t foaming methods . . . . . . . . . . . . . . . . . . . 52.3.1.2 Solid-gas eute ti solidi ation . . . . . . . . . . . . . . . 62.3.1.3 Investment asting . . . . . . . . . . . . . . . . . . . . . . 82.3.1.4 Synta ti foams using ller materials . . . . . . . . . . . . 82.3.2 Foams made from metal powders . . . . . . . . . . . . . . . . . . . 92.3.2.1 Powder metallurgy . . . . . . . . . . . . . . . . . . . . . . 92.3.2.2 Foaming of slurries . . . . . . . . . . . . . . . . . . . . . . 112.3.2.3 Gas entrapment . . . . . . . . . . . . . . . . . . . . . . . 112.3.2.4 Other Te hniques . . . . . . . . . . . . . . . . . . . . . . 112.3.3 Produ tion by Deposition Te hniques . . . . . . . . . . . . . . . . 122.4 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.1 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.4.2 Stru tural properties . . . . . . . . . . . . . . . . . . . . . . . . . . 152.4.3 Me hani al Properties . . . . . . . . . . . . . . . . . . . . . . . . . 172.4.3.1 Stress-strain behaviour under ompression . . . . . . . . . 17Linear elasti region . . . . . . . . . . . . . . . . . . . . . . 18Plateau region . . . . . . . . . . . . . . . . . . . . . . . . . . 20Densi ation region . . . . . . . . . . . . . . . . . . . . . . . 212.4.3.2 Energy Absorption under ompression . . . . . . . . . . . 222.4.4 Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4.5 A ousti properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4.6 Ele tri al properties . . . . . . . . . . . . . . . . . . . . . . . . . . 282.5 Appli ations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.5.1 Automotive industry . . . . . . . . . . . . . . . . . . . . . . . . . . 32i

2.5.2 Aerospa e industry . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.5.3 Shipbuilding industry . . . . . . . . . . . . . . . . . . . . . . . . . 342.5.4 Railway industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.5.5 Building industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 Experimental Pro edures 373.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.1.1 Powder spe i ations . . . . . . . . . . . . . . . . . . . . . . . . . . 373.1.2 Preparation of blowing agents powders . . . . . . . . . . . . . . . . 383.1.3 Preparation of pre ursor materials . . . . . . . . . . . . . . . . . . 393.1.4 Preparation of Al-alloys foams . . . . . . . . . . . . . . . . . . . . 403.2 Chara terization Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2.1 Powder analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2.1.1 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2.1.2 Parti le size analysis . . . . . . . . . . . . . . . . . . . . . 423.2.1.3 Thermal analysis . . . . . . . . . . . . . . . . . . . . . . . 433.2.1.4 X-ray dira tion analysis . . . . . . . . . . . . . . . . . . 443.2.1.5 Hot-stage mi ros opy analysis . . . . . . . . . . . . . . . 443.2.2 Mi ros opi analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 453.2.2.1 Light mi ros opy (LM) analysis . . . . . . . . . . . . . . 453.2.2.2 S anning ele tron mi ros opy (SEM) analysis . . . . . . . 463.2.3 Me hani al analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 464 Results and Dis ussion 494.1 Powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494.1.1 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494.1.2 Parti le size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504.1.3 Morphology analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 514.1.4 Thermal de omposition and oxidation behaviour of the TitaniumHydride powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534.1.4.1 Thermal behaviour of TiH2 powders in Air atmosphere . 554.1.4.2 Thermal behaviour of TiH2 powders in Oxygen atmosphere 594.1.4.3 X-ray dira tion investigations . . . . . . . . . . . . . . . 614.1.5 Thermal Behaviour of the Al-alloy powders . . . . . . . . . . . . . 704.2 Produ tion of pre ursor material . . . . . . . . . . . . . . . . . . . . . . . 744.3 Produ tion of Al-alloys foams . . . . . . . . . . . . . . . . . . . . . . . . . 814.3.1 Preliminary tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.4 Properties of Al-alloy foams . . . . . . . . . . . . . . . . . . . . . . . . . . 844.4.1 Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844.4.2 Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 864.4.3 Compression behaviour . . . . . . . . . . . . . . . . . . . . . . . . 874.4.4 Energy absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 915 Con lusions 956 Future works 97ii

List of Tables2.1 Ranges for properties of ommer ial foams . . . . . . . . . . . . . . . . . . 143.1 Supplier spe i ation of the Al-alloy powders . . . . . . . . . . . . . . . . 373.2 Supplier spe i ation of the Titanium Hydride powders . . . . . . . . . . 383.3 Manufa turing parameters used to produ e Al-alloy foams . . . . . . . . . 474.1 Density of the Al-alloy and TiH2 powders. . . . . . . . . . . . . . . . . . . 494.2 Parti le size analysis by laser dira tion. . . . . . . . . . . . . . . . . . . . 504.3 Optimum ompa tion onditions . . . . . . . . . . . . . . . . . . . . . . . 774.4 Foamable pre ursor materials . . . . . . . . . . . . . . . . . . . . . . . . . 784.5 Densities and relative density of Al-alloy foams . . . . . . . . . . . . . . . 864.6 Inuen e of the foam omposition and density on me hani al results . . . 894.7 Energy absorption apability and absorption energy per unit volume at50% strain and at densi ation strain . . . . . . . . . . . . . . . . . . . . . 91

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iv

List of Figures2.1 Metalli foams. a) open- ell stru ture. b) losed- ell stru ture . . . . . . . 42.2 Manufa turing pro esses of metalli foams . . . . . . . . . . . . . . . . . . 52.3 Dire t foaming of melts by gas inje tion . . . . . . . . . . . . . . . . . . . 62.4 Dire t foaming melts by adding blowing agents . . . . . . . . . . . . . . . 72.5 Solid-gas eute ti solidi ation . . . . . . . . . . . . . . . . . . . . . . . . 72.6 Investment asting pro ess . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.7 Syntati foams using ller metal . . . . . . . . . . . . . . . . . . . . . . . . 92.8 Powder metallurgy pro ess . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.9 Gas entrapment te hnique . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.10 Properties of ellular and dense engineering materials . . . . . . . . . . . . 132.11 The range of ell size and relative density for the dierent metalli foammanufa turing methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.12 Young modulus for dierent foam . . . . . . . . . . . . . . . . . . . . . . . 152.13 List of parameters to des ribe the stru ture of metalli foam . . . . . . . . 162.14 Closed- ell Aluminium foam . . . . . . . . . . . . . . . . . . . . . . . . . . 162.15 Closed- ell foam with gradient density . . . . . . . . . . . . . . . . . . . . 172.16 Compressive stress-strain urves of a) elastomeri , b) elasti -plasti and ) elasti -brittle foam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.17 Ee t of foam density on ompressive stress-strain urves . . . . . . . . . 182.18 Deformation me hanisms of foams: a) open- ell foam, sequentially ellwall bending, ell wall axial deformation and uid ow between ells andb) losed- ell foams, sequentially ell wall bending and ontra tion, mem-brane stret hing and en losed gas pressure . . . . . . . . . . . . . . . . . . 192.19 Cubi unit ell models of open- ell and losed- ell foams developed byGibson and Ashby . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.20 Variation of Young's module with density . . . . . . . . . . . . . . . . . . 212.21 Stress-strain urves for both an elasti solid and a foam showing the dif-feren e in energy per unit volume absorbed at the same stress level . . . . 222.22 Stress-strain urves of foams for three dierent density . . . . . . . . . . . 232.23 Comparison of the ideal absorber and a real absorber . . . . . . . . . . . . 242.24 Maximum stress o urring when a given deformation energy is absorbedby Aluminium foams with various densities . . . . . . . . . . . . . . . . . 242.25 Thermal ondu tivity plotted against volumetri spe i heat for urrentlyavailable metalli foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262.26 Sound absorption oe ient of various types of ellular Al foams in om-parison with ber lass sound absorbing material . . . . . . . . . . . . . . 27v

2.27 Sound absorption oe ient as fun tion of opened surfa e area for Alulightfoam (8.9 mm thi k AlSi12 foam, density 580kg/m3, holes drilled throughthe sample, 20 mm air gap between sample and rigid wall) . . . . . . . . . 272.28 Damping fa tor and vibration behavior for Alulight foam . . . . . . . . . . 282.29 The power law dependen e of normalized ele tri ondu tivity of Al-foamson the relative density for Alulight foam . . . . . . . . . . . . . . . . . . . 292.30 Magneti eld shielding ee tiveness as a fun tion of frequen y for Alu-minium foam (Alulight) and steel samples of the same weight and a mas-sive Al sheet of the same thi kness (t=8.5 mm) as the Aluminium foam.Sample size: 140x140 x t, density of foam 500kgm−3 . . . . . . . . . . . . 302.31 Appli ations of ellular metals grouped a ording to the degree of open-ness needed and whether the appli ation is more fun tional or stru tural 312.32 Appli ations elds for losed- ell metalli foams . . . . . . . . . . . . . . . 312.33 Automotive omponents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.34 Implementation of metalli foams in a ar . . . . . . . . . . . . . . . . . . 332.35 Real automotive appli ations for Al-alloy foams . . . . . . . . . . . . . . . 332.36 Ariane ro ket one: prototype assembly. 3D foamed AFS 6061 + AlCu6Cu6 342.37 Sandwi h panel metal foams . . . . . . . . . . . . . . . . . . . . . . . . . . 342.38 Building appli ations for Al-foams . . . . . . . . . . . . . . . . . . . . . . 353.1 Titanium Hydride powders: as-re eived (left side) and treated (right side) 383.2 Plasti bottles ontaining the dierent prepared powders mixture . . . . . 393.3 Tumbling mixer used in this resear h to mix the powders . . . . . . . . . . 393.4 Overview of the system in the laboratory to manufa ture the pre ursormaterials. a) Uni-axial pressing devi e with the die and the heating sys-tem. b) Pressing devi e used for the pressing the powder mixture . . . . . 403.5 Furna e used for the manufa ture of Al-alloy foams . . . . . . . . . . . . . 413.6 Py nometer used during analysis . . . . . . . . . . . . . . . . . . . . . . . 423.7 LS 230 Laser Dira tion Parti le Size Analyze from Be kman Coulter . . 433.8 (a) Thermal analyser LabsysTM TG-DTA16. (b) Simultaneous ThermalAnalyser STA 449 C Netzs yh . . . . . . . . . . . . . . . . . . . . . . . . . 443.9 Hot-Stage Mi ros opy (Leitz, model 2A) . . . . . . . . . . . . . . . . . . . 453.10 light mi ros ope Nikon E lipse LV150 . . . . . . . . . . . . . . . . . . . . 453.11 S anning Ele tron Mi ros ope (HR-FESEM Hita hi SU-70) . . . . . . . . 463.12 Universal testing ma hine (Shimadzu AG-50kNG) . . . . . . . . . . . . . . 473.13 Al-alloy foams tested . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474.1 SEM mi rograph of Al-alloy powders at dierent magni ations. Left side:overview. Right side: parti les details. . . . . . . . . . . . . . . . . . . . . 524.2 SEM mi rograph of as-re eived TiH2 powders at dierent magni ations.Left side: overview. Right side: parti le details. . . . . . . . . . . . . . . . 534.3 Thermo analysis of the TiH2 powder samples performed at 10k/min. . . . 544.4 TG/DTG urves measured on as-re eived and treated TiH2 powders inAir at 10k/min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 564.5 TG urves measured on as-re eived and treated TiH2 powders in air at10K/min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.6 TG/DTA urves measured on as-re eived TiH2 powder in oxygen at 10k/min. 59vi

4.7 TG/DTA urves measured on as-re eived TiH-2 powder in Air and inOxygen at 10k/min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604.8 XRD-measurements on TiH2 powder samples as-re eived and treated at480C during 3 hours. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624.9 Referen e spe tra for TiH2, TiO2, TiO3 and TiN0.3. . . . . . . . . . . . . . 634.10 XRD-measurements on as-re eived TiH2 powder samples and after sub-mitted in Air and Oxygen atmosphere. . . . . . . . . . . . . . . . . . . . . 644.11 SEM mi rographs of surfa e of a parti le of TiH2 powder submitted atdierent onditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654.12 SEM mi rographs of surfa e of a parti le of TiH2 powder submitted atdierent onditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664.13 EDX spe trum of TiH-1 powder . . . . . . . . . . . . . . . . . . . . . . . . 674.14 EDX spe trum of TiH-2 powder . . . . . . . . . . . . . . . . . . . . . . . . 684.15 EDX spe trum of TiH-2 powder after submitted at air atmosphere in Fig.4.12b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.16 TiH2 oxidised at dierent onditions. The olour of the powder dependson the thi kness of the Oxygen layer. a ) as-re eveid b) treated at 480Cfor 3h ) submitted in Oxygen atmosphere d) submitted in Air atmosphere. 694.17 Hot-stage mi ros opy images of a sample of Al-A alloy powder at dierenttemperatures (before, during and after melting) . . . . . . . . . . . . . . . 714.18 Hot-stage mi ros opy images of a sample of Al-B alloy powder (32.5µm)at dierent temperatures (before, during and after melting) . . . . . . . . 724.19 Hot-stage mi ros opy images of a sample of Al-C alloy powder (240.9µm)at dierent temperatures (before, during and after melting) . . . . . . . . 734.20 S hemati representation of experimental tests to adjust the ompa tionparameters using hot ompa tion stage (a) and a ombination of old andhot ompa tion stages (b) . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.21 Ee t of ompa tion parameters on the density of the pre ursor material . 754.22 Low-quality of pre ursor materials whi h was obtained by hot pressing atdierent manufa turing parameters . . . . . . . . . . . . . . . . . . . . . . 764.23 High-quality of pre ursor materials obtained by a ombination of old andhot pressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764.24 Mi rostru ture of pre ursor materials without blowing agent . . . . . . . . 774.25 Pre ursor materials manufa tured . . . . . . . . . . . . . . . . . . . . . . . 784.26 Mi rostru ture of pre ursor materials prepared of dierent Al-alloy (mag-ni ation of x100) ontaining 0.6wt.%.of TiH2 . . . . . . . . . . . . . . . . 794.27 Mi rostru ture of pre ursor materials prepared of Al-A alloy ontaining0.6wt.%.of treated and as-re eived TiH2 powders . . . . . . . . . . . . . . 804.28 Foaming behaviour of pre ursor material with dierent Al-alloys ontain-ing 0.6 wt.% TiH-1 as-re eived powder, using a pre-heated furna e at 750C 814.29 Foaming behaviour of Al-A alloy pre ursor material ontaining 0.6 wt.%TiH-1, using a pre-heated furna e at dierent foaming temperatures . . . 824.30 Foaming behaviour of pre ursor material with dierent Al-alloys ontain-ing 0.6 wt.% TiH-1 treated powder, using a pre-heated furna e at 750C . 824.31 Foaming behaviour of pre ursor material with dierent Al-alloys ontain-ing 0.6 wt.% TiH2 treated powder, using a pre-heated furna e at 750C . 82vii

4.32 Foaming behaviour of Al-A pre ursor material ontaining 0.6 wt.% TiH-1Ttreated powder, using a pre-heated furna e at 750C . . . . . . . . . . . . 834.33 Foaming behaviour of Al-A pre ursor material ontaining 0.6 wt.% TiH-1T treated powder or TiH-2T treated powder, using a pre-heated furna eat 750C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.34 Pre ursor materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844.35 Mi rostru ture of the dierent pre ursor materials. TiH2 parti les (white olour) is distributed into the Al-A alloy matrix (light gray olour) . . . . 854.36 Al-alloy foams prepared to me hani al hara terisation . . . . . . . . . . . 854.37 Stress-strain behaviour as obtained in uniaxial ompression tests for bothAlSi1Mg and AlSi7 foams ( ylindri al samples ∅ = 30mm x 30 mm) . . . 884.38 Stress-strain urve in the linear elasti region . . . . . . . . . . . . . . . . 894.39 Average plateau stress for both studied Al-alloy foams . . . . . . . . . . . 904.40 Energy absorption apabilities of Al-alloy foams . . . . . . . . . . . . . . . 924.41 Energy absorption per unit volume of Al-alloy foams . . . . . . . . . . . . 93

viii

Nomen latureA Se tion area [m2E Young modulus [N.m2E∗ Metalli foams Young modulus [N.m2Es Base material Young modulus [N.m2Ev Absorbed energy per volume unit [MJ/m3F Compression for e applied [N K Thermal ondu tivity [W.m−1.K−1Ks Thermal ondu tivity [W.m−1.K−1P1 Referen e pressure [PaP2 Real pressure in the sample [PaR∗ Foam resistivity [Ω.mRs Base material resistivity [Ω.mT Temperature [KVc Constant for the alibration of equipmentVp True volume of the sample [m3Vr Constant for the alibration of equipmentηef Energy absorption e ien y [%φ Contribution of the edges of the ells onstantρ∗ Foam density [Kg/m3ρr Relative densityρs Density of foam base metal [Kg/m3σ Compression stress [Paσpl Plateau stress [Paix

σys Yield strength stress [Paε Compressive strain [%q Heat transfer gradient [W Al AluminiumC CarbonCa Cal iumCu CopperFe IronH HydrogenMg MagnesiumMn ManganeseN NitrogenNi Ni kelO2 OxygenSi Sili onT i TitaniumAl2O3 AluminaSiC Sili on CarbideT iHx(0 < x < 2) Titanium HydrideT iN0.3 OsborniteT iO2 RutileZrO2 Zi ornia

x

Chapter 1Introdu tionMetalli foams are urrently being onsidered for a number of appli ations in the marine,aerospa e and automotive industries. Re ently a number of metal foams have been de-veloped to repla e polymer foams in appli ations where multi-fun tionality is important.Closed Al-alloy foams oer a unique ombination of properties su h as low density, highstiness, strength and superior energy absorption. In addition, this material oers im-proved soundproong hara teristi s, low thermal ondu tivity hara teristi s, and lowtoxi ity under re onditions. Al-alloy foam has been used in omposite integral armourin order to develop a ballisti material. Here, it was found that the foam exhibitedsigni ant non-linear deformation under loading and stress wave attenuation [1.In ad-dition, the Al-alloy foam armours also resulted in a redu ed dynami dee tion in theba king plates. At present, there is an in reasing interest in the potential oered bymetalli foams for use in high performan e sandwi h stru tures. Unfortunately, in many ases, the skins are adhesively bonded to the metalli ore, a pro edure that in reasesthe length of the manufa turing y le and its asso iated osts. Many manufa turingpro esses, reported in the literature review are still being improved, as it is the aseof powder metallurgi al method. The urrent resear h studies the onne tions betweente hnologi al fa tors and the properties of the obtained foams in order to improve theirquality. In order to obtain a high quality material with high te hnologi al performan esit is ne essary the understanding, orrelation and ontrol of te hnologi al parameters.The aim of this thesis was to develop pre ursor material with high quality to obtain Al-alloy foams, using ommer ial powders of Al-alloy and titanium hydride. In addition, themain me hanisms responsible for the formation of foam, as the thermal de ompositionand oxidation behaviour of the titanium hydride and the thermal behaviour of the Al-alloy were investigated in detail.This dissertation is organised in hapters in order to fa ilitate its understanding. Thus,Chapter 2 gives an overview of the urrent state of the art with regard to the manu-fa turing pro esses, properties and appli ations. Experimental pro edures used in thepresent work for pre ursor material produ tion, foam produ tion and the used experi-mental methods are des ribed in Chapter 3. Results and dis ussion are given in Chapter4 followed by the on lusions in Chapter 5. The future work is outlined in Chapter 6.1

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Bruno Pinto Lopes MIEM Dissertation

Chapter 2State of the Art2.1 Introdu tionHoney ombs, foams, wood, ro k, plant stems, bone and tissue engineering s aolds allhave a ellular stru ture that gives rise to unique properties that are exploited in engineer-ing and in medi ine. Nature, too, uses ellular materials to provide stru tural supportas well as to ondu t uids. Cellular materials onstitute attra tive lass of materialswith a wide variety of stru tural and fun tional appli ations. These materials an bemade from metal, erami and polymer. In addition to their low spe i weight, metalli foams feature a series of me hani al, thermal and a ousti properties that make themparti ularly well suited appli ations in the automotive, biome hani al and onstru tionindustries [2, 3.Metalli foams belong of this lass of ellular materials that are used in engineering andmedi ine appli ations. Its advantages are that these materials are lighter than tradi-tional solid forms (solid sheets), more fun tional than traditional ellular materials (hon-ey ombs, polymer foams), and therefore an be more ost-ee tive and environmentallyfriendly than any ompetitor material [4.Closed- ell metalli foam was rst reported in 1926 by Meller in a Fren h patent wherefoaming of light metals either by inert gas inje tion or by blowing agent was suggested.The next two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and1951 who applied mer ury vapour to blow liquid Aluminium [5. Closed- ell metalli foams have been developed sin e about 1956 by John C. Elliott at Bjorksten Resear hLaboratories [6. Although the rst prototypes were available in the 50s, ommer ialprodu tion was started only in the 90s by Shinko Wire ompany in Japan. Metalli foams are ommonly made by inje ting a inert gas or mixing a blowing agent (usually,Titanium Hydride) into molten metal [7.Nowadays, there are a number of su essful manufa turing pro esses for metalli foams.Some of these pro esses has been ommer ialised in Europe, Asia and North-Ameri a by ompanies like Alantum (Korea and Germany), Cymat (in Canada), Dunlop Equipment(in England), Glei h GmbH (in Germany), Mepura (in Austria), Shinko wire (in Japan)and Re emat (in Netherland) [813. Dierent types of metalli foams are ommer iallyavailable su h as, Ni kel, Copper, Zin , Steel, Aluminium and Gold. Among the metalli foams, Al-alloys foams are ommer ially the most exploited ones due to their low density,3

4 2.State of the Arthigh du tility, high thermal ondu tivity, and ompetitive ost of the metal [2.The most important onferen e in this eld is the international onferen e on PorousMetals and Metalli Foams series that is held regularly at two-years intervals sin e 1997.The onferen es have been s heduled to provide a forum for resear hers a tive in theelds of porous and foam materials and for industrial materials engineers and produ tdesigners seeking new materials [7.2.2 Classes of metalli foamsMetalli foams belong to a group of materials alled ellular solids. Cellular solids aredened as having a porosity > 0.7 [3. Natural foams are produ ed by plants and animalssu h as ork or bone. Man made foams an be manufa tured from a variety of materialssu h as erami s, polymers and metals. Cellular metalli materials are being in reasinglyregarded as a solution for problems of light weight onstru tion, passive safety, sounddamping or ltering purposes. Foams are three-dimensional arrays of ells that an bedivided into two ategories: open- ell and losed- ell metalli foams [3, as shown in Fig.2.1. In the open- ell foams, the ells are inter onne ted by ell edges, or ligaments. In the losed- ell foams, the ells are predominantly isolated from ea h other by solid fa es. This hara teristi an dire tly be observed by opti al mi ros ope or an also be determinedfrom the permeability of foam to a gas or a liquid. Open- ell metalli foams allowthe passage of liquids and gases for dierent appli ations ranging from ltering to heatex hange and give the foam its in reased surfa e area while the losed- ell ongurationis optimal for energy absorption and stru tural appli ations like in ar bumpers, bridgesand buildings. Open- ell metalli foams are usually used to enhan e heat transfer inappli ations su h as ryogeni heat ex hangers, heat ex hangers for airborne equipment, oal ombustors, ompa t heat sinks for high power ele troni devi es, heat shielding forair raft exhaust, ompa t heat ex hangers, liquid heat ex hangers, air- ooled ondenser- ooling towers and regenerators for thermal engines[2, 4, 7, 14, 15. A uid media annot pass through losed- ell foam. Closed- ell foams are being used in the transportindustry, as lightweight stru tures, energy-absorption stru tures and damping stru tures[7. In this work the foams of interest are losed- ell Aluminium alloy foams.

(a) (b)Figure 2.1: Metalli foams. a) open- ell stru ture (NICOFOAM Ni kel foam) [16. b) losed- ell stru ture (Aluminium foam) [17.Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 52.3 Manufa turing Pro essesNowadays, there are many ways to produ e metalli foams or similar porous metal stru -tures. The manufa turing pro esses an be lassied a ording to the state of the startingmetal: liquid, powdered and ionised - as shown in Fig. 2.2 [3, 4, 7, 14, 15, 1820.Themost important advantages and disadvantages for the most ommon methods of thesematerials are presented.

Figure 2.2: Manufa turing pro esses of metal foams [3, 4, 7.2.3.1 Produ tion of foams from metalli meltsMetalli melts an be dire tly foamed by the inje ting gases or by adding a blowing agentthat releases gas through its thermal de omposition. Other indire t methods an also beused through a polymeri foam or by asting the liquid metal around solid ller materialswhi h reserve spa e for the pores or whi h remain in the foam [3, 4, 7.2.3.1.1 Dire t foaming methodsThere are two ways for dire tly foaming metalli melts whi h are already in the stage ofa large-s ale ommer ial exploitation. The rst one is exploited by Cymat AluminiumCorporation (in Canada) in whi h metalli melt is foamed dire tly by inje ting gasesinto the molten metal (Fig. 2.3). The gases (e.g. Air, Nitrogen, Argon) are inje ted intothe molten metal using spe ially designed rotating impellers. These impellers have toprodu e very ne gas bubbles in the molten melt and distribute them homogeneously.The gas bubbles whi h are then formed into the molten metal tend to rise to its surfa equi kly due to the high buoyan y for es in the high-density liquid but this rise an beimpeded by in reasing the vis osity of the molten metal. This an be done by addingne erami powders (e.g. Alumina, Sili on Carbide and Zir onia) or alloying elementswhi h form parti les in the melt. The formed foamy mass is ontinuously drawn oby means a onveyor. An additional upper onveyor is used to reate a at surfa eon the top of the foam panel. This pro ess allows for manufa turing large volume ofBruno Pinto Lopes MIEM Dissertation

6 2.State of the Artfoams, ontinuously. A possible disadvantage is the eventual ne essity for utting thefoam due the high ontent of erami parti les (10-30 vol. %) used in the pro ess. Themain disadvantage of this pro ess is the poor quality of the foams produ ed. The ellsize is large and often irregular, and the foams tend to have a marked density gradient.Foam panels with 1m in width and thi kness range of 25-150 mm, an be produ ed in ontinuous length at produ tion rates of 900 kg/hour. The relative densities range ofthese foams is 0.05-0.55 g/ m3. The average ell size is 2.5-30 mm [3, 7.

Figure 2.3: Dire t foaming of melts by gas inje tion [3.The se ond alternative method is exploited by Shinko-Wire Company (in Japan) usingAlporas as the trade name (Fig. 2.4). This manufa turing pro ess, the metalli melt isfoamed by adding blowing agent parti les. The blowing agent parti les (e.g. TitaniumHydride) de ompose under the inuen e of heat and releases gas whi h then propels thefoaming pro ess. Cal ium is previously added to the molten metal for adjust the meltvis osity. After the vis osity has rea hed the desired value, Titanium Hydride (TiH2) isadded (typi ally 1.6 wt.%), as a blowing agent by releasing Hydrogen (H2) gas in the hotvis ous liquid. The melt starts to expand slowly and gradually lls the foaming vessel.After ooling the vessel below the melting point of the alloy, the liquid foam turns intosolid Al foam. Foam blo ks is is removed from the mould and are are ut into sheetsof the required thi kness. This pro ess is apable of produ ing large blo ks of goodquality. Blo ks with 450 mm in wide, 2050 mm in length and 650 mm in height an beprodu ed. These foams have uniform pore stru ture and do not require the addition of erami parti les, whi h makes it brittle. However, the method is more expensive thanfoaming melts by gas inje tion method requiring more omplex pro essing equipment.The density range of these foams is 0.18-0.24 g/ m3, and the mean ell size is about 4.5mm.2.3.1.2 Solid-gas eute ti solidi ationThis method whi h was developed some years ago at the Dnepropetrovsk Metallurgi alInstitute in the Ukraine exploits the fa t that some liquid metals form a eute ti systemBruno Pinto Lopes MIEM Dissertation

2.State of the Art 7(a) (b)( ) (d)Figure 2.4: Dire t foaming melts by adding blowing agents [3.with Hydrogen gas. The metal to be foamed is melted in an auto lave with a ontrolledpressure of Hydrogen, so that the melt be omes saturated with Hydrogen. The melt isthen dire tionally solidied and as it ools through the solid-gas eute ti point, it be omessupersaturated. A two phase solid/gas mixture is simultaneously formed from the melt,yielding an anisotropi porous solid with ylindri al pores oriented in the solidi ationdire tion. Metalli foams manufa tured by this pro ess are alled as GASARs [3. Themain advantage of this pro ess is to allow a ontrol of the nal properties of foamedmaterials, namely the geometry, the size and the orientation of the ellular pores. Radialand axial pores an be produ ed using a ylindri al asting vessel. The possibility ofsolidifying the liquid dire tionally oers the advantage of making foams with elongatedpores. However, the maximum porosities whi h an be a hieved by this pro ess are notvery high (5-75%) but metals with medium and high melting points su h as Copper andNi kel an be foamed. Layered stru tures with alternating bands of solid and porousmaterials an also be produ ed [3.

Figure 2.5: Solid-gas eute ti solidi ation [3.Bruno Pinto Lopes MIEM Dissertation

8 2.State of the Art2.3.1.3 Investment astingAnother method of foaming of metalli melts applies investment- asting te hnique foropen- ell metalli foam produ tion (Fig. 2.6). Commer ially known DUOCELTM foamused in heat ex hangers is manufa tured by the investment asting. The ERG Materialsand Aerospa e Corporation has been manufa tured for the aerospa e, national defense,semi ondu tor manufa turing, biote h and other high te hnology industries. A erami repli ate is produ ed using a polymeri foam pre ursor and then the liquid metals arepenetrated and repla e the polymeri foam. A wide regular shape and uniformity of foamstru ture an be possible with this method. The pro ess produ es relatively small quan-tities of expensive, high-quality foams with reliable material properties. The porosities an be as high as 98%, with pore size between one and several millimeters. Any metalor alloy an be foamed; however, its widespread use is limited due to the omplexity, ost and di ulty of s aling up the pro ess [3, 4, 7. Duo elR Aluminium foam is ourmost popular metalli foam, followed by opper foam. Other Duo elR foam metals thatwe have produ ed, but are not urrently ommer ial produ ts in lude tin, Zin , Ni kel,In onel, Silver, and Gold [2.(a) (b)

( ) (d)Figure 2.6: Investment asting pro ess [3.2.3.1.4 Synta ti foams using ller materialsThe most ommon manufa turing method for metalli synta ti foams is melt inltration,where the molten metal is pressure inltrated into a random pa k of erami spheres orhollow spheres (Fig. 2.7). A va uum or an external pressure an be reated to fa ility themelt inltration. The volume per entage of the metal matrix is thus determined by theamount of inter-parti le spa e of the erami mi rospheres. If the erami spheres have asimilar parti le size, the metal volume per entage is xed at about 37%. Therefore, thefabri ated synta ti foams (fabri ated with a similar parti le size of erami spheres) havea similar volume fra tion of metal matrix. The volume per entage of metal matrix of thefoam an be de reased by embedding erami spheres with multimodal size distributions,thus in reasing the porosity of the foam. A wide of metals an be produ ed by this way,Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 9in luding Aluminium, Zin , Magnesium, Lead, Silver et . [7. Parts of a predened shape an be produ ed using a mould of the appropriate geometry. The erami spheres ouldbe removed by lea hing them in suitable solvents or a ids or thermal treatment [4.

Figure 2.7: Syntati foams using ller metal [4.2.3.2 Foams made from metal powdersMetalli foams an be also produ ed using metal powders. Some of these manufa turingpro esses are also in the state of ommer ial exploitation, but in small-s ale.2.3.2.1 Powder metallurgyThe Powder Metallurgi al method (PM method) is one of the ommer ially exploitedmethods to produ e losed- ell metalli foams. This manufa ture pro ess was developedand patented at Fraunhofer Institute in Bremen (Germany) [3, 4. This PM method anbe used to produ e foams of dierent metals and its alloys [21, su h as Aluminium andits alloys, Tin, Zin , Lead, Steel and Gold whi h is one of the advantage of this PMmethod.Metal powders (elementary metal powders, alloy powders or metal powder blends) aremixed with a small fra tion of a suitable powdered blowing agent by onventional mixers(e.g. turbula mixer). The blowing agents usually used for produ ing Al-alloy foamsusing the PM method are metal hydrides, su h as Titanium Hydride (TiH2), Zir oniumHydride (ZrH2) and Magnesium Hydride (MgH2). The employment of others blowingagent powders, su h as the arbonates, as a ost-ee tive alternative to metal hydridesblowing agent has been also investigated. If metal hydrides are used as blowing agents,a ontent of less than 1% is su ient in most ases [7, 22, 23.In this simple manner, a homogeneous powder mixture is obtained. Subsequent to mix-ing, the powder blend is hot ompa ted by onventional pressing te hniques (e.g. uniaxialpressing, isostati pressing, extrusion) to a dense, semi-nished produ t alled foamablepre ursor material or pre ursor material in whi h the blowing agent parti les mustbe homogeneously distributed into the metalli matrix. The temperature of this om-pa tion is near to the initial of thermal de omposition temperature of the blowing agent[7, 22, 23. This is the rst main step of this pro ess, the produ tion of the pre ursorBruno Pinto Lopes MIEM Dissertation

10 2.State of the Artmaterial. This foamable pre ursor material an be pro essed into sheets, rods, proleset . by onventional te hniques like rolling or extrusion.

Figure 2.8: Powder metallurgypro ess [24.

Finally, this foamable pre ursor material is heated totemperatures above the melting point of the matrixmetal, resulting in the foam itself. The metal expandsdeveloping a highly internal porous stru ture of losed- ells due to the simultaneous o urren e of the meltingof the metal and thermal de omposition of the blow-ing agent with the release of a gas (e.g. H2 in the aseof metal hydrides). The liquid foam is then ooled inAir, resulting in a solid foam with losed ells and with avery thin dense external skin that improves the me han-i al properties of these materials. The foaming pro essusually takes pla e into the losed moulds with a samedesign and dimensions of the nal foam part [2, 3.The density of metalli foams an be ontrolled by ad-justing the ontent of blowing agent and several otherfoaming parameters. The high quality foams of thesedierent metals an be obtained by hoosing the ap-propriate blowing agent. Moreover, the manufa turingparameters of mixture, the ompa tion and the foaming y le, all have to be hosen arefully [7, 22, 23. Thispro ess an produ e foams with porosities between 75%and 90%.The main advantage of this PM method is the possibil-ity to produ e omponents of metalli foams with dif-ferent ar hite tures (e.g. sandwi h systems, lled pro-les and 3D omplex shaped stru tures) in ompari-son to the others [25, 26. The materials an be mixedduring the foaming step without joining adhesives [22.Other advantage lies in the fa t that the addition of erami parti les are not required, avoiding the brit-tle me hani al behaviour that this parti les infer to thefoams. Moreover, the foam parts are overed by an ex-ternal dense metal skin that improves its me hani albehaviour, providing a good surfa e nish. The disadvantage of this PM pro ess is thehigh ee tive ost of the pro ess whi h is mainly asso iated to the ost of the powders.Another disadvantage is the di ulty manufa ture large volume foam parts. Neverthe-less, sandwi h panel of 2mx1mx1 m an already be manufa tured. Furthermore, it mustbe pointed out that during PM method it is still rather di ult to fully ontrol thefoaming pro ess, whi h results in la k of uniformity of the pore stru ture. The foamsobtained by this manufa turing pro ess present a losed- ell stru ture and an externaldense skin that in reases the me hani al properties of this material [4, 7.Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 112.3.2.2 Foaming of slurriesMetals an be foamed by preparing a slurry of metal powder mixed with a blowing agent.The slurry is poured into a mould after mixing and dried there at elevated temperatures.The slurry be omes more vis ous and starts to foam as gas begins to evolve. If su ientstabilising measures have been taken the expanded slurry an be dried ompletely thusobtaining a metalli foam. Su h foams have been produ ed from aluminium powdersusing orthophosphori a id with aluminium hydroxide or hydro hlori a id as a blowingagent. Relative densities down to 7% have been a hieved but there are problems withlow strength and ra ks in the foamed material [7, 20.2.3.2.3 Gas entrapmentMetals an be foamed without using a blowing agent by ompressing powders to a pre ur-sor material and allowing gas to be entrapped in the metal stru ture during ompa tion(Fig. 2.9). Heating the pre ursor material then leads to an expansion of the metal dueto the internal pressure reated by the entrapped gas. The pro ess has mainly been de-signed for making porous titanium stru tures for some air raft appli ations manufa turerBoeing (USA). For this titanium powder is lled into a ontainer whi h is then eva uatedand relled with argon gas. The lled ontainer is then densied by hot isostati pressing,subsequently worked and nally foamed by means of an appropriate heat treatment.

Figure 2.9: Gas entrapment te hnique. [3.2.3.2.4 Other Te hniquesThere are many ways to make porous metalli produ ts from metal powders, bers, orhollow spheres. The easiest method is to sinter loose powder llings in a anister to givea porous material with open porosity. Mixtures of metal powders and polymer binders an be extruded and then heat treated to produ e porous parts. In this way porousmaterials with ylindri al pores an be produ ed. Rea tion sintering of metal powdermixtures an also yield porous produ ts. Hollow spheres made of titanium or steel an beused to form highly porous stru tures by sintering. Ordered and disordered arrangements an be realized. The density of su h stru tures is extremely low with strengths that areBruno Pinto Lopes MIEM Dissertation

12 2.State of the Artstill su ient for many appli ations. By inltrating the intersti es between the hollowspheres, the strength an be in reased [3.2.3.3 Produ tion by Deposition Te hniquesDeposition te hniques start from the ioni state of metals. The metal is galvani allydeposited on a polymer foam with open ells. This pro ess and the investment astingpro ess therefore have in ommon that the a tual foaming does not take pla e in themetalli state but with a polymer whi h is then repla ed by a metal. Galvani depositionon a polymer foam requires some ele tri al ondu tivity of the initial polymer foam. Thisis a hieved by dipping the polymer foam into graphite solutions or by oating it witha thin ondu tive layer by metal vaporisation. After ele troplating the polymer an beremoved from the metal/polymer omposite by thermal treatment. Foams of variousgrades an be fabri ated ranging from 2 to 30 ells per m (6 to 70 ppi). The preferredmetal is ni kel or a ni kel- hrome alloy but opper foams an also be fabri ated. Foamshave been oered on a ommer ial basis under the name RETIMET (Dunlop Ltd., GB)and CELMET (Sumitomo, Japan). This method is involves high produ tion osts [2.2.4 PropertiesMetalli foams ombine properties of ellular materials with those of metals. For thisreason, metalli foams are advantageous for lightweight onstru tions due to their highstrength-to-weight ratio, in ombination with stru tural and fun tional properties like, rash energy absorption, sound and heat management. Fig. 2.10 shows an overview ofproperties of the metals foams and its position in relation with other materials. In om-parison to polymer foams (for uses in automobiles), metalli foams are stier, stronger,and more energy absorbent. They are more re resistant, and have better weatheringproperties when onsidering UV light, humidity, and temperature. However, they areheavier, more expensive, and non-insulating.Many metals and their alloys an be foamed. Among the metalli foams, Al-alloys foamsare ommer ially the most exploited ones due to their low density, high du tility, highthermal ondu tivity, and ompetitive ost of the metal. The ranges of main propertiesoered by urrently available metalli foams are listed in Table 2.1:Relative density, foam morphology and pore size depend on the manufa turing pro essused to obtain a metalli foam, as shown is Fig. 2.11. These hara teristi s ae t physi aland me hani al properties of the foams [2.

Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 13

Figure 2.10: Properties of ellular and dense engineering materials [2.

Figure 2.11: The range of ell size and relative density for the dierent metalli foammanufa turing methods [27.Bruno Pinto Lopes MIEM Dissertation

14 2.State of the ArtTable 2.1: Ranges for properties of ommer ial foams [2.

Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 152.4.1 DensityThe relative density is the most important hara teristi whi h ae ts me hani al proper-ties of foams, su h as Young modulus, stress plateau and energy absorption. The relativedensity (ρr) is the ratio of foam density (ρ∗) to solid material density (ρs) and is givenbyρr = ρ∗/ρs (2.1)and relates to porosity simply by:

Porosity = 1− (ρ∗/ρs) (2.2)There is no doubt that both the density of a metalli foam and the matrix alloy propertiesinuen e, e.g. modulus and strength of the foam. Figure 2.12 shows the ee t of thedensity on the Young modulus. Density variation and imperfe tions yield a large s atterof measured properties, whi h is detrimental for the metalli foams reliability. Me hani alstudies demonstrate that sele tive deformation of the weakest region of the foam stru tureleads to rush-band formation.

Figure 2.12: Young modulus for dierent foam.2.4.2 Stru tural propertiesThere are several stru tural parameters for hara terising the metalli foams (Fig. 2.14),namely number, size-pore distribution, average size, shape and geometry of the pores,thi kness, interse tions and defe ts in the ell-walls and thi kness, defe ts and ra ksBruno Pinto Lopes MIEM Dissertation

16 2.State of the Artof the external dense surfa e for des ribing the ellular ar hite ture of the foams. Theproperties of the metalli foams are inuen ed by these morphologi al features. Fig. 2.15show some defe ts or imperfe tions of a losed- ell foam that de rease the me hani alproperties [2, 3.

Figure 2.13: List of parameters to des ribe the stru ture of metalli foam [21.

Figure 2.14: Closed- ell Aluminium foam [21.Progress has been made in understanding the relationship between properties and mor-phology. Although this exa t interrelationship is not yet su iently known, one usuallyassumes that the properties are improved when all the individual ells of a foam havesimilar size and a spheri al shape. This has not really been veried experimental. Allstudies indi ate that the real properties are inferior than theoreti ally expe ted due tostru tural defe ts (Fig. 2.14). This demands a better pore ontrol and redu tion instru tural defe ts [3.Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 17Curved, wiggled or missing ell-walls (see Fig. 2.14) mi ropores on the ell edges and ellwalls and non uniformities density are the further imperfe tions degrading the strength,and in turn, result in a redu ed deformation energy absorbed under ompression. Cellmorphology and inter onne tion ould also ae t thermal and a ousti properties. Itis widely a epted that foams with a uniform pores-distribution and defe ts free aredesirable. This would make the properties more predi table. Only then, metalli foamswill be onsidered reliable materials for engineering purposes and will be able to ompetewith lassi al materials. Despite their quality improvement in the last 10 years theresulting metalli foams still suer from non-uniformities. The ell size is large and oftenirregular, and the foams tend to have a marked density gradient (Fig. 2.15). S ientistsaim to produ e more regular stru tures with fewer defe ts in a more reprodu ible way,whi h will be one of the motivations of the urrent resear h in this eld [2, 3.

Figure 2.15: Closed- ell foam with gradient density [21.2.4.3 Me hani al PropertiesThe me hani al properties of the metalli foams have been evaluated using both stati and dynami ompression tests. A simple way to obtain material deformation behaviourunder an imposed load is to perform a stati test. The most ommon types of testingapparatuses employ hydrauli , pneumati or servo-me hani al power to ompress or ex-tend the spe imen. Typi ally, to be onsidered a stati test, the strain rate must beless than 10−1 s−1. The engineering strain rate is a fun tion of rosshead speed andspe imen height. The important properties obtained from the stati tests are the elasti modulus, yield strength, plateau strength and the absorbed energy at a ertain stress orstrain. Dynami testing is performed to determine the behaviour of materials at highstrain rates. This knowledge is important when designing for rash or blast loadings.The deformation under ompressive loads, elasti deformation, ollapse, plateau stressand energy absorption are among widely studied properties of foams [3.2.4.3.1 Stress-strain behaviour under ompressionFoams show a hara teristi stress-strain behavior under ompressive loads. Compressivestress-strain urve onsists of three distin t regions: linear elasti , plateau or ollapse andBruno Pinto Lopes MIEM Dissertation

18 2.State of the Artdensi ation, as illustrated in Fig. 2.16 . The elasti modulus, the plateau stress and thedensi ation strain are the most important me hani al parameters whi h are determinedusing these urves. These me hani al parameters in rease with in reasing the foamdensity, while the defe ts or imperfe tions have detrimental ee ts on the me hani alproperties. The ompressive stress behavior in reases with in reasing the foam density(Fig. 2.17).

Figure 2.16: Compressive stress-strain urves of a) elastomeri , b) elasti -plasti and )elasti -brittle foam.

Figure 2.17: Ee t of foam density on ompressive stress-strain urves [14.Linear elasti region The rst region of the stress-strain urve is linear elasti regionin whi h the stress in rease almost linearly with the strain. The deformation in thisregion is ontrolled by ell wall bending and/or stret hing depending on the stru ture ofthe foams: open or losed ell foam. Open- ell foam of low relative densities (the ratiobetween foam density and solid foam material density (ρ∗/ρs), deforms primarily by ellBruno Pinto Lopes MIEM Dissertation

2.State of the Art 19wall bending. With in reasing relative density (ρ∗/ρs>0.1), ell edge ompression playsa signi ant role. Fluid ow through open- ell foam ontributes to the elasti module ifthe uid has a high vis osity or the strain rate is ex eptionally high. Besides ell edgedeformation, the thin membranes of the losed ell foams, whi h form the ell fa es,stret h normal to the ompression axis and therefore ontribute to the modulus. If themembranes do not rupture, the ompression of the ell uid trapped within the ellsalso in reases the modulus. Ea h of these me hanisms ontributing to the linear-elasti response of the foams is shown s hemati ally in Fig. 2.18 for open and losed- ell foams[2, 4, 7.(a) (b)Figure 2.18: Deformation me hanisms of foams: a) open- ell foam, sequentially ell wallbending, ell wall axial deformation and uid ow between ells and b) losed- ell foams,sequentially ell wall bending and ontra tion, membrane stret hing and en losed gaspressure [7.Gibson and Asbhy (1997) develop a simple ubi unit ell model to predi t the normalizedme hani al properties of the foams, as shown in Fig. 2.19 However, the stru ture andshape of the ells are more omplex than those of the ubi model. The deformation andfailure me hanisms of the ubi model are however quite similar to those of real foamsand therefore it is very useful in predi ting me hani al properties.

(a) Cubi unit ell models of open- ell. (b) Cubi unit ell models of losed- ell.Figure 2.19: Cubi unit ell models of open- ell and losed- ell foams developed byGibson and Ashby [2.Bruno Pinto Lopes MIEM Dissertation

20 2.State of the ArtThe elasti modulus is an important parameter to determine in this region whi h is theslope of this urve region. There are many models to predi t this elasti modulus. Basedon this simple ubi unit ell model, Gibson and Asbhy proposed the following equationof the elasti modulus of the open ell foams (E∗), whi h is al ulated from the linear-elasti dee tion of a beam of length l loaded at its mid point by a load F, is givenas,E∗

Es= C1

ρ∗

ρs(2.3)where s refers to the solid material from whi h the foam is made and C1 is a onstant.The experimental elasti modulus of open- ell foams showed that this onstant (C1) isnearly equal to unity. Gibson and Asbhy proposed the following equation for the modulusof imperfe t losed- ell foams

E∗

Es≈ φ2

(

ρ∗

ρs

)2

+ (1− φ)ρ∗

ρs(2.4)where φ is the fra tion of the material ontaining on ell edges. The elasti modulus ofthe losed- ell foams based on this ubi model that in ludes en losed gas pressure isgiven by

E∗

Es= C1φ

2

(

ρ∗

ρs

)

+ C1′ (1− φ)ρ∗

ρs+

P0 (1− 2ν∗)

Es

(

1− ρ∗

ρs

) (2.5)where φ is the fra tion of the solid whi h ontained in the ell edges having a thi kness(te) and the remaining fra tion (1-φ) in the ell fa es of a thi kness (tf), P0 is the initialpressure of the ell uid and C1 and C1′ are the onstants. The rst, se ond and thirdterms of these equation are the ontribution of ell wall bending, membrane stret hingand en losed gas pressure, respe tively.The Young modulus depends above all on the foam density in whi h their values in reasewith the in rease of the density, as shown in Fig. 2.20.Plateau region Collapse region pro eeds with a stress plateau either with a onstantvalue or in reasing slightly with strain. Linear elasti ity is generally limited to smallstrains. Elastomeri foams an be ompressed mu h larger strains. Deformation is stillre overable, but non-linear. In ompression, the stress-strain urve shows an extensiveplateau at the elasti ollapse stress (σ∗el), see Figure 2.16 (a). Foams made frommaterial that have a plasti yield point su h as rigid polymers and du tile metals ollapseplasti ally when loaded beyond the linear-elasti region. Several deformation me hanismso ur is this region su h as elasti bu kling and brittle rushing of the ell walls andformation of plasti hinges. Plasti ollapse gives a long horizontal plateau in the stress-strain urve similar to the elasti bu kling, but the strain is no longer re overable. Bothelasti bu kling and plasti failure are lo alized; a deformation band is usually formedtransverse to the loading axis and propagates through undeformed se tions of the foamwith in reasing strain until all the foam se tion is lled with the band.The plateau tension (σpl) is determined taking into a ount the beginning of the plateauBruno Pinto Lopes MIEM Dissertation

2.State of the Art 21

Figure 2.20: Variation of Young's module with density [3.region where it initiates the plasti regime. Trying to estimate the value of the plateautension, an equation was introdu ed by Gibson and Ashby. This equation takes intoa ount the study of a foam with a ubi geometry and with a losed stru ture. Thisstress depends on the density of the foam and the yield strength (σys) of the solid [3, 7, 14.σpl

σys≈ 0.3

(

φρ∗

ρs

)2

3

+ 0.4 (1− φ)ρ∗

ρs(2.6)Where σpl is the plateau stress.The plateau stress also in reases with the foam density in rease (see Fig. 2.17). The de-termination of the plateau stress value is important to hara terize the energy absorption apa ity of foams [21.Densi ation region Following the plateau region, at a riti al strain, the ell wallsstart to tou h ea h other and the densi ation region begins. The stress in this regionin reases rapidly and approa hes to the strength of the solid foam material. Foam den-si ation o urs at the densi ation strain (εD). The point at whi h the foam starts thedensi ation is not well dened and the denition is found to vary between resear hers.Common methods of determining the densi ation strain (or stress) based on the stress-strain response are by visual inspe tion, at a stress 1.5 times the stress value at a strain of0.5. Other method utilizes the point of interse tion of the slope of the plateau region andthat of the densi ation region as the point of densi ation. Other resear hers hoose anarbitrary strain and utilize this as the densi ation point. In stru tural appli ations thedensi ation point is not as important as the yield point. However in energy absorptionappli ations the point of densi ation is important and ompression beyond densi ationis to be avoided due to the sharp in rease in stress. The densi ation strain (εD) alsoin reases with in reasing of the relative density, as illustrated in Fig. 2.17).Bruno Pinto Lopes MIEM Dissertation

22 2.State of the Art2.4.3.2 Energy Absorption under ompressionThe possibility of ontrolling the stress-strain behaviour by an appropriate sele tion ofthe matrix material, ellular stru tures and relative density makes metalli foams anideal material for energy absorbing stru tures. The quality of the energy absorbers isdened by the ability to absorb energy without the maximum or the highest o urringa eleration ex eeding the upper limiting at damages o ur.The long stress plateau typi al of the stress-strain urves of metalli foams gives rise toex ellent energy absorption properties [2. Damage to an obje t is aused when a riti alfor e (or a eleration) level is ex eeded [2. The ability to absorb energy at a for e belowthis riti al level is paramount to the prote tion of the obje t [2. The for e of an impa tis dire tly related through geometry to the stress in the foam [2.Figure 2.6 shows whyfoams are mu h better than solid materials at providing damage prote tion. For a givenstress foams always absorb more energy than a solid due to the bending, bu kling andfra ture of the foam ell walls [2.

Figure 2.21: Stress-strain urves for both an elasti solid and a foam showing the dier-en e in energy per unit volume absorbed at the same stress level [2.The ompression of Aluminium foam under an applied for e results in work. The workper unit volume (W) up to a strain of ε is the area under the stress-strain urve. It is al ulated in the following manner:W =

∫ ε

0

σ (ε) dε (2.7)Little energy absorption o urs in the initial linear elasti region. Energy absorption inthe plateau region a ounts for the majority of energy absorbed by the spe imen. Plasti deformation of Aluminium foams at near onstant stress in the plateau region translatesinto energy dissipated at near onstant stress. Ideally for better energy absorption thestress-strain urve would be perfe tly horizontal from ε = 0 to εD. After densi ationBruno Pinto Lopes MIEM Dissertation

2.State of the Art 23in rease in energy absorption is a ompanied by large stress in reases.Considering impa t onditions (i.e. strain rates in the range of 101 to 104 s−1), there isan optimum foam density to absorb energy e iently [2. This is illustrated in Figure2.22 where three foams of dierent relative densities. The stress and strain at whi h anamount of energy W is absorbed is shown for all three densities. This shows that if tooweak of a material is hosen the required amount of energy absorbed is more than thatunder the plateau, the foam densies and the for e in reases sharply before all energyis absorbed. If however too strong of a material is hosen the load be omes too largebefore all the required energy is absorbed. The most e ient foam is shown to have themiddle density where the full plateau is employed in energy absorption. The middle foamabsorbs the amount of energy W at the lowest peak stress.

Figure 2.22: Stress-strain urves of foams for three dierent density [28.The e ien y of the foams energy absorption an be al ulated omparing the obtained urve of foam ompression with an ideal urve (Fig. 4.40). This ideal urve has are tangular urve that represents always the same ompressive stress. The determinationof the absorption e ien y is a ne essary pro edure to hara terize the energy absorptionin foams. Energy absorption e ien y (ηef ) an be al ulated by the following equation[7, 21:ηef =

∫ s′

0F(

s′

)

ds

Fmax (s′) ds

(2.8)where F is the applied for e, s' is the deformation and Fmax is the maximum for e appliedabove the deformation s′.The hoi e of energy absorbers an be made through the energy absorption diagrams (Fig.2.24). For that, samples of foam with a range of densities are tested in ompression, at axed strain rate and temperature. The area under ea h urve is measured and representsthe energy absorbed per unit volume (W/V). Thus, the diagram onsist to plot the value(W/V) against σp, for ea h urve, normalising both by the solid modulus measures atBruno Pinto Lopes MIEM Dissertation

24 2.State of the Art

Figure 2.23: Comparison of the ideal absorber and a real absorber [7.standard strain rate and temperature. Ea h foam has a ompressive stress for whi h itis the best hoi e. It is assumed that a near ideal foam absorbs a given at a minimumstress.

Figure 2.24: Maximum stress o urring when a given deformation energy is absorbed byAluminium foams with various densities [23.Energy absorption is ae ted by the shape of the stress-strain urve. The in rease inplateau stresses due to in reased relative densities result in higher values of energy ab-sorbed. However, the energy absorption e ien y is unae ted by relative density but hanges with loading dire tion in anisotropi foams. The defe ts in losed ell foamsde rease the energy absorption e ien y. Density gradients within the foam also de- rease the e ien y of energy absorbed. By areful ontrol of foam produ tion are ableto produ e losed- ell AlporasR with small ell sizes and, redu ed density gradients, ellwall urvature and orrugation. The modied foam with fewer defe ts not only showsan in rease in energy absorption over the unmodied foam with similar relative density,it also displays an in rease in energy absorption e ien y.Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 252.4.4 Thermal propertiesThe main thermal properties are the melting point, spe i heat, thermal expansion oe ient, thermal ondu tivity, surfa e emissivity and thermal sho k resistan e. Theseproperties in the foams are similar to the base material properties. However, there isfoams that these properties depend on their ellular stru tures.The metalli foams an be asso iated to a low thermal ondu tivity due to their ellularstru ture parameters. The pore size and the volume fra tion lead to a suppression of the onve tion and radiation ee ts, whi h makes metalli foams an interesting material touse in appli ations on erning thermal insulation [7. Thermal insulation is the redu -tion of the ee ts of the various pro esses of heat transfer between obje ts in thermal onta t or in range of radiative inuen e. The thermal ondu tivity of metalli foamsis dire tly related to the outer surfa e of the foam and is, thus, also dire tly related tothe ondu tivity of the base material. The value of thermal ondu tibility is also relatedto the density value, and the value in reases with in reasing foam density. The thermal ondu tivity (K) is given by the Fourrier law [29:q = −K.A

dT

dx(2.9)Where (q) is the rate of heat transfer, (dT

dx) is the heat transfers gradient and A is these tion area of the heat ondu tion. As referred above, thermal ondu tivity (K) analso be given approximately by an equation relating with foam density and base materialdensity [3:

K = Ks

(

ρ∗

ρs

)r (2.10)Where Ks is thermal ondu tivity of the base metal, K∗ is the thermal ondu tivity ofmetalli foams and r is the value between 1.65 and 1.8. Figure 2.25 shows the thermal ondu tivity of available metalli foams.2.4.5 A ousti propertiesThe ombination of good a ousti and me hani al properties makes metalli foams par-ti ularly attra tive produ ts. Three ases an be distinguished with respe t to the noiseattenuation materials:(i) materials for sound insulation: any means of redu ing the intensity of sound. Theperforman e of sound insulation material is des ribed in terms of sound redu -tion index whi h is proportional to the re ipro al value of the sound transmission oe ient in logarithmi s ale.(ii) materials for sound absorption: Absorbing sound spontaneously onverts part ofthe sound energy to a very small amount of heat in the intervening obje t (theabsorbing material), rather than sound being transmitted or ree ted. There areseveral ways in whi h a material an absorb sound. The hoi e of sound absorbingmaterial will be determined by the frequen y distribution of noise to be absorbedand the a ousti absorption prole required.Bruno Pinto Lopes MIEM Dissertation

26 2.State of the Art

Figure 2.25: Thermal ondu tivity plotted against volumetri spe i heat for urrentlyavailable metalli foams [3.(iii) materials for damping an redu e the a ousti resonan e in the air, or me hani alresonan e in the stru ture of the room itself or things in the room.The a ousti absorption properties of metalli foams depend mainly on foam properties(parameters) su h as porosity, pore morphology, pore size and air-ow resistan e. Closed- ell metalli foams are poor sound absorbers but both open metalli foams are good soundabsorbers and they are urrently used for noise redu tion. Open- ell foams are highlyee tive noise absorbers a ross a broad range of medium-high frequen ies. Performan e islower at low frequen ies. The best sound performan e was obtained using metal spongeswith open- ell stru ture (Fig. 2.26).The losed- ell metalli foams are too sti to onvert sound energy into heat by vibratingof their ell walls. However, the sound absorption performan e of losed- ell aluminumfoams an be signi antly improved by optimal opening of losed- ell stru ture usingpro esses like hole drilling, rolling and ompression (Fig. 2.27). In order to maintain thebest absorption values of the hosen materials, the air hannels should all be open to thesurfa e so that sound waves an propagate into the material. If pores are losed, as in losed- ell foam, the material is generally a poor absorber.Sound absorption materials ontrol airborne noise by redu ing the ree tion of soundfrom the surfa e boundaries thus redu ing the overall noise levels. Sound is attenuatedby vibration and fri tion losses by the repeated ree tions within the ell stru ture wherefull absorption is possible. Sound absorption properties of foams however an be variedsu h that foams an only be used to eliminate ertain frequen ies. Metalli foams also an be used in a wide variety of noise ontrol treatments su h as ma hinery en losures,plant ma hinery, walls, in automobiles for engine, tire, door noise and sides of roads toBruno Pinto Lopes MIEM Dissertation

2.State of the Art 27

Figure 2.26: Sound absorption oe ient of various types of ellular Al foams in om-parison with ber lass sound absorbing material [3.

Figure 2.27: Sound absorption oe ient as fun tion of opened surfa e area for Alulightfoam (8.9 mm thi k AlSi12 foam, density 580kg/m3, holes drilled through the sample,20 mm air gap between sample and rigid wall) [3.Bruno Pinto Lopes MIEM Dissertation

28 2.State of the Artredu e tra noise. Al-ally foam is normally employed as an internal lining to ma hineryguards or en losures.The damping apa ity of foamed Aluminium has been shown to be an order of magnitudehigher than that of the bulk metal. Stru tural damping is the ee t of internal fri tionwithin hanging a material vibration energy into heat. This redu es ex essive noise andvibration by onverting them into to be expelled into the surrounding area. Vibrationenergy in ellular stru tures is dissipated by the slight plasti deformation of the thinwalls separating the pores. It an also be redu ed by fri tion between the surfa es of ra ks appearing in the pore walls. The damping fa tor is usually determined from thede ay of the vibration response amplitude at resonant frequen y as shown Figure 2.28.

Figure 2.28: Damping fa tor and vibration behavior for Alulight foam [3.The Alporas foams already has been applied ommer ially in sound absorbing stru turesfor underside monorails, soundproong auditorium, sound absorbing stru ture, soundabsorbing wall panels at the entran e of tunnels in Japan. Internal noise in all vehi lesis primarily a question of omfort for drivers and passengers. The Al-alloy foams is apossible sound absorbing materials.2.4.6 Ele tri al propertiesThe metal omposition of metalli foams makes possible their ele tri al ondu tivity, butthe existen e of pores auses the ele tri al ondu tivity to be onsiderably lower thanthe ondu tivity of the metal from whi h it originates. Nevertheless, the foam metal ontinues to make a good earth onne tion.The main ele tri al property to retain is the foam's resistivity (R∗). Depending on thefoam's density and the base material resistivity (Rs), the resistivity of metalli foam isshown in following equation [3, 7:Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 29R∗ ∝ Rs

ρsρ∗

(2.11)Where the ρs and ρ∗ are the base metal material and metalli foam density respe tively.The ele tri ondu tivity is a non linear fun tion of density, as illustrated in Fig. 2.29.

Figure 2.29: The power law dependen e of normalized ele tri ondu tivity of Al-foamson the relative density for Alulight foam [3.Closed- ell metalli foams overed with dense skin have a good ele tri al ondu tivitythat minimises the penetration of ele tromagneti waves in to the stru ture. This fa t an be su essfully used for the prote tion of ele troni devi es or humans for the ee t ofele tromagneti noise. The main advantage of metalli foam is the possibility to a hieve athi kness required for low magneti permeability at lower weight omparing with the bulkmetals. The results have been demonstrated that the shielding ee tivity of the metalli foam is even slightly better than the metal plate of the same thi kness, as illustrated inFig. 2.30.2.5 Appli ationsDepending of the ellular stru tures- open or losed ell - metalli foams have a large rangeof industrial appli ations. Moreover, the size of the ellular pores ould be adjusted forea h appli ation. Generally, the identi ation of the feasibility of a possible appli ationhas taken into a ount the following required fa tors: Morphology: ellular stru ture (open or losed- ell), porosity level, total internalsurfa e area; Metallurgy: hemi al omposition of the metal or the alloy and mi rostru ture ofthe base material; Pro essing: Choi e of manufa turing pro ess attending the geometry, dimensionsrequired for the omponent (3D-parts, panels or proles); E onomy: ost issues, suitability for large volume produ tion;Bruno Pinto Lopes MIEM Dissertation

30 2.State of the Art

Figure 2.30: Magneti eld shielding ee tiveness as a fun tion of frequen y for Alu-minium foam (Alulight) and steel samples of the same weight and a massive Al sheetof the same thi kness (t=8.5 mm) as the Aluminium foam. Sample size: 140x140 x t,density of foam 500kgm−3 [3.The type of base material to be used depends on the nal appli ation. Aluminium alloysare widely used in engineering stru tures and omponents where light weight or orrosionresistan e is required. Titanium and its alloys are preferred for medi al appli ations dueof its ompatibility with tissue. The manufa turing pro ess must be hosen the a ordingto the omplexity of the nal omponent. The PM method allows the possibility to pro-du e omponents of metalli foams with dierent ar hite tures (e.g. sandwi h systems,lled proles and 3D omplex shaped stru tures). But, the foaming melt te hniques anprodu e a large blo ks of foams in ontinuous produ tion rates [4.The main appli ation areas of metalli foams are related to their ellular stru ture (open- ell and losed- ell stru tures). Metalli foams an also be lassied by their stru turalor fun tional appli ation, as shown in Fig. 2.31 [7, 14.The open- ell metalli foams are usually used in fun tional appli ations in the hemi alindustry as lters, storage reservoirs uids, heat transfers, heat shielding for air raftexhaust, ompa t heat ex hangers, liquid heat ex hangers, air- ooled ondenser- oolingtowers and regenerators for thermal engines.The losed- ell metalli foams are mainly used in stru tural appli ations in the transportindustry as lightweight, energy-absorption and damping stru tures (Fig. 2.32). Metalli foams in parti ular, Al-alloy foams are used in stru tural appli ations where weightis of parti ular on ern, e.g. ar bodies, doors and foot panels or portable ele troni devi es. Other potential appli ations in lude ships, buildings, aerospa e industry and ivil engineering. Examples of the foam panels produ ed to be used in automobiles areshown in Figure 2.33 [4, 14.This work was planned to investigate about losed- ell foams, therefore some appli ationsof this kind of foams are presented:Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 31

Figure 2.31: Appli ations of ellular metals grouped a ording to the degree of opennessneeded and whether the appli ation is more fun tional or stru tural [2.

Figure 2.32: Appli ations elds for losed- ell metalli foams [4.Bruno Pinto Lopes MIEM Dissertation

32 2.State of the Art

Figure 2.33: Automotive omponents [14.2.5.1 Automotive industryDue to several issues, mainly e onomi fa tors and governmental treaties, the implemen-tation of new te hnologies and weight redu tion in the manufa ture of automobiles hasbeen heavily studied. To follow all the standards required, automobile manufa turershave been implementing metal foams in the automobiles manufa ture to de rease theoverall weight of automobiles (Fig. 2.34) , to in rease passive safety with the large a-pa ity of absorbing energy that metal foams present, and in rease omfort by the abilitythat foams have to absorb the sound. Figure 2.35 shows some real appli ations of theAluminium alloy foams into transport se tor. [7, 23, 30.Currently, many ities are fa ing the problem of ar tra , whi h does not fa ilitate thelives of the people living in these big ities. To solve all these problems ar manufa turershave redu ed ars size. But the redu tion in the size of ars, although not ae ting thepassengers omfort, has been ae ting the dimensions of the luggage ompartment andthe engine ompartment. The de rease of the dimensions of the engine ompartmenthas brought some problems with the heat of the engine, whi h ould ompromise thesafety of the ar. The appli ation of metal foams in these ompartments with en losedstru ture is believed to be a viable solutions to the existing problems [2, 4, 7, 23.The high safety fa tors imposed by various agen ies lead automobile manufa turers toanalyse new te hniques and new materials in the manufa ture of automobiles. Theex ellent energy absorption by the metal foam formant losed, was taken into a ount inthe implementation of metal foam blo ks at spots/parts of potential impa t [7, 23.The use of metal foams in a ar at a stru tural level (inside the frame) leads to severalimprovements on omfort and safety derived from the properties presented on foams(a ousti damping, heat dissipation and longer listed apa ity of energy absorption)[7, 23.2.5.2 Aerospa e industryThe aerospa e industry is a major leader on the development of a wide range of endurable(material). The resear h developed in the aerospa e industry ontributes greatly to thedevelopment and implementation of new materials and te hnologies in various appli ationBruno Pinto Lopes MIEM Dissertation

2.State of the Art 33

Figure 2.34: Implementation of metalli foams in a ar [31.

(a) Front impa t energy absorber for COM-BINO tram. (b) Stiener in door sill ar.

( ) Crash element in safety guard.Figure 2.35: Real automotive appli ations for Al-alloy foams [32.Bruno Pinto Lopes MIEM Dissertation

34 2.State of the Art

Figure 2.36: Ariane ro ket one: prototype assembly. 3D foamed AFS 6061 + AlCu6Cu6[32.areas [7.When it omes to the aerospa e industry, the foam honey omb stru ture has been inlieu of Titanium and Aluminium foams. The possibility of obtaining three-dimensionalgeometry foams makes this substitution a big advantage ompared to the honey ombstru tures applied to turbine blades and seals (Fig. 2.36) [4, 7.The metal foams are also used as landing pads for spa e vehi les and the strengthening ofsample stru tures in satellites due to its apa ity of energy absorption and its resistan eto adverse environmental onditions [4, 7.2.5.3 Shipbuilding industryNowadays, the use of extruded Aluminium, Aluminium foil and honey omb Aluminiumstru tures for the manufa ture of passenger ships is quite large. The use of foam sandwi h ore with Aluminium has been tested for the repla ement of the urrently used materials.There are many uses for metal foams in stru tural appli ations of this type of industry,su h as platform lifts, stru tural bulkheads, de ks and ompartments of pyrote hni antennas [4, 7, 33.

Figure 2.37: Sandwi h panel metal foams [33.Bruno Pinto Lopes MIEM Dissertation

2.State of the Art 352.5.4 Railway industryThe appli ations in this industry are similar to the automobile industry. The passengers omfort obtained through noise isolation and the lightweight stru ture are used to themanufa ture of trains. In Japan trains are being equipped with foam blo ks of Aluminiumin absorbing energy zones to de rease the damage possibility in ase of an a ident [7, 14.2.5.5 Building industryThere is a wide range of possible appli ations in the buildings industry. As modern o eare made of on rete, their façades are de orated with panels whi h hide the on reteand improve the appearan e of the building 2.38a. Aluminium foams or foams panels ould be very helpful in redu ing the energy onsumption of elevators. The lightweight onstru tion obtained with this material is an important issue to this appli ations duethe a eleration whi h the elevators are submitted. In building industry metalli foamsalso are used in lightweight redoors and hat hes make use of the relatively poor thermal ondu tivity and re resistan e of some of the low density Aluminium foams [4. Metalfoams an be used as soundproong materials for building appli ations (Fig. 2.38b.

(a) Wall Cladding into an apartment [34 (b) Sound Proof Panels of AluminiumFoam [35Figure 2.38: Building appli ations for Al-foams.

Bruno Pinto Lopes MIEM Dissertation

36 2.State of the ArtThis page intentionally left blank

Bruno Pinto Lopes MIEM Dissertation

Chapter 3Experimental Pro edures3.1 Materials3.1.1 Powder spe i ationsThree ommer ial Aluminium alloy (Al-alloy) powders and two Titanium Hydride (TiH2)powders (blowing agent) were used in this resear h work. Their main hara teristi sprovided by the manufa turers are summarized in tables 3.1 and 3.2, respe tively. TheAl-alloy and TiH2 powders are identied with a nomen lature a ording to its size rangeto fa ilitate the presentation and interpretation of results. Thus, Al-A alloy, Al-B alloy,and Al-C alloy represent the Al-alloy powders (Table 3.1) and TiH-1 and TiH-2 are usedto identify TiH2 powders (Table 3.2). These powders were sele ted in order to studythe possibility to use urrent ommer ial powders of low ost and thus ontribute to theredu ing of the overall ost of the PM pro ess used to produ e Al-alloy foams (i. e. oneof the disadvantages of this PM pro ess) As it is known, the appropriate sele tion ofthe metal and blowing agent powders with respe t to purity, parti le size and alloyingelements is essential for su essful foaming. Thus, it what this work is on erned toadjust the manufa turing parameters to produ e Aluminium foams with good quality,using these powders be omes a hallenge. The powders were oered by the ompaniesAlpo o and Chemetall, for the Al-alloy and TiH2, respe tively.Table 3.1: Supplier spe i ation of the Al-alloy powders.

37

38 3.Experimental Pro eduresTable 3.2: Supplier spe i ation of the Titanium Hydride powders.

3.1.2 Preparation of blowing agents powdersBoth Titanium Hydride powders were studied as untreated (as re eived) and pre-treated onditions. The heat treatments of TiH2 powders were performed in order to shift thede omposition temperature to higher levels. The main obje tive was to ompare the har-a teristi s of gas release under dierent onditions, and the morphology of the formedfoam. For the both TiH2 studied, the optimized oxidation pre-treatment reported byMatijasevi was used [36. During the heat treatments of TiH2, the oxidation of surfa eo urs and the oxide ompounds of titanium overs the parti les. This oxide lm a ts asa barrier and hinders the de omposition of hydrogen [3638.In the literature, heat treat-ments are performed with loose TiH2 powders as well as the powder ompa ts of blowingagents. Bat hes of 3 g of untreated powder of blowing agent inside an alumina ru iblewere introdu ed into a pre-heated furna e at 480C during 3 hours. The powders wereoxidized in air. After this, the ru ible ontaining the TiH2 powder was removed and ooled down to ambient temperature. Figure 3.1 shows the hange of the olours for theboth titanium hydride studied. Thermal analysis of these TiH2 powders untreated (as re- eived) and pre-treated was performed in order to study the de omposition temperaturesat whi h H2 is de omposed.(a) TiH-1 (b) TiH-1 (T)( ) TiH-2 (d) TiH-2 (T)Figure 3.1: Titanium Hydride powders: As-re eived (left side) and treated (right side).Bruno Pinto Lopes MIEM Dissertation

3.Experimental Pro edures 393.1.3 Preparation of pre ursor materialsFoamable pre ursor materials were prepared by mixing Al-alloy and TiH2 powders ineither the untreated state and after treated. For that, 50g of a powder mixture ontaining0.6 wt.% TiH2 were prepared into a plasti bottle (Fig. 3.2). The plasti bottle ontainingthe powder mixture was pla ed into a tumbling mixer (g.3.3) in order to produ e ahomogeneous distribution of all the omponents during 30 minutes.

Figure 3.2: Plasti bottles ontaining the dierent prepared powders mixture.

Figure 3.3: Tumbling mixer used in this resear h to mix the powders.After mixing, the powder blend was subje ted to uni-axial ompression ( old and hot)using a hydrauli press with a apa ity of 200bar (Fig. 3.4). The pressing devi e anda sket h of the die are shown in Fig. 3.12a and 3.12b, respe tively. To avoid the riskof sti king of the pre ursor to the inner wall of the mould during hot ompa tion, a oating spray namely Molykote (supplier Kraft) was used and dried before pouring thepowder into the 30 mm diameter ylindri al steel die, to form a lm between the pre ursormaterial and mould wall that redu e fri tion and wear of the die. The pressing parameterswere adjusted in order to obtain a foamable pre ursor material with a theoreti al densityBruno Pinto Lopes MIEM Dissertation

40 3.Experimental Pro edures losed to 100% theoreti al density of the Al-alloy matrix, to guarantee that there isno residual open porosity. In this resear h, dierent ombinations of the old and hotpressing parameters were tested using various ompa tion onditions.The mixture of powder, was old pressed by applying 200bar during a given time. Afterthat, the old-pressed sample, die and pun hes together were heated up to 400C using anele tri heater. Temperature was measured via a k-type thermo ouple in a hole lo atedbetween the inner and outer diameter of the die. The heating time was approximately25 minutes. Upon rea hing 400C the temperature was maintained at that temperatureusing a Eurotherm temperature ontroller during a xed time, and the sample was againpressed using the same pressure for a given time. The parameters of pressing wereadjusted using an Al-alloy powders without blowing agent.After the onsolidation of the powder mixture, the densities of the pre ursor materialswere al ulated using geometri method.

(a) (b)Figure 3.4: Overview of the system in the laboratory to manufa ture the pre ursormaterials. Uni-axial pressing devi e with the die and the heating system. Pressingdevi e used for the pressing the powder mixture.3.1.4 Preparation of Al-alloys foamsThe preparation of the Al-alloy foams was made by heating the pre ursor material ata temperature up to the liquidus temperature of the metalli matrix inside a preheatedfurna e supplied by M.J.Amaral (Fig. 3.5). The foaming time varies between 7 to 10minutes depending on the pre ursor weight and the foaming temperature. This expansiono urs due to the release of Hydrogen from thermal de omposition of blowing agent andthe metal melts. The gas released by T iH2 forms bubbles inside the vis ous melt andexpands the metalli matrix. The foaming tests were arried out in a pre-heated furna eat dierent temperature. For ea h pre ursor material, the furna e temperature wasadjusted.Bruno Pinto Lopes MIEM Dissertation

3.Experimental Pro edures 41

Figure 3.5: Furna e used for the manufa ture of Al-alloy foams.3.2 Chara terization MethodsThe hara terization of the powders be omes an essential step towards a better under-standing of the powder hara teristi s and their properties. The hara terization of thepre ursor material is important to assess the quality of the obtained samples. The pres-en e of Hydride Titanium distributed evenly in the metal matrix and its high densityare good indi ators of a good quality pre ursor material. The quality of the Aluminiumfoams is determined by their me hani al and morphologi al hara terization.To a hieve all these properties through the hara terization of both the powders andthe pre ursor material and foam, several devi es were used and will be dis ussed furtherahead.3.2.1 Powder analysisThe hara terization of the powders was arried out on various parameters, su h as den-sity, parti le size analysis and morphologi al analysis. These pro esses will be explainedin the following se tions.3.2.1.1 DensityThe density of the powders was measured in an existing equipment Multipy nometerfrom Quanta rome (g. 3.6). This equipment measure the true volume of the powders.This te hnique employs the Ar himedes prin iple of the uid displa ement to determinethe volume. The displa ed uid is Helium gas whi h an penetrate the nest pores toassure maximum a ura y. This equipment determines the true density of the powdersamples by measuring the pressure dieren e when a known quantity of Helium underpressure is allowed to ow from a pre isely known referen e volume (Vr) into a sample ell ontaining the powered solid. The al ulation formula for determining the volume isas follows in equation 3.1:Vp = Vc − Vr

[

P1

P2

− 1

] (3.1)Bruno Pinto Lopes MIEM Dissertation

42 3.Experimental Pro edureswhere the Vp is the true volume of the sample, the Vc and Vr are onstant for the alibration of equipment, the P1 is the referen e pressure and the P2 is the real pressurein the sample.Two measurements were measured for ea h powder.

Figure 3.6: Py nometer used during analysis.3.2.1.2 Parti le size analysisParti le size analysis of the powders was performed by light s attering using a Parti leSize Analyzer (Coulter LS 230, Miami, FL, Fraunhofer opti al model). This equipmentmeasures parti le size distribution using the prin iple of laser dira tion. Fraunhofermathemati al model was used by default to al ulate the size of parti les based on the on entri ring pattern of light-s atter from the laser beam. The parti les dira t thelaser light that is dire ted through the ell. The s attered light then is dete ted and olle ted by a sensor to obtain the distribution of the parti les. The parti le sizes obtainedare re orded as mean diameters using volume distribution data.A sample pla ed in the uid module is ir ulated through a sample ell at onstant speed.A beam of laser light shone through the ell is dira ted by parti les within the sample,and the forward s attered (or dira ted) light is olle ted by a series of dete tors. TheLS 230 in ludes another measurement assembly, alled polarization intensity dierentials attering (PIDS). The PIDS assembly onsists of an in andes ent light sour e and po-larizing lters, a PIDS sample ell and an additional seven photodiode dete tors (six tomeasure s attered light plus one to monitor the beam strength).To measure the parti le size distribution, the powder suspension was prepared. A smallquantity of powder was suspended in 10ml of deionized water and was ultrasoni allydispersed for less than 5 min. The suspension was afterwards gently poured into theuid module lled with ltered and degassed water. The uid ir ulates through thesample ell during the analysis to make sure that all parti les are entrained into the owand ensuring a random orientation of the parti les. The adequate amount of sample wasdetermined by the obs uration per entage of the laser beam through the sample ell.Bruno Pinto Lopes MIEM Dissertation

3.Experimental Pro edures 43

Figure 3.7: LS 230 Laser Dira tion Parti le Size Analyze from Be kman Coulter.The parti le size distribution of the volume statisti was obtained dire tly from the Coul-ter LS 230 software. Range statisti al parameters to hara terise the parti le distributionwas obtained, in luding the mean, the median, mode, degree of symmetry or distortionto one side of the distribution average and kurtosis (degree of peakedness). In addition, umulative per entile values D10, D25, D50, D75 and D90 were obtained whi h orre-sponds to the parti le sizes below 10%, 25%, 50%, 75% and 90% of the total volume ofparti les analyzed, respe tively. The D90-D10 range is a measure of the dispersion ofthe sample distribution, with lower values indi ating a better-sorted material. Parti lesize and size distribution data an be represented in either a tabular or graphi al form.Graphi al representation of size and size distribution results were obtained as dierentialdistribution or umulative distribution of the size parameter, as a fun tion of the size.3.2.1.3 Thermal analysisThe thermal analysis of the powders was performed to study the de omposition of blowingagent powders as a fun tion of temperature or time under Air and Oxygen atmosphere.The tests were ondu ted using thermal equipments with a heating rate of 10C/minunder Air and Oxygen. A Setaram Labsys RO TG-DTA16 equipment (Fig. 3.8a) whi hallowed thermogravimetri (TG) measurements oupled with dierential thermal anal-ysis (DTA) were used for the tests arried out on an air atmosphere. A simultaneousThermal Analysis (STA) was also arried out on STA 449 C Netzs yh equipment (Fig.3.8a) to measure the simultaneous of Thermogravimetry (TGA) and Dierential s anning alorimetry (DSC) under Air atmosphere. The TGA, DTA and DSC are distinguishedfrom one another by the property whi h is measured the mass, dieren e temperatureand the heat dieren e. This last equipment was used to study the de omposition ofblowing agent powders as a fun tion of temperature or time under air atmosphere. Dur-ing measurements a onstant amount of 25 mg of either the untreated or pre-treatedblowing agent powders were lled into the Alumina ru ible whi h was pla ed on top ofa erami sample arrier, as shown in Fig. 3.8.Dierential s anning alorimetry (DSC) an be used to measure the enthalpy of transi-tions and the heating apa ity of materials. Thermogravimetri analysis (TG) or thermalgravimetri analysis (TGA) is a type of testing performed on samples that determinesBruno Pinto Lopes MIEM Dissertation

44 3.Experimental Pro edures

(a) TG-DTA (b) STAFigure 3.8: (a) Thermal analyser LabsysTM TG-DTA16. (b) Simultaneous ThermalAnalyser STA 449 C Netzs yh. hanges in weight in relation to hange in temperature. Su h analysis relies on a highdegree of pre ision in three measurements: weight, time, temperature, and temperature hange. As many weight loss/mass gain urves look similar, the weight loss/mass gain urve may require transformation before results may be interpreted. A derivative weightloss/mass gain urve an identify the point where gain or weight loss is most apparent.3.2.1.4 X-ray dira tion analysisA dira tometer model equipped with graphite mono hromator and a s intillation ounterwas used to determine the phases present in the TiH2 powders before and after pre-treatments in Air at 480C during 3 hours. The phases present in TiH2 before and afterpre-treatment in Air, as well as after the DTA/TG analysis were determined by XRD.Phase identi ation of the powders before and after pretreated was determined by X-raydira tion (XRD, using a high-resolution Rigaku Geigerex D/Ma , C Series dira -tometer, Tokyo, Japan). A s anned the dira tion angles (2θ) between 10 and 80 inthe steps of 0.02. An a quisition of 3/ min time using CuKα radiation was su ientfor su essful phase analysis. The XRD patterns were evaluated using a JPC standarddira tion database. Crystallographi identi ation of the phases of powders was a om-plished by omparing the experimental XRD patterns with standards ompiled by theJoint Committee on Powder Dira tion Standards (JCPDS).3.2.1.5 Hot-stage mi ros opy analysisThe thermal behaviour of the Al-alloy powders used in this resear h was ondu ted onHot stage mi ros ope (Leitz, 2A model) equipped with a furna e that working up to a1750C, with dire t and / or oblique lighting (Fig. 3.9). The heater is mounted on a ompound mi ros ope. The temperature is measured using a thermo ouple (PtRd 30-PtRd 6%, B type) inside a pyrometri reed on whi h to pla e the holder with the sample.The mi ros ope is equipped with three lenses with magni ations of 2.4, 3.2 and 7 timesrespe tively. The External and/or ba k-lighting is used to illuminate the sample. A CCD amera is mounted on the mi ros ope to visualize and to re ord the images. The sampleswere previous prepared using a old uni-axial pressing to obtain a ube with 3 mm inBruno Pinto Lopes MIEM Dissertation

3.Experimental Pro edures 45edge. The experiments are performed at a heating rate of 10C/min.

Figure 3.9: Hot-Stage Mi ros opy (Leitz, model 2A).3.2.2 Mi ros opi analysis3.2.2.1 Light mi ros opy (LM) analysisThe mi rostru ture of the pre ursor materials and the Al-alloy foams were observed onlight mi ros ope Nikon E lipse LV150, supported with software Perfe t-Image Version7.5 developed by Clara Vision (Fig. 3.10).

Figure 3.10: light mi ros ope Nikon E lipse LV150.Samples for LM were prepared by using standard metallographi preparation te hniques:grinding with a sili on arbide paper 500-4000. End polishing was done with 1µm dia-mond past at DiaPro (Polishing on MD-Nap and DP-Nap) by Struers.Bruno Pinto Lopes MIEM Dissertation

46 3.Experimental Pro edures3.2.2.2 S anning ele tron mi ros opy (SEM) analysisThe morphology of powders was hara terised by SEM using a Hita hi (SU-70 model)operated at 0, 1to30 kV (Fig. 3.11). Stru ture analysis of the powder parti le is very im-portant to evaluate and optimise the pro ess of the powder produ tion whi h ae ts theproperties of the nal produ t. This te hnique was also used to visualize the morphologyof the surfa e, the presen e of satellite, impurities and agglomerates on the pre ursormaterials. EDX analysis was used in order to get detail information about the ontent ofoxygen around individual TiH2 powder parti les. S anning Ele tron Mi ros opy oupledwith Energy Dispersive X-ray. Energy-dispersive X-ray spe tros opy (EDS or EDX orEDAX) is an analyti al te hnique used for the elemental analysis or hemi al hara -terization of a sample. Powders were observed from SEM, with elemental ompositionsbeing analyzed by EDX.

Figure 3.11: S anning Ele tron Mi ros ope (HR-FESEM Hita hi SU-70).3.2.3 Me hani al analysisThe me hani al properties of the Aluminium foams were performed using a universaltesting ma hine (Shimadzu AG-50kNG) with a load ell of 50kN at room temperature(Fig. 3.12). The rosshead rate was of 1mm/min. The stress was al ulated by dividingthe load by the total ross-head se tional area, and the engineering strain was al ulatedfrom the ross-head displa ement. The energy absorption per unit volume was omputedby evaluating the area under the stress-strain urve. The ompression test spe imens werere orded and weighed a urately to estimate the relative density.The Al-alloy foam samples were produ ed by heating the pre ursor material inside the ylindri al steel mould, using a pre-heated furna e at 750C in Air. The pre ursormaterial whi h obtained by the optimum ompa tion parameters whi h was adjustedin this resear h. The dimensions of the samples are 30 mm in diameter and 30 mmin height, approximately (Fig. 3.13). The pre ursor material was hosen to present aBruno Pinto Lopes MIEM Dissertation

3.Experimental Pro edures 47

(a) Testing ma hine (b) Load ellFigure 3.12: Universal testing ma hine (Shimadzu AG-50kNG).better foaming behaviour. Table 3.3 shows the used manufa turing parameters to obtainthe Al-alloy foams. The Al-alloy powders were Al-A, and two treated Titanium Hydridepowders were used (TiH-1T and TiH-2T). Two types of Al-alloy foams were tested, alledFoam-I and Foam-II, with dierent blowing agents. These foams have an external densesurfa e around the ellular stru ture. Five (5) samples for ea h type of Al-alloy foamswere used.

Figure 3.13: Al-alloy foams tested.Table 3.3: Manufa turing parameters used to produ e Al-alloy foams.

Bruno Pinto Lopes MIEM Dissertation

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Bruno Pinto Lopes MIEM Dissertation

Chapter 4Results and Dis ussion4.1 PowdersThe quality of the metal foams depends on the properties of the metal and the blowingagent used, su h as the size range, the parti le shape, the hemi al omposition and thedensity. Some of these properties were evaluated using the dierent methods des ribedpreviously (see se tion 3) for all powders used in this resear h. The thermal de omposi-tion and the oxidation behaviour of the Titanium Hydride and the thermal behaviour ofthe Al-alloy powders were also evaluated.4.1.1 DensityThe density of the Al-alloy and TiH2 powders were evaluated using a py onometer (seese tion 3.2.1.1), as shown in the Table 4.1.Table 4.1: Density of the Al-alloy and TiH2 powders.

* no values was provided by supplier.The results showed that the Al-alloy powders have dierent values of density. Al-A alloypowder presents the lowest value (2.10 g/ m3) while the Al-B alloy powder presents thehighest values (2.60 g/ m3). The density values of the TiH2 powders are lose. Thesemeasured values are slightly dierent to those provided by the supplier (3.76 g/ m3 for49

50 4.Results and Dis ussionboth TiH2 powders). For example, the measured values and values provided by supplierfor the TiH-1 powder are 3.79 g/ m3 and 3.76 g/ m3, respe tively.4.1.2 Parti le sizeThe parti le size and parti le distribution of the powders were analyzed by a parti lesize analyzer (COULTERTM LS 230 Parti le Size Analyzer, Coulter Corporation, Fuller-ton, California, USA), as des ribed in se tion 3.2.1.2. Parti le size and parti le sizedistribution results are summarised in Table 4.2.Table 4.2: Parti le size analysis by laser dira tion.

These results onrmed that three ommer ial Al-alloy powders have dierent parti lesizes and size distributions. Al-C alloy powder presented the greatest parti les and hadthe widest size distribution followed by Al-B alloy and Al-A alloy powders. The parti lesize parameters (e.g. D10, D50 and D90) of Al-A alloy powder are half of the parametersof the Al-B alloy powder. For example, the values of the mean parti le size are 16 µmand 32 µm for the Al-A alloy and Al-B alloy powders, respe tively. Al-C alloy powderhad the largest parti le size and widest parti le size distribution in whi h the values ofthe D10 and D90 are 44.78 µm and 1095.5 µm, respe tively. This powder had a largedieren e between the value of the mean parti le size (240,9 µm) and D50 (72,22 µm).As seen in Table 4.2, there are slightly dieren es between the measured values and thevalues given by the supplier. This ma be due to the dierent measuring pro esses used todetermine the parti le size and parti le distribution parameters. It is di ult to denethe term parti le size, be ause only the spheri al parti les an be dened ompletelyBruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 51by diameter alone. Most powders used in industry are not spheri al in shape. In orderto hara terise the parti le size, therefore, ertain quantiable physi al properties of theparti les used, su h as the length, the volume, the surfa e area, or the intera tion withan ele tri eld. Parti le shape and size inuen es ea h of the measuring pro esses sothat measurement by dierent methods an yield dierent results.The two ommer ial TiH2 powders have dierent parti les sizes and size distributions,in whi h the parameters of the TiH-1 powder is approximately a half of the parametersof the TiH-2 powder. More spe i ally, the TiH-2 powder has a mean parti le diameter(15.39 µm) twi e the other TiH-1 powder (6.89 µm).The parti le size and size distributions of Al-A alloy powder and TiH-2 powder are lose.However, the Al-A alloy powder has a smaller dispersion of the sample distribution thanthe TiH-2 powder. The values of dispersion of the sample distribution (D90-D10 range)are 24.10 and 27.59, respe tively for Al-A alloy and TiH-2 powders.4.1.3 Morphology analysisThe morphology of the Al-alloy and the TiH2 powders were observed using the SEManalysis (see se tion 3.2.2.2). Figures 4.1 and 4.2 show SEM images of the Al-alloypowders (Al-A alloy, Al-B alloy and Al-C alloy) and TiH2 powders (TiH-1 and TiH-2),respe tively.The parti les of the three ommer ial Al-alloy powders (Al-A alloy, Al-B alloy and Al-Calloy) used in this resear h have a quasi-spheri al shape or/and an oblong shape withvarying size distributions (Fig. 4.1). The surfa e of the parti les is irregular. Two ommer ial Al-alloy parti les (Al-B alloy and Al-C alloy) have an irregular shape, knownas nodular morphology typi al of the air atomised Al powders. The morphology of thepowders is typi al for gas-atomised powders from a liquid metalli melt, and it an bedes ribed as a bulk prolonged, semi-spheri al shape. This SEM data was onrmed bythe pro essing information obtained from the supplier who stated that these powderswere made by air atomised Al powders and milling. These two powders also ontainsome % of small spheri al parti les. The most of the parti les of the Al-A powder havea spheri al shape (Fig. 4.1a ), and had smaller and uniform parti les ompared to theother two Al-alloy powders (Fig. 4.1b and 4.1 for the Al-B alloy and Al-C alloy powders,respe tively). This last observation was onrmed by measured parti le size values (Table4.2). The Al-B alloy and Al-C alloy powders have two type of population of the parti leshape and size as shown in the Fig. 4.1b and in the Fig. 4.1 . The rst population isa group of small spheri al shaped parti les. The se ond population is a group of largeoblong shaped parti les with varying size distributions. The Al-C alloy powder had thelargest parti le size and widest parti le size distribution, in whi h the parti les with largedimension have nodular morphology typi al.The parti les of the as-re eived TiH2 powders show an angular shape with a variableparti le size (Fig. 4.2). The TiH2 powder was prepared by the ball milling of Titaniumin a Hydrogen atmosphere under pressure and it exhibits an irregular, sharp, polygonalmorphology.The parti les size of the TiH-1 powder are smaller than the TiH-2 powder parti le size, asBruno Pinto Lopes MIEM Dissertation

52 4.Results and Dis ussion

(a) Al-A alloy powder

(b) Al-B alloy powder

( ) Al-C alloy powderFigure 4.1: SEM mi rograph of Al-alloy powders at dierent magni ations. Left side:overview. Right side: parti les details.Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 53

(a) as-re eived TiH-1 powder

(b) as-re eived TiH-2 powderFigure 4.2: SEM mi rograph of as-re eived TiH2 powders at dierent magni ations.Left side: overview. Right side: parti le details.measured above through parti le size analysis using laser dira tion (Table 4.2). Theseimages show that the TiH2 powders have a tenden y to reate agglomerates of the par-ti les. The size and the morphology of the powder parti les an ae t de quality of themixture that will be made. Parti le surfa e of both TiH2 powders is irregular and at.4.1.4 Thermal de omposition and oxidation behaviour of the TitaniumHydride powdersThe TiH2 was shown to be a suitable blowing agent for Al-alloys, although other hydrides(e.g. ZrH2, MgH2) an also be used [22, 23. Hydrogen release from TiH2 starts around400C, whi h is markedly below the melting point of the most ommer ial Al-alloys. Thisdieren e between the de omposition temperature of the blowing agent and the meltingtemperature of the Al-alloy auses the formation of the irregularities and ra ks in theearly expansion stages whi h then an lead to foams with defe ts.To minimize this temperature mismat h the melting point of the alloy an be loweredby alloying elements or the de omposition threshold of the blowing agent an be raisedBruno Pinto Lopes MIEM Dissertation

54 4.Results and Dis ussionby thermal pre-treatment. The TiH2 powder is pre-heated in air to form an oxide layeron the surfa e of the powder parti les. This layer delays gas release from the parti les inorder the Hydrogen is released during foaming only the alloy has been rea hed the meltingpoint. During these treatments, there is a loss of the Hydrogen, but only 25% of thereleased Hydrogen is ee tive in forming gas-lled pores, whereas the rest is lost duringfoaming [23. Several authors have been made to optimise the oxidation pre-treatmentof the powders [37, 39. Both powders were oxidised at 480C during 180 minutes basedon studies whi h was reported by Matisjasevi -lux. The aim is studied the ee t of thistreatment on foaming behaviour. The TiH2 was subje ted to heat treatments prior topro essing to shift hydrogen release up into the melting range of the Al-alloy.The as-re eived and treated TiH2 was hara terized by thermal analysis in onjun tionwith X-ray dira tion, as des ribed in se tions 3.2.1.3 and 3.2.1.4, respe tively. TheSEM in ombination with EDX (Energy dispersive X-ray spe tros opy) was also used asadditional analysis. Figure 4.3 shows the overview of this study.

Figure 4.3: Thermo analysis of the TiH2 powder samples performed at 10k/min.Briey, two types of thermal analysis were made a ording to the atmosphere (Air orOxygen) due the experimental limitations. The rst one, dierential s anning alorimetry(DSC) and thermogravimetri analysis (TG) are arried out for both TiH2 powders (as-re eived and treated) under air atmosphere whi h measures both heat ow and weight hanges in a material as a fun tion of temperature or time in a ontrolled atmosphere.The se ond, a simultaneous TG- DTA measurements were performed for both as-re eivedBruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 55TiH2 powders under Oxygen atmosphere whi h measures both heat ow and weight hanges in a material as a fun tion of temperature or time in a ontrolled atmosphere.The as-re eived and treated TiH2 powders were hara terised and/or observed using theX-ray dira tion and SEM oupled with EDX te hniques. The powders submitted to anatmosphere (Air and Oxygen) during the thermal analysis were also analysed by X-raydira tion te hnique.4.1.4.1 Thermal behaviour of TiH2 powders in Air atmosphereUnderstanding the thermal de omposition me hanism of Titanium Hydride, espe iallydetermining of the best onditions of the heat-treatment of this material, would be animportant step on the pro ess of produ tion of Al foams. The thermal behaviour of the ommer ial TiH2 powders under non-equilibrium onditions similar to the industrial oneswas arried out by measuring in stati Air. Figure 4.4 (a and b) shows the TG urves andthe derivate of TG urve (DTG) for both as-re eived and treated TiH2 powders (TiH-1and TiH-2) heated at a onstant rate (10 k/min) under Air atmosphere. The DTG urveis used to determine ine tion points on the TG urve, to provide referen e points forweight hange measurements in systems where the weight hange are not ompletelyresolved.These thermal analysis results showed that the powders in rease their mass when heatedunder Air atmosphere at a onstant heating rate (10K/min), as shown in the obtainedTG urves for both as-re eived and treated TiH2 powders. The plot of TG urves startsat 100% whi h indi ate that there is no mass loss and/or mass gain. The sample oxidise,resulting in an in rease of the TG urve due to the mass gain for all the TiH2 powders.The TG urves show distin t hanges in slope whi h be ome visible when showing thederivative of TG urve (DTG). TiH2 powders are oxidised in Air at least three stages:an initial slow stage of the pro ess, rapid stage, and nal stage of the rea tion. TheTG results showed that the oxidation of as-re eived TiH-1 and TiH-2 powders startsat 240C (Fig. 4.4a) and 390C (Fig. 4.4b), respe tively and leads a ontinuous massin rease. The total mass gain from room temperature to 1000C is 30.8% (Fig. 4.4a)and 31.5% (Fig. 4.4b), respe tively for the TiH-1 and TiH-2 as-re eived powder. Theseresults also showed that the oxidation (mass gain) depends on the parti le size of TiH2.As lower the parti le size, lower is the temperature at whi h oxidation starts, as the aseof TiH-1 powder (6.89 µm). The kineti s of mass gain (oxidation of TiH2) is slightlyslower in the ase of the TiH2 with higher value of parti le size (TiH-2 powder, 15.89µm). The oxidation of the TiH-1 powder (6.89 µm) starts before TiH-2 powder. DTG urves show two peaks (ine tion points on the TG urve) at 480C and 590C and at490C and 645C for as-re eived TiH-1 powder (Fig. 4.4a) and as-re eived TiH-2 powder(Fig. 4.4b), respe tively.After the heat treating of the powders the rst peak disappeared and it an be observed awell-dened peak at 700C and 695C for the treated TiH-1 and TiH-2 powders, respe -tively (grey lines). These TG results have also revealed that the TG urves of treatedpowders (TiH-1T and TiH-2T) are shifted to the right of the TG urves of as-re eivedpowders. However, all TG urves present at least three stages. There is a delay to startthe in rease of the mass gain for both TiH2 powders. The mass gain starts at approx-Bruno Pinto Lopes MIEM Dissertation

56 4.Results and Dis ussion

(a) TiH-1 and TiH-1T powders.

(b) TiH-2 and TiH-2T powders.Figure 4.4: TG/DTG urves measured on as-re eived and treated TiH2 powders in Airat 10k/min.imately 240C and 555C for as-re eived and treated TiH-1 powder, respe tively (Fig.4.4a). The initial temperature of the mass gain in reased more than 310C, movingfrom 240C for the as-re eived powder to around 555C for treated TiH-1 powder. TheBruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 57TiH-2 powder has a similar behaviour. The mass gain starts at 390C and 595C for theas-re eived and treated TiH-2 powders, respe tively. There is a temperature dieren eis about 205C. The total mass gain from room temperature to 1000C is 24.9% (Fig.4.4a) and 25.7% (Fig. 4.4b), respe tively for TiH-1T and TiH-2T.TG-DSC urves for both as-re eived and treated TiH2 powders at 10 K/min under airare shown in Fig. 4.5 . The enthalpy hanges asso iated with the events o urring aregiven by the area under the peaks, as shown in Figures 4.5a and 4.5b.In the DSC urves measured in air for both as-re eived TiH2 powders (TiH-1 and TiH-2), two well-dened peaks are identied, a ompanied by a mass gain in the TG urves.The rst one is an exothermi peak at 585C and 630C, respe tively for TiH-1 andTiH-2 powders. The se ond is an endothermi peak at 660C and 695C, respe tivelyfor TiH-1 and TiH-2 powders. DSC urves of treated TiH-1 and TiH-2 powders exhibitan exothermi peak at 665C and 695C, respe tively. A small endothermi peak is stillobserved at 700C for treated TiH-2. But, the endothermi peak is not visible for treatedTiH-1 powder.The results showed that the values of the peaks from the DTG urves (Fig. 4.4) areapproximately the same tend as the peaks of the DSC urves (Fig. 4.5a).The DSC urves for heating in air learly show exothermi oxidation pro ess taking pla eand the peaks are in broad agreement with the work of others authors [38, 4043. Forthe non-isothermal measurements performed in air, exothermi peaks are observed onDSC urves that are a ompanied by mass in reases on TG urves. Heating of TiH2 inair results in the formation of oxyhydrides at lower temperatures and a mixture of oxidesof Ti above 470C. The end produ t of prolonged oxidation is TiO2 and the omplete onversion of the TiH2 to TiO2 is impossible. The shift in the endothermi to highertemperature shows the potential for delaying Hydrogen evolution. These observationsare assigned to the oxidation of the TiH2.The de omposition of the TiH2 under non-equilibrium onditions at linear heating is amulti-stage pro ess, but the me hanisms are s ar ely studied. The thermal desorptionspe tros opy (TDS) and DSC te hniques have been used to studing these me hanisms[38, 41, 4447. The details in the kineti s of the hydrogenation, as well as of the hightemperature oxidation of TiH2 powder was studied using the DSC te hnique by Liu [41.The me hanisms and its intera tion that o ur during the heating of the powders in airatmosphere is very omplex. In the presen e of air, Hydrogen omes out from TitaniumHydride during several stages. The thermal de omposition me hanism of TiH2 powderseems to be under ontrol of both internal diusion (diusion of Hydrogen and oxygenatoms within the layers of α solid solution, Titanium oxide and Titanium Hydride) andthe hemi al rea tion (Titanium oxidation) [48, 49. The oxidation of the powders is oneof the me hanisms that o urs due to the presen e of the Oxygen mole ules in the airthat forms with Titanium, the oxide surfa e layer. Thermal de omposition with release ofthe Hydrogen gas is another important me hanism. The Hydrogen mole ules an rea twith the Oxygen mole ules, leading to water mole ules. Thus, there is a ompetitionbetween the dehydrogenation and oxidation of TiH2 when the TiH2 powder is heated ata onstant heating rate under air atmosphere. In open air, ertain relation between theBruno Pinto Lopes MIEM Dissertation

58 4.Results and Dis ussion

(a) TiH-1 and TiH-1T powders.

(b) TiH-2 and TiH-2T powders.Figure 4.5: TG urves measured on as-re eived and treated TiH2 powders in air at10K/min.dehydrogenation and the oxidation of Titanium should exist.Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 594.1.4.2 Thermal behaviour of TiH2 powders in Oxygen atmosphereFigure 4.6 shows the DTA/TG urves from the linear heating measurement of TiH2powders in Oxygen atmosphere. Exothermi well-dened peaks were identied in DTA urves, a ompanied by the in rease of mass gain in TG urves. The shape of the DTA urves is similar for both powders. The mass gain due to the oxidation of the powder isslightly slower in the ase of the TiH-2 powder whi h has a higher parti le size (15.39µm).The oxidation of the TiH-1 powder starts before the TiH-2 powder. The total mass gainfrom room temperature at 1000C is 58.42 and 59.91% for TiH-1 and TiH-2 powders,respe tively. This value is lose to the theoreti al value to transform all the TitaniumHydride in Titanium Dioxide (TiO2). The XRD spe trum of the powder after su hDTA/TG measurements shows only the TiO2 lines (see Figs. 4.10 and 4.11). Two peaksare observed at 560C and 644C for the TiH-1 powder. Two exothermi well-denedpeaks are observed at 669 and 782C for the TiH-2 powder.

Figure 4.6: TG/DTA urves measured on as-re eived TiH2 powder in oxygen at 10k/min.The ee t of the atmosphere (Air and Oxygen) on the thermal behaviour an be observedin Fig. 4.7 for the as-re eived TiH-2 powder. A series of weakly resolved exothermi peakswith maxima at 669C and 782C is observed on DTA urve under air atmosphere. InDTA urves, two exothermi peaks were identied, a ompanied by a stepped mass gainin the TG urves. These are asso iated with the thermal de omposition and oxidation.Emphasis that both oxidation and dehydrogenation took pla e during the heating of thepowder at a onstant heating rate. The DTA urves in Oxygen and in the Air atmosphere ould be subtra ted and the dehydrogenation extra ted.The oxidation forms the ompli ated and large exothermi peaks. The size, shape, stru -ture of parti les and ontent of impurities in the metal and the quantity of Hydrogen areBruno Pinto Lopes MIEM Dissertation

60 4.Results and Dis ussion

Figure 4.7: TG/DTA urves measured on as-re eived TiH-2 powder in Air and in Oxygenat 10k/min.fa tors that exert great inuen e on the ignition temperature and the oxidation pro essof TiH2 powders. The global rea tion to transform the Hydride is as follows:T iH2 +

3

2O2 T iO2 +H2O (4.1)At temperatures of 1000C, the transformation of TiH2 to TiO2 is quasi ompleted, andthe maximum weight hanges of 58.42 and 59.91% orrespond to this point. This value isnear to the theoreti al one if the ideal stoi hiometry of the Hydride is taken into a ount.As shown above, the de omposition of TiH2 powder pro eeds independently of the at-mosphere. Contrawise, the oxidation is dependent on the a ess of Oxygen. The morehindered is the Oxygen inlet, the slower and more pe uliarly the oxidation manifestsitself. Thus, for example, in the ase of linear heating rate in air atmosphere. When thesample lies deeply in a profound ontainer, the oxidation starts regularly and then stopswhen all the Oxygen lo ated in the ontainer has been rea ted and the steam of liberatedHydrogen prevents further inlet of additional Oxygen. Later, when the dehydrogenationis ompleted (represented by the endothermi ) a new Oxygen mole ule might rea h thesample and the oxidation might ontinue at higher temperature again. The rea tion islimited by the partial diusivity of Oxygen mole ules in the air. Oxidation slightly shiftsthe dehydrogenation to higher temperatures. Limiting the supply of the Oxygen doesnot inuen e the dehydrogenation. However, it might ontrol the oxidation and its heatprodu tion.Hydrogen whi h was released during the heating of the samples rea ted with Oxygen toform water. The high exothermi ity of this pro ess a ounts for the samples aking. TheBruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 61dynami s hemes of TiH2 thermal oxidation has been reported by [50:T iHx −→ T ixHx−yOz −→ T i2O −→ T iO2 (4.2)Hydride oxidation is pre eded by its de omposition and only after that a separate oxi-dation of metal and Hydrogen takes pla e. The Titanium thermal oxidation shows thatrates of surfa e oxidation depend strongly on the oxidising atmosphere.4.1.4.3 X-ray dira tion investigationsThe as-re eived TiH2 powders were submitted to a heating treatment in whi h the pow-ders were heated inside a preheated furna e at 480C during 3 hours (se tion 3.1.2).The as-re eived and treated TiH2 powders were hara terised using SEM oupled withEDX te hniques and X-ray dira tion method. The X-ray dira tion is used to identi- ation and quantitative determination of various rystalline forms, known as 'phases' of ompound present in the dierent powders.Dira tion patterns of as-re eived and treated TiH2 powder are shown in Figures 4.8Both as-re eived powders were found to have a single phase, as shown in Figures 4.8aand 4.8b for TiH-1 powder and TiH-2 powder, respe tively. The TiH2 peaks appear in thedira tion patterns for both as-re eived powders. After heating treatment, three phaseswere identied for both treated powders. The ommon peaks for both treated powders orrespond to the TiO2 (Rutile) and TiH1.5 peaks. The dieren e, is the TiO2 (Rutile)peaks appears for the dira tion patterns of TiH-1 powder, while TiN0.3 (Osbornite)peaks appears for the TiH-2 powder. The XRD results revealed that the treated powdersstill ontain Titanium Hydride (TiH1.5) whi h is the su ient to foam the Al-alloys.

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62 4.Results and Dis ussion

(a) TiH-1 and TiH-1T powders.

(b) TiH-2 and TiH-2T powders.Figure 4.8: XRD-measurements on TiH2 powder samples as-re eived and treated at480C during 3 hours.The phase identi ation from powder dira tion data ompares the powder dira tionpattern of the powder sample to a database ontaining referen e patterns (Fig. 4.9) inorder to identify the phases whi h are present.Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 63

Figure 4.9: Referen e spe tra for TiH2, TiO2, TiO3 and TiN0.3.Dira tion patterns of both TiH2 powder after the thermal analysis are shown in Fig.4.10, whi h is observed the ee t of the atmosphere (Air and Oxygen atmosphere). Afterthe powders were heated to the Oxygen atmosphere up to 1000C, a single Rutile phasewas identied for both powders. The thermal analysis onrms that Titanium Hydridewas quasi ompletely transformed to the TiO2 (Rutile), whi h orresponds to the valuesnear to theoreti al value (60%). After the powders were heated to the Air atmosphereup to 1000C, the powders have dierent phases. The TiH-1 powder was identied twophases (TiO2 and TiO3). The TiH-2 powder was identied three phases (TiO2, TiN,TiNO3). The formation of the TiN, TiNO3 phase is due the presen e of the Nitrogenatoms. The presen e of the Nitrogen atoms into the TiH2 latti e an leads a formationof Titanium Nitride.

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64 4.Results and Dis ussion

(a) TiH-1 and TiH-1T powders.

(b) TiH-2 and TiH-2T powders.Figure 4.10: XRD-measurements on as-re eived TiH2 powder samples and after submit-ted in Air and Oxygen atmosphere.Titanium Hydride is well known for its stability in air be ause of formation of thin TiO2oxide layer on top of TiH2 powder parti les. The ee t of Oxygen atmosphere on parti lesurfa e of TiH2 is shown in Fig. 4.11. All parti le surfa es are overed of Titanium oxideBruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 65for both TiH2 powders. The surfa e parti le of as-re eived powders of TiH2 have loweramount of Titanium oxide when ompared with the parti le surfa e submitted at Oxygenatmosphere. Formation of the oxide layer is dire tly related to high anity of Titaniumto Oxygen and it an be formed in few se onds. The thi kness of TiO2 layer on the TiH2powder varies with the type of atmosphere (see Fig. 4.11 and Fig. 4.12). Existen e ofoxide parti les on surfa e of powder parti les was onrmed with SEM (Figures 4.11 and4.12) and EDXS spe trums (Figs 4.13 to 4.14).

(a) As-re eived TiH-1 parti le. (b) TiH-1 parti le after the thermal analysiswhi h is submitted at Oxygen atmosphere.

( ) As-re eived TiH-2 parti le. (d) TiH-2 parti le after the thermal analysiswhi h is submitted at Oxygen atmosphere.Figure 4.11: SEM mi rographs of surfa e of a parti le of TiH2 powder submitted atdierent onditions.The surfa e of all the parti le powder was oxidized with dierent levels. The parti le sub-mitted at Oxygen atmosphere show a higher thi kness of the oxide layer. The thi knessof the oxide layer depended on the anity of the basi material for Oxygen, whi h wasvery high for both TiH2. The oxide layer was examined and found to be one of the inu-ential parameters for the formation of metalli foams by the powder-metallurgy pro ess.The oxide layer on TiH2 powders aused a delayed gas evolution as demonstrated withthermal analysis, whi h had a favourable inuen e on the foaming pro ess. The amountof the TiO2 on the surfa e parti le in reases with the amount of the Oxygen mole ules.Bruno Pinto Lopes MIEM Dissertation

66 4.Results and Dis ussionThe ee t of atmosphere on the thi kness of the oxide layer is learly visible in Figure4.12, in whi h the thi kness is lower in the ase of the TiH2 submitted at Air atmosphere(Fig. 4.12b) than the TiH2 submitted at Oxygen atmosphere (Fig. 4.12 ) , or evenas-re eived powder.

(a) As-re eived TiH-2 parti le.

(b) TiH-2 parti le after the thermal analysiswhi h was submitted at air atmosphere. ( ) TiH-2 parti le after the thermal analysiswhi h was submitted at Oxygen atmosphere.Figure 4.12: SEM mi rographs of surfa e of a parti le of TiH2 powder submitted atdierent onditions.Figures 4.13 to 4.15 show the EDX spe trum for the parti le of as-re eived and treatedTiH2 powders, as well as the parti le submitted to dierent atmosphere. The qualitativeresults onrmed the others results. The values of % element Oxygen are higher in theEDX spe trum of both TiH2 powders submitted to the Oxygen atmosphere.The heating treatment resulted in olouration of the powders (see Fig. 4.16). Coloursobserved on oxidised titanium hydride are asso iated both with the thi kness and the omposition of oxide layers formed. The hange of the olour of the powders is a strongindi ation of a surfa e oxide being formed on the powders.Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 67

(a) EDX-spe trum results for the parti le of as-re eived TiH-1 in theframe area in Fig. 4.11a.

(b) EDX-spe trum results for the parti le of TiH-1 after submitted atOxygen atmosphere in Fig. 4.11b.Figure 4.13: EDX spe trum of TiH-1 powder.Bruno Pinto Lopes MIEM Dissertation

68 4.Results and Dis ussion

(a) EDX-spe trum results for the parti le of as-re eived TiH-2 powder inFig. 4.11 .

(b) EDX Spe trum of TiH-2 after submitted at oxygen atmosphere inFig. 4.11d.Figure 4.14: EDX spe trum of TiH-2 powder.Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 69

Figure 4.15: EDX spe trum of TiH-2 powder after submitted at air atmosphere in Fig.4.12b

Figure 4.16: TiH2 oxidised at dierent onditions. The olour of the powder depends onthe thi kness of the Oxygen layer. a ) as-re eveid b) treated at 480C for 3h ) submittedin Oxygen atmosphere d) submitted in Air atmosphere.Bruno Pinto Lopes MIEM Dissertation

70 4.Results and Dis ussion4.1.5 Thermal Behaviour of the Al-alloy powdersThe hot stage mi ros opy tests (HSM) were performed on the Al-alloy powder ompa tsamples at a onstant heating rate of 10k/min, as des ribed in se tion 3.3. The systemwas equipped with a mi ros ope for observation of the shape hanges. Figures 4.17 to4.19 show some sele ted images aptured during the heating of the samples up to 1000Cfor ea h Al-alloy powder studied. The sequen e of the sele ted images of ea h Al-alloypowder represents the main features that o urred during the heating of ea h Al-alloypowder. Hot stage mi ros opi investigation aimed at the ignition hara teristi s ofdierent Al-alloy powders (Al-A, Al-B and Al-C). The agglomeration phenomena of Al-alloy parti les during gradual heating was arried out using an opti al mi ros ope witha ontrolled hot stage.These observations revealed that the alloy omposition ae ts the thermal behaviour.The Al-A alloy presents the better thermal behaviour for produ tion Al-alloy foamswhen ompared to others Al-alloy (Al-B and Al-C). This Al-A alloy parti les in pre-pressed samples revealed the formation of large amount of Al-alloy liquid drops on thesample surfa e temperatures slightly ex eeding the Al-alloy melting point (Fig. 4.17).The Al-A alloy liquid drops appears at 555C. The Al-B alloy presents the worst thermalbehaviour for produ tion Al-alloy foams in whi h it was observed a ra k formation onthe powder ompa ts (Fig. 4.18). This ra k appears at 581C and propagates in thesample (Fig. 4.18). This is due to a rapid in rease in volume and release of energy, withthe temperatures and the release of gases. Su h defe ts may be una eptable for someappli ations even if their presen e does not ae t produ t performan e.The formation of the alloy drops and ra ks are not learly visible on the hot-stage ofthe Al-C alloy. To this alloy it was observed a large drop (Fig. 4.19). A large amountof the Al-alloy drops appears in the ase of Al-A alloy. The number of the small dropsin reases with in reasing temperature.Heating of the powder of Al-alloy shows thermal expansion in both dire tions along thex and y axes (in height and width) for all the Al-alloy powders. The expansion of thefree Al-alloy powder ompa t samples is observed at 200C, 400C and 450C for theAl-C, Al-B and Al-A, respe tively.

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4.Results and Dis ussion 71

Figure 4.17: Hot-stage mi ros opy images of a sample of Al-A alloy powder at dierenttemperatures (before, during and after melting).Bruno Pinto Lopes MIEM Dissertation

72 4.Results and Dis ussion

Figure 4.18: Hot-stage mi ros opy images of a sample of Al-B alloy powder (32.5µm) atdierent temperatures (before, during and after melting).Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 73

Figure 4.19: Hot-stage mi ros opy images of a sample of Al-C alloy powder (240.9µm)at dierent temperatures (before, during and after melting).Bruno Pinto Lopes MIEM Dissertation

74 4.Results and Dis ussion4.2 Produ tion of pre ursor materialFoaming behaviour of metals depends on the parameters of the powder ompa tion pro- ess. One has to ensure that the blowing agent parti les are uniformly distributed in themetal matrix and that no residual open porosity remains. Hot ompa tion or a ombi-nation of old and hot ompa tion stages were performed to obtain foamable pre ursormaterials with good quality. The applied pressure and the hot pressing temperature usedin this resear h were 200 bar and 400C, respe tively. The time of the old and/or hotpressing and the stabilisation time at 400C were varied. Figure 4.20 shows a globaloverview of the experimental pro edure to adjust these parameters. The nal densityand the distribution of the TiH2 in the Al-alloy matrix are the main fa tors to evaluatethe quality of ea h pre ursor. The nal density of ea h pre ursor was ompared with theinitial density of mixture powders whi h must have a lose values. The density of thepre ursor was measured the weight and the volume. The density of the initial mixturepowders was measured using py nometer method (se tion 3.2.1.1). The distribution ofthe TiH2 in the Al-alloy matrix was observed using light mi ros ope.

(a)

(b)Figure 4.20: S hemati representation of experimental tests to adjust the ompa tionparameters using hot ompa tion stage (a) and a ombination of old and hot ompa tionstages (b).Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 75Preliminary tests were made to adjust the ompa tion parameters in order to obtaina high quality of the foamable pre ursor material. For that, the Al-B alloy powderwithout blowing agent was used. The maximum applied pressure of the used equipmentto prepare the pre ursor material is 200 bar. Thus, the adjustment of the parameters torea h desirable dense pre ursor was arried out to in rease the time of the applied pressureduring the hot and old ompa tion. Figure 4.21 shows the series of experimental testswhi h were performed to obtain pre ursor material without residual porosity with adensity near to its theoreti al density.

Figure 4.21: Ee t of ompa tion parameters on the density of the pre ursor material.The results revealed that the old ompa tion should be done before the hot- ompa tionin order to obtain a high level of densi ation. This is due the dusty nature of the powderswhi h tends to adhere to the mould walls ae ting its densi ation levels. Lower levelsof densi ation were rea hed when the ompa ted was obtained only by hot pressing ofthe powder, as shown in the examples in Fig. 4.22. From these experimental tests it wasobserved that it is not ne essary to apply the pressure at room temperature ( old pressingstage) during a long period of time. The riti al times are the time of the stabilisationat 400C and the time of the applied pressure at 400C. The in rease of these times ondu ts to an in rease of the level density.

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76 4.Results and Dis ussion

(a) (b) ( )Figure 4.22: Low-quality of pre ursor materials whi h was obtained by hot pressing atdierent manufa turing parameters.The powder was heated up to 400C, and then was hot pressed at 200 bar during 1s (seeFig. 4.22a). The powder was heated up to 400C and remains during 10 min at 400C,and then was hot pressed at 200 bar during 1 min (see Fig. 4.22b). The powder washeated up to 400C and remains during 5 min at 400C, and then was hot pressed at200 bar during 2 min (see Fig. 4.22 ).The high-quality of pre ursor material was obtained using a ombination of a old press-ing and hot pressing. Table 4.3 summarises the optimum ompa tion onditions whi hwas obtained. The powder is ompa ted at room temperature during 5 min and thenis heated up to 400C and remains during 40 min at this temperature. Finally, thepre ursor material is obtained by hot pressing at 400C during 25 min (Fig. 4.23).

Figure 4.23: High-quality of pre ursor materials obtained by a ombination of old andhot pressing.Pre ursor materials of the dierent Al-alloys without blowing agent were prepared us-ing the optimum ompa tion onditions (Table 4.3). Mi rostru tures of these pre ursormaterials under opti al mi ros ope are shown in Fig. 4.24. The relative density ofthese ompa ts is more than 80% of the theoreti al density. Residual porosity is learlyBruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 77Table 4.3: Optimum ompa tion onditions.visible within these ompa ts (bla k points). These mi rostru tures show a typi al mi- rostru ture of Al-Si alloys. Al-Si alloys form an eute ti system where the two phasesin equilibrium are Aluminium and Sili on [22.

(a) Al-A alloy (b) Al-B alloy

( ) Al-C alloyFigure 4.24: Mi rostru ture of pre ursor materials without blowing agent.After these preliminary tests, several pre ursor materials of dierent powder mixtures(Al-alloy+TiH2) were prepared using the manufa turing parameters des ribed above (Ta-ble 4.3), as shown in Figure 4.25. Several powder mixtures were made using the experi-mental pro edure des ribed in se tion 3.1.3.Bruno Pinto Lopes MIEM Dissertation

78 4.Results and Dis ussion

Figure 4.25: Pre ursor materials manufa tured.Table 4.4 summarises the pre ursor materials fabri ation methodology. An importantfeature for foaming is to obtain pre ursor materials with a redu ed porosity. The relativedensity of pre ursor materials is more than 80% of the theoreti al density.Table 4.4: Foamable pre ursor materials.

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4.Results and Dis ussion 79Figure 4.26 shows opti al images of the mi rostru ture of dierent pre ursor materialsprepared in with blowing agent parti les. The darker TiH2 parti les an be seen in themetal matrix of the dierent alloys.

(a) Al-A alloy pre ursor materials.

(b) Al-B alloy pre ursor materials.

( ) Al-C alloy pre ursor materials.Figure 4.26: Mi rostru ture of pre ursor materials prepared of dierent Al-alloy (mag-ni ation of x100) ontaining 0.6wt.%.of TiH2.Bruno Pinto Lopes MIEM Dissertation

80 4.Results and Dis ussionDue to the di ulty in distinguished TiH2 parti les on opti al mi rographs, EDX wasused in further work to help in identifying these parti les. Aluminium matrix (light gray)and TiH2 parti les (white) an be seen in Figure 4.27.

(a) Al-A + 0.6wt.% TiH-1 (b) Al-A + 0.6wt.% TiH-1T

( ) Al-A + 0.6wt.% TiH-2 (d) Al-A + 0.6wt.% TiH-2TFigure 4.27: Mi rostru ture of pre ursor materials prepared of Al-A alloy ontaining0.6wt.%.of treated and as-re eived TiH2.powders.

Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 814.3 Produ tion of Al-alloys foams4.3.1 Preliminary testsThe foaming behaviour of the dierent Al-alloy powders (Al-A alloy, Al-B alloy and Al-C alloy) using two as-re eived TiH2 powders were evaluated. Pre ursor materials wereprepared using dierent Al-alloy (Al-A alloy, Al-B alloy, Al-C alloy) and 0.6 wt.% ofdierent type of TiH2 (TiH-1 and TiH-2) powders. A series of the tests were ondu tedto adjust the temperature for ea h Al-alloy, as well as, to evaluate the foaming behaviourof dierent Al-alloy. The ee t of the Al-alloy omposition on foaming behaviour isillustrated with the example whi h is shown in Figure 4.28. The results showed that thepre ursor of Al-A alloy ontaining 0.6wt.% of TiH2 present a good foaming behaviour.During the foaming pro ess, the pre ursor of Al-B alloy ontaining 0.6wt.% of TiH2 formsa ra k with similar hara teristi s of ra k whi h is observed in hot stage tests.(a) Al-A alloy +0.6wt.%TiH-1 (b) Al-B alloy +0.6wt.%TiH-1 ( ) Al-C alloy +0.6wt.%TiH-1Figure 4.28: Foaming behaviour of pre ursor material with dierent Al-alloys ontaining0.6 wt.% TiH-1 as-re eived powder, using a pre-heated furna e at 750C.The ee t of foaming temperature on foaming behaviour is presented in Fig. 4.29. Thebest foaming behaviour was a hieved at 750C. It is evident that the foaming hara ter-isti s depend sensitively on the furna e temperature hosen. If the sample temperatureis below the solidus temperature of the alloy there is not very mu h more of an ee t thata slight solid state expansion. If the nal temperature is in the solidus/liquidus intervalfoam formation an be observed. Apparently a ertain ex ess temperature above themelting point is needed to obtain full expansion. It is evident that the foaming pro ess.In Fig. 4.30, a omparison between samples produ ed with treated TiH1-T powder fordierent Al-alloys is shown. It is lear that the foam produ ed with pre ursor of Al-Aalloy have a mi rostru ture with spheri al pores. The other two foams show a ra kinside the foams whi h results in the ra ks inside the pre ursor during its heating. As a onsequen e, dierent morphologies observed between the foams produ ed with dierentalloys.In Fig. 4.31 shows the dierent morphologies of the Al-A alloy foams whi h are produ edwith dierent treated TiH2 powders (TiH-1T and TiH-2T). Foams have morphologieswith spheri al pores. The diameter of the pores is 4mm and 8mm for Al-A alloy foamswhi h was produ ed by 0.6wt.% TiH-1T and TiH-2T, respe tively. The TiH-2 powderhas a mean parti le diameter (15.39 µm) twi e the other TiH-1 powder (6.89 µm) (Table4.2).Bruno Pinto Lopes MIEM Dissertation

82 4.Results and Dis ussion(a) 675C (b) 700C( ) 750C (d) 800CFigure 4.29: Foaming behaviour of Al-A alloy pre ursor material ontaining 0.6 wt.%TiH-1, using a pre-heated furna e at dierent foaming temperatures.

(a) Al-A alloy+0.6wt.%TiH-1T (b) Al-B alloy+0.6wt.%TiH-1T ( ) Al-C alloy+0.6wt.%TiH-1TFigure 4.30: Foaming behaviour of pre ursor material with dierent Al-alloys ontaining0.6 wt.% TiH-1T treated powder, using a pre-heated furna e at 750C.

(a) Al-A alloy +0.6wt.%TiH-1T (b) Al-A alloy +0.6wt.%TiH-2TFigure 4.31: Foaming behaviour of pre ursor material with dierent Al-alloys ontaining0.6 wt.% TiH2 treated powder, using a pre-heated furna e at 750C.Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 83In Fig. 4.33 shows the Al alloy foams whi h are produ ed with dierent treated TiH2powders (TiH-1T and TiH-2T). The results demonstrated that the Al-alloy ompositionae ts the foaming behaviour. Foams of Al-A alloy with TiH2 treated powders ould beused to produ e foams with quality. To make foams with the others two type of Al-alloyshould be ne essary to adjust the parameters of the pro ess.

Figure 4.32: Foaming behaviour of Al-A pre ursor material ontaining 0.6 wt.% TiH-1Ttreated powder, using a pre-heated furna e at 750C.

(a) Al-A alloy+0.6wt.% TiH-1T (b) Al-B alloy +0.6wt.% TiH-1T ( ) Al-C alloy +0.6wt.% TiH-1T(d) Al-A alloy+0.6wt.% TiH-2T (e) Al-B alloy +0.6wt.% TiH-2T (f) Al-C alloy +0.6wt.% TiH-2TFigure 4.33: Foaming behaviour of Al-A pre ursor material ontaining 0.6 wt.% TiH-1Ttreated powder or TiH-2T, using a pre-heated furna e at 750C.Bruno Pinto Lopes MIEM Dissertation

84 4.Results and Dis ussion4.4 Properties of Al-alloy foams4.4.1 PreparationThe Al-alloy foams were prepared using adjusted parameters of the dierent manufa -turing steps whi h was demonstrated above. Series of two type of pre ursor materialsof Al-A alloy powder ontaining 0.6wt.% of treated TiH2 powders used in this resear h(TiH-1T and TiH-2T) were prepared (Fig. 4.34). Figure 4.35 show the mi rostru tureof the dierent used pre ursor material. Then, the Al-alloy foams were manufa turedusing a pre-heated furna e at 750C inside a losed steel mould. The resulting foamshave an internal losed- ells and a dense external skin around the samples (Fig. 4.36).The presen e of external skin inuen es the me hani al properties, results its in rease.In order to make a hara terization of the manufa tured foams, ve sample foams of ea htype of mixture was produ ed. As an be seen all samples are very similar geometri alyall samples took the shape of the expansion mould.

Figure 4.34: Pre ursor materials.

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4.Results and Dis ussion 85

(a) Al-A+0.6wt.%TiH-1T (b) Al-A+0.6wt.%TiH-2TFigure 4.35: Mi rostru ture of the dierent pre ursor materials. TiH2 parti les (white olour) is distributed into the Al-A alloy matrix (light gray olour).Two types of Al-A alloy foams were produ ed (Fig. 4.36). First type, alled Foam I wasobtained using a pre ursor material of Al-A alloy ontaining 0.6wt.% of TiH-1T treatedpowder. The se ond type alled Foam II was obtained using a pre ursor material ofAl-A alloy ontaining 0.6wt.% of TiH-2T treated powder. Five samples were produ edfor ea h foam for evaluating of the me hani al properties.

Figure 4.36: Al-alloy foams prepared to me hani al hara terisation.

Bruno Pinto Lopes MIEM Dissertation

86 4.Results and Dis ussion4.4.2 DensityIn Table 4.5 are summarized the densities of the dierent samples prepared, whi h weremeasured by geometry method. The density of sample is dened as its mass per unitvolume. The mass was measured with an appropriate balan e (A& D Instruments witherror=1mg, deviation= 0,1mg). The volume was measured dire tly from the geometryof the sample using a vernier alliper with 0,05mm pre ision. Table 4.5 also lists therelative density whi h is the ratio of the sample density (mass of a unit volume) to thesolid material density.Table 4.5: Densities and relative density of Al-alloy foams.

Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 874.4.3 Compression behaviourMetal foams are lightweight materials and are identied as materials of great interestdue to their unique ombinations of properties derived from their ellular stru ture andmetalli behaviour. The ompressive behaviour of Al-alloy foams has been the subje tof thorough investigations be ause of their en ouraging energy dissipation apability.Stress-strain urves are an extremely important graphi al measure of a materials me- hani al properties. Fig. 4.37 shows results of the ompression tests of both studiedfoam types (I and II - see Table 4.5).The typi al stress-strain urve of the Al-alloy foam an be divided into three stages:(i) in the rst stage the load in reases with in reasing ompression displa ement almostlinearly (elasti dee tion of the pore walls), (ii) followed by a deformation "plateau"with a nearly steady ompression load (pore walls yield or fra ture, whereas in reasingdeformation does not require an in rease of the load) and nally (iii) there is a rapidin rease on the load after the ell walls rushed together (material densi ation).These me hani al results revealed that the ompressive behaviour of these Al-alloy foamsdepends mainly on their density and on their Al-alloy omposition. The stress-strain urves rise with the in rease of the foam density for both Al-alloy foams.The ompression results show that there are dieren es between the Al-alloy foams stud-ied. The Foams I show a signi ant in rease of the stress in the "plateau" region of the ompression urves (Fig. 4.37a). The other type of foams (Foams II) with low densitiespresents a slight in rease of the stress value during its plateau. This an be observed omparing the values of the stress at 20%, 30% for both Al-alloy foams.

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88 4.Results and Dis ussion

(a) Foams I

(b) Foams IIFigure 4.37: Stress-strain behaviour as obtained in uniaxial ompression tests for bothAlSi1Mg and AlSi7 foams ( ylindri al samples ∅ = 30mm x 30 mm).Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 89

Figure 4.38: Stress-strain urve in the linear elasti region.The evaluation of the me hani al properties was based on the Japanese standard (JIS H7902, 2008) and German standard (DIN 50134, 2008) [51.The average plateau stress was al ulated using the stress values of 20% and 30% strain. The beginning strain for den-si ation was measured orresponding to a strain 1.3 times the average of plateau stressbased on the German standard: DIN 50134. Table 4.6 summarises the me hani al re-sults of these two types of foams. These me hani al results revealed that the ompressivebehaviour of these Al-alloy foams depends mainly on their density.Table 4.6: Inuen e of the foam omposition and density on me hani al results.

The elasti ity modulus was al ulated a ording to these international standards. Theplateau stress and densi ation strain are two important parameters to hara terise theme hani al properties, whi h have been used widely in the design of ellular solids. Themodulus of elasti ity, m, the slope of the quasi-elasti region of the Foam I are higherBruno Pinto Lopes MIEM Dissertation

90 4.Results and Dis ussionthan the values of the Foams II. The range of the elasti ity is 147-106 MPa and 64-27MPa, respe tively for the foams I and foams II foams (Table 4.6). The beginning strainfor densi ation of type II is higher than the values of the Foams I. The average plateaustress in rease with the in reasing of the foam density for both Al-alloy foams (Fig. 4.39).The results also revealed that the plateau and the end of its region is mu h shorter in thefoams-I than the foams-II. The foams present values below 50% strain in the beginningof the densi ation regions.The ompression behaviour of these Al-alloys foams depends on parameters su h as thedensity, the foam morphology ( ell size range), the defe ts of ellular stru ture and the hara teristi s of the external surfa e skin. However, the ompression results show thatthere are dieren es between the Al-alloy foams studied. Foams I present a slight in reaseof the stress value during its plateau (Fig. 4.37a). For example, omparing the values ofthe stress for the values of 20%, 30% of strain in the ase of these foams, the dieren eis signi ant. Foams II exhibits a plateau region where the stress value is pra ti ally onstant during the whole plateau.

Figure 4.39: Average plateau stress for both studied Al-alloy foams.

Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 914.4.4 Energy absorptionThe energy absorption apability of the foams was estimated from the area under theload-displa ement urve, as shown in Figure 4.40. The absorption energy per unit volumewas determined from the area under the stress-strain urve, as shown in Figure 4.41 Table4.7 present the energy absorption apability and absorption energy per unit volume at50% strain for both foams.Table 4.7: Energy absorption apability and absorption energy per unit volume at 50%strain and at densi ation strain.

As foamed materials often exhibit a steady load "plateau", they an absorb higher levelsof energy than dense alloys [7. The energy absorption in reases with the in reasing ofthe foam density. Foams I exhibit higher energy absorption apabilities when omparedto foams II, as it an be seen from the results in Table 4.7 and Figs 4.40 and 4.41.The dieren e between these two behaviours is due to the dierent stru tural propertiesof these foams su h as, the mi rostru ture of the base material whi h dene the ell walls,the presen e of the mi ro pores and/ or ra ks, the thi kness of the dense surfa e skin ofthese foams.The shape of the energy urves of both foams studied is similar. On e more, these resultsreveal that these values rise with the in rease of the foam density. Of the Al-alloy foamsstudied, the foams-I have higher energy absorption apabilities than the Foams-II.Bruno Pinto Lopes MIEM Dissertation

92 4.Results and Dis ussion

(a) Foams I

(b) Foams IIFigure 4.40: Energy absorption apabilities of Al-alloy foams.Bruno Pinto Lopes MIEM Dissertation

4.Results and Dis ussion 93

(a) Foams I

(b) Foams IIFigure 4.41: Energy absorption per unit volume of Al-alloy foams.Bruno Pinto Lopes MIEM Dissertation

94 4.Results and Dis ussionThis page intentionally left blank-

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Chapter 5Con lusionsSeveral on lusions were drawn during the exe ution of this work for the produ tion ofAluminium alloy foams by powder metallurgy pro ess:(i) Aluminium-alloy powder: Not all ommer ial Aluminium alloys exhibit a thermalbehaviour that gives high quality foams. Some of the Al-alloy powders form gaseousprodu ts during its heating in air at a onstant rate whi h auses the formation of ra ks in the ompa ted material that propagates to lead omponents with large ra ks, or even its disintegration.(ii) Titanium Hydride powder: The me hanisms of thermal de omposition of the Ti-tanium Hydride and the melting pro ess of the alloy ould be ontrolled throughheating treatments that promote the formation of an oxide layer on the surfa e ofthe parti les of Hydride Titanium that delay the gas release from the parti les.During the heating treatment of the Titanium Hydride powders at 480C during180 min o urs the oxidation of the powders, as well as the loose of Hydrogen.However, the experimental results revealed that the remaining parti les of TiHxstill are su ient for the formation of foam. The oxidation of Titanium Hydridepowder (mass gain in the thermogravimetri urves) o urs more rapidly in nepowders. During heating of Titanium Hydride in Air at a onstant rate, there is omplex ompetition between the dehydrogenation and the oxidation of the TiH2.The oxidation of the powders o urs due the presen e of oxygen mole ules in the Airthat forms with Titanium the oxide surfa e layer. The release Hydrogen mole ules an rea t with Oxygen mole ules, leads to water mole ules. Limiting the supply ofthe Oxygen does not inuen e the hydrogenation, but it an ontrol the oxidation.(iii) Pre ursor material: Dense pre ursor material with density between 80% and 100% ould be obtained with a ombination of old and hot pressing at room temperatureand 400C during 5 min and 25 min, respe tively. The applied pressure was 200bar for both old and hot pressing.(iv) Aluminium-alloy foams: High quality Al-alloy ould be produ ed with an Al-alloypowder with a stable thermal behaviour (in this ase it was the Al-A alloy powder)and a Titanium Hydride powder with a de omposition temperature near to themelting point of the alloy. The results also have revealed that the size of the ellularpores depends on the parti le size of the Titanium Hydride used. Coarse ellular95

96 5.Con lusionspores are orresponding to the oarser parti le sizes of TiH2. The optimization ofthe foaming thermal y le ould be made for ea h metal/blowing agent system.The optimal temperature to obtaining quality foam was 750C.(v) Properties of Al-alloy foams: The density range of Al-alloy foams obtained was527.44-652.28 [kg/m3 and 421.8-487.8 [kg/m3 for the pre ursor material ontaining0.6 wt% TiH-1T and TiH-2T treated powders, respe tively. The properties of thesefoams are identi al to those obtained for other Al-alloy foams produ ed by the samemanufa turing method. The me hani al properties, su h as the plateau stress, theYoung's modulus, the energy absorption in reases with in reasing density.

Bruno Pinto Lopes MIEM Dissertation

Chapter 6Future worksDuring the elaboration of this thesis, some te hni al and experimental limitations on- erning the available equipment were en ountered. The following areas are worthy offurther resear h: The development of a new automated pressing devi e to produ e Al-alloy foamsmust be a omplished to implement the pro ess into an industrial s ale taking intoa ount the te hni al problems and limitations asso iated with the safety of theworkers. For example, the extra tion of the pre ursor tablet from the mould at theend of the pro ess must be improved and he handling of the mould at 400C mustalso be avoided. The improvement of the produ tion of the material pre ursor to de rease its time of y le of produ tion and to obtain pre ursor materials with higher density (100% ofthe theoreti al density). The ee t of the applied pressure used in ompa tion stephigher than the 200 bar used in this resear h on the nal properties of pre ursormust be studied. The study of the real intera tion between the thermal de omposition and oxidationof the blowing agents under air atmosphere in order to ontrol the ellular stru tureof the foams is an important issue whi h will be addressed in future work. The use of heaper raw materials to produ e Al-alloy foams must be investigatedin order to redu e the material and pro ess ost issues.

97

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Bibliography[1 BA Gama, TA Bogetti, BK Fink, C-J Yu, TD Claar, HH Eifert & JW Jr. Gille-spie. Aluminum foam integral armor: a new dimension in armor design. CompositeStru tures, 52:38195, (2001).[2 LJ Gibson & MF Ashby. Cellular Solids. Se ond Edition. (1997).[3 M.F. Ashby, A. Evans, N.A. Fle k, L.J. Gibson, J.W. Huthinson & H.N.G Wadley.Metal foams - a design guide. (2000).[4 J. Banhart. Manufa ture, hara terisation and appli ation of ellular metals andmetal foams. Progress in Materials S ien e, 46(6):559632, (2001).[5 B. Sosnik. Us patent 2 434 775, (1948).[6 J. C. Elliot. Us patent 2 751 289, (1956).[7 I. Duarte. Espumas metáli as pro esso de fabri o, ara terização e simulaçãonuméri a. PhD thesis, FEUP Fa ulty of Engineering of university of Porto, (2005).Portugal.[8 Glei h gmbh - alminium [internet. visited at (02/11/2011) available from:http://www.glei h.de.[9 Cymat [internet. visited at (02/11/2011) available from: http://www. ymat. om.[10 Alulight - international gmbh [internet. visited at (05/03/2011) available from:http://www.alulight. om.[11 Retimet - dunlop equipment [internet. visited at (23/04/2011) available from:http://dunlop-equipment. om/pro_retimet.htm.[12 Alantum innovations in alloy foam [internet. visited at (02/11/2011) available from:http://www.alantum. om.[13 Re emat international, metal foam, porous metal [internet. visited at (24/03/2011)available from: http://www.re emat. om.[14 J. Tavares. Inuên ia das ondições ambientais nas espumas de alumínio. Mas-ter's thesis, DEM Department od Me hani al Engineering of University of Aveiro,(2008). Portugal. 99

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BIBLIOGRAPHY 101[32 J. Banhart. Metalli foams ii: Properties and apppli ations. (presentation). FourthMarie Curie Summer S hool. Porous and Aqueous Materials, (2008) Trest, Cze hRepubli .[33 F.G. Figueiras. Espumas metáli as : Poten ial de inovação na onstrução naval.Jornadas do Mar, pages 232239, (2006).[34 Alusion appli ations. visited at (28/11/2011) available from:http://www.alusion. om/appli ations.aspx.[35 Proofs panels, aluminium foam. visited at (28/11/2011) available from:http://noisebarrier.org/soundbarrier/sound-proof-panels-aluminum-foam.html.[36 B. Matijasevi . Chara terization and optimization of blowing agent for making im-proved metal foams. PhD thesis, Te hnis he Universität Berlin, (2006). Germany.[37 B. Matijasevi -Lux, J. Banhart, S. Fie hter, O. Görke & N. Wanderka. Modi ationof titanium hydride for improved aluminium foam manufa ture. A ta Materialia,54:18871900, (2006).[38 B. Matijasevi & J. Banhart. Improvement of aluminium foam te hnology by tai-loring of blowing agent. S ripta Materialia, 54:503508, (2006).[39 M. Mukherjee. Evolution of Metal Foams during Solidi ation. PhD thesis, Te h-nishe niversität Berlin, (2009). Germany.[40 A.R. Gromov, N.N. Kouznetsova, S.L. Yudina & VN Lunin. The investigation oftitanium hydride oxidation pro ess. Journal of Alloys and Compounds, 261:269272,(1997).[41 H. Liu, P. He, J.C. Feng & J. Cao. Kineti study on nonisothermal dehydrogenationof tih2powders. International Journal of hydrogen energy, 34:30183025, (2009).[42 E. Illeková, J. Harnuskova, R. Florek, F. Siman ik, I. Ma'ko & P. Sve . Pe uliaritiesof tih2 de omposition. Journal of Thermal Analysis and Calorimetry, 105(2):583590, (2010).[43 M. B. Novikova & A. M. Ponomarenko. Kineti s of oxidation of titanium hydridepowder. Metal S ien e and Heat Treatment, 50:78, (2008).[44 C. Jiménez. Chara terization and modi ation of powders used to make aluminium-based metal foams. PhD thesis, Te hnique Universität Berlin, (2010).[45 A.R. Kennedy. The ee t of tih2 heat treatment on gas release and foaming inal-tih2 preforms. S ripta Materialia, 47:763767, (2002).[46 A. Kennedy & V. Lopez. The de omposition behavior of as-re eived and oxidizedtih2 foaming-agent powder. Materials S ien e and Engineering A, 357:258263,(2003).[47 M.T. Mala hevsky & C.A. D'Ovidio. Thermal evolution of titanium hydride opti-mized for aluminium foam fabri ation. S ripta Materialia, 61:14, (2009).Bruno Pinto Lopes MIEM Dissertation

102 BIBLIOGRAPHY[48 H. Wipf, B. Kappesser & R. Werner. Hydrogen diusion in titanium and zir oniumhydrides. Journal of Alloys and Compounds, 310:190195, (2000).[49 I. Paulin, C. Donik, D. Mandrino, M. Von ina & M. Jenko. Surfa e hara terizationof titanium hydride powder. Va uum, pages 16, (2011).[50 A. Ibrahim, K. Koerner & R. Singer. The ee t of tih2 parti le sizes on the morphol-ogy of al-foam produ ed by the powder ompa t melting pro ess (pm). Advan edEngineering Materials, No. 9:845848, (2008).[51 N. Kanetake, U. Hrupp, A. Ohrndorf, T. Guillen, T. Hipke, J. Hohlfeld, J. Aegerter,A. Danninger & M. Reinfried. Development of a Standard for Comression Testingof Cellular Metals. DESte h Publi ations, In , (2007).

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