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Soldagem Insp. So Paulo, Vol. 13, No. 3, p.255-263, Jul/Set 2008 255

Microstructure and Heat Treatment Response of 2014-T6 GMAW Welds Obtained with a Novel Modified IndirectElectric Arc Joint

(Microestructura y Respuesta al Tratamiento Trmico de Soldaduras GMAW de 2014-T6 Obtenidas con la Junta de Arco

Elctrico Indirecto Modificado)

Ricardo R. Ambriz

1

, Gerardo Barrera

1

, Rafael Garca

1

, Victor H. Lpez

1

1Instituto de Investigaciones Metalrgicas, Universidad Michoacana de San Nicols de Hidalgo, A.P. Post 888, Morelia, Michoacn,

Mxico. C.P. 58000. Tel./Fax; 52 443 3168355.

E-mail; [email protected],gbar r [email protected], r ga r cia@jupite r .umich.mx, [email protected]

Abstract

In this study, plates of a 2014-T6 aluminium alloy with a thickness of 12.7 mm were welded with the GMAW process using a single

V groove joint and a novel modified indirect electric arc (MIEA) joint. Whilst the later enables welding of the plates in a single pass

with a non-heat treatable ER4043 electrode, the first needs a multi-pass procedure. These joint designs were compared in terms of

microstructure and mechanical behavior in the as-welded and post weld heat treated (PWHT) conditions. It was found that the single

V groove joint failed in the heat affected zone (HAZ) with a tensile strength of 159 MPa whereas the novel joint failed in the weld

metal with values ranging from 257 to 240 MPa depending on the preheating temperature of the joint. PWHT of the welds increasedthe tensile strength of the single V groove joint to 268 MPa and to 396 343 MPa for the MIEA joint with failure occurring

consistently in the weld metal. As the mechanical strength of the ER4043 electrode (190 MPa) is well below these values, the results

are discussed in terms of dilution ratios, microstructure and microhardness profiles across transverse sections of the welds.

Key Words: 2014-T6 aluminum alloy; Modified indirect electric arc (MIEA); Heat treatment; Mechanical properties; Dilution ratios.

Resumen: En el presente estudio, se soldaron placas de la aleacin de aluminio 2014-T6 con un espesor de 12.7 mm por medio del

proceso de soldadura GMAW utilizando la junta en simple V y la junta del arco elctrico indirecto modificado (AEIM). Se encontr

que el nuevo diseo de junta permite soldar las placas en una sola pasada de soldadura con un electrodo ER-4043 mientras que la

junta en simple V requiere un procedimiento de pasadas mltiples. El diseo de las juntas se comparo en trminos de la

microestructura y el comportamiento mecnico en las condiciones de tal como fue soldado y con un tratamiento trmico posterior a lasoldadura (TTPS). Las soldaduras con la junta en simple V fallaron en la zona afectada por el calor (ZAC) con una resistencia a la

tensin de 159 MPa, mientras que en la nueva junta las probetas fallaron en el metal de la soldadura con valores de 257 a 240 MPa

dependiendo de la temperatura de precalentamiento de la junta. TTPS de las soldaduras incremento la resistencia a la tensin de

la junta en simple V a 268 MPa y a 396 343 MPa para la junta AEIM, para ambas juntas la falla ocurri consistentemente en el

metal de la soldadura. Como los valores estn por encima de la resistencia mecnica del electrodo ER-4043 (190 MPa), los

resultados se discuten en trminos de la dilucin del material base, microestructura y perfiles de microdureza en las secciones

transversales de las soldaduras.

Palabras clave: Aleacin 2014-T6; Arco elctrico indirecto modificado (AEIM); Tratamiento trmico; Propiedades mecnicas,

Dilucin.

1. Introduction.

Heat treatable Al-Cu alloys are widely used in truck frames,

aircraft parts, tanks for cryogenic applications and so forth.

In particular, the 2014-T6 aluminium alloy is used in aircraft

components under tension loads, such as wings, due to its good

mechanical properties. Although t he mechanical properties of

the 7075-T6 alloy are higher than the 2014-T6 alloy, the first is

more sensible to cracking and therefore its fracture propagation

is rapid [1-3]. Owing to its high specific properties, the 2014-

(Recebido em 29/01/2008; Texto Final em 03/08/2008).

T6 alloy is often used in structural applications where joining is

required. Welding of aluminium alloys is not, however, an easy

task, there are a number of issues that determine the mechanical

behavior of both fusion zone and HAZ. To start with, the high

solubility of hydrogen in liquid aluminium might lead to a

porous weld metal if care is not taken to avoid sources of

hydrogen [4-7]. The thermal cycle imposed during fusion

welding of heat treatable aluminium alloys is known to

dramatically change the metallurgical conditions of the base

metal due to the generation of an overaged zone with very low

mechanical properties [8-10]. The 2014-T6 alloy is not an

exemption to this phenomenon [11,

12]. The high thermal conductivity of aluminium and its alloys

also contributes to the later phenomenon, since the heat input
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by the electric arc is dissipated very quickly by conduction in


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256 Soldagem Insp. So Paulo, Vol. 13, No. 3, p.255-263, Jul/Set 2008

Microstructure and Heat Treatment Response of 2014-T6 GMAW Welds Obtained with a Novel Modified Indirect Electric Arc Joint

the bulk of the base material, the energy required to succeed

welding is therefore increased.

The 2014 alloy is mainly alloyed with copper and in order

to achieve its maximum hardening via a T6 heat treatment, it

follows a complex microestructural evolution which depends

on the supersaturation grade of the solid solution, , and the

aging temperature as well as holding time [13]. It is established

that the hardening mechanism by precipitation in Al-Cu alloysproceeds as follows;

Supersatured Solid Solution (SSSS) GP zones (1)

The above precipitation sequence is completed when the

alloy has been aged below the Guinier-Preston (GP) solvus

temperature [14, 15]. In a welding process, however, the

temperature in the HAZ exceeds by far the GP solvus line and

the mechanical properties are modified due to profound

metallurgical transformations. These transformations are further

enhanced by the high energy input when welding thick

aluminium sections with a multi-pass procedure [16].

In recent studies, the authors envisaged a new joint design,named modified indirect electric arc joint (MIEA), which

enables welding of Al plates up to 12.7 mm thick in a single

pass with an ER4043 filler wire [17]. This is also achieved with

the original MIG-IEA joint but there is the need of removing

the residual feeding strips on top of the weld [18]. A

comparison of the mechanical properties of 6061-T6 welded

joints obtained

using the GMAW process with the single V groove and MIEA

joints [17] showed that the later joint design yields the highest

tensile strength in the as welded condition. Besides, the high

dilution rates of the MIEA joint give rise to alloying of the weld

metal, making it sensible to heat treating [19].

In this context, it is worth to assess the use of the MIEA

joint to weld high strength heat treatable 2000 series aluminium

alloys in which the loss of mechanical properties after fusionwelding is very large and the recovery with a PWHT is

limited when welding with a non heat treatable filler wire.

Thus, the goal of this investigation is to compare the

mechanical properties 2014- T6 GMAW welds obtained with

the MIEA and single V groove joints in the as-welded and

PWHT conditions.

2. Experimental.

Plates of a commercial 2014-T6 aluminium alloy with a

thickness of 12.7 mm and 150 mm in length were used in this

study along with a non-heat treatable ER4043 filler wire, 1.2

mm in diameter, which was employed to weld the plates. The

chemical composition of both materials is specified in Table 1.

Figure 1 shows the configuration of the joints; 2a) the typical

single V groove butt joint [16] and 2b) the novel MIEA joint

design which has a little lash in the upper side of the plates.

The gap of the joints was aligned with a groove on the backing

plate.

Table 1. Chemical composition of the materials employed (%wt.).

2014-T6

Cr Cu Fe Mg Mn Ni Pb Si Ti Zn Zr Al

0.020 4.148 0.418 0.660 1.162 0.028 0.014 0.792 0.028 0.165 0.012 Bal.

Electrode ER4043

Cr Cu Fe Mg Mn Ni Pb Si Ti Zn Zr Al

-- 0.300 0.800 0.050 0.050 -- -- 5.250 0.200 0.100 -- Bal

Figure 1. Joint designs; a) traditional single V groove and b) modified indirect electric arc joint (MIEA).

A semi-automatic GMAW process was employed with a

constant-voltage power source of 300 amperes and a voltage

range of 0 to 50 volts. A spray transfer mode was obtained with

the welding parameters shown in Table 2. Measurement of the

current was performed using a Hall effect sensor. Digitalization

of the signal was carried out with a data acquisition unit, NI

USB-6800, at a sampling speed of 100 readings each second.

A program in language G was used for data collection. Direct

current-reverse polarity and Ar shielding gas, flowing at 23.6

L min-1, were employed to weld the plates at a travel speed of

3.6 mm s-1 for both joint designs. To achieve full penetrated

welds in a sole welding pass, the MIEA joint requires

preheating [17, 18, 20] and a traveling speed faster than 3.6 mm

s-1

yields partially penetrated welds. To study the effect of


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256 Soldagem Insp. So Paulo, Vol. 13, No. 3, p.255-263, Jul/Set 2008

preheating

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