artigo sobre alumínio osmar

8/12/2019 Artigo Sobre Alumnio Osmar

1/10

Aluminum-Silicon Alloys

Abstract:Castings are th e main use o aluminum-silicon alloys, although some sh eet or wire ismade for welding and brazing, and some o the piston alloys are extruded for forgingstock. Often the brazing sheet has only a cladding o aluminum-silicon alloy and thecore consists o some other high melting alloy.The copper-free alloys are used for low- to medium-strength castings with goodcorrosion resistance; the copper-bearing for medium- to high-strength castings, wherecorrosion resistance is not critical. Because o their excellent castability, it is possible toproduce reliable castings, even in complex shapes, in which the minimum mechanicalproperties obtained in poorly fed sections are higher than in castings made from higherstrength but lower-castabili ty alloys.

Castings are the main use of aluminum-silicon alloys, although some sheet or wire ismade for welding and brazing, and some of the piston alloys are extruded for forgingstock. Often the brazing sheet has only a cladding o aluminum-silicon alloy and thecore consists of some other high melting alloy.The copper-free alloys are used for low- to medium-strength castings with goodcorrosion resistance; the copper-bea ring for medium- to high-strength castings, whereco rrosion resistance is not critica l. Because of their excellent castabi li ty, it is possible toproduce reliable castings, even in complex shapes, in which the minimum mechanicalproperties obtained in poorly fed sections are higher th an in castings made from higher-strength but lower-castability alloys. The alloys o this group fall within the compositionlimits:

[Si]5-25% IMn, Cr, Co, Mo Ni, Be, Zr]cu ]0-5% ] eIMg lo-2% INa , SrZrllo-3% P

up to 3]up to 3I< .o2%1


2/10

Cobalt, ch romium, manganese, molybdenum and nickel are sometimes added asco rrectives for iron; their addition also improves strength at high temperature. Copper isadded to increase the strength and fatigue resistance without loss of castability, but atthe expense of corrosion resistance. Magnesium especially after heat treatment,increases substantially the strength, but at the expense of ductility.Zinc is a tolerated impurity in many alloys, often up to 1.5-2 Zn because it has nosubstantial effect on room-temperature properties. Titanium and boron are sometimesadded as grain refiners, although grain size in these alloys is not too important, becausethe properties are mainly controlled by the amount and structure of the silicon, asaffected by modification produced by sodium additions or by phosphorus additions.A distinction between dissolved and 'graphitic' si licon is sometimes made by dissolvingthe alloy in acids, in which the dissolved silicon transforms in Si 2 whereas the graphiticremains uncombined. Prolonged or repeated heating tends to spheroidise the silicon.This spheroidising is faster in mod ified alloys and results in a coarseni ng of the silicon toa size very close to that of non modified material. In the absence of copper the iron isusually in the AI-FeSiAI5-Si eutectic as thin platelets interspread with the silicon needlesor rods. If there is more than 0.8 Fe primary FeSiAI5 crystals appear.Titanium and boron are usually added in amounts well within their solid solubility and donot form any separate phase. Iron reduces their solubility, so that less is needed forgrain refinement; 0.1-0.2 Vis reported to refine the FeMn compounds. Tin and lead, ifpresent together with magnesium, tend to enter the Mg2Si phase. All the phases formedtend to concentrate at the grain boundaries, in the form of complex eutectics, more orless coupled.The lattice parameter is decreased slightly by sil icon in so lution and somewhat moreby copper; none of the other elements affects it apprec iably. Thus, the parameter of thealloys is between a= 4.045 x 10-10m and a= 4.05 x 10- m, depending on compositionand treatment.Thermal expansion is reduced substantially by si licon and much less pronouncedly byall other additions except magnesium, which tends to increase it slightly. Expansioncoefficients at subzero temperatures also are some 10-20 lower than those for purealuminum. A reduction of expansion coefficient by titanium and zirconium additions isreported, but it is very doubtful that it can be appreciable. Alloys produced by powdermetallurgy containing up to 50 Si have even lower expansion coefficients. Permanentexpansion accompanies precipitation out of solution of silicon, magnesium and copper;the amount varies but maybe as high as 0. 15 .Thermal conductivity is of the order of 1.2-1 .6 x 1o2W/m/K the lower values being forthe alloys cast in metallic molds or heat treated to re tain silicon, copper or magnesiumin solution.Electric conductivity depends mostly on the amount of silicon in solution; copper andmagnesium also affect it. Values of the order of 35-40 lACS for annealed materialsand of 22-35 lACS for solution treated alloys are reported. In the liquid state resistivityis some 10-15 times the resistivity at room temperature. Manganese, chromium ,titanium, zirconium also reduce conductivity, and so does modification.Magnetic susceptibility is only slightly decreased by si licon, copper and magnesium,but depends mostly on manganese content.Mechanical properties. Alloys prepared from powders exhibit somewhat higherstrengths, especially at elevated temperatures. In wrought products ultimate tensile


3/10

strengths of 200-400 MPa wi th elongation correspondingly from 20 to 2-3 areobtained. Poor casting technique may reduce the properties, although the aluminumsilicon alloys are among the least sensitive to such variables as gas content, design ofcastings, rate of cooling and feeding. High purity find special treatments can produceproperties some 10-20 better than average, and, conversely, secondary alloys tend tohave lower ductility than do primary ones. Casting under pressure improves propertiestoward those of forgings.Increasing si licon content increases strength at the expense o ductility, but this effect isnot very marked. Modification by sodium produces a limited increase o strength, but theincrease o ductility is substantial, especially in sand castings. At the higher coolingrates, normal with metal mold castings, the silicon is already somewhat refined withoutmodification and the improvement from modification is reduced. The effect o cell sizeand dendritic arm spacing on mechanical properties o alloys with Si > 8 is not veryma rked , but in lower-silicon alloys, in which the aluminum dendrites predominate , theeffect is normal.Iron may slightly increase the strength, but drastically decreases the ductility, especiallyif above 0.7 Fe and not corrected by manganese, cobalt, etc. Beryllium, manganese,chromium, molybdenum, nickel , cobalt and zirconium all slightly increase the strength;manganese, cobalt, nickel and molybdenum, i needed to correct for the iron, can alsoincrease the ducti lity; otherwise all of them reduce it. Beryllium is also reported tocorrect the iron effect. Copper and zinc increase the strength at the expense o ductility,but the most effective strengthener is magnesium, especially after heat treatment.provided that the amount and distribution of the magnesium are correct.Grain refinement by titanium , boron and zirconium additions has only a limited effecton mechanical properties. Silver additions are reported to increase the elongation.Antimony, tin, lead and cadmium decrease all properties, and antimony, by combiningwith magnesium, may reduce response to heat treatment. Calcium may increasestrength and decrease elongation in straight aluminum-silicon alloys, but it has adeleterious effect on piston alloys.Compressive strength is higher than tensile by some 10-15 . Shear strength isapproximately 70 of the tensile strength.Impact resistance is low, but so is notch sensitivity, as is to be expected in alloys thatcontain a large amount of hard, brittle second phase, often with sharp angles. Impactresistance is improved by spheroidising the silicon.The modulus o elasticity is o the order of 85-95 GPa, changing with temperature asdoes tensile strength. A decrease in damping capacity with aging is reported.Properties at cryogenic temperatures are higher than at room temperature; th ere islittle or no increase down to 170 K but at 70 K the strength has become some 20higher than at room temperature, with little or no decline in ductility. Notch strength doesnot change substantially at cryogenic temperatures. The effect o alloying elements oncryogenic properties is not too well established, but probably it is negligible.t high temperature the strength declines and the ductility increases. The decline isregular and more rapid than for other aluminum alloys except the aluminum-zincmagnesium group. The slight increase in strength shown by heat treatable alloys,especially i only naturally aged, is only temporary, once the averaging stage is reached ,there is a sharp drop and then the decline o strength with temperature becomesregular. Impact resistance increases with increasing temperature. At the higher


4/10

temperatures elements with high melting points (copper, iron, manganese, nickel,cobalt, chromium, tungsten) reduce to some extent the decline in strength, althoughtheir effect is not substantia l. Beryllium, too, is reported to improve the high-temperaturestrength. In spite of their poor high-temperature strength and fatigue resistance,aluminum-silicon alloys are used extensively for pistons because of their low expansioncoefficient, good wear resistance and good castability. Hypereutectic alloys with up to 2-3 additions o copper, nickel, iron, manganese, chromium or magnesium arepreferred, although good performance has been obtained also with hypoeutectic alloysand alloys low in heavy metals. Zinc, lead and tin decrease the high-temperaturestrength. Modified alloys have sl ightly lower high-temperature strength.Creep resistance is not particularly good. Silicon increases the creep resistance oaluminum much less than do most other alloying elements. Copper, iron, manganese,nickel, cobalt, chromium, etc. , increase it, as is to be expected, and so do magnesiumand rare earths.Fatigue resistance is re latively low, especially i the silicon is not modified or isspheroid ised by heat treatment. Cobalt and manganese may improve the fatigueresistance. Pressure during freezing increases the fatigue strength and wear resistance;surface defects and comp lex loads reduce it, especially at high temperature. Fatiguestrength drops gradually with temperature in straight aluminum-silicon, but there is nodrop up to 500 K in aluminum-copper-silicon alloys. The alloys are susceptible tothermal fatigue because of the substantial difference in expansion coefficient of thematrix and silicon particles.Wear resistance is very good, especia lly in hypereutectic alloys in which the hardsilicon particles are well distributed either by phosphorus nucleation or by powdermetallurgy fabrication, or in alloys to which bismuth has been added. Wear resistance ohigh-silicon alloys (20-25 is 10 times better than that of plain steel and comparablewith that of su rface hardened steel. Friction in couples o steel again st aluminum-siliconal loys decreases with surface perfection and hardness of the steel; however, aluminumsilicon alloys for bearings have not been successful unless they contain substantial tin.Corrosion resistance. Aluminum-silicon alloys without copper have good corrosionresistance in most reagents; only in alkaline solutions which attack silicon as well asaluminum their performance is poor. Copper reduces appreciably the corrosionresistance and so does iron, unless corrected with manganese or chromium. Zinc up to2-3 has no effect. Tin and calcium also have a deleterious effect on corrosionresistance. Porosity decreases corrosion resistance. Corrosion by flowing water is morerapid than in sti ll water, but of the same type. Aluminum-silicon alloys with iron andnickel have particularly good resistance to high-temperature water or steam. Insecondary alloys, where many elements are present in small amounts, zinc andmanganese compensate for copper and nickel , and corrosion resistance is reported asvery close to that of primary alloys. Contact corrosion is especia lly poor in aluminumsi licon-copper alloys, but even copper-ree alloys are worse in this respect thanaluminum 99.8 .Machinability is poor, because the extreme hardness of the silicon combined with therelative softness o the matrix tends to wear the tools very rapidly. In hypereutecticalloys phosphorus additions that improve the silicon distribution improve machinability;but in hypoeutectic alloys phosphorus tends to reduce it, whereas sodium improves it.Copper reduces further the machinab ility for the same silicon content, especially after


5/10

heat treatment but same o the copper-silicon alloys with low silicon may havemachinability equal to or better o high that o high-sil icon copper-free alloys. Ironmanganese nickel zinc titanium etc. do not decrease machinability


6/10

luminumAlloys Effects of lloying ElementsAbstract:The important alloying elements and impurities, that effect on aluminium alloys arelisted here alphabetically as a concise review of major effects. Some of the effects,particularly with respect to impurities, are not well documented and are specific toparticular alloys or conditions.The important alloying elements and impurities are listed here alphabetically as aconcise review of major effects. Some of the effects, particularly wi th respect toimpurities, are not well documented and are specific to particular alloys or conditions.Antimony is present in trace amounts (0.01 to 0.1 ppm) primary in commercial-gradealuminum. Antimony has a very small solid solubility in aluminum (


7/10

properties. Calcium combines with silicon to form CaSi2, which is almost insoluble inaluminum and therefore wi ll increase the conductivity o commercial-grade metalslightly. In aluminum-magnesium-silicon alloys, calcium will decrease age hardening. Itseffect on aluminum-silicon alloys is to increase strength and decrease elongation, but itdoes not make these alloys heat treatable.Carbon may occur infrequently as an impurity in aluminum in the form o oxycarbidesand carbides, o which the most common is AI4C3, but carbide formation with otherimpurities such as titanium is possible. A14C3 decomposes in the presence of waterand water vapor, and this may lead to surface pitting.Cerium, mostly in the form o mischmetal (rare earths with 50 to 60 Ce), has beenadded experimentally to casting alloys to increase fluidity and reduce die sticking.Chromium occurs as a minor impurity in commercial-purity aluminum (5 to 50 ppm). Ithas a large effect on electrical resistivity. Ch romium is a common addition to manyalloys of the aluminum-magnesium, aluminum-magnesium-silicon , and aluminummagnesium-zinc groups, in which it is added in amounts generally not exceeding0.35 . In excess of these limits, it tends to form very coarse constituents with otherimpurities or additions such as manganese, iron, and titanium. Chromium has a slowdiffusion rate and forms fine dispersed phases in wrought products. These dispersedphases inhibit nucleation and grain growth. Chromium is used to control grain structure ,to prevent grain growth in aluminum-magnesium alloys, and to prevent recrysta llizationin aluminum-magnesium-sil icon or aluminum-magnesium-zinc alloys during hot workingor heat treatment.Cobalt is not a common addition to aluminum alloys. It has been added to somealuminum-silicon alloys containing iron, where it transforms the acicular f3 (alum inumiron-silicon) into a more rounded aluminum-cobalt-iron phase, thus improving strengthand elongation. Aluminum-zinc-magnesium-copper alloys containing 0.2 to 1.9 Co areproduced by powder metallurgy.Copper. Aluminum-copper alloys containing 2 to 10 Cu , generally with other additions,form important families of alloys. Both cast and wrought aluminum-copper alloysrespond to solution heat treatment and subsequent aging with an increase in strengthand hardness and a decrease in elongation. The strengthening is maximum between 4and 6 Cu, depending upon the influence o other constituents present.Copper-magnesium. The main benefit of adding magnesium to aluminum-copper alloysis the increased strength possible following solution heat treatment and quenching. Inwrought material o certain alloys o this type, an increase in strength accompanied byhigh ductility occurs on aging at room temperature. On artificial aging, a further increa sein strength, especia lly in yield strength can be obtained, but at a substantial sacrifice intensile elongation.Copper-magnesium plus other elements. The cast aluminum-copper-magnesium alloyscontaining iron are characterized by dimensional stability and improved bearing


8/10

characteristics, as well as by high strength and hardness at elevated temperatures.However, in a wrought AI-4 Cu-0.5 Mg alloy, iron in concentrations as low as 0.5lowers the tensile properties in the heat-treated condition, if the silicon content is lessthan that required to tie up the iron as the aFeSi constituent.Ga llium is an impurity in aluminum and is usually present at levels of 0.001 to 0.02 .Atthese levels its effect on mechanical properties is quite small. At the 0.2 level, galliumhas been found to affect the corrosion characteristics and the response to etching andbrightening of some alloys.Hydrogen has a higher solubility in the liquid state at the melting point than in the solidat the same temperature. Because of this, gas porosity can form during solidification.Hydrogen is produced by the reduction of water vapor in the atmosphere by aluminumand by the decomposition of hydrocarbons. In addition to causing primary porosity incasting, hydrogen causes secondary porosity, blistering, and high-temperaturedeterioration (advanced internal gas precipitation) during heat treating. It probably playsa role in grain-boundary decohesion during stress-corrosion cracking. Its level in meltsis controlled by fluxing with hydrogen-free gases or by vacuum degassing.Indium. Small amounts (0.05 to 0.2 ) of indium have a marked influence on the agehardening of aluminum-copper alloys, particularly at low copper contents (2 to 3 Cu).Iron is the most common impurity found in aluminum. It has a high solubility in moltenaluminum and is therefore easily dissolved at all molten stages of production. Thesolubility of iron in the solid state is very low -0.04) and therefore, most of the ironpresent in aluminum over this amount appears as an intermetallic second phase incombination with aluminum and often other elements.Lead. Normally present only as a trace element in commercial-purity aluminum, lead isadded at about the 0.5 level with the same amount as bismuth in some alloys (2011and 6262) to improve machinability.Lithium. The impurity level of lithium is of the order of a few ppm, but at a level of lesstha n 5 ppm it can promote the discoloration (blue corrosion) of aluminum fo il underhumid conditions. Traces of lithium greatly increase the oxidation rate of moltenaluminum and alter the surface characteristics of wrought products.Magnesium is the major alloying element in the 5xxx series of alloys. Its maximum solidsolubility in aluminum is 17.4 , but the magnesium content in current wrought alloysdoes not exceed 5.5 . The addition of magnesium markedly increases the strength ofaluminum without unduly decreasing the ducti lity. Corrosion resistance and weldabilityare good.Magnesium-Manganese. In wrought alloys, this system has high strength in the workhardened condition, high resistance to corrosion , and good welding characteristics.Increasing amounts of either magnesium or manganese intensify the difficulty of


9/10

fabrication and increase the tendency toward cracking during hot rolling, particularly itraces of sodium are present.Magnesium-Silicon. Wrought alloys of the 6xxx group contain up to 1.5 each ofmagnesium and silicon in the approximate ratio to form Mg2Si, that is, 1.73:1. Themaximum solubil ity of Mg2Si is 1.85 , and this decreases with temperature.Precipitation upon age hardening occurs by formation o Guinier-Preston zones and avery fine precipitate. Both confer an increase in strength to these alloys, though not asgreat as in the case of the 2xxx or the 7xxx alloys.Manganese is a common impurity in primary aluminum, in which its concentrationnormally ranges from 5 to 50 ppm. It decreases resistivity. Manganese increasesstrength either in solid solution or as a finely precipitated intermetallic phase. It has noadverse effect on corrosion resistance. Manganese has a very limited solid solubility inaluminum in the presence of normal impurities but remains in solution when chill cast sothat most o the manganese added is substantially retained in solution, even in largeingots.Mercury has been used at the level o 0.05 in sacrificial anodes used to protect steelstructures. Other than for this use, mercury in aluminum or in contact with it as a metalor a salt will cause rapid corrosion o most aluminum alloys.Molybdenum is a very low level (0.1 to 1.0 ppm) impurity in aluminum. It has been usedat a concentration of 0.3 as a grain refiner, because the aluminum end of theequilibrium diagram is peritectic, and also as a modifier for the iron constituents, but it isnot in current use for these purposes.Nickel. The solid solubility o nickel in aluminum does not exceed 0.04 . Over thisamount, it is present as an insoluble intermetallic, usually in combination with iron.Nickel (up to 2 ) increases the strength o high-purity aluminum but reduces ductility.Binary aluminum-nickel alloys are no longer in use but nickel is added to aluminumcopper and to aluminum-silicon alloys to improve hardness and strength at elevatedtemperatures and to reduce the coefficient of expansion.Niobium. As with other elements forming a peritectic reaction, niobium would beexpected to have a grain refining effect on casting. It has been used for this purpose,but the effect is not 11_1arked.Phosphorus is a minor impurity 1 to 10 ppm) in commercial-grade aluminum. Itssolubility in molten aluminum is very low (-0.01 at 660oC) and considerably smaller inthe solid.Silicon, after iron, is the highest impurity level in electro lytic commercial aluminum 0.01to G.15 ). In wrought alloys, silicon is used with magnesium at levels up to 1.5 toproduce Mg2Si in the 6xxx series of heat-treatable alloys.


10/10

Vanadium. There is usually 10 to 200 ppm V in commercial-grade aluminum, andbecause it lowers conductivity, it generally is precipitated from electrical conductoralloys with boron.Zinc. The aluminum7zinc alloys have been known for many years, but hot cracking othe casting alloys and the susceptibility to stress-corrosion cracking of the wroughtalloys curtailed their use. Aluminum-zinc alloys containing other elements offer thehighest combination of tensile properties in wrought aluminum alloys.Zinc-Magnesium. The addition of magnesium to the aluminum-zinc alloys develops thestrength potential o this alloy system, especially in the range o 3 to 7,5 Zn.Magnesium and zinc form MgZn2, which produces a far greater response to heattreatment than occurs in the binary aluminum-zinc system. The strength o the wroughtaluminum-zinc alloys also is substantially improved by the addition of magnesium.Increasing the MgZn2, concentration from 0.5 to 12 in cold-water quenched 1.6 mmsheet continuously increases the tensile and yield strengths. The addition of magnesiumin excess (1 00 and 200 ) o that required to form MgZn2 further increases tensi lestrength.Zinc-Magnesium-Copper. Th e addition of copper to the aluminum-zinc-magnesiumsystem, together with small but important amounts of chromium and manganese,results in the highest-strength aluminum-base alloys commercially available. In this alloysystem, zinc and magnesium control the ag ing process. The effect of copper is toincrease the aging rate by increasing the degree of supersaturation and perhapsthrough nucleation of the CuMgAI2 phase. Copper also increases quench sensitivityupon heat treatment. In general, copper red uces the resistance to general corrosion ofaluminum-zinc-magnesium alloys, but increases the resistance to stress corrosion. Theminor alloy additions, such as chromium and zirconium, have a marked effect onmechanical properties and co rrosion resistance.Zirconium additions in the range 0. 1 to 0.3 are used to form a fine intermetallicprecipitate that inhibits recovery and recrys tallization. An increasing number of alloys,particularly in the aluminum-zinc-magnesium family, use zirconium additions to increasethe recrystallization temperature and to control the gra in structure in wrought products.

artigo sobre alumínio osmar

Documents