Heat Treating of Aluminum Alloys

ASM Handbook, Volume 4: Heat Treating Copyright © 1991 ASM International® ASM Handbook Committee, p 841-879 All rights reserved. DOI: 10.1361/asmhba0001205 www.asminternational.org Heat Treating of Aluminum Alloys HEAT TREATING in its broadest sense, • Aluminum-copper-magnesium systems The mechanism of strengthening from refers to any of the heating and cooling (magnesium intensifies precipitation) precipitation involves the formation of co- operations that are performed for the pur- • Aluminum-magnesium-silicon systems herent clusters of solute atoms (that is, the pose of changing the mechanical properties, with strengthening from Mg2Si solute atoms have collected into a cluster the metallurgical structure, or the residual • Aluminum-zinc-magnesium systems with but still have the same crystal structure as stress state of a metal product. When the strengthening from MgZn2 the solvent phase). This causes a great deal term is applied to aluminum alloys, howev- • Aluminum-zinc-magnesium-copper sys- of strain because of mismatch in size be- er, its use frequently is restricted to the tems tween the solvent and solute atoms. Conse- specific operations' employed to increase quently, the presence of the precipitate par- strength and hardness of the precipitation- The general requirement for precipitation ticles, and even more importantly the strain hardenable wrought and cast alloys. These strengthening of supersaturated solid solu- fields in the matrix surrounding the coher- usually are referred to as the "heat-treat- tions involves the formation of finely dis- ent particles, provide higher strength by able" alloys to distinguish them from those persed precipitates during aging heat treat- obstructing and retarding the movement of alloys in which no significant strengthening ments (which may include either natural aging dislocations. The characteristic that deter- can be achieved by heating and cooling. The or artificial aging). The aging must be accom- mines whether a precipitate phase is coher- latter, generally referred to as "non-heat- plished not only below the equilibrium solvus ent or noncoherent is the closeness of treatable" alloys, depend primarily on cold temperature, but below a metastable miscibil- match or degree of disregistry between work to increase strength. Heating to de- ity gap called the Guinier-Preston (GP) zone atomic spacings on the lattice of the matrix crease strength and increase ductility (an- solvus line. The supersaturation of vacancies and on that of the precipitate. These nealing) is used with alloys of both types; allows diffusion, and thus zone formation, to changes in properties result from the forma- metallurgical reactions may vary with type occur much faster than expected from equi- tion of solute-rich microstructural domains, of alloy and with degree of softening desired. librium diffusion coefficients. In the precipi- or GP zones. Except for the low-temperature stabilization tation process, the saturated solid solution The exact size, shape, and distribution of treatment sometimes given for 5xxx series first develops solute clusters, which then be- GP zones depend on the alloy in which they alloys (which is a mill treatment and not come involved in the formation of transitional form and on the thermal and mechanical discussed in this article), complete or partial (nonequilibrium) precipitates. history of the specimen. Their shape can annealing treatments are the only ones used for non-heat-treatable alloys. A general overview of these heat treatments is covered in the article "Principles of Heat Treating of Nonferrous Alloys" in this Volume. 8°°/ L ,400 Precipitation from Solid Solution One essential attribute of a precipitation- hardening alloy system is a temperature- ////I////I/.'////////I/I///Jf..z,~/7/////x/~ - 1000 dependent equilibrium solid solubility char- 0 / I I I Temperaturerange for u_ acterized by increasing solubility with ! AI ]~ J i i solution heat treating ~_ increasing temperature (see, for example, the phase diagrams in Fig 1 and 2). Al- though this condition is met by most of the ~~~'~ L Temper;turerange E binary aluminum alloy systems, many exhibit very little precipitation hardening, and ~~'~~"~ J ~i~ iiai[ilran efor -- 600 1- these alloys ordinarily are not considered heat treatable. Alloys of the binary alumi- 200 precipitationheat num-silicon and aluminum-manganese sys- ~ J treating I tems, for example, exhibit relatively insig- nificant changes in mechanical properties as A,+ooA,2 I I i 200 a result of heat treatments that produce considerable precipitation. The major alu- ol I l t I minum alloy systems with precipitation 0 2 8 10 12 hardening include: Copper, % Portion of aluminum-copper binary phase diagram. Temperature rangesfor annealing, precipitation heat • Aluminum-copper systems with strength- Fig 1 treating, and solution heat treating are indicated. The range for solution treating is below the eutectic ening from CuAI 2 melting point of 548 °C (1018 °F) at 5.65 wt% Cu. 842 / Heat Treating of Nonferrous Alloys 700 The GP zones are characteristically meta- nar aggregates (GP zones), which form on 1200 stable and thus dissolve in the presence of a particular crystallographic planes of the alu- 600 more stable precipitate. This dissolution minum matrix. These aggregates create co- Solidus j~__ 1000 causes a precipitate-free, visibly denuded herency strain fields that increase resis- region to form around the stable precipitate tance to deformation, and their formation is 500 / 595 °C u_ particles. The final structure consists of responsible for the changes in mechanical ? J at 1.85% 800 oa; equilibrium precipitates, which do not con- properties that occur during natural aging. 40o ~.~ Mg2Si _ tribute as significantly to hardening. More At higher temperatures, transition forms of Solvus detailed information about preprecipitation approximate composition AI2Cu develop ~- 300 6oo & E E phenomena can be found in the article and further increase strength. In the highest / "Structures Resulting From Precipitation strength condition, both the 0" and 0' tran- 2OO 400 From Solid Solution" in Volume 9 of the sition precipitates may be present. When I 9th Edition of Metals Handbook. time and temperature are increased suffi- Precipitation in Aluminum-Copper Alloys. ciently to form high proportions of the equi- 100 I 200 I Figure l, which illustrates the required sol- librium 0, the alloy softens and is said to be Mg-Si ratio of 1.73:1 ubility-temperature relationship needed in "overaged." I I I 0.5 1.0 1.5 2.0 precipitation strengthening, shows the tem- The commercial heat-treatable aluminum Mg2Si, % perature ranges required for solution treat- alloys are, with few exceptions, based on (a) ment and subsequent precipitate hardening ternary or quaternary systems with respect in the aluminum-copper system. The equi- to the solutes involved in developing Temperature, °F librium solid solubility of copper in alumi- strength by precipitation. Commercial al- 570 660 750 840 930 10201110 num increases as temperature increases-- loys whose strength and hardness can be 1.0 1.4 from about 0.20% at 250 °C (480 °F) to a significantly increased by heat treatment maximum of 5.65% at the eutectic melting include 2xxx, 6xxx, and 7xxx series wrought 0.8 Solvus with silicon _ ~ 1.2 o~ and Mg2Si present /0_ 1.0 temperature of 548 °C (1018 °F). (It is con- alloys (except 7072) and 2xx.0, 3xx.0, and siderably lower than 0.20% at temperatures 7xx.O series casting alloys. Some of these o.6 0.8 below 250 °C.) For aluminum-copper alloys contain only copper, or copper and silicon, 0.6 § containing from 0.2 to 5.6% Cu, two distinct as the primary strengthening alloy addi- == 0.4 equilibrium solid states are possible. At tion(s). Most of the heat-treatable alloys, 0.4 ~ temperatures above the lower curve in Fig 1 however, contain combinations of magne- 0.2 .o.~ 0.2 (solvus), the copper is completely soluble, sium with one or more of the elements and when the alloy is held at such temper- copper, silicon, and zinc. Characteristical- o 0 " atures for sufficient time to permit needed ly, even small amounts of magnesium in 300 350 400 450 500 550 600 diffusion, the copper will be taken com- concert with these elements accelerate and Temperature, °C pletely into solid solution. At temperatures accentuate precipitation hardening, while (b) below the solvus, the equilibrium state con- alloys in the 6xxx series contain silicon and Equilibrium solubility as function of tempera- sists of two solid phases: solid solution, ct, magnesium approximately in the propor- Fig 2 ture for (a) Mg2Si in aluminum with an Mg-Si plus an intermetallic-compound phase 0 tions required for formulation of magnesium ratio of 1.73-to-1 and (b) magnesium and silicon in solid (AIECU). When such an alloy is converted to silicide (MgESi). Although not as strong as aluminum when both Mg2Si and silicon are present all solid solution by holding above the sol- most 2xxx and 7xxx alloys, 6xxx series al- vus temperature and then the temperature is loys have good formability, weldability, ma- sometimes be deduced by refined studies of decreased to below the solvus, the solid chinability, and corrosion resistance, with diffuse x-ray scattering. Under favorable solution becomes supersaturated and the medium strength. conditions, GP zones can be seen in trans- alloy seeks the equilibrium two-phase con- In the heat-treatable wrought alloys, with mission electron micrographs. Spherical dition; the second phase tends to form by some notable exceptions (2024, 2219, and solute-rich zones usually form when the solid-state precipitation. 7178), such solute elements are present in sizes of the solvent and solute atoms are The preceding description is a gross over- amounts that are within the limits of mutual nearly equal, as in the aluminum-silver and simplification of the actual changes that solid solubility at temperatures below the aluminum-zinc systems.

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