Thermomechanical Treatments of Aluminum Alloys

Thermomechanical processes are defined in the broadest sense to include any combination of thermal or deformation processes that give rise to interactive microstructural features. The varieties of mechanisms involving the creation, rearrangement and elimination of dislocations and grain boundaries are reviewed to show the range of possibilities in microstructure and property production.

The interactions of dislocations and grain boundaries with solutes and second phase particles are examined, and the opportunities for synergistic combinations are discussed. The primary concern is for aluminum alloys, but attention is paid to contrasting and comparative alloy systems. The processes result in improvements in yield strength, toughness and resistance to stress corrosion cracking, fatigue and creep.

Traditionally the thermal treatment and deformation processing of metals were kept separate in both practice, and theory. This arose on one hand because austenitizing completely eradicated any worked structure and on the other because the martensitic steel vas very difficult to work.

Non-ferrous alloys were largely not heat-treatable and the introduction of ones that were did not immediately change preconceived notions. The shaping of metals, especially hot working, was considered solely as a mean of changing shape and metallurgical effort was directed at improving size capability, rate of processing and yield. Cold working and annealing were recognized as means of controlling the strength and the grain size but were considered techniques reserved for metals that were not heat treatable.

As knowledge grew of properties, microstructures, substructures, and mechanisms active during heat treatment and during deformation, and as the concept of strengthening mechanisms developed, the idea sprang up of superimposing mechanistic effects by combining processes. The term "thermomechanical processing" was coined to cover manufacturing in which a heat treatment and deformation were combined so that microstructural changes wrought by each interacted.

For the purposes of this paper, thermomechanical processing includes all combinations of thermal and mechanical treatments, irrespective of their order, which produce microstructural changes that do not obliterate each other. Another aspect which distinguishes such processing from simple forming operations is that the primary goal is product microstructure and properties, with the shape production and flow stresses being secondary.

The microstructures produced by forming processes over a broad range of temperatures are examined. This includes the effects on dynamic restoration mechanisms of solute, particle dispersions and massive second phase.

Cold and Warm Working. Deformation at room temperature is a simple technique which can be carried out with great precision and good surface finish. Even with good lubrication, very high forces are developed at high strains and, depending on the process, there are limits to ductility. When the rate of working is increased, adiabatic heating may raise the temperature so that warm working ensues. The limits of warm preheating for forging or impact extrusion are, set by the lubricant stability.

Hot Working - Dynamic Recovery. Deformation at temperatures above 0.5 Tm, above which dynamic recovery is dependent on climb and vacancy migration, gives rise to a polygonized substructure. During the initial strain-hardening phase, dislocations accumulate in regular tangles and sub-boundaries. In distinction from cold or warm working, stable equiaxed subgrains form and persist through steady state deformation (temperature, strain rate and stress constant) without change in size or wall density.

As the temperature is raised or the strain rate lowered, the steady state subgrains become larger and more perfect. As the degree of dynamic recovery increases, the hot flow stress decreases and the ductility increases.

Hot Working - Dynamic Recrystallization. The degree of high temperature dynamic recovery depends upon the stacking fault energy of the metal much as in cold working. Thus, for metals such as α-Fe and Zr, the substructures are very similar to those of Al. On the other hand, metals such as Ni and Cu have much smaller less perfect subgrains which have a higher dislocation density.

The recrystallization results in work softening to a new steady state in which the microstructure consists of equiaxed grains with a distribution of sizes. The larger grains are those which nucleated the longer time and strain before the point of observation and consequently have developed a high density dynamically recovered substructure. The preferred orientation after dynamic recrystallization appears to be the same as that developed with only dynamic recovery because of the continuing deformation.

The yield strength at room temperature follows the normal Petch relationship for grain size, but is stronger than recrystallized material of the same grain size because of the substructure.

Hot Working-Effects of Alloying. Solid solution alloying is likely to decrease the amount of dynamic recovery as it lowers the stacking fault energy as observed in Cu-Zn and Zr-Sn alloys. The effect on microstructure is not nearly so clear in other cases where a well defined substructure is produced, e.g. Al-Mg alloys, ferritic Fe-Si, Fe-Cr and Fe-Ni alloys, austenitic stainless steels, and nickel superalloys.

In the case of Al-Mg alloys the substructure depends on the absence or presence of dynamic strain aging. If the alloy composition, thermal history and deformation temperature and strain rate are such that the impurity atoms interact dynamically with the dislocations, they inhibit subgrain formation and change the activation energy. Somewhat similar effects are observed in Al-Cu alloys and in α-Fe -C-N alloys. Solute addition frequently delays dynamic recrystallization to higher strains since it inhibits grain boundary migration.

Fine, well-distributed particles can interact with the dislocations increasing their density, pinning the cell walls and in some cases may define the substructure. In hot working, the effect is more noticeable than in cold working, since the metal, without the particles, would have much larger subgrains. Particles which shear may prevent the formation of a substructure. Dynamic precipitation may also occur with one or other of the effects already mentioned. Fine particles on the sub-boundaries stabilize them slowing static recovery, and delaying or preventing static or dynamic recrystallization.

Static Recovery and Substructure Strengthening. If a metal containing a substructure is heated, the stored energy gives rise to static restoration processes. Recovery involves individual dislocation motions within the existing grains whereas recrystallization involves displacements of grain boundaries which eliminate the deformed structure entirely. Recovery usually always occurs first to some degree but can be the sole mechanism if a critical stored energy dependent on the annealing temperature is not reached. This critical stored energy is higher for metals which have dynamically recovered more at higher temperatures.

Product Strengthening by Recovered Substructure. Cold deformation followed by static recovery is a thermomechanical process used in industry mainly for stress relief anneals in which extensive recovery is not the object. It does appear, however, that in preparation for deep drawing of aluminum cans, annealing to polygonize the cold work substructure is employed to raise the strain hardening coefficient. Hot working is almost always followed by static recovery during the period after deformation until the temperature is too low for vacancy migration. In Al-alloys and ?-Fe alloys, the dynamically recovered structure is such that moderate rates of cooling can avoid recrystallization and permit static recovery that is limited to annihilation of the dislocations within the sub-grains.

Deformation Substructures and Precipitates. Precipitates or dispersed particles can be present at the start of deformation and help to define the substructure. The powder metallurgy pressing and sintering of Ni-ThO₂ alloys and the internal oxidation of Cu-Al₂O₃ alloys are the initial phases of thermomechanical treatments. To improve high temperature stability, CuAl₂ has been precipitated in Al-Cu alloys and Fe-Al₆ and (FeCo)Al₉ have been included in aluminum prior to working and annealing. However, as an example of a different possible phenomenon, the Cu precipitates in Fe-Cu alloys prevent the formation of a substructure.

Precipitates may form on a substructure created in the first step of a thermomehanical treatment, e.g. Cu precipitates on the dislocations of a cold worked and recovered substructure in Fe. The prior deformation usually accelerates the precipitation by providing more sites for heterogeneous nucleation, e.g. in austenite the rate of precipitation of Nb is increased an order of magnitude.

Thermomechanical Treatments for High Strength Aluminum Alloys. These processes have been called ITMT, intermediate thermomechanical treatments. The research in this area also indicates that improved properties can be achieved by extending structure control to earlier stages of processing.

Rapid cooling produces very fine dendrite arm spacing and thus finer inclusions and constituent particles which do not reduce the toughness as do coarser distributions. Powder metallurgy fabrication also reduces the problems of inclusions and large dendrite spacing. High purity alloys with reduced inclusion contents exhibit improved ductility, toughness, and resistance to fatigue and stress corrosion cracking.

For many years, a simple press heat treatment has been practiced, in which the hot working served as the solution treatment and the hot worked product had to be rapidly quenched. This gave fairly satisfactory results for Al-Cu (2000 series) and Al-Mg-Si alloys (6000 series) but was in general inappropriate for the Al-Zn-Mg-Cu alloys (7000 series with exception possibly of 7005 and 7039). The problems arise from three areas:

1. the hot working is not of adequate duration and precision for a solution treatment;
2. the quenching rates, because of the forming process are not rapid enough and
3. the heterogeneous nucleation on dislocations supplants the very fine uniform, partially-coherent precipitate needed for highest strength.

The high strength aluminum alloys of the Zn-Mg-Cu and Zn-Mg classes can have their toughness and resistance to fatigue and stress corrosion cracking improved by what is known as FTMT, final thermomechanical treatment. This consists of solution, preaging, deformation and final aging.

The preaging plays a very important role: carried out near or slightly below the temperature and time for normal aging, it provides a set of uniformly distributed nuclei which guarantees that homogeneous nucleation can compete with heterogeneous nucleation on dislocations during final aging.

Such preaging also provides for more uniform deformation in contrast to the heavy narrow slip bands which occur in slightly aged material. The deformations utilized have been in the range 10-50% and may be cold or warm. The substructure produced by the working should be uniform with slight cellularity being acceptable. The optimum appears to be deformation at or slightly above the aging temperature to strains of about 20%. Without the final aging the strength would be superior to simple aged material with ductility and toughness the same.

The second aging is frequently an overaging which lowers the strength to the normal value but increases the resistance to stress-corrosion cracking thus being somewhat similar to the second in a double aging treatment. Similar treatments have been worked out for Al-Mg-Si (6000 series), with similar restrictions on acceptable deformation structures. The Al-Cu alloys can usually be improved by working after the solution treatment but the conditions are not as restrictive as for the alloys with Zn and Mg.

In examining potential thermomechanical treatments, it is useful to include processing which involves only dislocation accumulation and restoration. Through manipulation of many variables, they are capable of yielding a spectrum of product microstructures either as complete processes or in conjunction with phase transformations.

Some of these dislocation and grain boundary manipulating treatments have been applied to Al-alloys, the most important being:

The high strength aluminum alloys can have their ductility, toughness and resistance to stress corrosion cracking greatly improved by a special preaging, warm working and post aging which does not detract from the normal precipitate distribution.