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
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
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
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
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
- the hot working is not of adequate duration and precision for a solution treatment;
- the quenching rates, because of the forming process are not rapid enough and
- 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:
- preservation of the hot worked substructure to strengthen non heat-treatable alloys,
- use of a particle stabilized subgrain structure to influence the evolution of the
substructure during cold drawing and annealing to produce electrical wire with improved
resistance to thermal softening,
- the establishment through control of ingot homogenization, rolling and annealing
of an equiaxed microstructure with well distributed constituent particles which on
further rolling to plate gives improved short transverse properties.
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.