Heat treating processes for aluminum are precision processes.
They must be carried out in furnaces properly designed and built
to provide the thermal conditions required, and adequately equipped
with control instruments to insure the desired continuity and
uniformity of temperature-time cycles. To insure the final desired
characteristics, process details must be established and controlled
carefully for each type of product.
The general types of heat treatments applied to aluminum and its alloys are:
- Preheating or homogenizing, to reduce chemical segregation of cast structures and to improve their workability
- Annealing, to soften strain-hardened (work-hardened) and heat treated alloy structures, to relieve stresses, and to stabilize properties and dimensions
- Solution heat treatments, to effect solid solution of alloying constituents and improve mechanical properties
- Precipitation heat treatments, to provide hardening by precipitation of constituents from solid solution.
INGOT PREHEATING TREATMENTS (HOMOGENIZING)
The initial thermal operation applied to ingots prior to hot working is
referred to as "ingot preheating", which has one or more purposes
depending upon the alloy, product, and fabricating process involved.
One of the principal objectives is improved workability. The
microstructure of most alloys in the as-cast condition is quite
heterogeneous. This is true for alloys that form solid solutions
under equilibrium conditions, and even for relatively dilute alloys
The distorted, dislocated structure resulting from cold working of
aluminum is less stable than the strain-free, annealed state, to which
it tends to revert. Lower-purity aluminum and commercial aluminum alloys
undergo these structural changes only with annealing at elevated
temperatures. Accompanying the structural reversion are changes in the
various properties affected by cold working. These changes occur in
several stages, according to temperature or time, and have led to the
concept of different annealing mechanisms or processes.
Recovery. The reduction in the number of dislocations is greatest
at the center of the grain fragments, producing a subgrain structure
with networks or groups of dislocations at the subgrain boundaries.
With increasing time and temperature of heating, polygonization becomes
more nearly perfect and the subgrain size gradually increases.
In this stage, many of the subgrains appear to have boundaries
that are free of dislocation tangles and concentrations.
Recovery annealing is also accompanied by changes in other properties
of cold worked aluminum. Complete recovery from the effects of cold
working is obtained only with recrystallization.
Recrystallization is characterized by the gradual
formation and appearance of a microscopically resolvable grain structure.
The new structure is largely strain-free-there are few if any dislocations
within the grains and no concentrations at the grain boundaries.
Grain Growth After Recrystallization. Heating after
recrystallization may produce grain coarsening. This can take one of
General Principles of Precipitation Hardening. The heat
treatable alloys contain amounts of soluble alloying elements that exceed
the equilibrium solid solubility limit at room and moderately higher
temperatures. The amount present may be less or more than the maximum that
is soluble at the eutectic temperature.
Nature of Precipitates and Sources of Hardening. Intensive
research during the past forty years has resulted in a progressive
accumulation of knowledge concerning the atomic and crystallographic
structural changes that occur in supersaturated solid solutions during
precipitation and the mechanisms through which the structures form and
alter alloy properties. In most precipitation-hardenable systems, a complex
sequence of time-dependent and temperature-dependent changes is involved.
Kinetics of Solution and Precipitation. The relative rates
at which solution and precipitation reactions occur with different solutes
depend upon the respective diffusion rates, in addition to solubilities and
alloy contents. Bulk diffusion coefficients for several of the commercially
important alloying elements in aluminum were determined by various
Nucleation. The formation of zones can occur in an
essentially continuous crystal lattice by a process of homogeneous
nucleation. Recent investigations provide evidence that a critical
vacancy concentration is required for this process and that a nucleation
model involving vacancy-solute atom clusters is consistent with certain
effects of solution temperature and quenching rate.
The nucleation of a new phase is greatly influenced by the existence
of discontinuities in the lattice. Since in polycrystalline alloys grain
boundaries, subgrain boundaries, dislocations, and interphase boundaries
are locations of greater disorder and higher energy than the solid-solution
matrix, they are preferred sites for nucleation of precipitates.
Quenching is in many ways the most critical step in the sequence of heat
treating operations. The objective of quenching is to preserve as nearly
intact as possible the solid solution formed at the solution heat treating
temperature, by rapidly cooling to some lower temperature, usually near
Critical Temperature Range. The fundamentals involved in
quenching precipitation-hardenable alloys are based on nucleation theory
applied to diffusion-controlled solid state reactions. The effects of
temperature on the kinetics of isothermal precipitation depend principally
upon degree of supersaturation and rate of diffusion.
Quenching Medium. Water is not only the most widely used
quenching medium but also the most effective. It is apparent that in
immersion quenching, cooling rates can be reduced by increasing water
temperature. Conditions that increase the stability of a vapor film
around the part decrease the cooling rate; various additions to water
that lower surface tension have the same effect.
Aging at Room Temperature (Natural Aging)
Most of the heat treatable alloys exhibit age hardening at room temperature
after quenching, the rate and extent of such hardening varying from one
alloy to another. No discernible microstructural changes accompany the
room-temperature aging, since the hardening effects are attributable
solely to the formation of zone structure within the solid solution.
Since the alloys are softer and more ductile immediately after quenching
than after aging, straightening or forming operations may be performed
more readily in the freshly quenched condition.
Precipitation Heat Treating (Artificial Aging)
The effects of precipitation on mechanical properties are greatly
accelerated, and usually accentuated, by reheating the quenched material
to about 100 to 200oC. The effects are not entirely attributable to a
changed reaction rate; as mentioned previously, the structural changes
occurring at the elevated temperatures differ in fundamental ways from
those occurring at room temperature. These differences are reflected in
the mechanical characteristics and some physical properties.
A characteristic feature of elevated-temperature aging effects on
tensile properties is that the increase in yield strength is more
pronounced than the increase in tensile strength. Also ductility, as
measured by percentage elongation, decreases. Thus, an alloy in the
T6 temper has higher strength but lower ductility than the same alloy
in the T4 temper.
Precipitation Heat Treating Without Prior SoIution Heat Treatment
Certain alloys that are relatively insensitive to cooling rate during
quenching can be either air cooled or water quenched directly from a final
hot working operation. In either condition, these alloys will respond
strongly to precipitation heat treatment.
Precipitation Heat Treating Cast Products
The mechanical properties of permanent mold, sand, and plaster castings
of most alloys are greatly improved by solution heat treating, quenching,
and precipitation heat treating, using practices analogous to those employed
for wrought products.
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