Heat treating in its broadest sense, refers to any of the heating and
cooling operations are performed for the purpose of changing the mechanical
properties, the metallurgical structure, or the residual stress state of a
When the term is applied to aluminum alloys, however, its use frequently
is restricted to the specific operations employed to increase strength and
hardness of the precipitation-hardenable wrought and cast alloys. These
usually are referred to as the "heat-treatable" alloys to distinguish them
from those alloys in which no significant strengthening can be achieved by
heating and cooling. The latter, generally referred to as "non heat-treatable"
alloys depend primarily on cold work to increase strength. Heating to decrease
strength and increase ductility (annealing) is used with alloys of both types;
metallurgical reactions may vary with type of alloy and with degree of
One essential attribute of a precipitation-hardening alloy system is a
temperature-dependent equilibrium solid solubility characterized by
increasing solubility with increasing temperature. The mayor aluminum
alloy systems with precipitation hardening include:
- Aluminum-copper systems with strengthening from CuAl2
- Aluminum-copper-magnesium systems (magnesium intensifies precipitation)
- Aluminum-magnesium-silicon systems with strengthening from Mg2Si
- Aluminum-zinc-magnesium systems with strengthening from MgZn2
- Aluminum-zinc-magnesium-copper systems
The general requirement for precipitation strengthening of supersaturated
solid solutions involves the formation of finely dispersed precipitates during
aging heat treatment (which may include either natural aging or artificial aging).
The aging must be accomplished not only below the equilibrium solvus temperature,
but below a metastable miscibility gap called the Guinier-Preston (GP) zone
The commercial heat-treatable alloys are, with few exceptions, based on ternary
or quaternary systems with respect to the solutes involved in developing strength
by precipitation. Commercial alloys whose strength and hardness can be significantly
increased by heat treatment include 2xxx, 6xxx, and 7xxx series wrought alloys and 2xx.0,
3xx.0 and 7xx.0 series casting alloys.
Some of these contain only copper, or copper and silicon as the primary strengthening
alloy addition. Most of the heat-treatable alloys, however, contain combinations of
magnesium with one or more of the elements, copper, silicon and zinc.
Characteristically, even small amounts of magnesium in concert with these elements
accelerate and accentuate precipitation hardening, while alloys in the 6xxx series
contain silicon and magnesium approximately in the proportions required for
formulation of magnesium silicide (Mg2Si). Although not as strong as most 2xxx and
7xxx alloys, 6xxx alloys have good formability, weldability, machinability, and
corrosion resistance, with medium strength.
In the heat-treatable wrought alloys, with some notable exceptions (2024, 2219,
and 7178), such solute elements are present in amounts that are within the limits
of mutual solid solubility at temperatures below the eutectic temperature
(lowest melting temperature).
In contrast, some of the casting alloys of the 2xx.0 series and all of the 3xx.0
series alloys contain amounts of soluble elements that far exceed solid-solubility
limits. In these alloys, the phase formed by combination of the excess soluble
elements with the aluminum will never be dissolved, although the shapes of the
undissolved particles may be changed by partial solution.
Heat treatment to increase strength of aluminum alloys is a three-step process:
- Solution heat treatment: dissolution of soluble phases
- Quenching: development of supersaturation
- Age hardening: precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment).
Solution Heat Treating
To take advantage of the precipitation hardening reaction, it is necessary
first to produce a solid solution. The process by which this is accomplished
is called solution heat treating, and its objective is to take into solid
solution the maximum practical amounts of the soluble hardening elements
in the alloy. The process consists of soaking the alloy at a temperature
sufficiently high and for a time long enough to achieve a nearly homogeneous
Precipitation Heat Treating without Prior Solution 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 respond strongly
to precipitation heat treatment. This practice is widely used in producing
thin extruded shapes of alloys 6061, 6063, 6463 and 7005.
Upon precipitation heat treating after quenching at the extrusion press,
these alloys develop strengths nearly equal to those obtained by adding
a separate solution heat treating operation. Changes in properties
occurring during the precipitation treatment follow the principles
outlined in the discussion of solution heat-treated alloys.
Quenching is in many ways the most critical step in the sequence of
heat-treating operations. The objective of quenching is to preserve
the solid solution formed at the solution heat-treating temperature,
by rapidly cooling to some lower temperature, usually near room temperature.
In most instances, to avoid those types of precipitation that are
detrimental to mechanical properties or to corrosion resistance,
the solid solution formed during solution heat treatment must be
quenched rapidly enough (and without interruption) to produce
supersaturated solution at room temperature - the optimum condition
for precipitation hardening.
The resistance to stress-corrosion cracking of certain copper-free
aluminum-zinc-magnesium alloys, however, is improved by slow quenching.
Most frequently, parts are quenched by immersion in cold water, or in
continuous heat treating of sheet, plate, or extrusions in primary
fabricating mills, by progressive flooding or high-velocity spraying
with cold water.
After solution treatment and quenching hardening is achieved either at
room temperature (natural aging) or with a precipitation heat treatment
(artificial aging). In some alloys, sufficient precipitation occurs in
a few days at room temperature to yield stable products with properties
that are adequate for many applications. These alloys sometimes are
precipitation heat treated to provide increased strength and hardness in
wrought or cast products. Other alloys with slow precipitations reactions
at room temperature are always precipitation heat treated before being used.
In some alloys, notably those of the 2xxx series, cold working or freshly
quenched material greatly increases its response to later precipitation
Natural Aging. The more highly alloyed members of the 6xxx wrought series, the copper-containing
alloys of the 7xxx group, and all of the 2xxx alloys are almost always solution
heat treated and quenched. For some of these alloys, particularly the 2xxx alloys,
the precipitation hardening that results from natural aging alone produces useful
tempers (T3 and T4 types) that are characterized by high ratios of tensile to yield
strength and high fracture toughness and resistance to fatigue. For the alloys that
are used in these tempers, the relatively high supersaturation of atoms and vacancies
retained by rapid quenching causes rapid formation of GP zones, and strength increases
rapidly, attaining nearly maximum stable values in four or five days. Tensile-property
specifications for products in T3- and T4-type tempers are based on a nominal natural
aging time of four days. In alloys for which T3- or T4-type tempers are standard,
the changes that occur in further natural aging are of relatively minor magnitude, and
products of these combinations of alloy and temper are regarded as essentially stable
after about one week.
In contrast to the relatively stable condition reached in a few days by 2xxx alloys
that are used in T3- or T4-type tempers, the 6xxx alloys and to an even greater degree
the 7xxx alloys are considerably less stable at room temperature and continue to exhibit
significant changes in mechanical properties for many years.
Precipitation heat treatments generally are low-temperature, long-term processes.
Temperatures range from 115 to 190°C; times vary from 5 to 48 h.
Choice of time-temperature cycles for precipitation heat treatment should receive
careful consideration. Larger particles of precipitate result from longer times
and higher temperatures; however, the larger particles must, of necessity, be
fewer in number with greater distances between them.
The objective is to select the cycle that produces optimum precipitate size and
distribution pattern. Unfortunately, the cycle required to maximize one property,
such as tensile strength, is usually different from that required to maximize others,
such as yield strength and corrosion resistance. Consequently, the cycles used represent
compromises that provide the best combinations of properties.
Production of material in T5- through T7-type tempers necessitates precipitation heat
treating at elevated temperatures (artificial aging).
Differences in type, volume fraction, size, and distribution of the precipitated
particles govern properties as well as the changes observed with time and temperature,
and these are all affected by the initial state of the structure. The initial structure
may vary in wrought products from unrecrystallized to recrystallized and may exhibit
only modest strain from quenching or additional strain from cold working after solution
heat treatment. These conditions, as well as the time and temperature of precipitation
heat treatment, affect the final structure and the resulting mechanical properties.
Precipitation heat treatment following solution heat treatment and quenching produces
T6- and T7-type tempers. Alloys in T6-type tempers generally have the highest strengths
practical without sacrifice of the minimum levels of other properties and characteristics
found by experience to be satisfactory and useful for engineering applications. Alloys in
T7 tempers are overaged, which means that some degree of strength has been sacrificed or
"traded off" to improve one or more other characteristics. Strength may be sacrificed to
improve dimensional stability, particularly in products intended for service at elevated
temperatures, or to lower residual stresses in order to reduce warpage or distortion in
machining. T7-type tempers frequently are specified for cast or forged engine parts.
Precipitation heat-treating temperatures used to produce these tempers generally are higher
than those used to produce T6-type tempers in the same alloys.
Two important groups of T7-type tempers -- the T73 and T76 types -- have been developed
for the wrought alloys of the 7xxx series, which contain more than about 1.25% copper.
These tempers are intended to improve resistance to exfoliation corrosion and
stress-corrosion cracking, but as a result of overaging, they also increase fracture
toughness and, under some conditions, reduce rates of fatigue-crack propagation.