Magnesium alloys usually are heat treated either to improve mechanical properties
or as means of conditioning for specific fabricating operations. The type of heat
treatment selected depends on alloy composition and form (cast or wrought), and on
anticipated service conditions.
Solution heat treatment improves strength and results in maximum toughness and
shock resistance. Precipitation heat treatment subsequent to solution treatment
gives maximum hardness and yield strength, but with some sacrifice of toughness.
As applied to castings, artificial aging without prior solution treatment or
annealing is a stress-relieving treatment that also somewhat increases tensile
properties. Annealing of wrought products lowers tensile properties considerably
and increases ductility, thereby facilitating some types of fabrication.
Modifications of these basic treatments have been developed for specific alloys,
to obtain the most desirable combinations of properties.
The basic temper designations for magnesium alloys, the same as those applied to
aluminum alloys, are used throughout this article to indicate the various types of
The mechanical properties of most magnesium casting alloys can be improved by
heat treatment. Casting alloys can be grouped into six general classes of
commercial importance on the basis of composition, as follows:
- Magnesium-rare earth metal-zinc-zirconium
- Magnesium-rare earth metal-silver-zirconium, with or without thorium
- Magnesium-thorium-zirconium, with or without zinc.
In most wrought alloys, maximum mechanical properties are developed through strain
hardening, and these alloys generally are either used without subsequent heat
treatment or merely aged to a T5 temper. Occasionally, however, solution treatment,
or a combination of solution treatment with strain hardening and artificial aging,
will substantially improve mechanical properties.
Wrought alloys that can be strengthened by heat treatment are grouped into four
general classes according to composition:
Types of Heat Treatment
Wrought magnesium alloys in various conditions of
strain hardening or temper can be annealed by being heated at 290 to 455°C
(550 to 850°F), depending on alloy, for one or more hours. This procedure
usually will provide a product with the maximum anneal that is practical.
Because most forming operations on magnesium are done at elevated temperature,
the need for fully annealed wrought material is less than with many other metals.
Stress Relieving of Wrought Alloys. Stress relieving is used to
remove or reduce residual stresses induced in wrought magnesium products by cold
and hot working, shaping and forming, straightening, and welding.
When extrusions are welded to hard rolled sheet, the lower stress-relieving
temperature and the longer time should be used to minimize distortion-for example,
150°C (300°F) for 60 min, rather than 260°C (500°F) for 15 min.
Stress Relieving of Castings. The precision machining of castings
to close dimensional limits, the necessity of avoiding warp age and distortion,
and the desirability of preventing stress-corrosion cracking in welded
magnesium-aluminum casting alloys make it mandatory that cast components be
substantially free from residual stresses. Although magnesium castings do not
normally contain high residual stresses, the low modulus of elasticity of
magnesium alloys means that comparatively low stresses can produce appreciable
Residual stresses may arise from contraction due to mold restraint during
solidification, from no uniform cooling after heat treatment, or from quenching.
Machining operations also can result in residual stress and require intermediate
stress relieving prior to final machining.
Solution Treating and Aging. In solution treating of
magnesium-aluminum-zinc alloys, parts should be loaded into the furnace at
approximately 260°C (500°F) and then raised to the appropriate
solution-treating temperature slowly, to avoid fusion of eutectic compounds and
resultant formation of voids. The time required to bring the load from
260°C to the solution-treating temperature is determined by the size of
the load and by the composition, size, weight and section thickness of the parts,
but 2 h is a typical time.
During aging, magnesium alloy parts should be loaded into the furnace at the
treatment temperature, held for the appropriate period of time, and then cooled
in still air. There is a choice of artificial aging treatments for some alloys;
results are closely similar for the alternative treatments given.
Reheat Treating. Under normal circumstances, when mechanical
properties are within expected ranges and the prescribed best treatment has been
carried out, reheat treating is seldom necessary. However, if the microstructures
of heat treated castings indicate too high a compound rating or if the castings
have been aged excessively by slow cooling after solution treating, reheat
treating is called for. Most magnesium alloys can be reheating treated with
little danger of germination.
Effects of Major Variables
Casting size and section thickness, relation of casting size to volume capacity
of the furnace, and arrangement of castings in the furnace are mechanical
considerations that can affect heat treating schedules for all metals.
Section Size and Heating Time. There is no general rule for
estimating time of heating per unit of thickness for magnesium alloys. However,
because of the high thermal conductivity of these alloys, combined with their
low specific heat per unit volume, parts reach soaking temperature quite rapidly.
The usual procedure is to load the furnace and then begin the soaking period when
the loaded furnace reaches the desired temperature.
In the heat treating of magnesium alloy castings with thick sections a good
rule is to double the time at the solution treating temperature. For example,
the usual solution treatment for AZ63A castings is 12 h at about 385°C
(725°F), whereas 25 h at about 385°C is suggested for castings with
section thickness greater than 50 mm.
Similarly, the suggested solution-treating schedule for preventing excessive
grain growth in AZ92A castings is 6 h at about 405°C, 2 h at about 350°C
and 10 h at about 405°C; but for castings with sections more than 50 mm
thick, it is recommended that the last soak at 405°C be extended from 10 h to
19 h. The best way to determine whether or not additional solution treating
time is required is to cut a section through the thickest portion of a scrap
casting and examine the center of the section microscopically: if heat treatment
is complete, this examination will reveal a low compound rating.
Protective Atmospheres. Although magnesium alloys can be best
treated in air, protective atmospheres are almost always used for solution
treating. Government specification for heat treating of magnesium castings
requires a protective atmosphere for solution treating above 400°C (750°F).
Protective atmospheres serve the dual purpose of preventing surface oxidation
(which, if severe, can decrease strength) and of preventing active burning should
the furnace exceed proper temperature.
The two gases normally used are sulfur dioxide and carbon dioxide. Inert gases
also may be used; however, in most instances, these gases are not practical
because of higher cost. Sulfur dioxide is available bottled, while carbon dioxide
may be obtained either bottled or as the product of recirculated combustion gases
from a gas-fired furnace. A concentration of 0.7% (0.5% min) sulfur dioxide
will prevent active burning to a temperature of 565°C (1050°F), provided
that melting of the alloy has not occurred. Carbon dioxide in a concentration of
3% will prevent active burning to 510°C (950°F), and a carbon dioxide
concentration of 5% will provide protection to about 540°C (1000°F).
Equipment and Processing
In solution treating and artificial aging of magnesium alloys, it is standard
practice to use an electrically heated or gas-fired furnace equipped with a
high-velocity fan or comparable means for circulating the atmosphere and promoting
uniformity of temperature. However, because the atmosphere for solution treating
sometimes contains sulfur dioxide, only furnaces that are gastight and that provide
an inlet for introducing protective atmosphere are suitable.
Quenching Media. Magnesium alloy products normally are quenched
in air following solution treatment. Still air usually is sufficient; forced-air
cooling is recommended for dense loads or for parts that have very thick
In normal service up to approximately 95°C (200°F), all magnesium casting
alloys exhibit good dimensional stability and can be considered free from
additional dimensional changes.
Some cast magnesium-aluminum-manganese and magnesium-aluminum-zinc alloys in
certain tempers exhibit slight permanent growth after relatively long exposure
to temperatures exceeding 95°C. This growth, although slight, can give rise
In contrast to the growth characteristics of the magnesium-aluminum-zinc alloys
are those of the magnesium alloys containing thorium, rare earth metals and
zirconium as major alloying elements. These alloys normally are used in the
T5 or T6 temper, and they shrink, rather than grow, on exposure to elevated