Heat Treating of Magnesium Alloys

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 heat treatment.

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:

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

Annealing. 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 elastic strains.

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 sections.

Dimensional Stability

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 to problems.

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 temperatures.