Heat Treating of Magnesium Alloys

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Introduction to Magnesium Alloy Heat Treatment

Magnesium alloys typically undergo heat treatment either to improve mechanical properties or to condition them for specific fabricating operations. The selection of the most appropriate heat treatment depends on the alloy's composition, its form (whether cast or wrought), and the anticipated service conditions.

Solution heat treatment significantly improves strength while maximizing toughness and shock resistance. When precipitation heat treatment follows solution treatment, it produces maximum hardness and yield strength, though with some reduction in toughness. For castings, artificial aging without prior solution treatment or annealing serves as a stress-relieving treatment that also moderately increases tensile properties. In wrought products, annealing substantially reduces tensile properties while increasing ductility, which facilitates certain types of fabrication. Specialized modifications of these basic treatments have been developed for specific alloys to achieve the most desirable combinations of properties.

Throughout this article, the basic temper designations for magnesium alloys, which are identical to those used for aluminum alloys, are employed to indicate the various types of heat treatment.

Magnesium Alloy Classifications and Heat Treatment Potential

Cast Magnesium Alloys

Heat treatment can improve the mechanical properties of most magnesium casting alloys. Based on composition, commercially important casting alloys can be classified into six general groups:

1. Magnesium-aluminum-manganese
2. Magnesium-aluminum-zinc
3. Magnesium-zinc-zirconium
4. Magnesium-rare earth metal-zinc-zirconium
5. Magnesium-rare earth metal-silver-zirconium, with or without thorium
6. Magnesium-thorium-zirconium, with or without zinc

Wrought Magnesium Alloys

Most wrought alloys develop maximum mechanical properties through strain hardening and are typically used either without subsequent heat treatment or with aging to a T5 temper. However, in some cases, solution treatment or a combination of solution treatment with strain hardening and artificial aging can substantially improve mechanical properties.

Wrought alloys that can be strengthened through heat treatment fall into four general compositional classes:

1. Magnesium-aluminum-zinc
2. Magnesium-thorium-zirconium
3. Magnesium-thorium-manganese
4. Magnesium-zinc-zirconium

Types of Heat Treatment Processes

Annealing

Wrought magnesium alloys in various conditions of strain hardening or temper can be annealed by heating at temperatures ranging from 290 to 455°C (550 to 850°F), depending on the specific alloy, for one or more hours. This process typically provides a product with the maximum practical annealing effect.

Because most forming operations on magnesium are performed at elevated temperatures, the demand for fully annealed wrought material is less than for many other metals.

Stress Relieving Procedures

Stress Relieving of Wrought Alloys

Stress relieving is employed to remove or reduce residual stresses induced in wrought magnesium products by various processes including cold and hot working, shaping and forming, straightening, and welding.

When extrusions are welded to hard-rolled sheet, it is advisable to use a lower stress-relieving temperature and longer time to minimize distortion—for example, 150°C (300°F) for 60 minutes, rather than 260°C (500°F) for 15 minutes.

Stress Relieving of Castings

The precision machining of castings to close dimensional tolerances, the need to avoid warpage and distortion, and the importance of preventing stress-corrosion cracking in welded magnesium-aluminum casting alloys make it essential 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 relatively low stresses can produce significant elastic strains.

Residual stresses may develop from contraction due to mold restraint during solidification, from uneven cooling after heat treatment, or from quenching. Machining operations can also introduce residual stress, necessitating intermediate stress relieving before final machining.

Solution Treating and Aging

In the solution treating of magnesium-aluminum-zinc alloys, parts should be loaded into the furnace at approximately 260°C (500°F) and then gradually raised to the appropriate solution-treating temperature. This slow heating prevents fusion of eutectic compounds and the resulting formation of voids. The time required to bring the load from 260°C to the solution-treating temperature depends on the size of the load and the composition, size, weight, and section thickness of the parts, but 2 hours is typical.

During the aging process, magnesium alloy parts should be loaded into the furnace at the treatment temperature, held for the appropriate period, and then cooled in still air. Some alloys offer a choice of artificial aging treatments; the results are quite similar for the alternative treatments provided.

Reheat Treating

Under normal circumstances, when mechanical properties fall within expected ranges and the prescribed optimal treatment has been carried out, reheat treating is rarely necessary. However, if microstructural examination of heat-treated castings indicates too high a compound rating, or if the castings have been aged excessively by slow cooling after solution treating, reheat treating is recommended. Most magnesium alloys can be reheat treated with minimal risk of germination.

Effects of Major Variables on Heat Treatment

Casting size and section thickness, the relationship between casting size and furnace volume capacity, and the arrangement of castings within the furnace are mechanical factors that can affect heat treating schedules for all metals.

Section Size and Heating Time

There is no universal rule for estimating heating time per unit of thickness for magnesium alloys. However, due to the high thermal conductivity of these alloys, combined with their low specific heat per unit volume, parts reach soaking temperature relatively quickly. The standard procedure is to load the furnace and begin the soaking period once the loaded furnace reaches the desired temperature.

For heat treating magnesium alloy castings with thick sections, a good practice is to double the time at the solution treating temperature. For example, while the standard solution treatment for AZ63A castings is 12 hours at approximately 385°C (725°F), 25 hours at about 385°C is recommended for castings with section thickness exceeding 50 mm.

Similarly, the recommended solution-treating schedule for preventing excessive grain growth in AZ92A castings is 6 hours at approximately 405°C, 2 hours at approximately 350°C, and 10 hours at approximately 405°C. However, for castings with sections more than 50 mm thick, extending the final soak at 405°C from 10 hours to 19 hours is advised. The most effective way to determine whether additional solution treating time is necessary is to cut a section through the thickest portion of a scrap casting and microscopically examine the center of the section: if heat treatment is complete, this examination will reveal a low compound rating.

Protective Atmospheres

Although magnesium alloys can be heat treated in air, protective atmospheres are almost always employed for solution treating. Government specifications for heat treating magnesium castings require 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 inhibiting active burning should the furnace exceed the proper temperature.

The two gases commonly used are sulfur dioxide and carbon dioxide. Inert gases may also be employed; however, in most cases, these gases are not practical due to higher costs. Sulfur dioxide is available in bottled form, 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% minimum) sulfur dioxide will prevent active burning up to a temperature of 565°C (1050°F), provided that melting of the alloy has not occurred. Carbon dioxide at a concentration of 3% will prevent active burning up to 510°C (950°F), while a 5% carbon dioxide concentration provides protection up to approximately 540°C (1000°F).

Equipment and Processing Considerations

For solution treating and artificial aging of magnesium alloys, standard practice involves using an electrically heated or gas-fired furnace equipped with a high-velocity fan or comparable mechanism for circulating the atmosphere and promoting temperature uniformity. However, because the atmosphere for solution treating sometimes contains sulfur dioxide, only gastight furnaces that provide an inlet for introducing protective atmosphere are suitable.

Quenching Media

Magnesium alloy products are normally quenched in air following solution treatment. Still air is usually sufficient; however, forced-air cooling is recommended for dense loads or for parts with very thick sections.

Dimensional Stability Factors

In normal service up to approximately 95°C (200°F), all magnesium casting alloys demonstrate 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. Although slight, this growth can lead to problems in precision applications.

In contrast to the growth characteristics of magnesium-aluminum-zinc alloys, magnesium alloys containing thorium, rare earth metals, and zirconium as major alloying elements behave differently. These alloys, normally used in the T5 or T6 temper, shrink rather than grow when exposed to elevated temperatures.

Conclusion

Heat treatment of magnesium alloys is a critical process for optimizing their mechanical properties for specific applications. By carefully selecting and controlling heat treatment parameters based on alloy composition, form, and intended service conditions, manufacturers can achieve the desired balance of strength, ductility, and dimensional stability. Understanding the effects of variables such as section size, protective atmospheres, and equipment requirements ensures successful heat treatment outcomes and the production of high-quality magnesium alloy components.