Practical Heat Treatment of Magnesium Alloys

---

Introduction to Magnesium Alloy Heat Treatment

Both cast and wrought magnesium-alloy products are heat treated in practice to alter their mechanical properties. The type of treatment depends upon the nature and extent of the changes to be made. By one treatment, the tensile strength and elongation may be substantially raised. By another treatment, the yield strength and hardness may be considerably increased but with appreciable loss of ductility.

The alloys responsive to such heat treatments are those in which the principal alloying constituent shows a marked increase in solid solubility at elevated temperatures compared to room temperature. Most magnesium casting alloys are heat-treatable. However, among current wrought compositions, only AZ80 and ZK60 fall into this category.

Basic Heat Treatment Processes for Magnesium Alloys

Three basic types of thermal treating processes are commonly applied to magnesium alloys:

• Solution heat treatment
• Precipitation or aging
• Annealing

Additionally, stabilizing and stress-relieving treatments are used in practice. Stabilizing is essentially a type of precipitation, while stress-relieving is related to annealing.

Solution Heat Treatment (T4)

Solution heat treatment consists of heating the alloy to a temperature at which certain constituents dissolve into solution, followed by quenching to maintain these constituents in solution during cooling. For magnesium alloys, quenching is typically done in still or moving air rather than liquids.

For magnesium-alloy castings, the solution heat-treating temperature ranges from approximately 650°F to 1050°F (340°C to 565°C), with the precise temperature depending upon the composition and operational details. Exposure periods typically range from 16 to 24 hours. Solution heat-treated material carries the T4 temper designation (e.g., AZ63-T4). The T4 treatment is recommended for castings requiring high strength and maximum toughness.

Precipitation and Aging Treatment (T6)

Aging treatment involves heating the quenched alloy at a moderately elevated temperature to precipitate the constituents held in solid solution. Treatment temperatures typically range from 300°F to 500°F (150°C to 260°C), with exposures from 3 to 16 hours, depending on the alloy composition and other factors.

The temper designation for solution heat-treated and aged material is T6. This treatment develops high yield strength and hardness, making it ideal for highly stressed castings.

Annealing Process

Annealing consists of heating the alloy at a moderate temperature to effect recrystallization, agglomerate precipitate, or remove internal strains. For most cold-worked magnesium alloys, the typical annealing temperature is approximately 650°F (340°C).

Modified Precipitation Treatments

Precipitation treatment may sometimes be applied to material that has not received definitive solution treatment. Certain manufacturing processes for wrought products—such as extruding and forging—naturally retain alloy constituents in solid solution at concentrations substantially greater than equilibrium solubility at room temperature. This occurs because hot extrusion or forging products are air quenched when removed from the press.

For example, AZ80-F forgings can be fully aged by heating for 24 hours at 350°F (175°C). The resulting material, designated AZ80-T5, exhibits maximum tensile properties, compressive yield strength, and hardness. Various property combinations can be obtained through different aging cycles with F or T4 material, whether cast or wrought.

Stabilization Treatments (T5, T7)

Stabilization is primarily used to prevent or minimize growth of cast alloys during service at elevated temperatures. This treatment simultaneously provides stress relief and improves creep strength. Growth occurs due to volume changes resulting from the coalescence of constituents precipitated from solid solution. By producing much of this coalescence before the casting enters service, stabilizing treatment minimizes subsequent growth.

Although the total growth amount is generally small, suitable stabilizing treatments practically eliminate any growth tendency at temperatures normally encountered by magnesium castings in service.

Stabilization can be applied to castings in either the F or T4 temper, typically involving heating the product for 4 hours. This treatment has minimal effect on the original mechanical properties of the F temper. Stabilization of as-cast (F) material results in the T5 temper, while stabilization of T4 material produces the T7 temper.

Room Temperature Aging Effects

It's important to note that magnesium alloys tend to age-harden very slowly at room temperature. Alloys in the T4 temper are particularly susceptible to this phenomenon. For instance, aging AZ63-T4 for five years at 75°F to 90°F (24°C to 32°C) resulted in:

• 14% increase in tensile strength
• 68% increase in yield strength
• 22% increase in Brinell hardness
• 32% reduction in ductility
• 57% reduction in impact resistance

Similarly, aging AZ63-F for five years at room temperature increased yield strength by approximately 20% and decreased ductility by 33% without significantly affecting other properties. AZ92-T4 demonstrates even greater susceptibility to room-temperature aging than AZ63-T4.

While all commercial solid-solution type casting alloys in the F and T4 conditions can potentially age-harden over years at room temperature, this tendency has limited practical importance except for AZ92 and AZ63 type alloys. All alloys become insensitive to room-temperature aging when converted to T5, T6, or T7 tempers.

Stress Relief Treatments

Stress relief heat treatments are applicable to both cast and wrought magnesium-alloy products. Casting strains are typically removed by solution or stabilizing treatments, though quenching may introduce new internal stresses. Aging at moderately elevated temperatures to produce the T6 temper tends to relieve quenching strains. For parts supplied in the F temper, casting strains can be removed by heating for 1 to 2 hours at 400°F to 500°F (205°C to 260°C), depending on the alloy, followed by slow cooling.

The same heat treatments can be used for both sand and permanent-mold castings. It should be noted that die castings are generally not heat treated.

Stress Relief for Formed Parts

Parts formed from wrought AZ31, AZ61, and other high-strength alloys at room temperature or slightly elevated temperatures may retain internal working stresses that could cause stress-corrosion cracking under certain conditions.

To prevent such failure, formed parts should receive thermal treatment to relieve these stresses. A treatment of 1 hour at 300°F (150°C) is typically effective. Parts formed at 500°F (260°C) or higher generally don't require this treatment since forming stresses at these temperatures are very low. It's worth noting that forming does not make M1 alloy susceptible to stress-corrosion cracking.

Welding Stress Relief

Stress-corrosion cracking can occur in some magnesium alloys due to internal strains created during fusion welding. Appropriate thermal treatments to relieve welding stresses are therefore advisable. These treatments typically involve heating for 15 minutes to 1 hour at 265°F to 500°F (130°C to 260°C), depending on the alloy and temper, followed by air cooling.

Thermal Flattening

Thermal flattening of magnesium-alloy sheet is performed at 300°F (150°C) for AZ31 and 400°F (205°C) for M1 alloy. When applying thermal flattening to hard-rolled sheet, a slight reduction in strength occurs, but elongation improves considerably.