Heat Treatment of Magnesium Alloys

Heat Treatment of Magnesium Alloys

The conventional heat treatment processes for magnesium alloys are categorized into annealing and solution treatment with aging. Annealing involves heating and slow cooling to eliminate internal stresses and restore plasticity, while solution treatment and aging enhance strength by dissolving phases at high temperatures (solution treatment) followed by rapid cooling and subsequent precipitation of strengthening phases (aging). These two methods form the core of magnesium alloy heat treatment. Reducing casting or quenching-induced residual stresses in magnesium alloy castings improves dimensional stability. The effectiveness of heat treatment strengthening depends entirely on whether the solid solubility of alloying elements varies with temperature. When the solubility changes with temperature, magnesium alloys can be strengthened via heat treatment.

Heat-Treatable Cast Magnesium Alloys

The primary heat-treatable cast magnesium alloys include the following series:

Mg-Al-Mn Series (e.g., AM100A): A foundational cast alloy balancing strength and formability.

Mg-Al-Zn Series (e.g., AZ91D, AZ92A): Al/Zn solid-solution strengthening; widely used in automotive components, with AZ91D as a representative alloy.

Mg-Zn-Zr Series (e.g., ZK51A, ZK61A): Zn/Zr precipitation strengthening, with Zr refining grains; suitable for high-strength aerospace structural parts.

Mg-RE-Zn-Zr Series (e.g., EZ33A, ZE41A): Rare earth (RE) elements improve heat resistance; applied in engine brackets.

Mg-Ag-RE-Zr Series (e.g., QE22A): Ag enhances aging response via β' phase precipitation; used in precision instrument components.

Mg-Zn-Cu Series (e.g., ZC63A): Cu improves mechanical properties but requires additional corrosion protection; ideal for marine applications.

Mg-Y-Nd-Zr Series (e.g., WE43): Y/Nd synergistically enhance high-temperature strength (>80% retention at 300°C); employed in aero-engine casings.

Mg-Gd-Y-Zr Series (e.g., GW103K): T6-state yield strength >300 MPa, strengthened by β'-Mg₅Gd nanoprecipitates; used in satellite load-bearing structures.

Mg-Al-Ca Series (e.g., AXJ530): Ca/Sr refine grains and suppress β-Mg₁₇Al₁₂ phases, improving creep resistance; applied in lightweight transmission housings.

Mg-RE-Sc Series (e.g., Sc-ZE62): Sc inhibits brittle phases, increasing elongation to 8%; utilized in optical equipment supports.

Mg-Zn-Al Series (e.g., ZA85): A cost-effective alternative to AZ series; hardness reaches HV 110 after solution aging.

Mg-Nd-Zn-Zr Series (e.g., VW63): Excellent corrosion resistance (>500 h in salt spray tests); suitable for marine propeller components.

These alloys achieve precipitation strengthening through solution treatment and aging, with selection criteria based on service conditions (temperature, load, corrosion) and cost. Aerospace and automotive industries are key application areas.

Heat-Treatable Wrought Magnesium Alloys

Key heat-treatable wrought magnesium alloys include:

Mg-Al-Zn Series (e.g., AZ80A, AZ61A): Al/Zn contribute to solid-solution and precipitation strengthening. AZ80A offers high strength and corrosion resistance for aerospace structures and bicycle components.

Mg-Zn-Zr Series (e.g., ZK60A): Zn enables solid-solution strengthening, while Zr refines grains and enhances thermal stability. ZK60A exhibits superior fatigue strength for aerospace and high-performance sports equipment.

Mg-Zn-Cu Series (e.g., ZC71A): Zn/Cu composite strengthening for high-strength structural parts. However, excessive Cu may reduce corrosion resistance, necessitating surface protection.

The most significant characteristic of magnesium alloy heat treatment is the prolonged solution and aging treatment time, which is due to the extremely slow diffusion of alloying elements and the decomposition process of alloy phases. For the same reason, rapid cooling is not required during quenching of magnesium alloys; they are usually cooled in still air or forced airflow. 

For certain magnesium alloys with insignificant heat treatment strengthening effects, annealing is typically chosen as the final heat treatment process.