Heat Treatment of Magnesium Alloys
Key Takeaways
Understanding proper heat treatment methods is crucial for unlocking magnesium alloys' full potential in lightweight engineering applications.
• Solution treatment (T4) at 340-565°C for 16-24 hours maximizes strength and toughness by dissolving constituents into solid solution
• Aging treatment (T5/T6) at 150-260°C precipitates strengthening phases, with T6 achieving up to 53% tensile strength increases
• Protective atmospheres with 0.7% SO2 or 3-5% CO2 prevent oxidation during high-temperature processing up to 565°C
• Thick sections require doubled treatment times, while stress relief at 150°C prevents stress-corrosion cracking in formed parts
• Room temperature aging significantly affects AZ-series alloys, increasing strength by 14-68% but reducing ductility by 32%
The key to successful magnesium alloy heat treatment lies in matching the specific thermal cycle to both alloy composition and intended service conditions. Proper implementation of these methods enables manufacturers to achieve optimal mechanical properties while maintaining dimensional stability and corrosion resistance in critical applications.
Magnesium Alloy Classifications for Heat Treatment
Magnesium and its alloys are classified into two main categories based on their manufacturing process: cast alloys and wrought alloys. The manufacturing process determines their heat treatment response and subsequent mechanical properties. These materials offer exceptional strength-to-weight ratios with a density of 1.74 g/cm3 that make them attractive for applications requiring lightweight structural components.
Cast Aluminum Magnesium Alloys (AZ, AM Series)
Cast aluminum magnesium alloys represent the most significant group in commercial use, with the AZ designation that indicates aluminum-zinc compositions. AZ91 alloys contain 9 wt% aluminum and 1 wt% zinc, with the remaining composition being magnesium and trace amounts of manganese, iron, and copper. This composition serves structural component manufacturing due to its balanced mechanical properties. AZ31 alloys feature 3 wt% aluminum and 1 wt% zinc and find extensive use in biomedical applications for orthopedic inserts and medical devices.
The AM series has aluminum and manganese in varying proportions, tailored to achieve specific mechanical properties and performance characteristics. Manganese boosts corrosion resistance and improves mechanical properties, making AM series alloys valuable in applications where durability proves critical. AM90 alloy shows the highest values in terms of hardness and wear resistance among these alloys, though it exhibits the lowest machinability properties.
Wrought Magnesium Based Alloys (AZ80, ZK60)
Wrought alloys develop maximum mechanical properties through strain hardening and respond to heat treatment differently than cast counterparts. AZ80, composed of Mg-8.0Al-0.5Zn-0.11Mn, stands out as a high-performance wrought alloy suitable for extrusion and forging operations. AZ80 achieves tensile strengths exceeding 400 MPa after room temperature multidirectional forging followed by heat treatment, substantially superior to hot-deformed states.
ZK60 alloy contains 4.8-6.2% zinc and a minimum of 0.45% zirconium. This composition offers the best combination of strength and ductility at room temperature among wrought magnesium alloys when aged to T5 temper.
Rare Earth and Zirconium-Containing Alloys
Rare earth elements such as yttrium, gadolinium, and neodymium result in improved properties at raised temperatures. Zirconium serves as an effective grain refinement agent and reduces grain size by an order of magnitude compared to non-zirconium-containing alloys. The Mg-Gd-Y-Zr system demonstrates excellent high-temperature resistance, making these alloys attractive for aerospace and military applications.
Common Magnesium Alloys in Aerospace Applications
Aerospace applications just need materials that maintain structural integrity under demanding conditions. Magnesium alloys containing thorium, rare earth metals, and zirconium as major alloying elements exhibit superior dimensional stability at elevated service temperatures compared to conventional aluminum-zinc systems.
Core Heat Treatment Methods for Magnesium Alloys
Three primary thermal treatment processes apply to magnesium and its alloys. Each serves distinct metallurgical objectives and produces specific property modifications. Heat treatment selection depends on alloy composition, product form and intended service requirements.
Solution Heat Treatment (T4 Temper)
Solution treatment involves heating alloys to temperatures between 340°C and 565°C. This dissolves certain constituents into a solid solution. Air quenching follows to retain these elements during cooling. Magnesium-aluminum-zinc alloys require parts to be loaded into furnaces at around 260°C. Temperature then rises to the solution-treating level over roughly 2 hours. This prevents eutectic compound fusion. Exposure periods span 16 to 24 hours.
This process substantially improves strength while maximizing toughness and shock resistance. Selective laser melted AZ91D alloy undergoes solution treatment at 420°C for 1 hour. The β-Mg17Al12 phases transform from networks into diffuse dots and eventually dissolve. The T4 designation applies to solution heat-treated material. Applications requiring high strength combined with maximum toughness benefit from this treatment.
Precipitation Aging Treatment (T5, T6 Tempers)
Aging treatment heats quenched alloys at elevated temperatures. This precipitates constituents held in solid solution. Treatment temperatures range from 150°C to 260°C with exposures lasting 3 to 16 hours. T5 temper indicates aging without prior solution treatment. T6 designates solution-treated and aged material.
Hot-ring-rolled AZ80-Ag alloy reaches peak aging at 175°C for 36 hours. This produces discontinuous precipitates occupying around 64.9% volume fraction. T5 treatment improves tensile strength at ambient temperature but demonstrates reduced performance at elevated temperatures ranging from 120°C to 175°C. Hot-rolled WE43 alloy exhibits superior properties with T5 temper compared to T6. Combined strain hardening and age hardening effects account for this.
Annealing Process for Wrought Products
Wrought magnesium alloys undergo annealing at temperatures between 290°C and 455°C for one or more hours. Specific composition determines the exact parameters. This process effects recrystallization, agglomerates precipitates and removes internal strains. AZ31 typically undergoes annealing at around 340°C. Annealing greatly reduces tensile properties while increasing ductility. Subsequent fabrication operations become easier as a result.
Stress Relieving Procedures
Stress relief treatments remove residual stresses induced by cold working, hot working, forming, straightening and welding. Wrought alloys require heating at 150°C for 60 minutes or 260°C for 15 minutes. Formed parts from AZ31 and AZ61 alloys need 1 hour at 150°C to prevent stress-corrosion cracking. Aluminum/magnesium laminates fabricated by roll bonding undergo stress relief at 200°C for 1 hour. This reduces internal residual stress sufficiently.
Critical Process Variables and Equipment Requirements
Successful heat treatment outcomes depend on precise control of multiple interrelated process variables and specialized equiPment configurations.
Temperature and Time Parameters by Alloy Type
Solution treatment temperatures span 340°C to 565°C depending on alloy composition, with exposure periods ranging from 16 to 24 hours. Aging occurs at 150°C to 260°C for 3 to 16 hours. Thick-section castings exceeding 50 mm need double the solution treatment duration. AZ63A requires 25 hours instead of the standard 12 hours at 385°C.
Protective Atmospheres (SO2, CO2 Concentration)
Sulfur dioxide at 0.7% minimum concentration prevents active burning up to 565°C. Carbon dioxide at 3% provides protection to 510°C, while 5% CO2 extends coverage to 540°C. These atmospheres form protective MgO and MgSO4 surface films during treatment
Quenching Media and Cooling Rates
Air quenching is enough for most magnesium alloys. Water quenching at 100°C reduces residual stress by about 30% on surfaces and 40% in centers compared to 20°C water quenching. Transfer from furnace to quench media must occur within 30 seconds.
Section Thickness Effects on Heat Treatment Cycles
Magnesium's high thermal conductivity makes rapid heating possible, yet thick sections need extended exposure. Standard holding periods apply for sections up to 50.8 mm.
Furnace Equipment and Circulation Requirements
Gas-tight furnaces equipped with high-velocity circulation fans maintain temperature uniformity within ±6°C. Automatic controls with manual reset safety cutouts prevent overheating beyond 6°C above maximum treatment temperature.
Mechanical Property Optimization and Dimensional Stability
Heat treatment profoundly influences mechanical performance and long-term stability in magnesium and its alloys, with specific protocols optimizing strength, ductility, and resistance to environmental degradation.
Tensile Strength and Yield Strength Boost
T6 treatment improves mechanical properties to a high degree. The Mg-3.2Nd-2.5Gd-0.4Zn-0.5Zr alloy achieves ultimate tensile strength of 322 MPa and yield strength of 220 MPa at room temperature. This represents increases of 53% and 88% compared to as-cast conditions. This alloy maintains UTS of 292 MPa and YS of 185 MPa at 150°C.
Hardness and Ductility Trade-offs
Precipitation strengthening increases hardness but often reduces ductility. T6-treated specimens exhibit quasi-cleavage fracture at room temperature due to fine β′ precipitates pinning dislocations. Elevated temperature testing at 150°C produces ductile fracture with abundant dimples, as thermal activation boosts plasticity.
Room Temperature Aging Effects (AZ63, AZ92)
AZ63-T4 aged for five years at 75°F to 90°F demonstrates 14% tensile strength increase and 68% yield strength increase, yet 32% ductility reduction. AZ92-T4 exhibits even greater susceptibility to room-temperature aging than AZ63-T4.
Dimensional Stability at Elevated Service Temperatures
Magnesium-aluminum-zinc alloys exhibit slight permanent growth after extended exposure above 95°C. Alloys containing thorium and rare earth metals shrink rather than grow at elevated temperatures.
Stress-Corrosion Cracking Prevention in Formed Parts
Parts formed from AZ31 and AZ61 require 1 hour at 150°C to relieve stresses that prevent stress-corrosion cracking. T4 treatment improves SCC resistance. The Mg-6.7Zn-1.3Y-0.6Zr alloy shows SCC susceptibility index of 0.86 versus 0.95 for as-forged conditions.
Conclusion
Heat treatment protocols change magnesium alloy performance through control of temperature and time under specific atmospheric conditions. Solution treatment, aging, and annealing each serve different metallurgical objectives and let engineers optimize strength, ductility, and dimensional stability to match application needs. Proper heat treatment selection based on alloy composition and service requirements will give maximum mechanical properties while preventing stress-corrosion cracking. Manufacturers can then specify magnesium alloys for demanding aerospace, automotive, and biomedical applications where lightweight performance is critical.
FAQs
Q1. What are the main heat treatment processes used for magnesium alloys? Magnesium alloys undergo several key heat treatment processes including solution heat treatment (T4 temper), precipitation aging (T5 and T6 tempers), annealing, and stress relieving. Solution treatment dissolves constituents into solid solution at temperatures between 340°C and 565°C, while aging precipitates these constituents at 150°C to 260°C. Annealing recrystallizes the material and removes internal strains, whereas stress relieving eliminates residual stresses from forming and fabrication operations.
Q2. How does heat treatment improve the mechanical properties of magnesium alloys? Heat treatment significantly enhances mechanical properties by modifying the alloy's microstructure. T6 treatment can increase ultimate tensile strength by up to 53% and yield strength by 88% compared to as-cast conditions. Solution treatment maximizes toughness and shock resistance, while aging treatment optimizes hardness and strength. The specific improvements depend on alloy composition, with proper heat treatment enabling engineers to balance strength, ductility, and dimensional stability for particular applications.
Q3. Why are magnesium alloys considered advantageous for lightweight applications? Magnesium alloys offer exceptional strength-to-weight ratios due to their remarkably low density of 1.74 g/cm3, making them significantly lighter than aluminum (2.7 g/cm3) and steel (7.8 g/cm3). This low density combined with high strength after proper heat treatment makes magnesium alloys particularly attractive for aerospace, automotive, and biomedical applications where weight reduction is critical without compromising structural integrity.
Q4. What protective atmospheres are required during magnesium alloy heat treatment? Protective atmospheres are essential to prevent oxidation and burning during heat treatment. Sulfur dioxide (SO2) at minimum 0.7% concentration prevents active burning up to 565°C, while carbon dioxide at 3% provides protection to 510°C and 5% CO2 extends coverage to 540°C. These atmospheres form protective surface films of MgO, MgS, and MgSO4 that shield the material during high-temperature processing.
Q5. How does stress relieving prevent stress-corrosion cracking in formed magnesium parts? Stress relieving removes residual stresses induced by forming, welding, and fabrication processes that can lead to stress-corrosion cracking. For wrought alloys like AZ31 and AZ61, heating at 150°C for 1 hour effectively relieves these internal stresses. This treatment is particularly critical for formed parts, as it prevents stress-corrosion cracking and ensures long-term reliability in service environments.
