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Magnesium Alloys: Lightweight Solutions for Automotive, Aircraft, and Missile Applications
Magnesium is the lightest structural metal, with a specific gravity of about 1.8—roughly two-thirds that of aluminum and one-quarter that of steel. This SAE information report (J464) covers the magnesium alloys commonly used in the United States for automotive, aircraft, and missile applications. It provides foundational data on sources, properties, alloying elements, nomenclature, working methods, finishing, and testing, helping engineers leverage magnesium’s strength-to-weight advantage while avoiding common pitfalls.
Sources and Properties of Magnesium Alloys
Magnesium is the third most abundant structural element in the earth’s crust. Major sources include seawater, natural brines, magnesite, and dolomite. Three extraction methods are used in the U.S.: electrolytic reduction of magnesium chloride from seawater; electrolysis of anhydrous magnesium chloride from dehydrated brines; and the thermal ferro-silicon process for reducing magnesium oxide. Most magnesium ingot is sold at 99.80% purity, with higher purities (99.90–99.98%) reserved for nuclear and reduction uses.
Pure magnesium is not used for structural applications. Alloyed with elements such as aluminum, manganese, zinc, rare earths, silver, thorium, or zirconium, magnesium alloys achieve significantly higher strength. The table below summarizes typical physical and mechanical characteristics:
| Property | Typical Value |
|---|---|
| Specific Gravity | ~1.8 |
| Melting Point | 650°C (1202°F) |
| Coefficient of Thermal Expansion (20–100°C) | 26.1 × 10⁻⁶ /°C (14.5 × 10⁻⁶ /°F) |
| Modulus of Elasticity | 45 GPa (6.5 × 10⁶ psi) |
| Thermal Conductivity | Relatively high; some alloys approach aluminum values |
| Electrical Conductivity | Relatively high |
🔍 Engineering Design Insight: Magnesium alloys deliver an excellent strength-to-weight ratio, making them ideal for weight-critical components. However, designers must account for a higher coefficient of thermal expansion (over twice that of steel) and a lower modulus compared to aluminum. Working at elevated temperatures improves formability and eliminates springback.
Alloy Designation, Tempers, and Working
Magnesium alloys are designated per ASTM B275. The initial letter(s) indicate the major alloying element(s), the following numbers show nominal percentages by weight, and a final letter is assigned arbitrarily. For example, AZ91 contains ~9% aluminum and ~1% zinc. Temper designations follow ASTM B296, which is identical to the aluminum temper system (e.g., F, O, T4, T6).
Magnesium can be formed by die casting, sand casting, permanent mold, extrusion, forging, sheet rolling, drawing, spinning, and pressing. It is readily machinable at high speeds with excellent tool life. Joining methods include adhesive bonding, bolting, riveting, spot welding, and arc welding with inert gas shielding (TIG/MIG).
⚠️ Galvanic Corrosion Warning: When magnesium contacts dissimilar metals in the presence of moisture, galvanic attack can be severe. Connections must be adequately insulated or protected with coatings. In humid or marine environments, protective finishes are strongly recommended.
Finishing, Coating, and Testing
Bare magnesium is acceptable for many indoor applications. For outdoor or corrosive atmospheres, protective finishes (plating, painting, anodizing) prevent tarnishing and corrosion. Magnesium is tested using standard ASTM methods; tensile and compressive yield strengths are defined as the stress at 0.2% offset from the initial modulus line.
Frequently Asked Questions
Q: What are the most common magnesium alloys for structural use?
A: Alloys with aluminum and zinc (e.g., AZ31, AZ91), manganese (e.g., AM60), and rare earths or zirconium (e.g., WE43, ZK60) are widely used in automotive, aerospace, and defense.
Q: How are magnesium alloys designated and tempered?
A: Per ASTM B275, letters designate alloying elements and numbers indicate nominal percentages (e.g., AZ91D). Tempers follow ASTM B296, using the same codes as aluminum (F, O, T4, T6, etc.).
Q: What is the best method for welding magnesium?
A: Arc welding with inert gas shielding (TIG or MIG) is the most common fusion welding method. Spot welding, riveting, and adhesive bonding are also used effectively.
Q: Do magnesium alloys require corrosion protection?
A: Bare magnesium is suitable in dry, indoor conditions. In humid, marine, or industrial settings, protective coatings are necessary. Galvanic corrosion must be controlled by isolating magnesium from other metals.
