Aluminum & Magnesium Alloys
Aluminum is the second most-used structural metal after steel. At one-third the density of steel (2.7 vs 7.85 g/cm³), good corrosion resistance, and excellent recyclability, it's the workhorse lightweight metal for both automotive closures and aerospace structures.
Why Aluminum?
- Density: 2.7 g/cm³ — roughly 1/3 of steel
- Corrosion resistance: Forms a self-healing Al₂O₃ oxide layer
- Recyclability: ~95% recycling rate; recycling uses only 5% of the energy needed for primary production
- Formability: FCC crystal structure gives excellent ductility and formability
- Thermal conductivity: ~215 W/m·K — excellent for heat exchangers and engine components
Wrought Aluminum Alloy Series
Wrought alloys (rolled, extruded, forged) are classified by their primary alloying element:
| Series | Alloying Element | Heat Treatable? | Key Characteristics |
|---|---|---|---|
| 1xxx | Pure Al (≥99%) | No | Highest corrosion resistance, electrical conductivity. Bus bars, foil. |
| 2xxx | Copper | Yes | High strength, poor corrosion. Aerospace skins. |
| 3xxx | Manganese | No | Moderate strength, good formability. Heat exchangers, beverage cans. |
| 5xxx | Magnesium | No | Good weldability, marine corrosion resistance. Auto body panels, marine hulls. |
| 6xxx | Mg + Si | Yes | Good extrudability, moderate strength. Structural extrusions, bike frames. |
| 7xxx | Zinc | Yes | Highest strength aluminum alloys. Aerospace structures. |
Aerospace Workhorses
2024-T3 — σy ~345 MPa, σu ~485 MPa, elongation ~18%. The classic fuselage skin alloy. Excellent fatigue resistance and damage tolerance. Used for lower wing skins where tension loading dominates. 7075-T6 — σy ~505 MPa, σu ~570 MPa, elongation ~11%. One of the strongest aluminum alloys. Used for upper wing skins, spar caps, and bulkheads where compressive strength is critical. More notch-sensitive than 2024. 7050-T7451 — σy ~470 MPa, σu ~525 MPa. Thick-section aerospace alloy with superior stress-corrosion cracking resistance. Wing spars, bulkheads. Al-Li alloys (2195, 2050) — Lithium reduces density by ~3% per wt% Li and increases modulus by ~6%. Used in Airbus A380 fuselage panels and NASA Space Launch System fuel tanks. ~5% lighter and ~5% stiffer than conventional 2xxx/7xxx.Automotive Grades
5182-O — σy ~130 MPa, σu ~275 MPa, elongation ~25%. Excellent formability for inner body panels and closures. Used in the Audi A8 inner door panels. 6061-T6 — σy ~275 MPa, σu ~310 MPa. The most widely used aluminum alloy globally. Structural extrusions, crash management systems, bicycle frames. 6016-T4 — Auto body sheet alloy with good formability that bake-hardens during paint curing (~30 MPa strength gain at 180°C for 20 minutes).Temper Designations
The letter-number suffix after the alloy tells you the processing condition:
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| Temper | Meaning |
|---|---|
| O | Annealed — softest condition |
| H | Strain hardened (cold worked) — for non-heat-treatable alloys (1xxx, 3xxx, 5xxx) |
| T3 | Solution treated + cold worked + naturally aged |
| T4 | Solution treated + naturally aged |
| T6 | Solution treated + artificially aged — peak strength |
| T7 | Solution treated + overaged — improved stress-corrosion resistance, slightly lower strength |
| T76/T74 | Specific overaged tempers for aerospace |
Cast Aluminum
Cast alloys use a different numbering system (3-digit + decimal):
| Alloy | σy (MPa) | σu (MPa) | Application |
|---|---|---|---|
| A356-T6 | ~200 | ~275 | Wheels, suspension knuckles, control arms |
| A380 | ~160 | ~320 | Die-cast housings, transmission cases, engine brackets |
| A319-T6 | ~165 | ~250 | Engine blocks (Honda, Toyota, BMW) |
| AlSi10Mg (AM) | ~230 | ~380 | Additive manufactured aerospace brackets |
A356 is the dominant structural casting alloy in automotive — nearly every alloy wheel is A356.
Magnesium Alloys
Magnesium is the lightest structural metal: 1.74 g/cm³ — 35% lighter than aluminum, 78% lighter than steel. It offers the best strength-to-weight ratio of any common metal.
Key Grades
| Alloy | σy (MPa) | σu (MPa) | Process | Application |
|---|---|---|---|---|
| AZ91D | ~160 | ~240 | Die casting | Housings, covers, brackets |
| AZ31B | ~200 | ~260 | Sheet/extrusion | Laptop cases, camera bodies |
| AM60 | ~130 | ~225 | Die casting | Steering wheels, instrument panel beams |
| ZE41A | ~140 | ~205 | Sand casting | Helicopter gearbox housings |
Automotive Applications
- Instrument panel cross-car beam: BMW, Ford, GM use magnesium die castings (AM60) — saves ~3 kg vs steel
- Steering wheel armatures: Nearly all modern cars
- Seat frames: Some luxury vehicles
- Valve covers and oil pans: Several OEMs
- Wheels: Porsche, Corvette (forged magnesium — extremely expensive)
Challenges with Magnesium
Corrosion — Magnesium is the most anodic common structural metal. Galvanic corrosion is severe when in contact with steel or even aluminum without proper isolation (coatings, insulating washers). Flammability — Magnesium chips and powder ignite easily. Bulk magnesium requires special fire suppression (Class D extinguishers). This limits machining and recycling practices. Formability — HCP crystal structure means poor room-temperature formability. Sheet must be warm-formed at 200–300°C. Cost — Roughly 2× the price of aluminum per kg, though thin-wall die castings can be cost-competitive with aluminum on a per-part basis.Real-World Multi-Material Examples
Audi A8 (4th gen): Aluminum space frame with die-cast nodes, aluminum sheet panels, hot-stamped steel (rear), CFRP rear wall, magnesium dashboard beam — 58% aluminum by weight. Ford F-150 (2015+): Military-grade 6xxx and 5xxx aluminum body panels riveted and adhesive-bonded to a high-strength steel frame. Saved ~315 kg (700 lb), improving payload capacity and fuel economy. Tesla Model S: Aluminum-intensive body structure with boron steel reinforcements in the B-pillar and battery pack enclosure.Key Takeaways
- Aluminum alloy series are defined by primary alloying element (2xxx=Cu, 5xxx=Mg, 6xxx=Mg+Si, 7xxx=Zn)
- T6 temper means solution treated + artificially aged for peak strength
- 2024 and 7075 dominate aerospace; 5xxx and 6xxx dominate automotive
- Magnesium is 35% lighter than aluminum but has corrosion, formability, and cost challenges
- Cast aluminum A356-T6 is the standard for automotive wheels and structural castings
