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.

Wrought aluminum alloy series — UTS ranges and heat treatability. Hover bars for details.

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:

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