Aluminum alloys offer compelling advantages for structural applications: one-third the density of steel, excellent corrosion resistance, and high recyclability. However, the wide range of available alloys — each with distinct properties — makes selection critical for structural performance.
Alloy Designation System
Wrought aluminum alloys use a four-digit system where the first digit identifies the major alloying element: 1xxx (pure Al), 2xxx (Cu), 3xxx (Mn), 4xxx (Si), 5xxx (Mg), 6xxx (Mg+Si), 7xxx (Zn). Structural applications primarily use 5xxx, 6xxx, and 7xxx series.
5xxx Series — Marine and Industrial
5xxx alloys are strengthened by magnesium in solid solution (non-heat-treatable). They offer the best combination of strength, corrosion resistance, and weldability among structural aluminum alloys.
- **5083-H321**: The primary marine structural alloy. Tensile 305 MPa, yield 215 MPa. Excellent seawater corrosion resistance. Used for ship hulls, offshore platforms, and LNG tank construction.\n- **5754-H111**: Lower strength (tensile 190 MPa) but excellent formability. Automotive body panels, pressure vessels, flooring.\n- **Welding**: Excellent weldability with 5183 or 5356 filler. HAZ strength reduction approximately 10–15% — the lowest of all structural aluminum series.
6xxx Series — Architectural and General Structural
6xxx alloys are heat-treatable (Mg₂Si precipitation hardening) and are the most widely used for extruded structural profiles. They offer a good balance of strength, extrudability, and surface finish.
- **6082-T6**: The primary European structural alloy. Tensile 310 MPa, yield 260 MPa. Specified in Eurocode 9 (EN 1999-1-1) for structural design. Bridges, building frames, transport structures.\n- **6063-T6**: Lower strength (tensile 215 MPa) but superior surface finish and extrudability. Architectural profiles, curtain wall systems, window frames.\n- **Welding limitation**: HAZ softening is severe — strength drops to T4 temper level (approximately 40–50% reduction in proof stress). Structural design must account for HAZ properties per Eurocode 9.
7xxx Series — High-Strength and Aerospace
7xxx alloys (Al-Zn-Mg and Al-Zn-Mg-Cu) achieve the highest strengths of any aluminum series but with trade-offs in corrosion resistance and weldability.
- **7075-T6**: The classic aerospace alloy. Tensile 570 MPa, yield 505 MPa. Essentially non-weldable by fusion processes — joined by riveting or adhesive bonding. Aircraft wing skins, fuselage frames.\n- **7020-T6**: Weldable 7xxx alloy (no copper). Tensile 350 MPa, yield 280 MPa. Military vehicles, armored structures, bicycle frames. Naturally age-hardens after welding, partially recovering HAZ strength.\n- **7005-T6**: Similar to 7020. Bicycle frames, sports equipment, rail vehicle structures.
Eurocode 9 Design Framework
EN 1999-1-1 (Eurocode 9) provides the structural design framework for aluminum. Key differences from steel design (Eurocode 3): HAZ softening must be explicitly accounted for in joint design; local buckling is more critical due to lower elastic modulus (70 GPa vs 210 GPa for steel); and fatigue performance is generally lower than steel.
Material partial safety factors in Eurocode 9: γ_M1 = 1.10 for member resistance, γ_M2 = 1.25 for connections. These are higher than the corresponding steel factors, reflecting greater variability in aluminum mechanical properties.