Aluminum Alloys
Introduction
Aluminum alloys are a cornerstone of lightweight structural design. For civil applications—canopies, pedestrian bridges, façade frames, offshore platforms, transportation structures—they offer high specific strength, excellent corrosion resistance, and unrivaled extrudability for custom shapes. This guide explains which aluminum alloys belong in structural work, how they behave compared to steel, and how to detail connections, durability, and inspection so the load path is reliable from concept through maintenance. We connect alloy selection to credible loads, practical analysis, and field inspections.
Choose the alloy as a system decision—series + temper + fabrication + protection—so performance matches the environment and detailing.
Aluminum Alloy Series & Temper Systems
Aluminum is grouped by series (1xxx–7xxx) based on major alloying elements and by temper (e.g., -H, -T) indicating strain hardening and heat treatment. In civil structures, two families dominate:
- 5xxx (Al–Mg): Non-heat-treatable; strength from strain hardening (H tempers). Excellent weldability and marine corrosion resistance. Common plate/sheet for decks, tanks, and coastal details.
- 6xxx (Al–Mg–Si): Heat-treatable; strength from precipitation hardening (T tempers, e.g., T5/T6). Widely available extrusions; good balance of strength, corrosion performance, and formability—ideal for structural frames.
Other series appear in special cases: 3xxx (manganese) for formability, 7xxx (Zn–Mg–Cu) for very high strength but with stress corrosion cracking (SCC) concerns (use cautiously in civil works). Understanding temper is essential—H116/H321 in 5xxx indicate stress-relieved, marine-tempers; T6 in 6xxx marks peak-aged, high strength.
Typical Structural Grades
6061-T6, 6063-T6 (extrusions), 6082-T6 (higher strength extrusions in some markets), 5052-H32 (sheet), 5083-H116 (marine plate). Verify local availability and design data from the Aluminum Association.
Did you know?
Heat from welding can locally reduce strength in heat-treatable alloys (e.g., 6061-T6), creating a heat-affected zone (HAZ) that governs design near the joint.
Structural Properties: What Changes vs. Steel?
Aluminum’s density is about one-third of steel, but its elastic modulus is also roughly one-third. That means strength may check out while serviceability (deflection/vibration) governs member depth. Ductility is good, but post-yield behavior differs by series/temper; fatigue performance is detail-sensitive.
Specific Performance (Concept)
- Strength: 6xxx-T6 extrusions can achieve steel-like strength/weight. Use design values from recognized specifications (see references below).
- Stiffness: Expect deeper sections or sandwich action to meet deflection/vibration limits; check dynamic comfort for floors and pedestrian bridges.
- Fatigue: Detail governs. Avoid sharp re-entrant corners; grind/finish weld toes for high-cycle details; consider slip-critical bolting to lower stress ranges.
- Thermal Expansion: Coefficient is ~2× steel—plan movement joints and mixed-metal interfaces (see Thermal section).
Alloy Selection Framework
Choose the alloy/temper with a whole-system lens: loads, environment, fabrication, inspection, and lifecycle. The workflow below aligns choices with performance goals and constructability.
- Demand & Serviceability: Establish span, vibration, and deflection targets early; aluminum schemes often size on stiffness.
- Environment: Coastal/marine → 5xxx plates and 6xxx extrusions with protective details; industrial → consider crevice design and cleanability.
- Fabrication: Extrusions favor 6xxx; heavy plate weldments near splash zones favor 5xxx H-tempers.
- Connections: Prefer bolting for heat-treatable members where HAZ reduction is unacceptable; qualify welds where required.
- Lifecycle: Aluminum’s corrosion resistance reduces coatings/maintenance; plan inspection access and cleaning for deposits.
Important
Never swap 6061-T6 to 6063-T6 (or vice versa) late in design without re-checking section properties, weld behavior, and availability—extrusion limits and HAZ strengths differ.
Connections, Welding & Fasteners
Connections make or break aluminum structures. Balance welding and bolting with awareness of HAZ strength, galvanic isolation, and slip/fatigue performance.
- Welding: Gas metal arc (GMAW) and gas tungsten arc (GTAW) are common. Heat-treatable alloys (6xxx) lose strength in the HAZ; design with reduced allowable stresses near welds. Use appropriate filler (e.g., 5356 vs 4043) based on base alloy and service environment.
- Bolting: Use compatible fasteners (aluminum, austenitic stainless, or coated carbon steel) with isolation washers/sleeves to avoid galvanic couples; consider slip-critical joints for fatigue-sensitive locations.
- Mixed Metals: At aluminum–steel interfaces, add dielectric barriers and sealants; design for differential thermal movement.
- Detailing: Generous radii, avoid sharp copes, and provide smooth load paths; minimize eccentricities.
Thermal Movement (Interface Concept)
Corrosion, Durability & Surface Protection
Aluminum naturally forms a protective oxide film. In chloride or polluted environments, design details and cleaning access determine long-term performance as much as alloy choice.
- Galvanic Corrosion: Isolate from carbon steel and copper-bearing alloys; avoid water-trapping crevices; seal dissimilar joints.
- Crevice/Deposits: Provide drip edges, ventilation, and smooth, cleanable surfaces. Avoid pack-rust traps in mixed-metal assemblies.
- Finishes: Anodizing enhances oxide hardness and appearance; powder coating adds color and additional barrier—prep and sealing are critical near coastlines.
- Marine: Favor 5xxx plates and 6xxx extrusions with marine-proven fillers; keep welded zones drainable and inspectable.
Field Checklist
Confirm isolation hardware at steel interfaces, verify sealant compatibility, inspect weld toes for smoothness, and document cleaning schedules in the inspection plan.
Thermal, Fire & Serviceability Considerations
Aluminum’s higher thermal expansion and lower melting range require explicit strategies for service movement and fire.
- Movement Joints: Provide slip details, slotted holes, or bearings where aluminum connects to steel or concrete. Coordinate with façade anchors and canopy supports.
- Fire: Strength degrades rapidly at elevated temperature. Use rated assemblies, encapsulation, or performance-based evaluations where aluminum carries critical gravity loads during fire.
- Serviceability: Check wind deflection for canopies/frames—see wind design. For pedestrian bridges, verify vibration comfort using realistic damping.
Did you know?
Because extrusions are easy to customize, you can place material far from the neutral axis—raising I and stiffness without large weight penalties.
Fabrication, Erection & QA/QC
Aluminum enables fast, precise fabrication—especially with extrusions and modular assemblies. Quality assurance should tie submittals and mockups to structural assumptions in your analysis.
- Submittals: Alloy/temper certifications, extrusion drawings with section properties, weld procedures/fillers, fastener specs, coating/anodize details, and thermal movement provisions.
- Mockups: Verify tolerances, slip details, gasket compression, sealant compatibility, and drainage performance.
- Welding QA: Welder qualifications, procedure qualification records (PQRs), HAZ hardness/strength assumptions; surface cleanliness is crucial.
- Bolting QA: Torque/tension verification; isolation hardware inspected and documented; slip testing for critical joints where required.
- Erection: Protect surfaces from steel grinding dust (which can stain); avoid dragging parts; control clamping to prevent local denting.
Important
Do not modify heat-treatable members (e.g., heavy on-site grinding, local heating) without engineering review—local temper changes can invalidate design properties.
Standards & Trusted References
Use authoritative sources that maintain stable specifications and data:
- Aluminum Association: Design manuals, alloy data, and product standards. Visit aluminum.org.
- ASTM International: Material/product standards and testing (alloys, fasteners, coatings). Visit astm.org.
- NIST: Research on materials performance and durability. Visit nist.gov.
- FHWA Infrastructure: Guidance on corrosion, bridges, and materials. Visit fhwa.dot.gov.
For related structural context, see our pages on steel design, compare building materials, confirm loads and analysis, coordinate foundations, and plan inspections.
Frequently Asked Questions
6061-T6 vs 6063-T6—what’s the difference for structures?
6061-T6 generally provides higher strength and plate availability; 6063-T6 excels in extrusion finish and complex shapes. Select based on required properties and shape availability, then confirm weld design values at HAZ.
When should I pick 5xxx over 6xxx?
Choose 5xxx for welded plate work in marine or corrosive settings, where non-heat-treatable tempers keep weld-zone properties more uniform. Choose 6xxx for structural extrusions where shape efficiency and T-tempers are beneficial.
Do I need coatings on aluminum?
Often no for inland environments, but finishes improve appearance and cleanability. In coastal or industrial atmospheres, anodize or powder coat and design to shed deposits; isolate dissimilar metals.
Will thermal movement crack my connections?
Not if details allow slip or differential movement. Use slotted holes, sliding bearings, and flexible sealants where aluminum attaches to steel, concrete, or glass systems.
Is aluminum more “flexible” than steel?
It’s less stiff (lower modulus), so members may deflect more for the same depth. Combat this with efficient shapes/extrusions, composite action, or sandwich construction—and check vibration explicitly.
Key Takeaways & Next Steps
Aluminum alloys enable light, durable, and elegant structures—when you pick the right series/temper, design for stiffness and movement, and detail connections for galvanic isolation and fatigue resistance. 6xxx extrusions provide efficient shapes and good strength; 5xxx plates thrive in welded, marine-exposed work. Balance welding and bolting around HAZ behavior, and keep crevices clean and drainable for long life.
Continue with our guides on structural analysis, size to realistic loads, ensure a clean load path to foundations, and schedule inspections. For specifications and data, start at Aluminum Association, ASTM, NIST, and FHWA. Thoughtful alloy selection + efficient shapes + disciplined QA/QC = aluminum structures that build fast, look great, and last.