Why aluminium?
Aluminium has about one third the density of steel (2.7 against 7.85 g/cm³), conducts heat and electricity well, and forms a thin protective oxide film. Pure aluminium is weak, but its alloys reach strengths comparable to mild steel — giving the best strength-to-weight ratio for most airframe structure. Its weaknesses are a lower melting point, poor fatigue behaviour compared with steel (no endurance limit) and susceptibility to some forms of corrosion.
Alloy series
- 2024 (aluminium-copper-magnesium) — the classic fuselage skin and lower-wing alloy, with good fatigue and damage tolerance. Usually supplied as 2024-T3 or Alclad 2024-T3.
- 7075 and 7050/7150 (aluminium-zinc) — the highest-strength alloys, for upper wing skins, spars, stringers and fittings that carry compression. More notch-sensitive, and prone to stress-corrosion cracking and exfoliation; over-aged T73/T76 tempers improve this.
- 6061 (magnesium-silicon) — medium strength, good corrosion resistance and weldable: fittings, tubing, floats.
- 5052 (magnesium) — non-heat-treatable, easily formed and welded, good corrosion resistance: fuel tanks, fluid lines. 3003 — cowlings, fairings, non-structural parts.
- 2117 — the standard rivet alloy (AD), driven as received; 2017 and 2024 rivets are stronger "icebox" rivets; 5056 rivets are used with magnesium.
- Aluminium-lithium alloys (for example 2090, 2099, 8090) are lighter and stiffer and appear on newer airframes.
Cast aluminium alloys (wheels, housings, pistons) use a separate designation and are generally lower in strength and ductility than wrought alloys.
Temper designations
Alloys of the 1xxx, 3xxx and 5xxx series cannot be strengthened by heat treatment — only by cold working (strain hardening), shown by H tempers. The 2xxx, 6xxx and 7xxx series are heat-treatable and carry T tempers. Every heat-treatable alloy is also available annealed (O) for forming.
Alclad
Alclad sheet has a thin layer of pure aluminium (or a corrosion-resistant alloy such as 7072) rolled onto each side of a strong alloy core. The cladding is anodic to the core, so it corrodes sacrificially and protects the core even where scratched. It is marked "Alclad" or "clad" on the sheet. Avoid damaging it with scribes or abrasives, and remember that cladding lowers the overall strength slightly.
Heat treatment of aluminium alloys
- Solution heat treatment — heat to a precise temperature (about 490–500 °C for 2024) in a furnace or salt bath and soak so the alloying elements dissolve. Temperature control is critical: too low gives low strength, too high causes melting at grain boundaries.
- Quench immediately — the delay between furnace and quench is limited to seconds, otherwise strength and corrosion resistance (intergranular corrosion) suffer. Cold-water quenching gives the fastest cool; hot water or spray reduces distortion.
- Freshly quenched metal is soft (W condition) and can be formed or straightened. Strength then rises as fine particles precipitate: natural ageing at room temperature over a few days (T3, T4), or artificial ageing (precipitation heat treatment) in an oven at roughly 120–190 °C (T6).
- Annealing — heat (around 415 °C) and cool slowly to obtain the soft O condition for severe forming.
- Reheat treatment of clad sheet is limited, because each cycle lets core alloy diffuse into the cladding and reduces its protection.
- Correct heat treatment can be confirmed by hardness and electrical conductivity (eddy-current conductivity meter) checks.
Rivet alloys and icebox rivets
- 2117-T4 (AD) rivets are used as received — they are soft enough to drive and need no heat treatment.
- 2017-T (D) and 2024-T (DD) rivets are too hard to drive after ageing. They are solution heat-treated, quenched and then kept in a refrigerator (icebox) at about −18 °C or colder to delay ageing. Once removed, 2017 rivets must be driven within about an hour and 2024 rivets within about 10–20 minutes; otherwise they are re-heat-treated.
- 1100 (A) rivets are for non-structural parts; 5056 (B) rivets are used with magnesium to limit galvanic corrosion.
2024-T3 and 7075-T6 sheet look identical. Use the markings printed on the sheet, keep offcuts identified, and never substitute without approved data — 7075 is stronger in tension tables but less fatigue tolerant and cannot be bent as tightly.