Section 6.2

Non-Ferrous Materials

Aluminium, titanium, magnesium, copper and nickel alloys, age hardening and their aircraft uses.

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Aluminium alloys are the primary airframe material for their strength-to-weight ratio. 2024 (Al-Cu, 'duralumin') is a high-strength structural alloy; 7075 (Al-Zn) is stronger still; both need corrosion protection such as Alclad (a pure-aluminium cladding) or anodising.
Heat-treatable aluminium alloys gain strength by precipitation (age) hardening: solution heat-treat, quench, then age naturally or artificially — strength develops over hours/days, which is why 2024 rivets are kept refrigerated to stay soft until driven.
Titanium alloys give an excellent strength-to-weight ratio, outstanding corrosion resistance and retain strength at high temperature — used for firewalls, engine parts and highly loaded structure; they are expensive and harder to machine.
Magnesium is the lightest structural metal, used in castings (gearboxes, wheels), but corrodes readily and is flammable as fine chips or powder — it needs careful protection and special fire precautions.
Copper alloys (brass, bronze) serve as bushes, bearings and electrical parts; nickel alloys (Inconel, Nimonic) handle the highest engine temperatures.
⚠ Exam trap: a magnesium fire must NOT be fought with water (it reacts and spreads) — use a dry powder/class-D agent. And heat-treatable alloys (2024/7075) age with time, whereas non-heat-treatable alloys (1100/5052) are strengthened only by work (cold) hardening.

Solved examples

Why are 2024-T (DD) aluminium alloy rivets stored in a freezer, and what happens if they are left on the bench too long?
2024 is a heat-treatable alloy that age-hardens at room temperature. After solution treatment it is kept refrigerated ('ice-box' rivets) to slow ageing so it stays soft enough to drive. Left out, it hardens; once it has aged past the drive-time limit it must be re-solution-treated or it will crack when bucked.

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