Describe the production, heat treatment and key properties of aluminium alloys 2024 and 7075, identify their use in airframe structures, and compare them with structural steel and titanium

What this dot point is asking

NESA wants you to describe how aluminium alloys are produced and heat-treated, identify the specific properties of 2024 and 7075 (the two airframe-grade alloys), explain how the properties dictate selection for fuselage skins versus wing spars, and compare aluminium with structural steel and titanium.

The answer

Production

Aluminium is smelted from alumina ($Al_2 O_3$, refined from bauxite by the Bayer process) using the Hall-Heroult electrolytic process at about 950 degrees C in molten cryolite. Australia is the world's largest producer of bauxite (Weipa in Queensland, Boddington in Western Australia) and the second largest producer of alumina. Smelting is energy-intensive: about 14 kWh per kg of aluminium.

The pure metal is then alloyed with copper, zinc, magnesium, silicon and manganese to produce the wrought aluminium alloy families used in aerospace.

Alloy families and tempers

The standard four-digit designation identifies the principal alloying element:

• 1xxx Pure aluminium (electrical conductors, foil)
• 2xxx Al-Cu (aerospace, 2024)
• 5xxx Al-Mg (marine, structural)
• 6xxx Al-Mg-Si (extrusions, 6061 for general engineering)
• 7xxx Al-Zn (aerospace, 7075)

The temper designation follows: T3 (solution treated, cold worked, naturally aged), T6 (solution treated and artificially aged), T7 (solution treated and over-aged for stress corrosion resistance).

Precipitation hardening

The strength of 2024 and 7075 comes from precipitation hardening:

1. Solution treatment at about 500 degrees C dissolves alloying elements into the aluminium lattice.
2. Rapid quenching freezes a supersaturated solid solution.
3. Ageing (natural at room temperature for T3, artificial at 120 to 175 degrees C for T6) lets fine intermetallic precipitates form within the grains.
4. These precipitates impede dislocation motion, raising yield strength by a factor of 3 to 5 above the annealed state.

The mechanism is similar to the way carbon steels are strengthened, but uses solid solution precipitation rather than martensite formation.

Property comparison

Aluminium is one-third the density of steel with comparable yield strength, giving 3 to 4 times the specific strength. Titanium has higher specific strength still but costs about 10 times more per kilogram.

Where each alloy goes

• Fuselage skin and frames. 2024-T3 sheet (often clad with pure aluminium for corrosion resistance), riveted in place. Boeing 737, 747, 767; Airbus A320, A330. Damage-tolerant under fatigue.
• Wing spars and ribs. 7075-T6 extrusions and machined parts. Higher strength means smaller cross-section for the same load.
• Skin around pressurised areas, doors and windows. Doubler plates and reinforcements in 2024 or 7075 depending on local stress.
• Engine pylons and landing gear. Often high-strength steel or titanium because of higher load and temperature.

Australian context

The Government Aircraft Factories (Port Melbourne and Fishermans Bend, 1936 to 1986) produced aluminium-airframe aircraft including the Avon Sabre (CAC Sabre), the Nomad and the Wirraway trainer. The current Hawker de Havilland operations at Bankstown supply aluminium parts to Boeing under offset agreements. Australian-mined bauxite from Weipa feeds smelters at Tomago (NSW) and Boyne Island (Qld), with much of the aluminium exported as ingot.