Quick questions on Aluminium alloys in airframes: HSC Engineering Studies Aeronautical Engineering

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 standard four-digit designation identifies the principal alloying element:

The strength of 2024 and 7075 comes from precipitation hardening:

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.

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.

T3 versus T6 affects yield strength by 30 percent or more. Heat treatment is not a finish; it is part of the material specification.

Aluminium has a less defined endurance limit than steel. Fatigue determines aircraft inspection intervals and service life.

2024 in particular has poor corrosion resistance because of the copper. Cladding with pure aluminium or anodising is standard. 7075-T7 (over-aged) trades some strength for better stress-corrosion resistance.