7000 series aluminium alloy

Aluminium with zinc, and usually magnesium and copper as well. These are the strongest aluminium alloys made, reaching tensile strengths that overlap structural steel at roughly a third of the density, and they are the reason an aircraft structure can be aluminium at all.

The strength is bought with difficulty everywhere else. They are hard to weld — the wide freezing range makes the weld crack — so 7000 series structure is riveted or bonded. They are susceptible to stress corrosion cracking, which is a failure that needs tension, a corrosive environment and time, and which produces no warning; the T73 and T76 overaged tempers exist specifically to trade some strength away for resistance to it. And their corrosion resistance is the worst of the aluminium families, which is why aerospace sheet is clad with a thin layer of pure aluminium on each face.

The stiffness is worth stating because it surprises people: it is the same as any other aluminium. Alloying and heat treatment move strength by a factor of three and Young's modulus barely at all, so an aluminium part cannot be made stiffer by choosing a stronger alloy — only thicker.

Rolled to plate and sheet, extruded with difficulty, and forged. Solution treated at close to the melting range — the window is narrow — then quenched and aged, either to peak strength or deliberately past it for stress corrosion resistance.

Joined by riveting and adhesive bonding rather than welding, in aerospace almost universally. Friction stir welding, which never melts the metal, is the process that has made welded 7000 series structure possible where fusion welding could not.

Aircraft structure — wing skins, spars, stringers and fuselage frames — which is the application the family was created for and still its largest by value. High-end bicycle frames, climbing hardware and ski poles. Rifle receivers. Mould tooling for plastics, where the high thermal conductivity shortens cycle times against steel.

And sports equipment generally, where the strength-to-weight ratio is worth the price and the corrosion environment is mild.

Developed in Japan in the 1930s — the alloy known as Extra Super Duralumin, used in the Mitsubishi Zero, is the ancestor of the family — and developed in parallel in the United States and Germany. 7075 was in American service by 1943.

Stress corrosion cracking in early 7000 series structure was a serious and repeated problem in service, and the overaged tempers introduced from the 1960s are the industry's answer to it: deliberately giving up perhaps ten per cent of strength for a failure mode that does not announce itself.

Recyclable as aluminium, and awkward as a stream: the zinc and copper content mean 7000 series scrap cannot be diluted into other wrought alloys, so it is either kept segregated or downcycled to castings.

Aircraft dismantling is now an organised industry rather than a scrapyard activity, largely because segregating alloys at end of life is what makes the metal worth recovering as itself.