5000 series aluminium alloy

The 5000 series alloys aluminium with magnesium, typically between half a per cent and five, and they are not heat-treatable: their strength comes from work hardening and from the magnesium in solution, not from precipitation.

That sounds like a limitation and is the reason they are chosen. A weld destroys precipitation hardening, so a welded 6000 or 7000 series joint is much weaker than the parent metal. A 5000 series weld is close to as strong as what it joins, which for a welded marine structure decides the question.

They also have the best corrosion resistance of the aluminium families in seawater, and the highest magnesium grades have a specific caveat: held warm for long enough, magnesium precipitates at the grain boundaries and the alloy becomes susceptible to stress corrosion, which is why the high-magnesium grades are restricted to service below about 65 °C.

Rolled to plate and sheet, extruded less readily than the 6000 series, and welded with a matching magnesium-bearing filler. Formed cold, and it work-hardens noticeably as it goes — deep drawing needs intermediate annealing.

It anodises well, and it is the aluminium family most tolerant of a marine environment before any treatment at all.

Boat and ship hulls, and superstructures where weight above the waterline matters. Pressure vessels and cryogenic tanks, because aluminium gets tougher rather than more brittle as it gets colder. Vehicle body panels and fuel tankers. Beverage can bodies, which are one of the largest single uses of aluminium there is.

Architectural and marine fittings, and welded structures generally where the joint has to carry the load.

Aluminium-magnesium alloys date from the early twentieth century and became important with welded aluminium construction from the 1940s and 1950s. The aluminium hull is essentially a post-war development, and it depended on having an alloy whose welds could be trusted.

Aluminium's environmental case is entirely about recycling, and it is a strong one: remelting uses roughly five per cent of the energy of primary production from bauxite, and the metal does not degrade in the process.

The complication is alloy mixing. A recycling stream of mixed wrought alloys cannot be returned to a wrought alloy without dilution by primary metal, because the magnesium, silicon, copper and zinc do not separate. Most mixed scrap is therefore downcycled into casting alloys, which tolerate a wider composition. Beverage cans are the exception and the model: a can body is 5000 series and its lid is a different alloy again, and the industry keeps the loop closed by collecting cans as cans.