6000 series aluminium alloy

The 6000 series adds magnesium and silicon, which combine as magnesium silicide and precipitate on ageing. It is moderately strong, heat-treatable, corrosion resistant, weldable with some loss of strength at the joint, and it extrudes better than any other aluminium family.

That last property is why it dominates. An extrusion die can produce a complex constant cross-section — a window frame with its own drainage channels and gasket grooves, a heat sink with fifty fins, a bicycle tube with internal ribbing — in one operation, and 6000 series alloys flow through a die at speeds the stronger families cannot approach.

It is the compromise alloy, and it is chosen for being adequate at everything rather than good at anything. Where more strength is needed the answer is 7000 series and a harder manufacturing problem.

Extruded above all, and also rolled, drawn and forged. Solution treated, quenched and artificially aged — the T6 temper — which roughly doubles the strength of the annealed material and is a matter of hours in an oven at around 175 °C.

It machines moderately well and welds with a strength loss in the heat-affected zone that has to be designed for: the weld anneals a band of the parent metal and no amount of care avoids it. Post-weld ageing recovers some of it. It anodises excellently, which is why architectural aluminium is this family.

Window and door frames, curtain walling and every architectural glazing system. Bicycle frames. Heat sinks and electronics enclosures. Structural profile and framing systems. Vehicle space frames and crash structures. Scaffolding, ladders and access equipment. Boat fittings, and marine hardware where 5000 series is not needed.

It is the aluminium a person is most likely to be within a few metres of.

Follows directly from Alfred Wilm's 1906 discovery of age hardening, though the magnesium-silicon system was developed later than the copper-bearing Duralumin that discovery produced. It became the dominant extrusion alloy after the Second World War, as aluminium extrusion capacity built for aircraft found a civil market in construction.

Recycles well and is the family closest to a genuine closed loop in construction: architectural aluminium arrives at demolition in identifiable profiles of known alloy, in quantity, and is worth separating.

Primary production remains extremely energy intensive — smelting is roughly forty per cent of the metal's cost as electricity — so the environmental case for aluminium in a building rests almost entirely on service life and on the metal being recovered at the end of it. A fifty-year window frame that is remelted is a very different proposition from a short-lived one that is not.