aluminum alloys

Pick up an aluminum ladder or a soda can and the first thing you notice is how light it is, aluminum weighs about a third of what steel does. Pure aluminum is too soft to be useful for structure, so we alloy it: adding a little copper, magnesium, silicon, or zinc turns a soft metal into one strong enough for airplane wings and bicycle frames while staying light and naturally corrosion-resistant.

Aluminum's density is about 2.7 g/cm^3 versus steel's 7.9, and it forms its own thin, protective oxide film so it does not rust. It is classed by a four-digit system, grouped by main alloying element: 1xxx pure, 2xxx copper (strong, aircraft), 3xxx manganese (cans), 5xxx magnesium (marine), 6xxx magnesium-silicon (extrusions, all-round), 7xxx zinc (highest strength, aerospace). Many of these are age-hardenable (also called precipitation-hardenable): you dissolve the alloying elements at high temperature, quench, then let fine particles precipitate (at room temperature or in a low-temperature aging oven) and those particles block dislocations and multiply the strength. This is why a temper designation follows the number: -O is annealed (soft), -H is strain-hardened, and -T tells the heat-treatment story (T6 = solution-treated and artificially aged to peak strength).

Aluminum alloys dominate wherever weight costs money or fuel: aircraft, cars, bikes, cans, and window frames. Two honest limits keep them from replacing steel everywhere. First, aluminum's Young's modulus (about 70 GPa) is only a third of steel's (about 200 GPa), so an aluminum part of the same shape flexes three times as much; you must add section to make it stiff, which eats into the weight saving. Second, unlike steel, aluminum has no fatigue endurance limit, so under enough repeated cycles it will eventually crack no matter how low the stress. And a strong aluminum alloy gets its strength from heat treatment that welding's heat can undo.