magnesium alloys
Magnesium is the lightest structural metal we use, lighter even than aluminum, about a quarter the weight of steel. Reach for it when shaving grams matters most: laptop and camera bodies, power-tool housings, car steering wheels and seat frames, and once upon a time the crankcases of air-cooled engines. If it has to be light and rigid and you can machine or die-cast it thin, magnesium is tempting.
Magnesium's density is only about 1.7 g/cm^3, and it has an excellent stiffness-to-weight and strength-to-weight ratio, plus it die-casts crisply and machines easily. It is hexagonal-close-packed, and here lies a catch: HCP metals have few slip systems, so magnesium is less ductile at room temperature than the cubic metals and is usually shaped by casting or hot working rather than deep cold forming. Alloys are named in a letter-number code (A = aluminum, Z = zinc): AZ91, the most common die-casting alloy, is magnesium with about 9 percent aluminum and 1 percent zinc.
Magnesium wins wherever mass is the enemy, but it comes with real, honest hazards you must respect. It is chemically active: fine magnesium powder or thin ribbon burns fiercely (though a solid casting is hard to ignite), so machining chips must be managed carefully. It corrodes readily, especially galvanically when coupled to other metals in damp conditions, so magnesium parts need coatings and careful isolation. And its low modulus and modest high-temperature strength limit it. These drawbacks, not weight, are why aluminum, not magnesium, remains the default lightweight metal.
AZ91 magnesium die-casts into a laptop chassis or a camera body at about 1.8 g/cm^3, roughly a third lighter than the same part in aluminum, but the part needs a protective coating because bare magnesium corrodes, especially where it touches steel screws.
Lightest structural metal (about 1.7 g/cm^3), but the HCP structure limits cold ductility and it corrodes and ignites more readily than aluminum.
The magnesium-burns fear is half-true: fine chips, powder, or ribbon ignite easily, but a solid magnesium casting is genuinely hard to set alight. The more everyday problem is corrosion, especially galvanic corrosion when magnesium touches steel or aluminum in the wet.