Magnesium Sheet Metal: Hot Forming, Flammability, and the Lightest Structural Metal

It comes down to crystal structure. Magnesium has a hexagonal close-packed (HCP) lattice with very few active slip planes at room temperature, so it has limited ductility and cracks under cold deformation, unlike aluminum's face-centered-cubic structure that slips easily and forms well cold. AZ31B magnesium sheet can take only gentle bends at room temperature and splits on anything aggressive. The fix is heat: warming the metal to roughly 200 to 300 C activates additional slip systems, and ductility rises dramatically, letting magnesium form into shapes that are impossible cold. So magnesium sheet forming normally uses heated dies, heated tooling, or a pre-heated blank, a process step aluminum and steel skip entirely, and that adds cost and complexity. The most common and expensive magnesium mistake is assuming it behaves like aluminum because it looks similar and is also light; it does not, and a print that calls for tight cold bends in magnesium needs to be re-engineered around hot forming or the part will crack.

Yes, and it is the main reason many shops decline the work, though the risk is specific and manageable. Solid magnesium sheet is actually difficult to ignite, but the fine chips, dust, and grinding swarf created during machining, drilling, and deburring ignite easily and burn at extreme temperatures. Worse, a magnesium fire cannot be extinguished with water, which reacts to release hydrogen and intensifies it; it requires Class D dry-powder extinguishing media. Shops that work magnesium properly use sharp tooling and correct speeds and feeds to produce chips rather than fine dust, keep swarf volumes controlled, segregate magnesium chips from other metals and combustibles, and handle wet grinding carefully because it can generate hydrogen. The aerospace industry has fabricated magnesium safely for decades, so it is far from impossible, but it demands a shop that is genuinely equipped, trained, and disciplined for it. A general fabricator handling magnesium casually is a real safety risk, which is exactly why you should source one with established magnesium capability rather than assuming any sheet shop can do it.

Treat corrosion protection as mandatory, because bare magnesium corrodes quickly in humidity and salt and is galvanically very active, so an untreated part is effectively unfinished. The standard system starts with a conversion coating, traditionally chromate (per specs like MIL-DTL-5541-style treatments adapted for magnesium) and increasingly chrome-free alternatives, which provides a base layer and paint adhesion. That is usually followed by a primer and a paint or powder-coat topcoat for real durability in service. For demanding applications, plasma electrolytic oxidation produces a hard ceramic-like protective surface. Equally important is galvanic isolation: magnesium corrodes aggressively wherever it electrically contacts steel, aluminum, or other metals in a moist environment, so any fastener or mating part needs isolating washers, coatings, and sealants to break the galvanic couple, or the magnesium will sacrificially corrode at every contact point. The drawing should specify the conversion coating, the paint system, and the galvanic isolation scheme together, and you should budget the finishing as a real cost and lead-time item, not an afterthought.

Magnesium is justified when weight is the dominant design driver and you have exhausted aluminum, since magnesium is about a third lighter than aluminum (density around 1.74 versus 2.70 g/cm3) but costs more, forms harder, and demands special handling and finishing. The clearest wins are aerospace and defense, helicopter and aircraft components, missile structures, and anywhere every gram of mass translates directly into fuel, range, or payload, plus high-performance automotive and portable electronics housings where a thin, light, rigid shell matters. Use higher-performance WE43 when AZ31B's elevated-temperature strength or corrosion behavior is inadequate, accepting its higher price and lower availability. Magnesium is the wrong choice when weight is not the deciding factor, because then aluminum delivers similar lightness with far easier, cheaper, safer fabrication and better corrosion behavior. It is also wrong where the part runs hot beyond the alloy's limit or where fire-safety, corrosion, or galvanic complexity outweigh the mass saving. The honest test: if you cannot meet the weight target in aluminum and the application can carry the cost and finishing burden, magnesium earns its place; otherwise it usually does not.