Magnesium Forging: AZ31B, AZ91D and WE43

The fire risk is real but specific and routinely managed by qualified shops. Solid magnesium billet and forgings do not casually catch fire; magnesium ignites at high temperature, and the genuine hazard is fine chips, grinding dust and machining turnings, which have huge surface area and can ignite and burn intensely. Critically, magnesium fires cannot be put out with water, which reacts and makes them worse, and require Class D dry-powder extinguishers. During the forging operation itself, the metal is solid and below ignition temperature, so the risk is lower than during grinding or machining, though warm billets, flash and fines are handled carefully and inhibitor coatings or cover gases are sometimes used at higher temperatures. The practical consequence for buyers is supplier selection: magnesium must go to a shop with dedicated equipment, swarf and dust management, flood-coolant or controlled dry-machining protocols, and trained operators, not to a general aluminum forge shop. That narrows the supplier pool and adds to cost and qualification time, but it makes magnesium entirely safe to design with and process. The metal's flammability is a processing-control issue, not a reason to avoid it for weight-critical parts.

Magnesium has a hexagonal close-packed crystal structure, which at room temperature offers very few active slip systems, so it has poor ductility cold and will crack if formed hard. Heating it above roughly 400-700°F activates additional slip and twinning systems, dramatically improving ductility, which is why production magnesium forging runs warm, typically in the 500-800°F range depending on alloy, much cooler than aluminum or steel forging. The other constraint is strain rate: the HCP structure cannot accommodate fast deformation, so magnesium must be deformed slowly to avoid surface cracking and internal shearing. That makes hydraulic presses, which apply load slowly and steadily, the preferred tooling over fast drop hammers. Dies are usually heated to keep the workpiece in its narrow warm-working window, because magnesium's high thermal conductivity rapidly chills the part against cold tooling. The combination of warm billet, heated dies and slow strain rate is standard practice and is what allows magnesium to forge to fine grain with good directional properties. Violating any of these, forging too cold or too fast, produces cracked scrap, which is why magnesium forging is a deliberate, lower-throughput process and costs more per part than the cheap, light raw material implies.

AZ91D is primarily a die-casting alloy and is rarely the right choice for forging. With 9% aluminum and 1% zinc, it is the most common magnesium casting grade, valued for excellent castability and, in the high-purity D form, good corrosion resistance, which is why it dominates die-cast housings, brackets and covers. That same high aluminum content, however, reduces hot ductility and makes it a poor wrought-forging stock compared with lower-aluminum alloys. If a buyer requests forged AZ91, the honest first question is whether they actually need a die-cast part, because casting is usually the better and cheaper route for AZ91 geometry. For a part that genuinely must be forged, to get grain flow, higher fatigue strength, or freedom from casting porosity, the better magnesium forging alloys are AZ31B for general weight-saving parts and WE43 for high-temperature or high-performance applications. So the guidance is: use AZ91D as a casting alloy where its castability and corrosion resistance shine, and switch to AZ31B or WE43 when forging is truly required. Specifying forged AZ91D without confirming the process fit often leads either to a poor forging or to a redesign back toward casting.

WE43, alloyed with yttrium and rare-earth elements, justifies its high cost in two distinct arenas. First, in aerospace it retains strength and resists creep at elevated temperatures far better than the standard AZ alloys, which lose properties above about 250-300°F. WE43 is heat-treatable to good strength and stays serviceable at higher operating temperatures, making it the magnesium of choice for gearbox and transmission housings, helicopter components and other hot, weight-critical structures where AZ31B or AZ91 would soften. Second, in medical devices, WE43 has become a leading bioresorbable implant material: it is biocompatible and corrodes in the body in a controlled, predictable way, so an implant such as a bone screw or stent can provide temporary structural support and then safely dissolve, eliminating a removal surgery. That controlled-degradation behavior, combined with mechanical properties closer to bone than titanium, is genuinely valuable and hard to replicate. The premium comes from the rare-earth alloying content, the tight process control WE43 forging demands, and the qualification rigor for aerospace and medical use. For ordinary weight savings you would never pay for WE43; you reach for it specifically when you need high-temperature performance or bioresorbable behavior.

Magnesium is about 35% less dense than aluminum (roughly 1.74 vs 2.70 g/cm3), so it is the lightest structural metal you can forge, and for parts where mass is the dominant driver it can beat aluminum on a strength-to-weight basis once optimized. However, the comparison is not all in magnesium's favor. Forged aluminum alloys like 7075 reach higher absolute strength, machine and weld more easily, resist corrosion better without coatings, and forge faster and cheaper because aluminum tolerates higher strain rates. Magnesium forges slowly and warm, requires fire-conscious processing, has poorer corrosion resistance (galvanic corrosion in contact with other metals is a real concern requiring isolation and coatings), and the supplier pool is smaller, all of which raise cost and lead time. So magnesium wins specifically when every gram counts and the cost premium is justified, classic cases being aerospace brackets and housings, helicopter transmission cases (WE43), high-end automotive and portable electronics. For most lightweight structural parts where moderate weight savings suffice, forged 6061 or 7075 aluminum is the more practical, lower-cost, more corrosion-tolerant choice. Choose magnesium when the weight target cannot be met any other way and you can manage its corrosion and processing demands.