🪶 MAGNESIUM
Magnesium Casting: The Lightest Structural Metal and Its Foundry Realities
Magnesium is the lightest structural metal cast in industry, about two-thirds the density of aluminum and a quarter that of steel, and that single property drives every reason to use it and every difficulty in making it. Casting AZ91D, AZ31B, and the high-performance WE43 means controlling an oxidation and ignition hazard that no other common casting metal presents, which is why magnesium foundries are a specialized breed.
AS9100ISO 9001NADCAP
Living with magnesium's reactivity: cover gas, not water, in the foundry
Molten magnesium oxidizes rapidly and, as fine chips or thin streams, can ignite and burn fiercely, a fire that water makes worse because magnesium reacts with water to release hydrogen. Foundries manage this with protective cover gases over the melt: historically SF6, increasingly replaced by SO2 or fluorinated alternatives like HFC-134a and Novec for environmental reasons, and with careful flux practice. The melt is never exposed to open air the way aluminum can be, and chips and dross are handled as fire hazards.
This reactivity shapes the whole operation. Die casting is the dominant magnesium process precisely because the metal is injected into a closed steel die with minimal air exposure, and magnesium's low heat content and excellent castability let it run fast cycles with long die life (it is gentler on dies than aluminum). Hot-chamber die casting, where the injection system sits in the melt, is feasible for magnesium because of its low melting point (about 650 C) and is widely used for small thin-wall parts.
For buyers, the message is that magnesium casting is safe and mature in the hands of a proper magnesium foundry, but it is not something a general aluminum shop bolts on casually. Source a foundry with dedicated magnesium melting, cover-gas systems, and fire-safety practice. The reactivity is a managed process variable, not a reason to avoid the metal, but it does narrow your supplier list and is why magnesium is most often die cast rather than sand cast.
Alloy selection: AZ91D for die casting, AZ31B for wrought, WE43 for performance
The AZ alloys (magnesium-aluminum-zinc) dominate commercial magnesium casting. AZ91D is the most-used die-casting alloy, about 9 percent aluminum and 1 percent zinc, offering good castability, strength (around 23 ksi yield, 33 ksi tensile as die cast), and, crucially, the high-purity 'D' version has tightly controlled iron, nickel, and copper to dramatically improve corrosion resistance over older AZ91. It is the default for electronics housings, brackets, covers, and automotive components.
AZ31B (3 percent aluminum, 1 percent zinc) is primarily a wrought alloy, used in sheet, plate, and extrusion rather than casting; it appears on this list but in practice AZ31B parts are formed or machined from wrought stock, not poured, because its lower aluminum content makes it less castable than AZ91. If a print calls AZ31B for a cast part, the realistic conversation is whether AZ91D (cast) or a wrought AZ31B route is correct, this is a genuine substitution discussion worth having up front.
WE43 is the high-performance member: a magnesium-yttrium-rare-earth alloy with no aluminum, developed for aerospace and motorsport. It retains strength at elevated temperature (good to about 250 C, far better than AZ alloys which soften earlier), offers excellent specific strength after T6 heat treatment, and is used for helicopter transmission housings and high-end powertrain parts. WE43 also has a bioresorbable variant used in medical implants. It is sand or investment cast for aerospace housings and carries a much higher cost than the AZ alloys. Match the alloy to the duty: AZ91D for general die-cast parts, wrought AZ31B for formed sheet, WE43 for high-temperature aerospace performance.
Corrosion, galvanic traps, and the finishing that magnesium demands
Magnesium's Achilles' heel is corrosion. It sits at the anodic end of the galvanic series, meaning it is highly reactive and, when coupled with almost any other metal in the presence of moisture, magnesium corrodes preferentially and fast. A steel fastener directly threaded into a magnesium housing in a humid environment is a galvanic battery that eats the magnesium. The high-purity D-grade alloys (AZ91D) reduced the impurity-driven corrosion that plagued early magnesium, but galvanic and environmental corrosion still demand engineering attention.
The defenses are coatings and isolation. Magnesium castings are almost always protected by conversion coatings (chromate historically, now chrome-free options), anodizing (such as the Tagnite or Keronite plasma electrolytic processes), or paint and powder coat. Fasteners and inserts are isolated with coatings, sealants, or compatible materials, and design rules call for aluminum or coated fasteners rather than bare steel. For any magnesium part that will see moisture, salt, or dissimilar-metal contact, the finishing system is not optional, it is part of the design.
Machining magnesium is, ironically, a pleasure, it has the best machinability of any structural metal (rated around 500 percent relative to free-machining steel), cuts fast with low power and excellent finish. The caveat is the fire hazard from fine chips and dust: dry-cutting fine magnesium can ignite, so shops control chip size, avoid dull tools that generate heat and fines, keep dust collected, and have Class D extinguishers ready. The combination of superb machinability and fire risk means magnesium is wonderful to machine in a shop that respects it. Budget the corrosion protection as a real cost, it often determines whether a magnesium part survives in service.
Frequently Asked Questions
The casting process carries a genuine fire hazard, but it is well-managed in proper magnesium foundries, and the finished parts are completely safe. Molten magnesium oxidizes rapidly and can ignite, and fine chips or dust burn fiercely; water makes a magnesium fire worse because magnesium reacts with water to release hydrogen. Foundries control this with protective cover gases over the melt (SO2 or fluorinated gases that have largely replaced SF6 for environmental reasons), careful flux practice, closed-die injection that minimizes air exposure, and Class D fire suppression. Machining shops control chip size and dust and avoid dull tools that overheat fines. This reactivity is why magnesium is a specialized foundry service, not a casual add-on for an aluminum shop. The finished solid castings, however, do not spontaneously ignite, a solid magnesium part requires extreme temperatures to burn and is used safely in cars, aircraft, laptops, and power tools every day. The real in-service concern is not fire but corrosion: magnesium is galvanically very reactive and must be coated and isolated from dissimilar metals. So source a qualified magnesium foundry, and design proper corrosion protection, but do not let the foundry fire hazard scare you off the material; it is routine for specialists.
AZ31B is primarily a wrought alloy, used for sheet, plate, and extrusion, not a casting alloy, so in practice magnesium parts are die cast in AZ91D and wrought parts are formed or machined from AZ31B stock. AZ31B contains about 3 percent aluminum, too little for the good castability and fluidity that die casting needs, whereas AZ91D's 9 percent aluminum gives it the wide freezing range and feeding behavior that make it the dominant magnesium casting alloy. If a drawing specifies AZ31B for a cast part, that is usually a sign the designer chose the alloy from a wrought-properties table without realizing the casting implication, and the right move is to discuss substitution: either cast the part in AZ91D (accepting its different properties and better corrosion resistance in the high-purity D grade) or produce it from wrought AZ31B by machining or forming if the wrought properties are genuinely required. AZ91D die cast delivers about 23 ksi yield and 33 ksi tensile with good castability and the tightly controlled impurities that give it far better corrosion resistance than older magnesium alloys. Always confirm the cast alloy and condition before tooling, because the wrought-to-cast translation for magnesium is a real and common source of confusion.
WE43 is chosen when you need magnesium's light weight plus performance the AZ alloys cannot deliver, specifically elevated-temperature strength and high specific strength after heat treatment. WE43 is a magnesium-yttrium-rare-earth alloy with no aluminum; the rare-earth additions form stable precipitates that resist coarsening at temperature, so WE43 retains useful strength up to about 250 C, whereas AZ91 and AZ31 soften and creep at much lower temperatures (the aluminum-containing alloys begin losing properties above roughly 120 to 150 C). After T6 heat treatment, WE43 reaches roughly 36 ksi yield and 36 to 40 ksi tensile with good ductility, an excellent strength-to-weight ratio. This makes it the magnesium alloy for aerospace and motorsport: helicopter and aircraft gearbox and transmission housings, high-performance powertrain components, and structural parts that see heat. WE43 also has a medical-grade bioresorbable variant used in temporary implants because magnesium dissolves harmlessly in the body. The trade-offs are cost (yttrium and rare earths are expensive, so WE43 costs several times more than AZ91) and that it is typically sand or investment cast rather than high-volume die cast. Use AZ91D for general lightweight die-cast parts and reserve WE43 for high-temperature or fracture-critical aerospace applications where its performance justifies the premium.
Corrosion protection for magnesium is a design requirement, not an afterthought, because magnesium is one of the most galvanically reactive structural metals and will corrode rapidly if exposed to moisture or coupled to dissimilar metals. Start with the alloy: the high-purity D-grade alloys like AZ91D have tightly controlled iron, nickel, and copper impurities, which dramatically reduces the impurity-driven corrosion that plagued older magnesium, so always specify the high-purity grade. Then apply a protective coating system: conversion coatings (chrome-free chromate alternatives) as a base, or more robust anodizing such as plasma electrolytic oxidation (Tagnite, Keronite, or MIL-anodize), often topped with paint or powder coat for parts in harsh environments. Critically, manage galvanic coupling: never thread a bare steel fastener directly into magnesium in a wet environment, it forms a corrosion battery that eats the magnesium. Use aluminum or coated fasteners, isolate dissimilar metals with sealants, washers, or compatible inserts, and seal joints against moisture intrusion. Design to shed water rather than trap it. With the right alloy, a proper anodize-plus-paint system, and disciplined galvanic isolation, magnesium castings serve reliably in automotive, aerospace, and consumer applications. Skip these steps and the part can corrode visibly within months in a humid or salty environment.
Magnesium metal costs more per pound than aluminum, but because it is so light, the cost per part can be competitive and the weight savings can be decisive. Magnesium ingot runs roughly $2 to $3 per pound, and finished die-cast AZ91D parts commonly land at $3 to $7 per pound, while sand or investment-cast WE43 aerospace parts cost far more due to the alloy price and lower-volume process. The key is that a magnesium part weighs about one-third less than the same part in aluminum, so on a per-part basis magnesium die castings are often cost-competitive with aluminum die castings while saving significant mass. Die-cast tooling runs $25,000 to $120,000; magnesium is actually gentler on dies than aluminum (lower heat content), giving long die life and fast cycle times that help unit economics at volume. The weight savings is worth paying for when mass directly drives value: aerospace (every pound off an aircraft saves fuel over its life), automotive (lighter cars meet efficiency targets, and magnesium is used in steering wheels, seat frames, instrument-panel beams, transmission cases), portable electronics and power tools, and motorsport. Add the corrosion-protection finishing cost (conversion coat or anodize plus paint) to your budget. Where weight does not drive value, aluminum is usually cheaper overall; where it does, magnesium frequently wins despite the higher metal price.
Last updated: July 2026
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