🪶 MAGNESIUM
Magnesium Assembly: Lightweight Joining with Galvanic Discipline
Magnesium is the lightest structural metal in common use, about a third lighter than aluminum, and every assembly decision around it is shaped by two unforgiving traits: it is the most anodic engineering metal, so it corrodes at almost any dissimilar-metal contact, and it galls and creeps badly at threaded joints. Assemble it with galvanic discipline and proper inserts and it delivers unmatched weight savings; ignore those rules and the structure corrodes and loosens fast.
AS9100ISO 9001IATF 16949
Magnesium sits at the anodic extreme of the galvanic series, more active than aluminum, zinc, or any common structural metal. That means almost any dissimilar metal in electrical contact with magnesium in the presence of moisture turns the magnesium into a sacrificial anode, and it corrodes rapidly, sometimes visibly within days in a salt environment. This single fact governs magnesium assembly more than strength or weight.
Every fastener and mating part is therefore isolated. Assemblers use aluminum or coated fasteners (never bare steel against bare magnesium), insulating washers and bushings, and barrier coatings, sealants, primers, and conversion coatings on the magnesium itself. The faying surfaces are sealed so no electrolyte reaches the interface. Aerospace magnesium assembly often specifies a chromate or modern conversion coating plus an epoxy primer and wet-installed fasteners.
Fastener selection narrows accordingly. Aluminum fasteners are closest to magnesium galvanically and are preferred where strength allows. Where steel strength is needed, the steel is coated (zinc, aluminum, or specialized coatings) and electrically isolated. The cardinal sin in magnesium assembly is a bare steel or stainless fastener driven straight into bare magnesium, which corrodes the joint quickly.
Threading, inserts, and preload retention in magnesium
Magnesium is soft and low in modulus, so tapped threads in magnesium strip easily and bolted joints lose preload through creep and relaxation more than aluminum joints do. Direct steel fasteners into tapped magnesium are avoided in any joint that matters.
The standard solution is threaded inserts: aluminum or coated-steel inserts (or specialized magnesium-compatible inserts) installed into the boss to provide a durable thread and distribute load. Inserts also help the galvanic problem when chosen and isolated correctly. Self-tapping into magnesium is reserved for low-load, non-critical fastening.
Preload retention is managed with the same tools used for aluminum, but more aggressively: locknuts, thread-locking adhesive, Belleville and conical washers to hold clamp load as the magnesium creeps, and controlled, reduced assembly torque well below steel values because the soft magnesium boss crushes easily. Designers also keep clamped magnesium thickness adequate and avoid point loads that would yield the soft metal under the fastener head, using larger washers to spread the load.
Grade selection and the special case of WE43
The three grades sourced for assembly span automotive, general, and high-performance roles. AZ31B is a wrought magnesium-aluminum-zinc alloy supplied as sheet, plate, and extrusion, with good formability and weldability, used for lightweight panels, brackets, and enclosures that are fastened or welded. It is the most assembly-friendly of the common grades.
AZ91D is the dominant die-casting alloy, used for the vast majority of magnesium castings in automotive and electronics: housings, brackets, steering and instrument-panel components. Assembled AZ91D parts arrive as net-shape castings with cast-in bosses, and the assembly task is fastening these castings while managing the same galvanic and creep constraints. Its corrosion resistance is better than older magnesium alloys thanks to tight impurity control (the D suffix).
WE43 is the high-performance, rare-earth-containing alloy (yttrium and neodymium) that retains strength at elevated temperature and offers better corrosion behavior than the AZ alloys. It is used in aerospace, motorsport, and, notably, bioresorbable medical implants, since magnesium dissolves harmlessly in the body. WE43 assemblies are higher-value, lower-volume, and built under tighter controls, with the medical variant assembled to implant-grade cleanliness and traceability.
Fire safety, cost, and when magnesium is the wrong choice
Magnesium machining and grinding produce fine chips and dust that are flammable, so shops handling magnesium maintain dedicated tooling, dust collection, and Class D fire procedures. This is a real consideration in selecting an assembly partner: not every shop is equipped or willing to machine and handle magnesium safely, and that limits the supplier pool and can affect cost.
Cost-wise, magnesium alloy is more expensive than aluminum per pound, and the added isolation hardware, coatings, and careful assembly raise the finished cost further. The justification is weight: where shaving mass has genuine value, in aircraft, motorsport, and portable electronics, magnesium pays for itself. Where weight is not critical, aluminum delivers most of the lightness with far fewer galvanic headaches and lower cost.
The honest guidance: choose magnesium only when its weight advantage over aluminum is worth the corrosion-control burden and higher cost. For most lightweight structural needs, aluminum is the safer, cheaper default. Reserve AZ91D for high-volume die-cast weight savings, AZ31B for formed lightweight panels, and WE43 for high-temperature aerospace or bioresorbable medical applications where no other material fits.
Frequently Asked Questions
Bringing bare dissimilar metal into direct contact with bare magnesium. Magnesium is the most anodic common engineering metal, more active than aluminum or zinc, so when a bare steel or stainless fastener, or even an aluminum part, contacts bare magnesium in the presence of any moisture, the magnesium becomes a sacrificial anode and corrodes rapidly, sometimes visibly within days in a salt-spray or marine environment. The joint then loosens and fails. Prevention is non-negotiable: apply a conversion coating and primer to the magnesium, use galvanically compatible fasteners (aluminum is closest; coated steel where strength demands), insulate with non-conductive washers and bushings, and seal the faying surface with a wet-install sealant so no electrolyte reaches the interface. Aerospace practice combines all of these. The second-biggest mistake is over-torquing into the soft magnesium boss, which crushes threads and the boss; use reduced torque, threaded inserts, and large washers to spread the load. Magnesium rewards discipline and punishes shortcuts faster than almost any other structural metal.
For any load-bearing or repeatedly serviced joint, use threaded inserts rather than tapping directly into magnesium. Magnesium is soft and low in elastic modulus, so direct tapped threads strip easily, and bolted magnesium joints lose preload through creep and relaxation faster than aluminum joints. Installing an aluminum or coated insert (or a magnesium-compatible specialty insert) gives a durable, reusable thread that distributes load over more material and, when chosen and isolated correctly, helps manage the galvanic problem. Direct self-tapping or formed threads in magnesium are acceptable only for low-load, non-critical fastening. Whichever you choose, control assembly torque well below the steel-equivalent value because the soft boss crushes easily, use locknuts or thread-locking adhesive to fight vibration loosening, add Belleville or conical washers to maintain clamp load as the magnesium creeps over time, and use generously sized washers under fastener heads to spread the point load and avoid yielding the soft metal. Keep the clamped magnesium section thick enough that the fastener cannot pull through.
Magnesium is flammable in finely divided form. The fine chips, turnings, and especially the dust produced during machining and grinding can ignite, and a magnesium fire burns extremely hot and cannot be extinguished with water (water can make it worse by releasing hydrogen). Shops that machine magnesium use sharp tooling and proper feeds to produce coarser chips rather than fine dust, maintain dedicated dust collection that keeps magnesium fines separate from other metals and from sparks, avoid letting chips accumulate, and keep Class D dry-powder extinguishers and procedures on hand. This absolutely affects sourcing: not every machine shop is equipped or willing to handle magnesium safely, so the qualified supplier pool is smaller than for aluminum or steel, and that can lengthen lead time and raise cost. When sourcing magnesium assembly, confirm the partner regularly machines magnesium and has the proper fire-safety setup. Bulk solid magnesium parts are not a significant fire hazard in normal handling and assembly; the concern is specifically the machining and grinding fines, so an assembly-only operation receiving finished castings faces less of this constraint than a full machining shop.
Choose magnesium only when its weight advantage genuinely justifies the added corrosion-control burden and higher cost. Magnesium is roughly 35 percent less dense than aluminum (about 1.8 versus 2.7 g/cc), so it offers real mass savings where every gram counts: aircraft and aerospace structure, motorsport components, and portable electronics housings. But magnesium alloy costs more than aluminum per pound, demands extensive galvanic isolation and coating, creeps and strips threads more easily, and requires fire-aware machining. For the majority of lightweight structural needs, aluminum delivers most of the lightness with far fewer assembly headaches, easier corrosion management, and lower cost, making it the safer default. Reserve AZ91D for high-volume die-cast parts where the weight saving multiplies across production (automotive housings, brackets), AZ31B for formed lightweight panels and enclosures, and WE43 for high-temperature aerospace applications or bioresorbable medical implants where magnesium's dissolving behavior in the body is actually the goal. If you cannot point to a specific, quantified value for the extra weight saved over aluminum, aluminum is usually the better engineering and economic choice.
Last updated: July 2026
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