🪶 MAGNESIUM
Milling Magnesium: Fast Cutting, Real Fire Risk, and Grade Choice
Magnesium is the fastest-cutting structural metal there is, and also the one most likely to set your shop on fire. Those two facts define every magnesium milling job: the machining itself is a dream of high speeds and low cutting forces, but the fine chips are genuinely flammable, so the real expertise is in chip and dust management rather than the cut.
AS9100ISO 13485ISO 9001
Magnesium has the best machinability of any structural metal, often rated even above free-machining brass. It is the lightest structural metal at about two-thirds the density of aluminum, cuts with very low force, conducts heat well, and produces clean chips at high speeds, so cycle times are short and tool life is excellent. Shops can run magnesium fast with relatively modest spindle power.
The hazard is fire. Bulk magnesium parts are hard to ignite, but the fine chips, dust, and fines produced in milling have enormous surface area and burn intensely once lit, with a flame that water makes worse by liberating hydrogen. The controls are non-negotiable: keep tools sharp and feeds high enough to make chips rather than fine dust, avoid heat-generating dull-tool rubbing, manage swarf so fines do not accumulate, and keep Class D extinguishing media or dry sand on hand rather than water. Many shops machine magnesium dry or with mineral-oil-based coolant, never water-based coolant, because water reacts with magnesium. Dust collection and good housekeeping are part of the process, not an afterthought.
AZ31B, AZ91D, and the WE43 Outlier
AZ31B is the wrought magnesium-aluminum-zinc alloy, available as sheet, plate, and extrusion, used where a light formable structural part is needed. It machines beautifully and is common in aerospace and electronics housings. AZ91D is the high-aluminum die-casting alloy, the most common magnesium casting grade, used for automotive and consumer die-cast parts; when it is milled it is usually a casting being machined to final features, and it cuts cleanly though castings bring their own porosity and inclusion considerations.
WE43 is the high-performance outlier and the most interesting magnesium for advanced applications. It is a magnesium-yttrium-rare-earth alloy that retains strength at elevated temperatures and offers good corrosion behavior, used in aerospace and motorsport for demanding structural parts. More notably, WE43 is the leading bioresorbable implant alloy: it dissolves in the body at a controlled rate, so it is used for medical implants like bone screws and stents that are meant to disappear after healing, eliminating a removal surgery. Milling WE43 for medical use carries the same fire-safety requirements plus the cleanliness, traceability, and validation demands of implant manufacturing.
Tolerances, Finish, and Cost Realities
Magnesium holds tight tolerances readily because of its low cutting forces and good machinability, with +/-0.001 in routine, though its relatively high thermal expansion and low stiffness mean thin parts can move with temperature and deflect under clamping, so fixturing and thermal awareness matter on precision work. Surface finish is excellent off sharp tooling, which is one reason magnesium is attractive for finished housings.
Cost has an unusual profile. The machining itself is cheap because cycle times are short and tooling lasts, but raw magnesium costs more than aluminum, and the fire-safety overhead, dedicated handling, dust collection, swarf management, and the fact that fewer shops are set up to machine it safely, can raise the effective price and limit your supplier options. WE43 in particular is expensive and specialized, especially medical-grade. Lead times for AZ31B parts are generally short given the fast machining, while medical WE43 work carries the long validation and documentation timelines of implant manufacturing. The honest framing: magnesium machines faster and easier than almost anything, but you pay for the safety infrastructure and the narrower pool of capable shops, and you choose it specifically when its light weight or, for WE43, its bioresorbability is the requirement.
Frequently Asked Questions
Yes, magnesium is milled safely every day, but only by shops that take the specific precautions, and it should not be treated as just another light metal. The risk is not the bulk part, which is hard to ignite, but the fine chips, dust, and fines produced during machining, which have enormous surface area and burn intensely once lit. The controls are well established: keep tools sharp and feeds high enough to produce chips rather than fine powder, avoid the heat from dull-tool rubbing, manage swarf so fines do not accumulate in the machine or collector, and keep Class D extinguishing media or dry sand on hand because water makes a magnesium fire worse by releasing hydrogen. Shops machine magnesium dry or with mineral-oil-based coolant, never water-based coolant, since water reacts with magnesium. Good dust collection and disciplined housekeeping are part of the process. For buyers, the practical point is to use a shop experienced with magnesium specifically, since the safety infrastructure and habits are what make it routine rather than dangerous. The pool of capable shops is narrower than for aluminum.
Magnesium has the best machinability of any structural metal, often rated even higher than free-machining brass. Several properties combine to make it so easy to cut. It is the lightest structural metal, about two-thirds the density of aluminum, and it cuts with very low cutting force, so machines do not need much spindle power and parts and tools see little stress. It conducts heat well, pulling heat away from the cutting zone. And it produces clean, well-formed chips at high cutting speeds, so shops can run it fast with excellent tool life and short cycle times. The result is that the machining operation itself is fast, cheap, and forgiving compared to almost any other metal. The catch, of course, is the fire safety of the fine chips, which is the real challenge of magnesium work and the reason it is not simply the obvious default despite its machining ease. If a part needs to be light and is going to be heavily machined, magnesium's combination of low density and superb machinability is genuinely compelling, provided you use a shop equipped to handle it safely.
WE43 is a high-performance magnesium alloy containing yttrium and rare-earth elements, which give it good strength retention at elevated temperatures and improved corrosion behavior compared to common magnesium alloys. It is used in aerospace and motorsport for demanding lightweight structural parts. Its most distinctive use, though, is in bioresorbable medical implants. Magnesium dissolves in the body, and WE43's controlled corrosion rate lets it be used for implants like bone screws, plates, and cardiovascular stents that are designed to provide support during healing and then gradually dissolve and be absorbed, eliminating the need for a second surgery to remove hardware. This is a major advantage over permanent titanium or stainless implants in certain applications. Milling WE43 for medical use combines the standard magnesium fire-safety requirements with the cleanliness, biocompatibility, traceability, and process-validation demands of implant manufacturing, so it is specialized and expensive work done by a small number of qualified suppliers. For non-medical use it is chosen when its elevated-temperature strength and light weight justify its higher cost over AZ31B or AZ91D.
The comparison is interesting because the two cost drivers pull in opposite directions. On machining alone, magnesium is cheaper than aluminum: cycle times are shorter because it cuts faster with lower force, and tool life is excellent, so spindle time and tooling cost per part are lower. But on raw material, magnesium costs more than common aluminum alloys per pound, and the fire-safety overhead adds real cost that aluminum does not carry, including dedicated handling, dust collection, swarf management, and the fact that fewer shops are equipped to machine magnesium safely, which limits competition and can raise quotes. For AZ31B parts the net result is often roughly comparable to or modestly above aluminum depending on the part and supplier, with the weight savings being the reason to choose it. WE43 is in a different cost tier entirely, expensive and specialized, especially in medical grade. Lead times for AZ31B work are generally short thanks to the fast machining, while medical WE43 carries long validation timelines. Choose magnesium when its low density, about two-thirds of aluminum, genuinely matters to the application, since that weight advantage is the payoff for the added handling cost.
Yes, and milling magnesium castings is common, especially with AZ91D, the most widely used magnesium die-casting alloy for automotive and consumer parts. When a magnesium casting is milled it is usually being machined to final features, sealing surfaces, bores, and mounting faces, while the bulk shape comes from the casting. The material cuts cleanly and quickly like other magnesium. The things to watch are the usual casting considerations layered on top of magnesium's fire safety. Castings can contain porosity, gas voids, and inclusions, which may be exposed when you machine into the surface, potentially affecting sealing surfaces or pressure integrity, so critical castings may need porosity inspection or pressure testing. Surface skin on die castings can also vary, and the first cut removes any as-cast contamination. The fire-safety controls for chips and dust apply exactly as they do to wrought magnesium. For buyers, specify any required leak-test or porosity standard for pressure-containing parts, and ensure the casting source and the machining shop both understand the magnesium handling requirements. Done right, machined magnesium castings deliver very light, complex parts efficiently.
Last updated: July 2026
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