Magnesium Waterjet Cutting

Karl Gillihan

🪶 MAGNESIUM

Waterjet Cutting Magnesium (AZ31B, AZ91D, WE43): The Safe Way to Cut It

Magnesium is the lightest structural metal and also the one that scares shops, because magnesium fines and chips can ignite and burn fiercely. Counterintuitively, waterjet is one of the safest ways to cut it, since the whole process is drowned in water and there is no heat source and no dry dust to catch fire.

AS9100ISO 9001ISO 14001

Magnesium's reputation comes from real hazard: fine magnesium chips, dust, and turnings can ignite, and a magnesium fire burns hot and cannot be put out with water once established, which is why dry machining magnesium demands careful chip control and fire precautions. Grinding magnesium is especially risky because of the fine dust. This understandably makes shops nervous about cutting it at all. Abrasive waterjet flips the hazard on its head. The cut happens underwater or in a flooded zone, the removed material is swept away as wet slurry mixed with garnet, and there is no flame, spark, or hot edge to provide an ignition source. The magnesium particles are immediately wet and entrained, so they never become airborne dry dust. For magnesium, waterjet is not just convenient, it is arguably the safest cutting method available, which is a major reason aerospace shops use it for Mg detail parts.

AZ31B, AZ91D, and WE43 in the abrasive stream

AZ31B is a wrought magnesium-aluminum-zinc alloy supplied as sheet and plate, the most common wrought grade, used where light weight and moderate strength are needed. It cuts very fast on a waterjet because magnesium is soft and low density, often faster than aluminum of the same thickness. AZ91D is primarily a die-casting alloy with higher aluminum content, harder and used for cast housings; waterjet trims and profiles AZ91D castings cleanly. WE43 is the premium grade, a magnesium-yttrium-rare earth alloy with good high-temperature strength and creep resistance, used in aerospace and increasingly in bioresorbable medical implants. It is more expensive and the cold cut protects its tailored microstructure. All three cut quickly and cleanly, and the cold process avoids any concern about heat altering the alloy or, more importantly, any ignition risk during cutting.

When waterjet is the clear call and when it is not

Waterjet is the clear call for cutting magnesium whenever safety, thick plate, intricate profiles, or low quantities are in play, because it removes the ignition hazard that makes other methods nerve-wracking and it needs no tooling. Aerospace bracketry, lightweight chassis components, and WE43 implant blanks all fit this profile, and the cold cut adds no thermal concerns. The honest caveat is the water-plus-reactive-metal interaction. Some shops are cautious about cutting magnesium on a shared waterjet because magnesium reacting with water can slowly generate hydrogen in the catch tank, and fine magnesium in the tank sludge needs careful disposal; not every shop will run it. For very high volume flat magnesium parts, a stamping die may be cheaper. But for the typical short-to-medium-run magnesium job, find a shop experienced with it, and waterjet is both the safest and a very economical route.

Edge quality, corrosion handling, and post-cut care

Magnesium cuts cleanly with a square edge and minimal burr, holding roughly +/-0.005 inch on 0.25 inch sheet. Because it is soft and cuts fast, the edge quality is good and the process is quick. The genuine post-cut concern is corrosion: magnesium is reactive and the cut leaves bare metal wet with water and garnet, so parts can corrode quickly if not dried and protected promptly. Reputable shops cutting magnesium dry the parts immediately, remove residual garnet, and may apply a protective treatment, and buyers should specify prompt drying and a corrosion inhibitor or downstream conversion coating such as a chromate or anodize. Embedded garnet should be cleaned out, as it can promote galvanic corrosion. With proper drying and protection the waterjet edge is clean and ready for the next operation; the key is not leaving wet magnesium sitting.

Frequently Asked Questions

Yes, waterjet is arguably the safest way to cut magnesium. The fire hazard with magnesium comes from fine dry chips, dust, and turnings that can ignite, especially during grinding or dry machining. Abrasive waterjet cuts underwater or in a flooded zone with no flame, spark, or hot edge, so there is no ignition source, and the removed magnesium is immediately wet and swept away as garnet slurry rather than becoming airborne dry dust. This eliminates the main ignition pathways. The one nuance is that magnesium reacts slowly with water to release hydrogen, so the catch tank and the magnesium-laden sludge need proper handling and disposal, and not every shop will run magnesium for that reason. But the cutting itself carries far less fire risk than dry machining or grinding, which is why aerospace shops favor waterjet for magnesium detail parts.

Very fast, often faster than aluminum of the same thickness, because magnesium is soft and the lightest structural metal. A 90,000 psi abrasive waterjet cuts 0.25 inch AZ31B at roughly 20-30 inches per minute at a Q3 finish, 0.5 inch at about 10-15 ipm, and 1 inch at 5-8 ipm. AZ91D die-cast alloy is a bit harder and cuts slightly slower, and WE43 is in a similar range. The fast cut makes magnesium economical to waterjet despite the special handling, and tolerances are good, around +/-0.005 inch on quarter-inch sheet. The cold cut adds no heat distortion and, critically, no ignition risk. The main thing to plan for is not cut speed but prompt drying and corrosion protection afterward, since bare wet magnesium corrodes quickly.

Yes if left untreated, because magnesium is a reactive metal and the cut leaves bare surfaces wet with water and garnet. Parts can develop corrosion quickly, so they must be dried immediately after cutting, have residual garnet removed, and be protected. Embedded garnet particles are a particular concern because they can drive galvanic corrosion against the magnesium. Reputable shops experienced with magnesium dry the parts right away and may apply a temporary protective treatment, and the parts should then move promptly to a permanent finish such as a chromate conversion coating, anodize, or paint. With proper immediate drying and protection the clean waterjet edge is ready for the next operation. The key rule is simple: never leave wet magnesium sitting, and specify prompt drying and corrosion protection in your purchase order.

Thermal cutting of magnesium is risky precisely because magnesium ignites readily, and a hot cutting process plus magnesium dust is a fire waiting to happen, so laser and plasma cutting of magnesium are uncommon and require serious precautions. Thermal cutting also creates a heat-affected zone that can alter the alloy, and on a reactive metal that is undesirable. Waterjet sidesteps the fire hazard entirely by cutting cold and underwater, with the magnesium immediately wetted and swept away as slurry, and it leaves no heat-affected zone. It also cuts thick magnesium plate and intricate profiles that thermal processes struggle with, with no tooling. The tradeoff is the post-cut corrosion handling and the need for a shop willing to run magnesium and manage the hydrogen and sludge, but on balance waterjet is the safer and usually preferred method for cutting magnesium.

Last updated: July 2026

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