Magnesium alloys in CNC processing: advantages and challenges

• 1
  Why magnesium behaves differently at the spindle
• 2
  AZ31B, AZ91D, and what the grade means for machining
• 3
  Cutting parameters, tooling, and the chip fire problem
• 4
  Distortion, thin walls, and holding tolerance
• 5
  Surface protection and corrosion control
• 6
  Where magnesium is the wrong answer
• 7
  Magnesium vs aluminium vs steel for CNC parts

Magnesium has a density near 1.74 g/cm³, about 35 percent lighter than aluminium 6061 and roughly a quarter the density of steel. For a housing, bracket, or cover that has to hit a stiffness target rather than a strength target, that difference alone can remove several hundred grams from a finished assembly. Engineers reach for it when the part is weight-critical and the loads are moderate.

The alloy also cuts fast. Its low hardness and high thermal conductivity let the tool push through at high surface speeds without building the same heat you see in steel or titanium. On a stable setup, magnesium can run at cutting speeds several times higher than aluminium with comparable tool life. Cycle time drops, and so does the thermal load on the workpiece.

That combination is the core advantage of magnesium alloys in CNC processing: less mass to move, less heat to manage, and shorter cycles. It is why the material shows up in aircraft interior hardware, handheld tool bodies, drone airframes, and EV brackets.

The catch is that magnesium does not tolerate sloppy process control. Chips ignite easily, the material galls against cutting edges, and thin walls move under clamping force. The rest of this page deals with those limits.

Two wrought and cast grades cover most CNC work: AZ31B and AZ91D. AZ31B is the wrought grade, supplied as plate, sheet, bar, and extrusion. It is the more ductile of the two, with good formability and weldability, which makes it the default choice for milled brackets, covers, and prototypes. Its aluminium and zinc content sits around 3 percent and 1 percent respectively.

AZ91D is a die-casting grade, higher in aluminium at roughly 9 percent. It offers better castability and higher room-temperature strength, but it is less ductile and more prone to hot cracking. When a part is machined from a casting rather than from plate, you are usually looking at AZ91D, and the machining strategy has to account for porosity near the skin.

The practical difference at the machine is chip formation. AZ31B produces longer, stringier chips that need aggressive chip breaking to clear the cut. AZ91D tends to break chips earlier but can leave a harder, more abrasive surface where the casting skin was not fully removed.

Neither grade is a drop-in substitute for the other. If a drawing specifies AZ31B for weldability and you quote AZ91D because the casting is cheaper, the weld joint will tell you about it.

• 1
  AZ31BWrought plate and extrusion. Best all-round machinability and weldability.
• 2
  AZ91DDie casting. Higher strength, lower ductility, watch for porosity.
• 3
  Skin removalTake a clean-up pass on castings before finishing cuts.
• 4
  CertificatesAsk for the mill certificate so the grade on the bench matches the drawing.

Magnesium machines with sharp, high-rake tooling and no coolant in many operations, or with a light mineral-oil mist. Water-based coolant is generally avoided because fine magnesium chips react with water to release hydrogen, and a pile of wet fines is a real hazard. The standard practice is dry cutting with high-volume chip evacuation, or a dedicated magnesium-rated oil.

Spindle speeds run high. Surface speeds in the range of 500 to 1,000 m/min are common for finishing passes on AZ31B with carbide, and feeds per tooth on a 12 mm end mill typically land between 0.05 and 0.15 mm. Depth of cut should stay moderate; the goal is to keep the chip thick enough to carry heat away rather than rubbing.

Chip management is the single biggest difference from aluminium. Magnesium fines ignite at low temperature, and a spark from a dry cut or a dropped tool can set them off. Class D extinguishers, dry sand, or a dedicated magnesium suppressant must be within reach. Never use water or CO₂ on a magnesium chip fire.

Tool geometry matters more than coating. High positive rake, polished flutes, and a sharp edge reduce galling. Coatings that work well on aluminium also work here, but a dull tool will rub, heat the chip, and raise ignition risk before it raises the surface finish number.

Magnesium has a low elastic modulus, around 45 GPa, roughly two-thirds that of aluminium. A part that deflects comfortably within tolerance in 6061 can spring past the limit in AZ31B under the same cutting force. Thin ribs and unsupported floors are where this shows up first, usually as chatter marks or a wall that measures over size after unclamping.

Clamping is the other pressure point. Magnesium dents and marks easily, so hard jaws tightened to steel-part force will deform the workpiece before the first cut. Soft jaws, low clamping pressure, and support under thin sections reduce the problem. For long parts, a fixture that follows the geometry is better than a vise and a prayer.

Residual stress from the plate or casting will release as material is removed. Rough the part, let it rest, then finish. On a part with a flatness callout tighter than 0.05 mm, a stress-relief step between roughing and finishing is often the difference between a passing part and a second setup.

None of this is exotic, but it does mean magnesium rewards shops that plan the setup before they cut metal. The tolerance itself is not the hard part; ±0.005 mm is achievable on the right machine. Keeping it across a thin-wall magnesium part is where the process earns its keep.

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  Rough, rest, finishLet stress release before the finishing pass on tight flatness.
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  Soft jawsLow clamping pressure; magnesium marks under hard jaws.
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  Wall supportBack thin ribs with a fixture or sacrificial material.
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  Light finishing cutsSmall depths keep deflection inside the tolerance band.

Untreated magnesium corrodes quickly in humid or salty air. The surface oxide that forms is not the protective barrier you get on aluminium. Any part that will see real service needs a coating, and the choice of coating affects the machining allowance you leave on the drawing.

Chemical conversion coatings are the usual base layer, followed by an epoxy or polyurethane paint for exposed parts. For conductive or EMI-shielded housings, a conductive anodizing or a chromate-free conversion layer is common. Plating is possible but requires a specific pretreatment sequence, and not every plating shop runs it.

Machined surfaces should be free of embedded iron. A tool that previously cut steel will leave iron particles in the magnesium surface, and those particles become corrosion initiation sites. Keep magnesium tooling separate, or clean it thoroughly before the run.

Bead blasting and tumbling are both used to even out the surface before coating. Laser marking for part numbers is fine, but character height should stay at 1.5 mm or more so the mark stays legible after the coating goes on top.

Magnesium is the wrong material when the part sees continuous moisture, salt spray, or galvanic contact with steel fasteners and no coating budget exists. The corrosion problem is not theoretical, and a part that needs re-coating every year is rarely cheaper than an aluminium one that does not.

It is also the wrong answer for high-cycle fatigue loading or impact. AZ91D is brittle relative to aluminium, and a bracket that takes repeated shock loads will crack where a 6061 part would bend. For structural parts with safety factors, aluminium or steel is the safer call.

High-volume production is another boundary. Magnesium plate costs more per kilogram than aluminium, and the fire-safety overhead adds fixturing and housekeeping cost. Below a few hundred parts, the weight saving usually justifies the premium. Above that, a die casting in AZ91D followed by finish machining is often the better route.

Finally, consider the supply chain. Not every machine shop runs magnesium, and fewer still run it with proper chip handling. If the shop cannot show you its chip evacuation and fire plan, the low quote is not a bargain.