CNC machining magnesium alloy: process basics for engineers

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  What makes CNC machining magnesium alloy different
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  Chip control and fire risk in CNC machining magnesium alloy
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  AZ31B vs AZ91D for machined parts
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  Tolerances, finish and tooling on magnesium
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  Boundaries: when magnesium is the wrong call
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  Magnesium alloys compared with aluminum for CNC work

Magnesium is the lightest structural metal we cut. Density sits around 1.74 g/cm³ for AZ31B, roughly two-thirds that of aluminum and a quarter that of steel. On a part where mass drives the design, that gap shows up immediately: a housing that weighs 300 g in 6061-T6 lands near 190 g in AZ31B with the same wall thickness.

The cutting mechanics are friendlier than the numbers suggest. Magnesium has a low shear strength and a hexagonal close-packed lattice, so it forms short, discontinuous chips instead of the long stringy swarf you fight with 304 stainless. Cutting forces drop, spindle load drops, and surface finish often comes off the tool at Ra 0.8–1.6 μm without a finishing pass.

The catch is heat concentration. Magnesium has high thermal conductivity, so heat leaves the shear zone quickly and flows into the chip and the workpiece. That sounds good until you look at ignition temperature. Fine magnesium dust can ignite from a single spark, and the powder burns hot enough to be difficult to extinguish.

So the process question is not whether magnesium machines well. It does. The question is whether your shop controls chips, coolant choice, and tool geometry tightly enough to keep the risk bounded. That is where most magnesium work succeeds or fails.

The rule is simple: never let fines accumulate. Magnesium chips that stay in a pile can trap heat, and a pile of fines plus a spark is the failure mode you are designing against. We run high-volume coolant or a correctly rated minimum-quantity lubrication setup so chips are flushed away from the cut immediately.

Coolant choice matters more than people expect. Water-based coolants work for many magnesium operations, but they can react with fine swarf to release hydrogen, and wet magnesium fines are harder to handle than dry ones. Mineral-oil-based coolant is the conservative pick for heavy removal, and it keeps chips coated and cool.

Tool geometry drives chip size. A high positive rake angle, typically 10–15°, produces a thin chip that exits cleanly. Zero or negative rake tools rub, generate heat, and produce dust instead of chips. We also keep depth of cut moderate and feed per tooth high enough that the tool cuts rather than polishes.

Housekeeping is a process step, not an afterthought. Chips are removed at the machine, stored in covered metal containers, and never mixed with steel or aluminum swarf. A dedicated area for magnesium keeps the whole shop out of the risk zone.

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  No fines pilesFlush chips at the cut and clear the tray between operations.
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  Oil-based coolant for roughingReduces hydrogen risk and keeps chip temperature down.
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  Positive rake, sharp edges10–15° rake produces chips, not dust.

Two magnesium alloys cover most of the work we see. AZ31B is the wrought grade, supplied as plate, bar and extrusion. It has better ductility and formability, machines cleanly, and welds reasonably well. If your part is a bracket, panel, or housing that needs some toughness, AZ31B is usually the starting point.

AZ91D is the die-casting grade, high in aluminum and zinc, with excellent castability and good corrosion resistance. It is harder and stronger in the as-cast state but less ductile. When you machine AZ91D, you are often finishing a casting rather than cutting from solid, so you inherit casting porosity and may need to plan around it.

For high-temperature service, neither alloy is a hero. Magnesium alloys lose strength above roughly 100–125 °C, and creep becomes a design concern. If the part sits near an exhaust manifold or a hot motor, aluminum or a different alloy may be the better call even at a weight penalty.

Corrosion is the other decision point. Magnesium sits low on the galvanic series and will sacrifice itself to protect steel fasteners. Isolate dissimilar metals with coatings, plated fasteners, or a proper conversion coating, and the part holds up. Skip that step and you get white corrosion product at the joint.

Magnesium holds tolerance well because it does not spring back like titanium and does not work-harden like stainless. We routinely hold ±0.005 mm (±0.0002 in) on critical features, and the material's stiffness-to-weight ratio helps on thin walls where aluminum would deflect under clamping.

Thermal expansion is the trap. Magnesium expands about 26 × 10⁻⁶ per °C, higher than aluminum. A part machined at 25 °C and inspected at 20 °C moves. On long features or tight bores, we let the part stabilize and measure at a controlled temperature rather than trusting the in-process reading.

Surface finish depends on alloy and tool path. Sharp tooling and light finishing passes get you to Ra 0.8–1.6 μm on AZ31B; Ra 0.2–0.8 μm is reachable on sealing faces with a dedicated finishing pass. AZ91D tends to leave a slightly more open surface because of its cast structure.

Tooling is ordinary carbide. No exotic substrate is required, though we keep coated carbide for longer runs and polish the flutes to reduce built-up edge. Feeds and speeds run faster than aluminum, often 500–1,000 m/min surface speed, which is why cycle times on magnesium parts are short.

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  Clamp gentlyThin magnesium walls mark easily; use soft jaws and distributed pressure.
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  Control measuring temperatureLet parts stabilize before final inspection on tight bores.
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  Finishing pass for sealsRa 0.2–0.8 μm on faces that carry gaskets or O-rings.

Magnesium is not a general replacement for aluminum. If the part sees salt spray, road splash, or humid marine air without a coating system you control, corrosion will find the weak points. Aluminum behaves better in those environments and costs less to protect.

High-temperature service is a second boundary. Above roughly 125 °C, magnesium alloys creep and lose strength. Exhaust-adjacent brackets, motor mounts near heat sources, and engine components in continuous hot zones usually belong in aluminum or steel.

Cost is not automatically lower. Raw magnesium plate often costs more per kilogram than 6061, and the safety controls add processing time. You win on weight and cycle time, not on material price. If weight is not a driver, the business case is thin.

Finally, abrasive or high-wear surfaces are a poor fit. Magnesium is soft. Threads, bearing bores, and sliding surfaces need inserts, bushings, or a hardcoat anodize. We flag those features during DFM review so they are designed in from the start.