Magnesium Aluminum Alloy Motor Casing: How the Two Metals Behave on a CNC

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  Key takeaways
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  Why magnesium aluminum alloy motor casing weight matters at all
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  Picking the grade: AZ31B, AZ91D, 6061-T6 or ADC12
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  How each metal behaves when the cutter touches it
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  Wall thickness, 5-axis workholding and the roundness problem
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  When magnesium is the wrong answer
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  Magnesium vs aluminum for a machined motor casing

A motor casing does three jobs at once. It holds the stator and rotor gap, it pulls heat out of the windings, and it carries the mount loads into the frame. Mass enters the picture because the casing sits at the end of a moment arm. Every gram removed from the housing also lets the bracket, the shaft and the bearing seats shrink, so a 35% density cut on the housing often turns into a 15–20% cut on the whole drive unit.

Magnesium sits at 1.74 g/cm³. Aluminum 6061 sits at 2.70 g/cm³. On a housing with 380 cm³ of material, that is roughly 660 g versus 1,025 g before any pocketing. On a drone arm or a handheld tool, that difference is the design. On a 400 mm industrial pump motor bolted to a concrete pad, it is noise.

The trade is stiffness. Magnesium alloys have a lower elastic modulus, around 45 GPa against 69 GPa for aluminum. A magnesium casing has to be thicker or more heavily ribbed to hit the same deflection target. That eats back some of the weight saving, and it changes the machining strategy because thick ribs and thin webs appear in the same part.

So the question is never whether magnesium is lighter. It is lighter. The question is whether your stiffness target, your temperature ceiling and your production volume let you use it.

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  Weight-driven designsHandheld tools, drone and eVTOL drives, robotics joints, portable pumps.
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  Stiffness-driven designsLarge frames, precision spindles, anything where deflection sets the air gap.
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  Heat-driven designsHigh duty-cycle motors where the casing is part of the thermal path.

Magnesium AZ31B is wrought. It machines cleanly, takes a good finish and welds. Use it for prototype casings and low-volume housings where you are milling from plate. Magnesium AZ91D is a casting alloy with better castability and higher strength, but it is more brittle and more reactive in chip form. Most production magnesium motor housings start as an AZ91D die casting and are then finish-machined on the bearing bores, the stator seat and the mounting faces.

On the aluminum side, 6061-T6 is the default for billet housings. It is weldable, anodizes well and holds ±0.005 mm without drama. 7075 gives higher strength but poorer corrosion behavior and a higher price. ADC12 is the die-casting grade that most automotive and industrial motor housings actually use, because it fills thin walls at 2–3 mm and takes a machined finish on the critical seats.

The decision tree is short. If the housing comes from a mold, you are choosing between AZ91D and ADC12. If it comes from a billet, you are choosing between AZ31B and 6061-T6. Mixing the two, such as a magnesium body with aluminum inserts, is common and brings its own problem: galvanic corrosion at the joint.

We machine magnesium AZ31B and AZ91D, and aluminum 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12. Grades outside that list get quoted case by case.

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  AZ31BWrought plate, best machinability, good for prototypes and small runs.
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  AZ91DDie casting alloy, higher strength, brittle chips, needs chip control.
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  6061-T6Billet default, stable, anodizes cleanly, easy to hold tolerance.
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  ADC12Aluminum die casting grade, thin walls, machined on critical faces.

Magnesium cuts fast and it cuts dry or near-dry. Specific cutting energy is low, so spindle load stays modest even at high feed. The chips are discontinuous and light. That is good for evacuation and bad for fire, because fine magnesium dust ignites at a low ignition temperature and burns hot. We run magnesium with high-volume air blast, no water-based flood coolant, and a dedicated chip collection path. Never let magnesium fines sit in a tray overnight.

Aluminum is the opposite kind of easy. It is gummy at low speed, so it wants surface speed and sharp geometry. Built-up edge is the usual cause of a torn finish on a 6061 bearing bore. Keep the tool sharp, run carbide with a polished flute, and use coolant or a heavy air-oil mist. Aluminum chips are heavy enough to evacuate with normal augers.

Thermal expansion drives the tolerance plan for both. Aluminum expands at roughly 23 × 10⁻⁶ per °C, magnesium at about 26 × 10⁻⁶ per °C. A 150 mm bearing bore measured hot can read 0.04 mm off after it cools. On a motor casing, where the stator seat sets the air gap, that is the difference between a quiet motor and a noisy one.

So we rough, let the part rest, then finish. On a thin casing the rest can be most of a shift.

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  MagnesiumDry or air-blast, high feed, strict chip control, no standing fines.
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  AluminumSharp polished carbide, coolant or mist, watch built-up edge.
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  BothRough, stabilize, then finish the bore and the mounting faces.

A motor casing is a tube with features on both ends and often on the side. Once you thin the wall to 1.5–2.5 mm, clamping force starts to deform it. A three-jaw chuck on a thin magnesium tube will squeeze it oval, and the bore will spring back out of round after unclamping. That is the most common failure we see on incoming drawings.

The practical answer is 5-axis with a rotary table and low-pressure workholding. We use the Ø400 mm rotary table so the part can be indexed to five faces without a refixture, and we hold on internal bosses or a sacrificial web rather than on the finished outer wall. One setup also keeps the bearing bore, the stator seat and the mounting face in the same datum chain, which is what controls the air gap.

Ribs are the other constraint. Thin, tall ribs chatter. We rough them with a smaller radial engagement and leave 0.3–0.5 mm for a light finishing pass at reduced feed. Ribs also trap chips, so the program needs to clear them before the finish pass, not after.

For a 4,000 mm envelope, we can machine large housings on the long-travel machines. For compact casings, the 500 × 500 × 450 mm and 500 × 310 × 200 mm travels cover most motor sizes with room for the fixture.

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  Hold on bosses, not wallsClamp on internal features or a sacrificial web.
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  One 5-axis setupKeeps bore, stator seat and mounting face in one datum chain.
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  Rough, rest, finishLeave 0.3–0.5 mm and let the part cool before finishing.

Magnesium is the wrong answer when the casing is part of the heat path. Its thermal conductivity is roughly half that of aluminum, so a magnesium housing runs the windings hotter for the same load. If your motor is already near its insulation limit, switching the housing to magnesium makes that worse, not better.

It is also the wrong answer above about 120 °C in continuous service. Magnesium alloys lose creep resistance quickly, and a bearing bore that creeps under load loses its fit. For a high-temperature or high-duty-cycle motor, aluminum wins on that single criterion.

Corrosion is the third boundary. Bare magnesium needs a coating, and it reacts badly with steel and stainless fasteners in a wet or salt environment. If the housing lives outdoors or near a wash-down area, the joint design has to isolate the magnesium from the steel, or you should go back to aluminum.

And if the part is a one-off bracket-shaped housing with no weight target, none of this matters. Choose 6061-T6, skip the coating argument, and spend the time on the bore.

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  Choose magnesiumWeight-critical, moderate temperature, dry environment.
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  Choose aluminumHeat-critical, hot duty cycle, wet or corrosive environment.
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  Either worksLow volume, no weight target, ambient conditions.