Application trend and development of magnesium alloy shells in new energy vehicles

Magnesium sits at roughly 1.74 g/cm³, about a third lighter than aluminum and a quarter of steel. A housing that weighs 8 kg in aluminum drops to around 5 kg in magnesium, and that saving sits high on the vehicle, where it also lowers the center of gravity and helps range.

Density alone is not the argument. Damping is. Magnesium AZ91D absorbs vibration noticeably better than aluminum, so an inverter or charger housing made from it runs quieter without extra acoustic padding. Thermal conductivity is also higher than steel, which matters on parts that sit near power electronics.

The trade is stiffness and corrosion. Magnesium has a lower elastic modulus than aluminum, so a thin flat cover can flex under load. It also corrodes fast in salt spray unless the surface gets a proper conversion coating or paint system. Those two limits decide where the material fits.

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  Best fitCover-like and mid-stiffness housings where mass and damping matter more than bending strength.
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  Poor fitLarge flat panels or parts that see road salt without a full coating stack.
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  WatchGalvanic contact with steel or carbon, which accelerates corrosion at the joint.

The clearest application trend is around the battery and drive electronics, not the body-in-white. Battery pack end plates and module housings use magnesium where the pack needs mass cut without losing stiffness at the mounting points. Inverter, OBC and DC-DC housings are a second volume application, because the parts are enclosed, coated, and not directly exposed to road spray.

Transmission and e-axle cases are the harder case. A case has to absorb torque, hold bearing bores in precise relative position, transfer heat, and damp gear noise. Magnesium handles the noise and heat side well, and the mass saving on a large casting is real. The stiffness limit is why many e-axle designs still use aluminum for the main case and magnesium for covers and smaller shells.

Development direction points to larger thin-wall castings and more integrated parts. Instead of a welded assembly of castings and stamped plates, one magnesium die casting carries the ribs, bosses and cooling channels. That cuts part count and joints, but it raises the bar on tooling and on the machining that follows.

Seat frames, steering column brackets and some interior structural shells also use magnesium, though these are often extruded or forged rather than die cast. For high-volume EV programs the die-cast shell remains the main route.

Almost no magnesium shell ships as-cast. Die casting gives the near-net shape, the ribs and the thin walls, but it cannot hold a bearing bore to ±0.005 mm or a sealing face flat enough to seat a gasket. Those features are cut after casting.

Micro-arc oxidation is the common surface step before or between machining operations. It builds a ceramic-like oxide layer that resists salt spray and gives paint something to bite. The layer is thin, usually tens of microns, so it does not rescue a badly cast surface, and machined faces often need a mask or a re-cut after coating.

Machining magnesium is not the same as machining aluminum. Chips are light and can ignite if they build up, so we run sharp tooling, high feed, no fine dust accumulation, and dedicated chip handling. Coolant choice matters too: water-based coolant can stain the surface if it sits on the part, so we keep parts dry and move them quickly between operations.

On our 5-axis centers we hold ±0.005 mm on bearing bores and sealing faces, with surface finish down to Ra 0.2–0.8 μm where a seal or a sliding fit needs it. General machined faces sit at Ra 1.6–3.2 μm. We check 100% of parts before shipment, and raw material, in-process and final inspection records can be supplied.

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  FixturesSoft jaws or cast-in datums, because thin magnesium walls distort under clamp pressure.
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  ToolingSharp carbide, no dwell, generous chip clearance to avoid heat build-up.
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  InspectionCMM on bore position and flatness; reports on request.

Start with the corrosion path. If the shell sits in a wet, salt-exposed area and you cannot guarantee a full coating stack, magnesium is the wrong choice and aluminum will cost less over the vehicle life. If the part is enclosed and coated, the mass case is strong.

Then check stiffness. Run the load case on the magnesium design, not on the aluminum drawing with a density swap. Ribs and wall thickness usually have to change, and that changes the casting and the tool.

Then talk to the machinist early. Datum strategy, coating sequence and which faces get cut after coating all affect the drawing. Getting that settled before tooling is cut avoids a rework loop later. We quote and return a DFM analysis within 12 hours, and production can start within 24 hours once the design is frozen.