What Are the Main Types of Magnesium Die Casting?

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  Key takeaways
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  Why magnesium die casting is its own process family
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  Hot chamber magnesium die casting for thin walls at volume
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  Cold chamber magnesium die casting for larger, thicker parts
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  Vacuum-assisted magnesium die casting and its porosity trade-off
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  AZ91D, AM60B, and AM50A: which alloy fits which part
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  Where CNC machining takes over from magnesium die casting
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  Magnesium die casting types compared

Magnesium is the lightest structural metal in commercial use. Its density is about 1.74 g/cm³, roughly two-thirds that of aluminum and one-quarter that of steel. A housing that weighs 1.2 kg in aluminum lands near 0.8 kg in magnesium at the same wall section. That gap is why the material keeps showing up in handheld tools, steering columns, and laptop chassis.

The useful part for a foundry is not the density. It is how magnesium behaves in the shot sleeve. Molten magnesium has low viscosity and low heat content compared with aluminum. It flows into thin sections quickly, and it releases heat fast once it touches the die. Thin walls fill before they freeze, which is why magnesium die casting can hold 0.9–1.2 mm walls where aluminum struggles below 1.5 mm.

That same fast freezing is the catch. Magnesium solidifies in a narrow temperature window, so the die must run hot and the shot must be timed tightly. A cold die or a slow plunger speed produces cold shuts and incomplete ribs. The process is less forgiving of setup drift than aluminum.

Magnesium also reacts with water and oxygen. Molten metal must be protected with SF₆ or a similar cover gas, and chips from later machining need dedicated handling. This is a safety and cost factor, not a technical barrier. Foundries that run magnesium daily treat it as routine.

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  DensityAbout 1.74 g/cm³, the lowest of any structural metal cast at scale.
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  FluidityFills thin ribs and small bosses that aluminum cannot reach.
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  SolidificationFast, narrow window. Die temperature control matters more than on aluminum.
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  HandlingCover gas at the melt, dry chip handling after machining.

In a hot chamber machine, the gooseneck and nozzle sit submerged in the molten metal pot. The plunger pushes metal directly into the die without a separate ladling step. Cycle times run short because there is no transfer loss and no open pour. For magnesium, this is the classic high-volume route.

The type suits small to medium parts with thin, detailed geometry: brackets, camera bodies, instrument housings, power tool shells. Walls of 0.9–1.5 mm are normal. Ribs, bosses, and molded-in threads are practical because the metal arrives hot and fast.

The limit is part size and wall mass. A hot chamber machine cannot push the same shot weight as a cold chamber machine, and heavy sections cool unevenly against a hot die. If your part weighs more than a few hundred grams or carries a 5 mm boss next to a 1 mm wall, hot chamber is the wrong machine.

Tooling cost is real but amortized fast. A simple single-cavity die pays back over a run of tens of thousands of parts. Below that volume, the economics tilt toward cold chamber with a simpler die, or toward CNC machining from billet.

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  Best forThin-wall housings, brackets, and small enclosures at high volume.
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  Wall rangeRoughly 0.9–1.5 mm with ribs and bosses molded in.
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  Watch outLarge shot weights and heavy sections that outrun the die's heat balance.

A cold chamber machine ladles molten magnesium into a horizontal shot sleeve, then the plunger drives it into the die. The metal never sits in a heated pot, so the machine can run larger shot weights and higher pressures. This is how you cast a transmission housing, a seat frame node, or an instrument panel beam.

Walls here typically run 2–4 mm. Thicker sections are possible but need care: magnesium shrinks about 1.3 percent as it solidifies, and a heavy section surrounded by thin walls will pull porosity toward the thick zone. Feed the thick section with a dedicated runner or accept a machined pocket later.

Cold chamber magnesium die casting tolerates more part size than the hot chamber route, but the cycle is longer and the scrap rate on thin walls is higher. If the part is mostly thin wall with one thick boss, designers often split it: cast the shell, then machine the boss flat and drill it.

Porosity is the main quality risk. Turbulence during fill traps gas, and gas pores open up when you machine into the casting. Vacuum assistance, covered next, is the standard answer when a sealing face or a pressure-tight bore is required.

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  Best forLarger structural housings, frames, and thick-section parts.
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  Wall rangeAbout 2–4 mm, with heavier bosses fed from the runner.
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  Watch outGas porosity that opens when you machine into the casting.

Vacuum-assisted casting pulls air out of the die cavity before the shot. Less trapped gas means less porosity, and that matters when a bore must hold pressure or a face must be sealed with an O-ring. The technique works on both hot and cold chamber machines.

The gain is not free. Vacuum systems add tooling complexity, need a sealed die and a reliable valve, and extend cycle time slightly. They also do not eliminate shrinkage porosity, which comes from thick sections cooling last. Vacuum fixes gas porosity, not feeding problems.

The practical rule: specify vacuum when the part has a pressure-tight requirement, a machined sealing surface, or a cosmetic Class A face. Skip it when the part is a bracket with generous walls and no fluid contact. The cost difference is not worth it there.

For parts that still show porosity after vacuum, the fix is often a process change rather than a new tool: adjust gate location, add overflow wells, or lower the die temperature in the thick zone. These are shop-floor decisions, and they need a foundry that tracks shot data.

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  Best forPressure-tight bores, sealing faces, and visible cosmetic surfaces.
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  FixesGas porosity from trapped air during fill.
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  Does not fixShrinkage porosity in heavy sections that cool last.

AZ91D is the workhorse. It carries the best combination of castability and room-temperature strength, and it machines cleanly. Most thin-wall housings and general brackets run in AZ91D. Its weakness is creep above about 125 °C, so it is a poor choice for a part that bolts directly to a hot engine block.

AM60B gives up some strength for elongation. It bends instead of cracking, which is why it appears in impact-prone parts: seat frames, instrument panel supports, airbag housings. If your part sees crash loads or drop tests, AM60B is the safer specification.

AM50A sits between the two. It offers better elongation than AZ91D with more strength than AM60B, and it is often chosen for large thin-wall structural parts where stiffness and toughness both matter.

All three are covered by standard specifications, and all three can be machined after casting. Wall thickness, draft, and corner radii still drive the design more than the alloy label does.

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  AZ91DHighest strength and castability. Not for sustained heat above ~125 °C.
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  AM60BHigher elongation for impact and crash-loaded parts.
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  AM50AMiddle ground for large thin-wall structural parts.

Casting gets you the shape. It does not get you a ±0.005 mm bore or a Ra 0.8–1.6 μm sealing face. Those features come off a CNC machine, and on magnesium that means cutting dry or with a controlled mist, keeping chips isolated, and using sharp tooling with high rake angles.

Typical post-cast operations are simple: face a mating surface, bore a bearing seat, drill and tap mounting holes, and trim flash from the parting line. A cast part with four machined faces usually needs one or two fixtures and a short cycle. That is well inside the range of a 4-axis or 5-axis machining center.

GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and 16 mill-turn centers, with a maximum processing size of 4,000 mm. Magnesium AZ31B and AZ91D are on the material list. Tolerances hold at ±0.005 mm, and finish reaches Ra 0.2–0.8 μm when the drawing calls for it.

The decision point is volume. Casting needs tooling and pays back on runs of thousands. Machining from billet needs no tooling and starts within 24 hours. For a prototype, a bridge quantity, or a part with a design still in motion, machining the whole part is usually the faster path.

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  Cast then machineTooling plus a short CNC pass on sealing faces, bores, and threads.
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  Machine from billetNo tooling, one-piece minimum, production can start within 24 hours.
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  HybridCast the shell, machine the critical features only.