Magnesium is the lightest structural metal at 1.74 g/cm³ — 33% lighter than aluminum and 77% lighter than steel. Die-cast magnesium components are essential in automotive, electronics, and aerospace applications. However, molten magnesium presents unique fire and explosion hazards requiring specialized safety protocols.
Fire Safety Fundamentals
Molten magnesium ignites at approximately 473°C and burns at temperatures exceeding 3,100°C. Magnesium fires cannot be extinguished with water — water decomposes into hydrogen and oxygen at these temperatures, causing explosive intensification.
**Class D extinguishing agents** are mandatory:
- **Dry sand or dry salt flux**: Cover the fire to exclude oxygen. Keep at least 200 kg of dry sand within 5 meters of every casting station.\n- **Foundry flux (MgCl₂/KCl mixture)**: Purpose-designed for magnesium fires, forms a smothering crust.\n- **D-class dry powder extinguishers**: Boron trifluoride or sodium chloride based agents. Standard ABC extinguishers are NEVER suitable.\n- **Argon flooding**: For enclosed furnace areas, argon inert gas systems provide effective suppression.
Never use water, CO₂, halon, or foam extinguishers on magnesium fires. Facility design must include fire-resistant walls between magnesium casting areas and other operations.
Alloy Selection for Die Casting
The two dominant magnesium die casting alloys are:
**AZ91D** (Mg-9Al-1Zn): The most widely used magnesium die casting alloy. Offers excellent castability, good corrosion resistance (when high-purity grade), and moderate strength (tensile 230 MPa, yield 160 MPa). Ideal for housings, covers, and structural brackets. Maximum service temperature approximately 120°C.
**AM60B** (Mg-6Al-0.3Mn): Higher ductility and impact resistance than AZ91D (elongation 8% vs 3%). Preferred for safety-critical automotive components — steering wheels, seat frames, instrument panel substrates. Lower strength (tensile 225 MPa, yield 130 MPa) but significantly better energy absorption.
Other alloys: **AE44** (Mg-4Al-4RE) for elevated temperature applications up to 175°C (engine components), and **AS41** (Mg-4Al-1Si) for moderate temperature creep resistance.
Die Casting Process Parameters
Magnesium die casting uses hot-chamber machines (up to 900 tonnes) for thin-wall components and cold-chamber machines for larger, thicker parts:
- **Melt temperature**: 650–680°C for AZ91D, 660–690°C for AM60B\n- **Die temperature**: 200–250°C (maintain uniformly to prevent cold shuts)\n- **Injection speed**: 4–6 m/s gate velocity for thin-wall (1.5–3 mm); 2–4 m/s for thicker sections\n- **Intensification pressure**: 30–70 MPa depending on wall thickness and part complexity
Protective Atmosphere
Molten magnesium must be protected from atmospheric oxidation. Two approaches are used:
**SF₆/CO₂ cover gas**: Traditionally effective but SF₆ has extremely high global warming potential (GWP 23,900). Being phased out under EU F-gas regulations.
**Alternative cover gases**: AM-cover™ (HFC-134a/CO₂), SO₂/air mixture, and solid flux covers are replacing SF₆. The EU Foundry Sector roadmap targets complete SF₆ elimination by 2027.
Machining and Finishing
Magnesium machines exceptionally well with high speeds and low cutting forces. However, fine chips and dust are highly flammable. Mandatory precautions: use mineral oil coolant (never water-based), maintain chip extraction systems, and store swarf in sealed steel containers away from moisture.