AZ91D vs A380: Mechanical Properties, Castability, and Real Tooling Costs

The two workhorse die casting alloys, compared the way a design engineer actually needs: numbers first, then what they mean for your die and your BOM.

Exclusive Magnesium Engineering Blog | 9 min read | For hardware product teams, design engineers and mechanical R&D professionals

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AZ91D is to magnesium what A380 is to aluminum: the default high-pressure die casting alloy that covers the majority of parts in its family - brackets, covers, cases and housings. Yet most alloy comparisons online stop at a datasheet screenshot. This article puts the key numbers side by side using International Magnesium Association data, then goes where datasheets don’t: what the two alloys do to your casting cell, your die life, and your cost structure.

Mechanical and physical properties, side by side

Property (die cast, room temp.)	AZ91D magnesium	A380 Aluminium
Density (g/cc)	1.81	2.74
Yield strength (MPa)	160	160
Ultimate tensile strength (MPa)	240	320
Elongation (%)	3	3.5
Elastic modulus (GPa)	45	71
Hardness (Brinell)	70	80
Thermal conductivity (W/m·K)	51	96
Melting range (°C)	435-598	540-595
Salt-spray corrosion rate (mg/cmsq/day)	0.05	0.34

Source: International Magnesium Association / Meridian Lightweight Technologies published data. Values are typical, not minimums; die casting properties depend strongly on process variables.

Three things jump out. First, yield strength is identical at 160 MPa - for the strength-dominated parts both alloys are used for, AZ91D gives you the same load capacity at 34% less mass. Divide yield strength by density and AZ91D delivers roughly 88 kN·m/kg of specific strength against A380’s 58: a 50% advantage. Second, general corrosion favors magnesium: in ASTM B117 salt spray, high-purity AZ91D corrodes several times more slowly than A380 - contrary to magnesium’s outdated reputation, which dates from low-purity alloys decades ago. Third, stiffness is the honest trade-off: at 45 GPa, AZ91D has about 63% of A380’s modulus. For a bending-stiffness-limited wall, matching aluminum means roughly 16% more thickness - and even with that added section, the magnesium part is still on the order of a fifth lighter. Good magnesium design uses ribs, not bulk, to buy stiffness even more cheaply.

Where the datasheet ends: castability

AZ91D is widely regarded as the more castable of the two, for reasons that show up directly in part quality and cost:

• Fluidity. AZ91D fills thin walls, long flow paths and fine ribs that A380 cannot reliably fill. Production magnesium housings run wall sections approaching 1 mm; the practical floor for aluminum is meaningfully higher. Thinner walls mean lighter parts and less material per shot.

• Thermal load per shot. Each magnesium shot carries less heat into the die (lower density and lower casting temperatures), so solidification is quicker and cycle times shorter. More parts per hour from the same press.

• Hot-chamber capability. Small and mid-size AZ91D parts can run on hot-chamber machines - impossible with aluminum, which attacks the immersed gooseneck. Hot-chamber cells cycle faster, hold tighter shot-to-shot consistency and waste less metal.

• Dimensional accuracy and draft. Magnesium’s fluidity and low die soldering allow minimum draft angles and near-net features, often eliminating machining operations that an A380 part would need.

The tooling cost story

An HPDC die for AZ91D costs about the same to build as the equivalent A380 die - same die steels, same toolroom hours. The difference is what happens after the first shot. Molten aluminum dissolves iron; over hundreds of thousands of cycles it washes out gates, solders to cavity surfaces and heat-checks the steel. Molten magnesium has very low solubility for iron, which the IMA lists as a major basis for superior tooling life, and its lower casting temperature reduces thermal fatigue on every cycle.

In production terms, a magnesium die typically outlasts its aluminum counterpart by a substantial multiple. Spread the same tool investment over several times the shots and the tooling amortization per part falls accordingly - while the program avoids the downtime, requalification and PPAP re-submission that a mid-life die replacement forces. For multi-year programs at volume, this is frequently a larger cost lever than the material price difference the comparison usually fixates on.

Where A380 keeps the advantage

Honesty matters in alloy selection. A380 retains strength better at elevated temperature: standard AZ91D loses yield strength above roughly 100-150°C and is not recommended for sustained service beyond about 150°C, while A380 tolerates under-hood-adjacent duty better. A380 also conducts heat nearly twice as fast, which matters when the casting is primarily a heatsink rather than a structure. And in assemblies with many steel fasteners in wet, salty environments, aluminum’s galvanic position is more forgiving - though well-established design rules (compatible washers, coated fasteners, drainage) manage this routinely on magnesium; steering wheel armatures and instrument panels have run in AZ91D and AM alloys for decades.

The selection rule of thumb

• Strength-dominated part near ambient temperature, weight matters: AZ91D.

• Higher ductility or crash energy absorption needed: consider AM50A/AM60B magnesium, whose ~10% elongation and impact strength exceed die cast A380.

• Continuous service above ~150°C or heatsink-first function: A380 (or creep-resistant Mg alloys for specific cases).

• Thin-wall electronic housing needing EMI shielding and minimum mass: AZ91D, decisively.

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