AZ91D Magnesium vs ADC12 Aluminium: Which Casting Alloy
Where each alloy earns its place
ADC12 (and its close relative ADC10) is the workhorse aluminium die casting alloy: excellent fluidity for thin and complex sections, good dimensional stability and a mature supply chain. It is the default choice for enclosures, heat sinks and structural brackets.
AZ91D magnesium is chosen when mass is the constraint. Mg is substantially lighter than aluminium for a given volume, and it also machines very well - shorter cycle times and less tool wear on the CNC operations that follow casting. That machinability partially offsets the higher material cost.
The decision points that usually surface late
Corrosion and galvanic compatibility. Magnesium is more anodic than most metals, so where it meets steel, copper or even aluminium fasteners in a damp environment, galvanic corrosion must be designed out - isolation, coating or managed drainage - not discovered in field returns.
Thermal role. If the casting is also a heat spreader, conductivity governs the choice. Our aluminium heat sink castings are specified in the 72 to 100 W/m·K range. Where the part carries structural load at elevated temperature, creep behaviour becomes relevant earlier than most designers expect.
Finishing compatibility. Anodising and powder coating behave differently on each alloy and change both appearance and surface properties. State the finishing requirement while the alloy is still open rather than after the tool is built.
Costing the finished part, not the ingot
Comparing alloy price per kilogram is misleading. Cycle time, machining time, scrap rate, finishing steps and any corrosion protection together dominate the landed cost. Magnesium often wins on machining and significant weight-sensitive applications; aluminium usually wins on raw material cost, corrosion behaviour and galvanic compatibility with the rest of the assembly.
We cast both, alongside Zamak zinc for very small precision parts, with five-axis CNC machining, anodising and powder coating in the same 22,000 square metre facility. Send the drawing in STEP, IGES or DXF for a free DFM review and we will quote each candidate alloy side by side.
FAQ
Which alloys do you cast?
Standard alloys are ADC10 and ADC12 aluminium, AZ91D magnesium and Zamak zinc alloys. Selection depends on wall thickness, structural load, thermal requirements and the finishing specified on the drawing.
Is magnesium difficult to machine?
Magnesium generally machines well, with good chip breaking and lower tool wear, which can shorten CNC cycle times. It does require appropriate chip handling and fire-safety discipline in the workshop; those controls form part of the process.
How does corrosion resistance compare?
Aluminium alloys generally have good atmospheric corrosion behaviour and accept anodising well. Magnesium is more prone to galvanic corrosion when coupled with dissimilar metals, which is why surface protection and isolation from the mating part must be specified in the design rather than added later.
Can you quote both alloys for one design?
Yes. Send the STEP, IGES or DXF drawing and the annual volume, and we will run a free DFM review comparing the candidate alloys on castability, machining and finishing before quoting each option.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.