Choosing an alloy is a balancing act between the part's service conditions and its castability. The alloy that looks best in the strength table may not flow well enough in a thin walled geometry; the one that flows best may not give the surface you expect. So an alloy decision cannot be made independently of the part.
The two most common groups in high pressure die casting are the AlSi9Cu3 (EN AC-46000) and AlSi10Mg (EN AC-43400) families. The basic difference is copper versus magnesium content, and that difference spreads through every heading below.
What drives the decision?
- Mechanical load. On load bearing connection parts, ductility and impact resistance lead; you do not want brittle behaviour. It is not only static strength but how the part fails that the alloy decides.
- Thermal conduction. On heat sinks, LED luminaire bodies and cooling plates, thermal conductivity decides. Copper and silicon content act directly on this property.
- Corrosion. On parts working outdoors or in humid conditions, copper content works against corrosion resistance; low copper alloys come forward there.
- Surface treatment. If the part will be anodised, alloy composition directly sets the resulting appearance. High silicon alloys anodise to a darker, more variable surface.
- Castability. On thin walled, complex geometries, fluidity becomes critical. Fluidity improves with silicon content, which is why high silicon alloys are preferred on thin walled parts.
- Pressure tightness. On parts that must hold pressure, the solidification range of the alloy matters; a wide range increases the risk of micro shrinkage and therefore of permeability.
The two main families side by side
| AlSi9Cu3 (EN AC-46000) | AlSi10Mg (EN AC-43400) | |
|---|---|---|
| Distinguishing element | Copper | Magnesium |
| Castability | Very good | Good |
| Corrosion behaviour | Weaker, due to copper | Better |
| Ductility | Lower | Higher |
| Thermal conductivity | Medium | Better |
| Typical use | General purpose housings and enclosures | Heat shedding parts, outdoor service |
A third family, AlSi12 (EN AC-44300), is considered where fluidity dominates, on very thin walled parts; its solidification range is narrow, which is an advantage on parts that must be pressure tight.
A heat treatment requirement changes the picture
Heat treatment cannot always be applied in high pressure die casting: gas trapped during rapid filling can expand at high temperature and blister the surface. So the assumption "we will raise strength with heat treatment" does not automatically hold in conventional HPDC.
If the strength requirement demands heat treatment, there are two routes: either vacuum assisted special casting processes are assessed, or alternatives such as low pressure and gravity casting are considered. That decision comes before the alloy, because a change of method changes the list of suitable alloys. See our casting methods article for the detail.
Recycling and scrap management
Casting alloys tolerate returned scrap far better than wrought alloys do; runners and overflows come back on every shot. But the composition of returned metal can drift over time — iron content in particular tends to rise through contact between molten metal and steel. Above a certain iron level, ductility falls.
So keeping the alloy the same throughout production matters as much as choosing it; that is what composition control is for. On the quality control and metrology side, the spectrometer serves exactly this purpose.
Specification, traceability and documentation
The alloy decision is not only a technical preference but also a matter of record. In automotive and appliance supply chains you can be asked which alloy, which batch and which records a part was produced from.
- The name in the specification. An alloy should be written with its standard code, not a local trade name. The same alloy carries different designations from country to country, and that breeds confusion down the supply chain.
- Composition control. That the composition stays within limits through production is verified with a spectrometer. Because of returned scrap, iron content in particular has to be watched.
- Batch traceability. Being able to trace which batch came from which melt narrows the scope of a problem when one appears — you deal with the batch concerned rather than the whole stock.
- Material declaration. Where the customer specification requires it, material information is documented per batch.
These headings look dull, but their function is clear: choosing the alloy once is not enough, you have to be able to show it stayed the same throughout production.
Alloy and surface treatment are chosen together
This link causes the most trouble in practice. The part is designed, the alloy is chosen for castability, the die is built, series production starts — and only then does the information arrive that "this part was going to be anodised". On a high silicon alloy, anodising will not give the clear appearance seen on wrought aluminium.
The remedy is simple but the order matters: the surface treatment expectation goes on the table before the alloy decision. Our surface treatment article covers the relationship in detail.
A common mistake
Choosing an alloy from the strength table alone. An alloy that looks good on paper may not flow well enough in a thin walled geometry; then you either thicken the wall (weight and cycle time go up) or fight filling problems. The alloy decision has to be made together with the part geometry.
The second mistake is delegating the alloy to the supplier without describing the service conditions. Rather than saying "cast it in AlSi9Cu3", describing where the part works, under what load, in what environment and with what appearance usually gives a better result — because then the alloy is chosen for what the part actually needs.
Note: the alloy codes in this article are common international standards; review them against your own product range and customer specification.