Aluminum casting alloys

Aluminum casting alloys are alloys intended for producing castings, as opposed to wrought alloys, which are used to make sheet and profiles. They contain more alloying additions, above all silicon, which improves castability and reduces shrinkage. The grades, chemical composition and minimum mechanical properties of aluminum alloy castings are specified in the PN-EN 1706 standard.

Pure aluminum is soft and fills the mold poorly. Only the alloying additions – silicon, magnesium, copper and, in smaller amounts, manganese, titanium or strontium – give the alloy the properties needed in casting and in the finished part.

Groups of aluminum casting alloys

Silumins is the common name for aluminum-silicon alloys. Near-eutectic alloys containing approx. 11–13% silicon fill the mold best – the aluminum-silicon eutectic occurs at approx. 12.6% Si.

How alloying elements affect alloy properties

• Silicon – improves castability and reduces shrinkage and hot tearing tendency. At high content it increases tool wear during machining.
• Magnesium – together with silicon, it enables precipitation hardening (solution treatment and aging). In larger amounts, as in Al-Mg alloys, it provides high corrosion resistance.
• Copper – increases strength and hardness, including at elevated temperatures, and improves machinability. However, it lowers corrosion resistance and ductility.
• Iron – usually an impurity that forms brittle phases. In die castings, a small addition of it reduces the alloy's tendency to stick to the die.
• Manganese – changes the shape of the iron-bearing phases to a less harmful one.
• Titanium, boron, sodium and strontium – refine the grain and modify the silicon in the eutectic, which improves the strength and ductility of the casting.

Casting shrinkage of aluminum alloys

Aluminum alloys shrink during solidification and cooling. The linear shrinkage, which is taken into account when making the pattern and mold, is usually approx. 1% for them, depending on the alloy, the process and the shape of the casting. Volumetric shrinkage during solidification is several times greater (approx. 6–7% for pure aluminum, less for silumins), which is why the casting is fed with risers, and alloys with a higher silicon content are easier to cast without shrinkage cavities.

Low-carbon casting alloys

The carbon footprint of a casting increasingly appears in inquiries. Producing primary aluminum requires a great deal of energy, while remelting recycled aluminum requires only a small fraction of it (often cited as approx. 5%). Alloys with a high share of secondary material, especially copper-bearing alloys, therefore have a lower carbon footprint. If CO₂ emission requirements are part of the specification, it is worth stating them in the request for quotation.

How to choose an alloy for a casting

• Technology – Al-Si-Cu alloys are most common for die casting, and Al-Si-Mg and Al-Mg alloys for permanent molds and sand molds.
• Strength – when high strength matters, a heat-treatable alloy is chosen, e.g. EN AC-42000 in the T6 temper.
• Corrosion – in aggressive environments, copper-free alloys perform better: Al-Mg and Al-Si-Mg.
• Pressure tightness and thin walls – near-eutectic silumins, e.g. EN AC-47100.
• Finishing – Al-Mg alloys are suitable for polishing and decorative anodizing, while castings from all groups can be powder coated.

Aluminum alloy castings from our foundry

We most often cast five alloys: EN AC-42000, 46200, 46500, 47100 and 51300 – their designations and chemical composition are given on the page aluminum casting alloys. We produce castings by permanent molds, in sand molds i high-pressure die casting, and on request also with heat treatment and CNC machining. We also help select the alloy to suit the requirements of the part.