The properties of every aluminium alloy ingot are set by the elements added to the aluminium in controlled amounts. Even a change of a few tenths of a percent in one element can alter the fluidity of the melt and the strength, machinability or corrosion resistance of the final part. That is why, at Almas Aluminium, the chemical composition of every melt is checked in the laboratory before casting.
Silicon (Si): the key element in casting alloys
Silicon is the most important alloying element in foundry ingot. It raises the fluidity of the melt, reduces shrinkage on solidification and the coefficient of thermal expansion, and makes thin-walled parts easier to cast. The eutectic point of the aluminium–silicon system is at about 12.6% silicon, which is why alloys such as LM6 and A413 (about 11–13% Si) have the best castability. Hypereutectic alloys such as LM28 (17–20% Si) have high wear resistance and are used for pistons, but they are harder to machine.
Copper (Cu): strength and hardness
Copper increases the strength and hardness of aluminium, even at high temperatures, and improves machinability. That is why common die-casting alloys such as ADC12, ADC10 and A380 contain 1.5–4% copper. On the other hand, copper reduces corrosion resistance and, in larger amounts, increases sensitivity to hot tearing.
Magnesium (Mg): heat treatability
Together with silicon, magnesium forms Mg2Si, which makes precipitation hardening (T6 heat treatment) possible. After heat treatment, alloys such as A356 and A357, with 0.25–0.7% magnesium, offer a good combination of strength and ductility and are used for car wheels and structural parts. In alloys such as ADC6, magnesium also improves corrosion resistance.
Iron (Fe): a controlled impurity
In most alloys iron is treated as an impurity, because it forms brittle needle-shaped phases (β-AlFeSi) that reduce ductility. That is why grades such as A356.2 keep iron below 0.12%. In high-pressure die casting, however, about 0.8–1.3% iron stops the melt from sticking to the steel die.
Manganese (Mn) and chromium (Cr)
Manganese changes iron-bearing phases from needles to a more compact form (α) and reduces the harmful effect of iron. Small amounts of chromium also help control the structure, but high combined levels of iron, manganese and chromium can form sludge in the furnace, so they must be balanced.
Zinc (Zn), nickel (Ni), tin (Sn) and lead (Pb)
Zinc is usually allowed up to a set limit in casting alloys and changes the properties at higher levels. Nickel increases strength at high temperatures and is found in piston alloys such as LM13 and LM28. Tin and lead are usually limited as impurities.
Titanium (Ti): the grain refiner
Titanium (usually with boron) creates nuclei for solidification and refines the grain structure. A fine-grained structure improves feeding, reduces hot tearing and makes mechanical properties more uniform.
Choosing the right grade
The choice of alloy ingot depends on the casting process (high-pressure, gravity or sand casting), the shape of the part and the properties required. The chemical composition of the grades Almas produces is on the products page, and our technical team is ready to help you choose the right alloy for your project.