Aluminium forms alloys with almost all metallic and non-metallic elements, but only a few are widely used. For the main part these are the metals iron, copper, magnesium, manganese and zinc and as well as the non-metal silicon; on a small scale it also forms alloys with lead, boron, chromium, nickel, titanium, bismuth, tin and zirconium. Even in small amounts, each of these elements improves certain aluminium properties, but often also adversely affects others, so that usually another element is added to compensate if possible for the deterioration.
- Lead, bismuth and tin improve the machinability by causing chips to break off readily. Such aluminium alloys are also called "free-cutting alloys".
- Boron increases the electrical conductivity of conductor-grade aluminium for use in electrical engineering, because when added too a melt, in quantities of 0.005 to 0.02 per cent, it forms chemical compounds with the elements chromium, titanium and vanadium (which impair the conductivity), which precipitate out in the solid metal as borides that do not affect the conduction of electricity.
- Copper increases the strength at room temperature but decreases corrosion resistance.
- Magnesium and manganese make alloys more corrosion resistant and increase the strength.
- Nickel increases the strength at higher temperatures.
- Silicon lowers the melting point and the viscosity of the melt (thus improving the castability), but has a detrimental effect on forming.
- In combination with boron, titanium refines the grain structure.
- In combination with magnesium, zinc results in the highest strength and high hardness but reduces formability.
Aluminium itself is also used as an alloying element to a significant extent:
- above all in copper, which on addition of about one per cent becomes harder, more corrosion resistant, easier to melt and tougher
- in magnesium and titanium alloys
- in steel production.