Different types of alloy

Alloy forms

Casting alloys are alloys that contain a total of up to 20% silicon, magnesium and copper as alloying elements and can only be formed by casting. The starting material is usually secondary aluminium (aluminium recovered from scrap). Here, the need for favourable casting properties is of primary concern. In addition, the compositions of casting alloys are tailored to the respective casting process (sand casting, permanent-mould casting or die-casting) to be used. Alloys with a silicon content of 5 to 20% have the most favourable casting properties.

Wrought alloys contain up to 10% alloying elements and are chosen for their good formability. One differentiates between age-hardenable and non-age-hardenable wrought alloys:

  • In non-age-hardenable alloys, all of the alloying elements are present in solid solution. These alloys are readily formable.
  • In age-hardenable alloys, the alloying elements are present at room temperature in the form of precipitates. Their distribution determines the strength of the alloy. By subjecting the alloy to a solution heat treatment, the elements are dissolved completely in solid solution and quenching then ensures that this condition is ‘frozen in’ at room temperature. Ageing then results in precipitates forming.

The addition of magnesium produces alloys that are non-age-hardenable but resistant to seawater. Duraluminium, the famous aluminium alloy developed by Alfred Wilm in 1909 using additions of copper, manganese and magnesium, is an example of an age-hardenable alloy.

With wrought alloys, plastic deformation is of primary concern. The most widely used age-hardenable wrought aluminium alloys are those that contain magnesium, silicon, manganese, copper and zinc.