Aluminium alloys: Master alloys

Aluminium master alloys are alloys that are added to molten metal to obtain the desired composition and/or influence the as-cast structure. Master alloys can contain more than 50 weight per cent of the main alloying element. They can be produced by melting or by mixing the individual constituents and then compacting them. One differentiates between two significantly different types of master alloy:

  • concentrated binary master alloys
  • grain-refining master alloys

Binary master alloys are used to finely adjust the alloy composition; grain-refining master alloys are used to influence the as-cast structure, i.e. the as-cast grain structure. The latter are only added in small quantities. When it comes to influencing the grain structure, one differentiates between the following terms:

Grain refinement: the aluminium grain structure is made finer by use of additions of master alloys such as Al-Ti-B, Al-Ti, Al-B oder Al-Ti-C.

Modification: this is refinement of the eutectic in Al-Si alloys by means of Al-Sr alloys or sodium.

Further information on modification

Silicon refining: this involves transformation of lamellar silicon into a granular shape and refinement of the primary silicon in hypereutectic aluminium-silicon alloys by means of a phosphorus treatment, i.e. the use of master alloys such as Cu-P, Fe-P or Ni-P, or phosphorus-containing additives.

The use of master alloys is indispensable in melting and casting plants. Master alloys make short alloying times possible, something that is important with alloying elements that have high melting points (iron, titanium, nickel, etc.). In addition, the use of master alloys reduces the loss of alloying elements in the melt. This is particularly important with expensive alloying elements and where metals have a high vapour pressure and/or high oxygen affinity. In addition, master alloys allow toxic metals to be added safely.