Modification of aluminium-silicon alloys

Modification is a treatment used with hypoeutectic or eutectic aluminium-silicon alloys. The addition of a modifier transforms the lamellar structure found in melts free from phosphorus or sodium.

Modification leads to an extremely high refinement of the aluminium-silicon eutectic during solidification and at the same time shifts the eutectic point to higher silicon contents and lower temperatures.

Aluminium-silicon alloys usually contain a small amount of phosphorus, which produces a granular structure with the formation of aluminium phosphite. Aluminium phosphite stimulates the crystallisation of primary silicon, which is then present in the structure in the form of polyhedral platelets. By contrast, during the classical modification using sodium, the sodium reacts with the phosphorus in the melt to form sodium phosphite. The first phase to crystallise out is then not silicon but the a phase solid solution in the form of dendrites.

Modification leads to a significant improvement in strength and ductility, especially in sand castings and thick-walled permanent mould castings. In thin-walled permanent mould castings and die-castings, the effect of modification is small because a fine grain size is obtained as a result of the high rate of solidification in the mould and the associated supercooling. With die-casting alloys, the addition of sodium can reduce the tendency to stick in the mould.