Effects of minor addition and cooling rate on the microstructure of cast magnesium-silicon alloys

Minor additions of antimony, calcium, sodium, and phosphorus were added to Mg-Al-Si alloys with the intention of refining/modifying the microstructure, thereby improving its mechanical properties. The high cooling rate of the die-casting process was simulated using a graphite wedge mold. The effects of cooling rate and minor addition on the microstructure of both Mg-Si hypoeutectic (AS21) and hypereutectic Mg-3%Si-2%Al cast alloys were examined. It was found that antimony does not show any obvious refinement or modification effect, sodium shows some limited modification, while phosphorus and calcium provide a significant refinement and modification effect on both primary and eutectic Mg2Si phase. This may lead to the development of low cost, high temperature creep-resistant and wear resistant magnesium alloys for the automotive industry.