Study of the Interaction Between Bismuth and Antimony on Solidification Kinetics, Eutectic Microstructure, and Mechanical Behaviour of Secondary Al–Si–Cu Alloys

This study investigates the interaction between bismuth (Bi) and antimony (Sb) additions and their influence on the microstructure and solidification behaviour of recycled Al-Si cast alloys. Microstructural analysis revealed a transition from flake to refined lamellar silicon when the combined Bi and Sb content reaches or exceeds 1 wt.%. Thermal analysis supported this, showing a reduction in the eutectic growth temperature (approximate to 3 degrees C) and an increase in recalescence (approximate to 4 degrees C). No change in silicon morphology and growth temperature was observed at higher concentrations of Bi and Sb. Electron backscatter diffraction (EBSD) analysis showed that the crystallographic orientation of eutectic aluminium is independent of primary aluminium nucleation in the refined silicon. Transmission electron microscopy (TEM) analysis of the refined silicon revealed a low density of twinning due to the coexistence of Bi and Sb, in contrast to the high twinning frequency observed in fully modified structures by strontium. Crucially, no evidence of a detrimental interaction between Bi and Sb was found. The convergence of results at higher Bi and Sb content, exceeding 1 wt.%, confirms a saturation phenomenon, which is attributed to the formation of the intermetallic Mg3(Sb,Bi)2 phase prior to the eutectic transformation. Consequently, high concentrations of Bi and Sb primarily augment the phase fraction of Mg3(Sb,Bi)2 without inducing further eutectic Si refinement.