Effects of Sr-Na interactions in hypoeutectic Al-Si foundry alloys

From recent published data, it is still unclear whether combined additions of Na and Sr have synergistic effects or deleterious interactions. This paper clarifies the interactions between these two modifiers and the effects of such interactions on alloy solidification and castability. Combined additions of Sr and Na do not appear to cause improvement of the modification of the eutectic microstructure even after only a short period after addition. Na addition may promote Sr vaporization and/or oxidation kinetically, leading to a quicker loss of both modifiers, which is blamed for the disappearance of the modification effect during holding. Quenching trials during the eutectic arrest indicate that addition of Sr into Na-modified melts does not alter the eutectic solidification behaviour. The effect of Na on eutectic solidification dominates, and the eutectic is observed to evolve with a significant dependency on the thermal gradient. Combined Sr and Na additions produced no beneficial effects on porosity and casting defects