Combining Sr and Na additions in hypoeutectic Al-Si foundry alloys

From recent published data, it is still unclear whether combining additions of Na and Sr have synergistic effects or deleterious interactions. This paper clarifies the interactions between these two modifiers and investigates the effects of such interactions on alloy solidification and castability. It was found that 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 rapid loss of the modification effect during melt 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. Combining Sr and Na additions produced no beneficial effects on porosity and casting defects.