Refinement of Primary Silicon Crystals by Novel Al-ZnS Master Alloy in Solidification of Hypereutectic Al-Si Alloys

Abstract

A novel Al-ZnS master alloy was developed by in situ reaction of Zn and Na₂S in the Al melt. The results from this study left little doubt that this novel Al-ZnS master alloy was a promising refiner in solidification of hypereutectic Al-Si alloys. It refined the primary silicon to the same extent as that achieved by adding P via Cu-P following the same refinement mechanism. The SEM work confirmed that there were many pre-formed ZnS particles contained in the master alloy. The optical metallography showed that the morphologies of the primary silicon crystals in the solidified Al-22Si alloy were drastically changed from irregular coarse morphology to fine regular particles. The primary silicon crystal was refined from 74 μm to 26, 22, 24 μm by adding 0.05, 0.1, 0.15 wt% ZnS respectively with holding time of 20 min, whereas the average particle size of the primary silicon refined by 200 ppm P reduced to 20 μm. With the increase of the holding time up to 1.5 hr, the primary silicon size will increase up to 33 μm in adding 0.1 wt% ZnS.