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 A1 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 nm to 26, 22, 24 |im 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 jim. With the increase of the holding time up to 1.5 hr, the primary silicon size will increase up to 33 urn in adding 0.1 wt% ZnS. Key words Hypereutectic Al-Si alloys, refinement, primary silicon, master alloy.