Studies On Modification Mechanisms Of Hypereutectic Al-Si Alloys

Al-Si hypereutectic alloys are widely used in industries and Hi-Tech areas, but coarse primary Si phase in casting is detrimental to the mechanical properties of the alloy. Therefore, refinement of primary silicon has been received great interest from metallurgists.The recent condition of modification and refinement of Al-Si alloys and the removal of impurities from Al alloys are described in this thesis. The modifying efficiency of vary modifier were compared in experiments. The main results and the conclusions are following:(1)The excellent modification efficiency on hyper-eutectic Al-Si alloys could be obtained through the addition of Cu-P master alloys. Examination of the microstructure and fine chemical compound of the alloy by SEM techniques shows that there is AlP compound in primary silicon. Analysis of effects of phosphorus element on silicon crystal in the hypereutectic Al-Si alloy shows that AlP acts as heterogeneous nuclei. It is consistent with many researcher's point.(2) In the experience Salt,Sb and thulium can change primary Si and eutectic Si through changing the morphology, but the effect of modification is very defferent. Salt is the best. Sb is better than thulium. The modification of modifying elements addition on Si takes effects through TPRE mechanics. The preferential absorption of modifying elements poisons the growth steps, preventing the anisotropic development of Si crystals, leading to multiple twining within Si crystals and the isotropic but still faceted growth. The difference in growth environment between primary Si and eutectic Si (cooperative growth withα(Al)) makes the experience different.(3)The effects of heat treatment on the microstructure of Al-20wt%Si were studied. The results show with the extension of solution time, the dissolution of a fraction of eutectic Si takes place through fragmentation of the phase.