Effect of ultrasonic melt treatment on intermetallic phase formation in a manganese-modified Al-17Si-2Fe alloy

Hypereutectic Al-Si based alloys are important automotive materials. Both manganese (Mn) addition and ultrasonic melt treatment (USMT) have each proved to be effective in modifying the embrittling platelet-like delta-Al3FeSi2 and beta-Al5FeSi intermetallic compounds (IMCs) in these alloys. However, previous studies have identified no clear synergy in combining Mn addition with USMT to control these IMCs. This work presents a systematic study of the individual and combined effects of different levels of Mn addition and USMT on the formation of Fe-containing IMCs in an Al-17Si-2Fe alloy. Increasing the Mn content without USMT resulted in macro-segregation and formation of coarse IMCs. In addition, not all delta-Al3FeSi2/beta-Al5FeSi IMCs transformed into the desirable alpha-Al-15(Fe,Mn)(3)Si-2 IMC. In contrast, the combination of USMT and Mn addition avoided all these issues. The resultant microstructure was featured by fine dispersed polyhedral alpha-Al-15(Fe,Mn)(3)Si-2 phase particles. The volume fraction of the IMCs was quantified and compared with predictions made from the Scheil equation and the equilibrium lever rule. In the absence of USMT, neither the Scheil equation nor the lever rule method offered reasonable predictions. Conversely, the measured volume fraction of the Fe-containing IMCs with USMT closely agreed with the lever rule predictions. The underlining mechanisms for the effect of Mn and USMT on Fe-containing IMC formation and the effect of USMT alone on the Fe-containing IMC selection were discussed and proposed.