Optimizing microstructures of hypereutectic Al–Si alloys with high Fe content via spray forming technique

Abstract

By using spray forming technique Fe-contained hypereutectic Al–Si alloys were prepared with different Mn/Cr additions for the study of their effects on the microstructures. The results show that adding 2 wt.% Mn/Cr separately can strikingly refine the Fe-bearing phase in spray-formed Al–25Si–5Fe–3Cu (wt.%) alloy into quantities of fine, uniformly distributed granular α-Al(Fe,Mn/Cr)Si phase, and Cr is more effective. But some short-plate Fe-bearing phases still exist in the spray-formed Al–Si alloys. Then, combined addition of Mn and Cr transforms these short-plate Fe-bearing phases into fine, granular α-Al(Fe,Mn,Cr)Si phase, promoting the appearance of almost single α-Al(Fe,Mn,Cr)Si phase in the spray-formed Al–Si alloys. Two mechanisms are proposed to elucidate the formation of α-Al(Fe,TM)Si phase (TM = Mn/Cr/(Mn + Cr)) during the solidification process: (1) transformed from metastable δ-Al(Fe,TM)Si phase in Mn/(Mn + Cr)-added alloys or (2) precipitated from liquids directly in Cr-containing alloys. Because the strong interactions and isomorphic substitution among different TM elements, the metastable δ-Al(Fe,TM)Si phase (clusters) can be precipitated from the liquids and transformed into stable α-Al(Fe,Mn,Cr)Si phase in Mn- or (Mn + Cr)-added alloys. The stable α-Al(Fe,Cr)Si phase can precipitate directly from the liquids because no metastable ternary intermetallics exist in Al–Cr–Si system and can be transformed into stable α-AlCrSi phase. Also the high segregation temperature of Cr in liquid Al melts promotes the microsegregation of Cr and formation of (AlCrSi) clusters/intermetallics in Cr-added alloys. As a result, both metastable δ-Al(Fe,TM)Si phase (clusters) and stable α-Al(Fe,TM)Si phase (clusters) can be present in (Mn + Cr)-added alloys. With further solidification, these clusters become the nucleation sites and grow up unceasingly. The coexistence of the nucleus of δ-Al(Fe,TM)Si and α-Al(Fe,TM)Si phases in the liquid alloys can induce the simultaneous presence of these two phases in atomized droplets or particles. With continuous deposition of atomized droplets or particles, the appearance of mushy zone on the surface of preforms promotes the transformation of metastable δ-Al(Fe,TM)Si phase into stable α-Al(Fe,TM)Si phase, especially inducing the ideal case featured by single α-Al(Fe,TM)Si phase. It is concluded that the improvements of the thermal stability of present hypereutectic Al–Si alloys are attributed to the increment of solidus temperatures and the presence of the granular α-Al(Fe,TM)Si phase with high dissolution temperature induced by TM addition.