Formation and sedimentation of Fe-rich intermetallics in Al-Si-Cu-Fe alloy

Formation and sedimentation of Fe-rich intermetallics were studied in a commercial Al-Si-Cu-Fe alloy with extra additions of Mn. It is found that the introduction of extra Mn is an effective approach to lower the Fe level in the equilibrium liquid phase after sedimentation of solid Fe-rich phase at a temperature between its liquidus and solidus. The higher Mn/Fe mass ratio results in the lower Fe content in the retained alloy, during which Mn is also consumed and settled at the bottom of the melt as solid Fe-rich intermetallics. Therefore, the final Fe content in the alloy can be controlled by the Mn content and the holding temperature of the melt. The results confirmed a good agreement of the theoretical calculation and the experimental test with a specially designed 50 mm cylindrical casting. The sedimentation of Fe-rich intermetallics in the Al-Si-Cu-Fe alloy is completed at 600 °C after 10 min. The reduction of Fe content in the retained alloy is 31.4% when m(Mn)/m(Fe)=0.5 and 53.3% when m(Mn)/m(Fe)=1.0 in comparison with that in the original alloy. The settled Fe-rich intermetallics were identified as α-Al₁₅(Fe,Mn)₃Si₂, which provided the lower balanced Fe concentration in the melt in comparison with other Fe-rich intermetallics.