Effects of ultrasonic vibration and manganese on microstructure and mechanical properties of hypereutectic Al–Si alloys with 2%Fe

The effects of ultrasonic vibration (USV) on the microstructure and mechanical properties of Al–17Si–2Fe–2Cu–1Ni (mass %) alloys with 0.4% or 0.8% Mn were studied. The results show that the average grain size of primary Si in the alloys treated by USV could be refined to 21–24 μm, whether with or without P modification. The P addition has no further refinement effect on the primary Si in the case of the combined use of USV with P addition. Without USV, the alloy with 0.4%Mn contains a large amount of long needle-like β-Al5(Fe,Mn)Si phase and coarse plate-like δ-Al4(Fe,Mn)Si2 phase. Besides, the alloy with 0.8% Mn contains a small amount of coarse dendritic α-Al15(Fe,Mn)3Si2 phase. With USV treatment, the Fe-containing compounds in the alloys are refined and exist mainly as δ-Al4(Fe,Mn)Si2 particles with average grain size of about 18 μm, and only a small amount of β-Al5(Fe,Mn)Si phase is remained. With USV treatment and without P modification, the ultimate tensile strengths (UTS) of the alloys containing 0.4% and 0.8% Mn are 271 MPa and 289 MPa respectively at room temperature, and the UTS are 127 MPa and 132 MPa at 350 °C. The Brinell hardness of the alloys are 131 HB and 139 HB respectively. It is considered that the modified morphology and uniform distribution of the Fe-containing intermetallic compounds and the primary Si phases, which are caused by USV process, are the main reasons for the increase of the tensile strength and hardness of these two alloys.