Combined effects of cobalt addition and ultrasonic vibration on microstructure and mechanical properties of hypereutectic Al–Si alloys with 0.7% Fe

Cobalt is generally considered as the element that can not only neutralize the effect of iron, but also promote the high-temperature mechanical properties of Al–Si alloys. Ultrasonic vibration (USV) is an effective mean for modifying morphology of the Fe-bearing intermetallic compounds. The combined effects of the Co addition and USV on microstructure and properties of Al–20Si–2Cu–1Ni–0.7Fe alloy were studied in this paper. The results show that, when added 0.3%, 0.7%, 0.91%, and 1.05% Co, respectively, into the alloy, the Fe-containing compounds changed from long acicular β-Al5(Fe, Ni)Si phases to Chinese-script, granular or rod-like α-Al15(Fe, Co, Ni)3Si2 phases with the increasing of Co content. The USV treatment applied around liquidus temperature of the alloys could not only lead to refinement and volume fraction reduction of primary Si, but also results in a great decrease of grain size and aspect ratio of Fe-containing compounds. The addition of equal or less than 0.91% Co, as well as application of USV has a positive effect on tensile strength of the Al–20Si–2Cu–1Ni–0.7Fe alloys. The maximum tensile strengths at room temperature and elevated temperature (350 °C) of alloys in traditional casting were 275 MPa and 101 MPa, respectively, with 0.91% Co, while the corresponding data were 284 MPa and 132 MPa, respectively, for alloys treated with USV.