Effect of ultrasonic melt processing and cooling rate on microstructure evolution of Al–Cu–Mn–Mg–Fe–Si alloy

Hard and brittle Fe-rich phases are formed during the solidification of recycled Al–Cu alloys, significantly decrease the mechanical properties of the alloy. In this study, optical microscopy (OM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), X-ray diffraction (XRD), synchrotron X-ray imaging and thermodynamic calculations were used to investigate the microstructure evolution influenced by ultrasonic melt processing (USMP) and cooling rate during solidification. The results show that increasing the cooling rate after USMP significantly refines the α-Al grain size and inhibited the growth of Fe-rich phase, Al2Cu and pores. The three-dimensional (3D) morphologies of the Fe-rich phase and Al2Cu changed from coarse dispersion to fine compact, their local thickness and mean radius decrease. The observed effects are attributed to the fact that both USMP and increased cooling rate reduce α-Al grain size, leading to the growth of the eutectic Fe-rich phase and Al2Cu within the confined spaces between the α-Al dendrites during the late solidification stage. The cooling rate of 1.2Fe alloy increases from 0.1 °C/s to 1.5 °C/s, there is a rise in the number of Fe-rich phases, accompanied by a decrease in their dimensions. Meanwhile, the increased cooling rate inhibits the diffusion of Fe elements, which in turn promotes the growth of the Fe-rich phase. Also, an increased increasing the cooling rate after USMP can inhibit the growth of pores, especially in 0.7FeU alloy, where the pores basically disappear.