Effect of Ultrasonic Melt Processing and Cooling Rate on Microstructure Evolution of an Al-Cu-Mn-Mg-Fe-Si Alloy

Abstract

Hard and brittle Fe-rich phases are formed during the solidification process of recycled Al-Cu alloys, significantly decreasing the mechanical properties. 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 under the influence of ultrasonic melt processing (USMP) and cooling rate during solidification. The results show that increasing the cooling rate after USMP significantly refined the α-Al grain size and inhibited the growth of Fe-rich phase, Al2Cu, and pores, and changed the Fe-rich phase and Al2Cu three-dimensional (3D) morphology from coarse dispersion to fine compact, decrease their surface thickness and mean radius. This is because USMP and increasing cooling rate reduce α-Al grain size, and the eutectic Fe-rich phase and Al2Cu grow between the narrow α-Al dendrites at the late solidification stage. The cooling rate of 1.2Fe alloy increases from 0.1 ℃/s to 1.5 ℃/s, and increase the number of Fe-rich phases and reduce the size. The increasing cooling rate is inhibited by the diffusion of Fe elements and promotes the growth of the Fe-rich phase. When increasing the cooling rate after USMP can inhibit the growth of pores, especially in 0.7FeU alloy, the pores basically disappear.

Keywords: Ultrasonic melt processing, Cooling rate, Fe-rich phase, Al-Cu alloys, Synchrotron X-ray radiography