Investigating the Grain Refinement Mechanisms of Pulsed Electric Current, Ultrasonic and Melt Stirring Solidification of Pure Aluminium

A comparison study between an electric current applied in the pulsed mode (ECP), ultrasonic treatment (UST), and melt stirring treatment (MST) was performed to understand the origin of equiaxed grains during the solidification of pure Al. ECP and UST were applied at 760°C and 700°C before the onset of nucleation, and at one temperature range after the onset of nucleation at 661°C. UST produces excellent refinement in all three temperature ranges compared to ECP. Interestingly, application of the MST process at 661°C over the surface of the solidifying melt also resulted in significant refinement comparable to that of UST (grain size of ~260–460 μm).ECP, UST, and MST techniques differ in terms of the dominant mechanism influencing the grain refinement. Therefore, the present work analyses and discusses the grain refinement mechanisms based on nucleation, fragmentation, and a crystal separation mechanism for the origin of fine grains.