Effect of electromagnetic stirring on grain refinement of Al-4.5%Cu alloy
There is considerable interest in electromagnetic stirring (EMS) of molten metal during solidification as a means to refine the grain structure and to modify the microstructure of the cast alloy by introducing a convective flow across the solid-liquid interface (SLI). . This convective flow induces changes in the thermal and solutal profiles in front of the SLI that affect both the morphology of the solidified metal as well as the composition variation throughout the metal when solidified. Morphology changes due to EMS consist of increased solutal fragmentation due to liquid with high solutal concentrations being forced into the SLI and deep into the mushy zone and causing the remelting of secondary dendrite arms (SDA's) which can be carried off in the convective flow created by the EMS. If the thermal undercooling conditions allow, these fragments serve as nuclei for new grains thereby increasing the nucleation rate during solidification. Another fragmentation mechanism caused by linear EMS, where recirculating flow loops of liquid are generated, is mechanical shearing. Albeit this mechanism appears to be minuscule compared to solutal fragmentation, but some evidence has been discovered supporting its existence.