The effect of electromagnetic stirring on macrostructure and macrosegregation in the aluminium alloy 7150

Ingots of aluminium alloy 7150 (Al-Zn-Mg-Cu) were unidirectionally solidified vertically downwards under conditions of natural convection and electromagnetic stirring of the bulk liquid. Increased fluid flow in the bulk liquid was found to promote the formation of equiaxed grains and hence the columnar-equiaxed transition (CET). Increased fluid flow was also associated with an increase in macrosegregation of the alloying elements. The extent of segregation was greatest for Cu and least for Zn. The experimental observations suggest that macrosegregation in the ingots was caused by the displacement of solute-enriched liquid from the mushy zone as a result of the electromagnetically stirred bulk liquid being driven into the solidification front.