Microstructure development and solute redistribution in aluminium alloys under low and moderate shear rates during rheo processing

Microstructural features and microsegregational behaviour of solute are studied in shear or stir-cast aluminium alloys under low and moderate shear rates. Alloys studied are Al-6.2% Cu, Al-7.3% Si and Al-13.2% Mg. In all the cases, microstructures of the primary pre-quench solid for stir-cast samples show rosette or ellipsoidal morphologies. Volume fractions of pre-quenched solid phase show significantly higher values for stir-cast alloys as compared to calculated. Microsegregation studies by microprobe analysis along the grains of the samples solidified under different treatment conditions show that stir casting changes the segregation pattern significantly. Except for Al-13.2% Mg alloys lower values than those calculated by Scheil's microsegregation equation are observed for other systems. A model for microstructure evolution during stir casting is presented. The microsegregation patterns have been discussed in terms of interaction between the diffusing solute and the vacancies migrating from solid into liquid.