Material flow, microstructure and mechanical properties of friction stir welded AA 2024-T3 enhanced by ultrasonic vibrations

Influence of stable ultrasonic power on the process effectiveness and weld quality are investigated in the friction stir welding of 3 mm thick AA 2024-T3 with and without ultrasonic vibrations. It is found that the ultrasonically enhanced process experiences a lower downward flow stress and higher material flow. The resulted weld exhibits higher refinement and distribution of precipitates, better mechanical properties and higher amount of ductile fracture. Variations in high angle grain boundary show the progress of microstructure evolution. The grain refinement and recrystallization are improved in the immediate deformation zone and stir zone of the weld with ultrasonic vibrations. Irrespective of workpiece thickness, ultrasonic effect is maximum at the center of stir zone and gradually decreases away from the center.