Experimental studies on ultrasonically assisted friction stir spot welding of AA6061

The present study introduces a new combination of two separate welding processes, i.e. friction stir spot welding (FSSW) and ultrasonic welding (USW). Here, in order to improve the weld quality, the friction stir spot welding is assisted by ultrasonic vibration of tool. To systematically analyze effect of process factors such as US vibration, tool rotary speed, tool plunge depth and dwell time on lap shear force and hardness, a L18 orthogonal array from Taguchi design of experiments is developed. Effects of process factors on responses were studied along with their percentage of contribution which was determined through analysis of variances. Results indicated that US vibration is a significant factor having positive influence on lap shear force and hardness. Next to the vibration, tool rotary speed, dwell time and plunge depth are also the important factors which affects mechanical properties significantly. Optimization of process factors by grey relational analysis showed that applying US vibration and selections of 1200 RPM tool rotary speed, 6 mm plunge depth and 6 s dwell time causes the highest value of grey relational grade and guarantee maximum lap shear force as well as maximum hardness.