Effect of ultrasonic vibration exerted at the tool on friction stir welding process and joint quality

To employ acoustic energy efficiently in friction stir welding, ultrasonic vibration was transmitted onto the tool along a direction perpendicular to the tool axis by a specially manufactured horn with a pair of rollers. The experimental results showed that the ultrasonic vibration exerted in this way decreased the torque and axial force acted on the tool, increased the welding speed, eliminated tunnel defects in welds, and improved the mechanical properties of the weld joints. The peak temperature of the thermal cycles was almost not changed by ultrasound except for a slight increase in heating rate and a slight decrease in cooling rate, which indicates that thermal effect due to ultrasonic vibration plays a minor role. The simultaneous interaction between the exerted ultrasonic vibration on the tool and the tool induced thermo-mechanical behavior intensifies the plastic deformation and material flow in the shear layer, which is the main reason why both the process and weld quality are improved.