Thermo-mechanical behaviors and microstructure characteristics of ultrasonic vibration enhanced friction stir welding

In order to utilize the ultrasonic energy to reduce the yield stress and flow stress of the metal materials during friction stir welding(FSW), the experimental system of the ultrasonic vibration enhanced friction stir welding(UVeFSW) is developed, and tests are conducted with 6061Al-T6 alloy plates. The welding load under the ultrasonic vibration are measured by real-time capturing the electrical parameters of the FSW machine and translating them to torque and force according to specific formula. Thermal cycle is measured by embedding thermal couples in specific positions. Weld cross-section size and microstructure in UVeFSW are also observed by stereomicroscope and metalloscope. They are all compared with those in conventional FSW at the same welding parameters. The results reveal that the ultrasonic vibration reduces the axial force and tool torque significantly, enlarges the weld cross-section size, refines and homogenizes the grains in weld nugget and thermo-mechanically affected zone. The measurements of the thermal cycle shows that the peak temperature of the measured points slightly decrease under ultrasonic vibration. It is inferred that the exerted ultrasonic vibration interacts with the plasticized material near the tool, decreases the yield stress and flow stress of the metals, and changes the temperature field, so that statisfactory results are achieved. © 2015 Journal of Mechanical Engineering.