Effect of ultrasonic vibration on thermal and material flow behavior, microstructure and mechanical properties of friction stir welded Al/Cu joints

Al/Cu dissimilar joint has been widely produced by using friction stir welding (FSW), but the weld quality remains limited. A recently developed ultrasonic vibration-assisted FSW (UVaFSW) technology has shown potential of enhancing the material flow and mechanical properties of Al/Cu joint. In this paper, a systematic study is presented to investigate the effect of ultrasonic energy on FSW Al/Cu joints with an optimized tool shoulder diameter of 16 mm and welding speed of 60 mm/min. It is found that the thermal effect of ultrasonic on the FSW process could be deemed negligible, which is distinguishing with other transmission method of ultrasonic energy. Correspondingly, the mechanical effect of ultrasonic is evident because of the dramatically decreased welding loads, the more intense intermixing between Al and Cu, and the more uniformly distributed micro-hardness in the stirring zone of UVaFSW Al/Cu joints. Moreover, the ultrasonic energy reveals a positive influence on the weld quality of Al/Cu joint; the maximum tensile strength by UVaFSW is achieved at the rotation speed of 500 rpm; and the value reaches 162.8 MPa, which improves by 60.7% compared with that by FSW. It is elucidated that the ultrasonic vibration effectively reduces the activation energy and flow stress of the material, stimulates stable plastic deformation in the vicinity of the tool, and is beneficial for the homogeneous of FSW Al/Cu joint.