Vertical compensation friction stir welding assisted by external stationary shoulder

Vertical compensation friction stir welding (VCFSW) was developed to solve the existence of a big gap at the interface between two workpieces to be welded. By a self-designed tool system with external stationary shoulder, VCFSW is successfully used to weld 6061-T6 aluminum alloy with the thickness of 4. mm, while 2024-T4 aluminum alloy is chosen as compensation material. When using a traditional tool, it is difficult for VCFSW to attain a sound joint if the width of compensation strip is no less than 1.5. mm. The self-designed stationary shoulder tool system is more suitable for VCFSW. Defect-free VCFSW joints using the self-designed stationary shoulder are attained when the widths of strip vary from 1. mm to 2. mm. For VCFSW process, increasing the rotational velocity and decreasing the welding speed are beneficial to break the compensation strip into pieces to get the higher quality welding joint. When the width of compensation strip is 1. mm, the tensile strength and elongation of joint by the stationary shoulder tool system reach the maximum of 75.1% and 58.8% of base material (BM) when using the welding speed of 50. mm/min and rotational velocity of 2000. rpm, respectively. The fracture surface morphology of VCFSW joint presents the typical ductile fracture.