Abstract: Vortex- Friction Stir Welding (VFSW) is a newly proposed modified process, which uses a stir bar made by the material identical with the workpiece and a non-consumable holder as the friction stir tool to perform FSW. It has great potential in welding high-melting point metals and welding repair. This paper investigated the effect of rotating speed on weld formation and joint mechanical properties in VFSW of 6061 aluminium alloy. The experimental results show that at a welding speed of 30 mm/min, if the rotation speed was too low, the "kissing bond" defect appeared in the weld bottom and thus the joint strength was relatively low; if the rotation speed was too high, it was difficult to form vortex material flow due to the interface slip between the workpiece and the stir bar, so that surface linear defects appeared as the welding process cannot run. At the rotation speeds of 400-800 r/min, good weld formation could be obtained. With the increase in rotation speed, the volume of weld nugget zone first increased and then decreased, but the volume of heat affected zone gradually increased. For the defect-free weld joint, the heat affected zone was the most severely softened region of the joint, and the fracture location of the tensile specimen. With the increase in rotation speed, the ultimate tensile strength of the joint increased first, and then decreased. On the contrary, the elongation first decreased and then increased. Under the experimental conditions, the maximum ultimate tensile strength of the joint was 174.8 MPa, and the corresponding elongation of the joint was 6.5%.

Key words: friction stir welding, internal friction, rotation speed, weld formation, vortex material flow