Effects of tool geometry and process conditions on material flow and strength of friction stir spot welded joints

In this study, the theoretical tool design was proposed based on the material flow during friction stir spot welding (FSSW). At first, the fracture mode in a cross tensile test and a tensile shear test was investigated in detail. Then, the ideal profile of the welded joint was proposed to accomplish the high joint strength. The high joint strength can be acquired when the tip of the partially bonded interface is located at distance from the periphery of the keyhole and the surface of the top sheet. Based on the material flow model, it became clear that the strong inwards flow produced by the shoulder or the increase of the heat input to soften material around the stir zone leaded to producing the ideal joint. Thus four types of tools with different shoulder profiles were investigated. The investigation of the material flow and tensile tests were carried out. 15mm-spiral tool, which was designed to facilitate the material flow beneath the shoulder efficiently, attained the high joint strength in both a cross tensile and a tensile shear test.