Abstract: Non-tilting friction stir welding (NT-FSW) is the critical technique for the joining of the plane two-dimensional and spatial three-dimensional complex structures. In order to illuminate the influence and affecting mechanism of tool pin geometrical configuration on the weld formation during NT-FSW and to provide technological guidance for the high-quality applications of NT-FSW process, the welding tools with smooth pin, smooth-flattened pin and threaded-flattened pin are utilized to carry out NT-FSW experiments in the present paper, and the weld morphology, defect distribution and joint properties are comparatively investigated. The results indicate that for NT-FSW process it is difficult to produce sound joints by using the smooth tool pin since the void defect tends to be formed at the weld root. The flat feature of tool pin enhances the material plastic flow at upper and middle parts of weld and thus indirectly promotes the filling of keyhole at the weld root, leading to the formation of defect-free joints at lower welding speeds. The threads of tool pin directly improve the material flow levels at the weld root and can produce defect-free joints in a wider process parameter range. Compared with the smooth tool pin, the flattened tool pin widens the softening region and improves the softening levels at the weld top, leading to a maximum strength efficiency of 86% for the defect-free joints. By contrast, the threads further machined on the flattened tool pin can control the local softening occurring in FSW, which narrows the softening region width and improves the hardness of nugget. Due to this, the maximum strength efficiency is improved up to 90%.

Key words: geometrical configuration, non-tilting friction stir welding, tool pin, weld formation