Effect of tool geometry on loads developing in friction stir spot welds of polycarbonate sheets

Friction stir spot welding (FSSW) experiments were carried out on polycarbonate sheets to assess the influence of the tool geometry on the joining loads and material flow. An instrumented drilling machine was used to measure the plunging load and the torque developing during the process. Different tool geometries were tested involving the variation of the tool pin shape (cylindrical or tapered), diameter of the tool pin, and diameter of the tool shoulder. The analysis of material flow enabled understanding the behavior of the load and torque trends measured during the process, such as the presence of load peaks during the plunging phase and reduced values of the axial pressure during the process. It was discovered that the average axial pressure was higher during pin plunging (such value was mainly determined by the pin diameter) while it dramatically decreased (almost to 1.2 MPa) during shoulder plunging owing to the higher heating resulting from higher tangential speed. The pin taper angle had negligible influence on the plunging load and torque. The tool shoulder influenced the plunging load and torque while it produced a negligible variation in the average axial pressure. On the other hand, the pin diameter influenced both the process loads and the axial pressure.