Development of friction stir spot welding using rotating tool without probe and its application to low-carbon steel plates

A friction stir spot welding process, in which a rotating tool without a probe was employed, was applied to a lap joint of low carbon steel plates with 0.5 mm thickness. In this process, the rotating tool of 3.6 mm diameter, rotating at 18,000 rpm, was plunged into the upper plate at a rate of 0.2 mm/s, and then kept at a maximum plunged depth of 0.05-0.25 mm for 0-1 s (dwell time). In the weld obtained by this process, a hole due to the impression of the plunged tool probe was not formed, although a slight depression by the tool plunging remained. At tool plunge depths of 0.05 mm or less, it was impossible to weld the plates. At tool plunge depths from 0.1 to 0.14 mm, joints were fractured at the interface in tensile test, and the failure load increased with tool plunge depth. At tool plunge depths from 0.16 to 0.22 mm, joints were fractured at an almost constant load along the periphery of the depression, leaving a part of the upper plate on the bottom plate surface. The maximum tensile failure load of 1.8 kN was obtained at a plunge depth of 0.2 mm. Based on the observation of the weld microstructure and measurement of the thermal cycle at various spots in the weld, controlling factors of the joint strength are discussed.