An embodiment of the friction stir spot welder having a shaft (1), a drive for the shaft (1) and a tool (2), which is arranged on the shaft (1), can be driven in rotation by the shaft (1), can be subjected to a force in the direction of the workpiece and has a substantially rotationally symmetrical tool tip (4), is distinguished by the fact that the ratio V/D of overall volume V of the tool (2), measured in mm3, to diameter D of the tool tip (4), measured in mm, is less than 50, in particular less than 20, and that the tool (2) is provided with thermal insulation against the shaft (1) and other parts of the welder.
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