in a device for butt joining workpieces by friction stir welding along a connecting area , including a shaft having a driven end and , at the opposite end , a pin - like projection to be disposed in the connecting area of the workpieces , whereby , upon rotation of the shaft , the workpiece material in contact with the pin - like projection is plasticized , first and second stops with first and second shoulders are provided on the device and disposed at opposite sides of the workpieces and at least one of the stops is movable relative to the other and biased toward the other with a controllable force for engaging the workpieces between the shoulders of the stops during the friction stir welding procedure .