Abstract: In a method and apparatus for joining at least two work pieces by friction stir welding wherein the work pieces are at least partially plasticized in the area where they are to be joined by a rotating friction tool provided with a projection which can be moved into the area of the joint to be formed, the projection on the rotating tool has a length corresponding about to the thickness of the work piece and a shoulder around the pin-like projection so that, upon movement of the tool toward the work pieces, the work piece material is plasticized by the rotating pin-like projection which moves into the work piece material until it reaches the surfaces of the lowermost