Abstract: The present application relates to a friction stir welding tool (20) for joining two adjacent work pieces (10), which work pieces comprise portions (6) to be welded and having a narrow space there between on one side, said tool having an axis of rotation, and comprising a first shoulder (1), a second shoulder (2), and a pin (3) arranged so as to connect the first and second shoulders, wherein said first shoulder has a first contact surface for friction heat generation to the work pieces to be joined during welding, said first contact surface having a first contact area, and said second shoulder has a second contact surface for friction heat generation to an opposite surface (5) of the same portions of the work pieces to be joined, said second contact surface having a second contact area, the first and second areas having different sizes, where the first area is a larger sized area than the second area.

Abstract: The present application relates to a friction stir welding tool (20) for joining two adjacent work pieces (10), which work pieces comprise portions (6) to be welded and having a narrow space there between on one side, said tool having an axis of rotation, and comprising a first shoulder (1), a second shoulder (2), and a pin (3) arranged so as to connect the first and second shoulders, wherein said first shoulder has a first contact surface for friction heat generation to the work pieces to be joined during welding, said first contact surface having a first contact area, and said second shoulder has a second contact surface for friction heat generation to an opposite surface (5) of the same portions of the work pieces to be joined, said second contact surface having a second contact area, the first and second areas having different sizes, where the first area is a larger sized area than the second area.