An apparatus for friction stir welding comprising: a pin configured to be driven in a rotating manner about an axis of rotation including a drive end and a free end, wherein between the drive end and the free end a cylindrical engaging portion is provided extending axially in the direction of the axis of rotation and having a circumferential surface configured for engagement with one or more workpieces, a first shoulder element arranged on the side of the engaging portion facing the drive end and which is arranged concentrically around the pin with respect to the axis of rotation, and including a first bearing surface facing the engaging portion and extending perpendicularly to the axis of rotation, and a second shoulder element arranged on the side of the engaging portion facing the free end and which is arranged concentrically around the pin with respect to the axis of rotation, and including a second bearing surface facing the engaging portion and extending perpendicularly to the axis of rotation, wherein the second bearing surface extends directly to the engaging portion and wherein the first shoulder element is configured to rest against the first surface and the second shoulder element is configured to rest against the second surface of the one or more workpieces, opposite the first surface, characterized in that, the second shoulder element is connected to the pin so as to be freely rotatable about the axis of rotation, so that the second shoulder element cannot perform a rotation with respect to the surface of the at least one workpiece, the second shoulder element comprises a second annular intermediate piece of ceramic material provided in the second bearing surface and the second intermediate piece surrounds the engaging portion. a pin configured to be driven in a rotating manner about an axis of rotation including a drive end and a free end, wherein between the drive end and the free end a cylindrical engaging portion is provided extending axially in the direction of the axis of rotation and having a circumferential surface configured for engagement with one or more workpieces, a first shoulder element arranged on the side of the engaging portion facing the drive end and which is arranged concentrically around the pin with respect to the axis of rotation, and including a first bearing surface facing the engaging portion and extending perpendicularly to the axis of rotation, and a second shoulder element arranged on the side of the engaging portion facing the free end and which is arranged concentrically around the pin with respect to the axis of rotation, and including a second bearing surface facing the engaging portion and extending perpendicularly to the axis of rotation, wherein the second bearing surface extends directly to the engaging portion and wherein the first shoulder element is configured to rest against the first surface and the second shoulder element is configured to rest against the second surface of the one or more workpieces, opposite the first surface, characterized in that, the second shoulder element is connected to the pin so as to be freely rotatable about the axis of rotation, so that the second shoulder element cannot perform a rotation with respect to the surface of the at least one workpiece, the second shoulder element comprises a second annular intermediate piece of ceramic material provided in the second bearing surface and the second intermediate piece surrounds the engaging portion.