An apparatus for friction stir welding comprising: a drive shaft extending along a longitudinal direction which coincides with an axis of rotation of the drive shaft, and having a drive end and a second end opposite the drive end, the drive end being adapted to be coupled to rotational drive means, a probe formed on the second end of the drive shaft, extending along the longitudinal direction, having a circumferential first friction surface extending along the longitudinal direction, and having a distal end remote from the drive shaft, and a shoulder element having a second annular friction surface extending away from the longitudinal direction and facing towards a free end and the distal end of the probe, wherein the shoulder element is supported on the drive shaft and wherein an annular cavity is formed between the drive shaft and/or the probe on the one hand and an inner surface of the shoulder element on the other hand, wherein the shoulder element comprises first and second through holes (33, 35) extending from an outer surface thereof to the inner surface, the first and the second through holes (33, 35) being spaced apart in the longitudinal direction, the annular cavity extending in the longitudinal direction between the first and the second through holes, which represent entrance and exit means of a cooling fluid, characterised in that an annular ring element is provided which surrounds and is rotatable with respect to the shoulder element around an axis that coincides with the longitudinal direction, the annular ring element being rotatably supported on the shoulder element by first and second support members (45, 47) between which an annular channel is formed, the annular channel being arranged such that the first through hole connects the annular channel with the annular cavity, and the annular ring element radially outwardly delimits the annular channel and comprises a through bore which extends to the annular channel, the shoulder element comprises a bearing member for supporting the probe and the bearing member is positioned with respect to the longitudinal direction between the first and the second through holes (33, 35). a drive shaft extending along a longitudinal direction which coincides with an axis of rotation of the drive shaft, and having a drive end and a second end opposite the drive end, the drive end being adapted to be coupled to rotational drive means, a probe formed on the second end of the drive shaft, extending along the longitudinal direction, having a circumferential first friction surface extending along the longitudinal direction, and having a distal end remote from the drive shaft, and a shoulder element having a second annular friction surface extending away from the longitudinal direction and facing towards a free end and the distal end of the probe, wherein the shoulder element is supported on the drive shaft and wherein an annular cavity is formed between the drive shaft and/or the probe on the one hand and an inner surface of the shoulder element on the other hand, wherein the shoulder element comprises first and second through holes (33, 35) extending from an outer surface thereof to the inner surface, the first and the second through holes (33, 35) being spaced apart in the longitudinal direction, the annular cavity extending in the longitudinal direction between the first and the second through holes, which represent entrance and exit means of a cooling fluid, characterised in that an annular ring element is provided which surrounds and is rotatable with respect to the shoulder element around an axis that coincides with the longitudinal direction, the annular ring element being rotatably supported on the shoulder element by first and second support members (45, 47) between which an annular channel is formed, the annular channel being arranged such that the first through hole connects the annular channel with the annular cavity, and the annular ring element radially outwardly delimits the annular channel and comprises a through bore which extends to the annular channel, the shoulder element comprises a bearing member for supporting the probe and the bearing member is positioned with respect to the longitudinal direction between the first and the second through holes (33, 35).