A friction stir welding tool with a base, a rotating friction stir slug mounted in the said base and at least one guide member connected to the said base, the welding tool being adapted to move from upstream to downstream; a welding tool having at least one displacement member of the guide member relative to the said base between at least one guiding position in which the guide member extends in a scout area located downstream and facing the friction stir slug and at least one retracted position in which the guide member is offset from the said scout area.