US Patent 11331747 Self-reacting friction stir welding tool

A self-reacting friction stir welding (SR-FSW) tool includes a crown shoulder having a central bore and a pin that extends through the crown shoulder's central bore. The central bore includes a first region at a first axial end of the crown shoulder, a second region axially adjacent to the first region, and a third region axially adjacent to the second region. The first region and third region have a diameter that provides sliding contact with the pin. The pin and crown shoulder have an annular gap there between at the second region of the central bore.