a friction stir welding pin having internal flow cavities . the pin is adapted to be driven by a conventional friction stir welding machine , and may include external threads for forcing plasticized material toward the weld root . an internal cavity located along the centerline and open to the distal end facilitates deformation of the workpiece material at the weld root . one or more flow channels extending from the sidewall of the pin to the internal flow cavity induce a continuous path of plasticized metal through the pin . the internal cavity may include internal threads to further help force plasticized material toward the root weld . the pin is particularly useful in welding aluminum work pieces where the tolerance of the workpiece thickness is not critical .