a friction stir rivet is rotated and driven through a first fusible workpiece into an engaged second fusible workpiece , causing local portions of the first and second workpieces to plasticize . a slideable cap contacts the exposed surface of the first workpiece shortly after the process begins . the contact causes the cap to act as a retaining element , limiting the escape of plasticized material from stir site . once the rivet is driven into the first and second workpieces , rotation ceases and the plasticized material hardens around the rivet . a weld is thus created , joining the workpieces and encompassing the rivet , which provides additional mechanical strength .