a friction stir rivet is rotated in a precessing motion and driven through a first fusible workpiece into an engaged second fusible workpiece , causing local portions of the first and second workpieces to plasticize . when the rivet is driven through the first workpiece and into the second workpiece , rotation is stopped and the plasticized material solidifies around the rivet creating an enlarged weld joining the metal workpiece and encompassing the rivet , which provides additional mechanical strength .