Abstract: A stir weld tool of the present invention is used to weld two pieces of metal together through frictional heating, plasticizing, mixing, and forging of the plasticized metal into a uniform weldment. The stir weld is accomplished by rotating the stir weld tool with a pin having at least one boss on its surface to create the mechanical friction in the metal in contact with the rapidly rotating pin. As the rotating pin travels along the weld joint, the plasticized metal of the work pieces is extruded to the back face of the pin while undergoing a mechanical stirring and forging from action imparted by the at least one boss on the surface of the pin, and while confined from above by the pressure exerted on the metal by a shoulder on the stir weld tool.