Material flow during friction stir spot welding

Material flow during friction stir spot welding (FSSW) is investigated using tracer material technique. Three distinct regions are developed in a weld after the rotating shoulder comes in close contact with the upper sheet. They are called flow transition zone, stir zone, and torsion zone which are evolved due to the combination of rotational, horizontal and vertical motions of the plasticized material. An incorporation of the upper and lower sheet materials takes place in the flow transition zone, and the intermingled materials flowing from the flow transition zone contribute primarily to the formation of the stir zone. A new model of material flow during FSSW is developed on the basis of experimental observations. It is believed that the intrinsic driving force for the downward motion of the plasticized material is originated from the material release from the rotating pin through an outward-spinning motion.