Material flow during friction stir spot welding

Material flow during friction stir spot welding is investigated. An examination of dissimilar Al 5754/Al 6111 spot welds was conducted to allow visualisation of material flow based on their differing etching characteristics. In addition, Al 6061-T6 spot welds containing Al2O3 tracer particles were examined to highlight the movement of material in different joint regions. It has been confirmed that upper sheet material is moved downwards into the lower sheet when a layer of upper sheet material is pushed ahead at the tip of the rotating pin, when upper sheet material becomes trapped at the root of the pin thread, and when an adhering layer of upper sheet material forms at the pin periphery during spot welding using a smooth pin. Lower sheet material is displaced upwards and outwards in a spiral motion when the rotating pin forms the keyhole. Two distinct zones of material flow are produced during friction stir spot welding: an inner flow zone close to the pin periphery where upper sheet material moves downwards in an anticlockwise direction with the rotating pin; and an outer flow zone where lower sheet material moves upwards and outwards in a spiral motion.