Mechanism of cracking in AZ91 friction stir spot welds

The mechanism of liquid penetration induced ( LPI) cracking is investigated in AZ91 friction stir spot welds. Liquid penetration induced cracking results from the following sequence of events: melted eutectic film formation in the periphery of the stir zone region, engulfment of melted eutectic films when the stir zone grows in width during the dwell period, penetration of alpha-Mg grain boundaries in the stir zone extremity and crack propagation when torque is applied by the rotating tool. Liquid penetration induced cracking occurs early in the dwell period during AZ91 spot welding and almost the entire stir zone is removed when the rotating tool is withdrawn. However, tool withdrawal does not provide the driving force for cracking during AZ91 spot welding; the upwards axial movement of the rotating tool at the end of the spot welding operation merely separates sections which are already cracked. The cracking tendency ( the amount of the stir zone material produced during spot welding, which is removed when the rotating tool is withdrawn) is determined by the competing effects resulting from dissolution of melted eutectic films at alpha-Mg grain boundaries and the incorporation of eutectic rich material into the stir zone.