Friction Stir Spot Welding Of Mg And Al-alloys

Welding is one of the key problems which limit the wide application of Mg and Al-alloys in the automotive weight reducing industry. Friction stir spot welding has become the first alternative for Mg and Al welding due to its intrinsic advantages, which has the potential to replace the traditional resistance spot welding to be the primary joining technique used in the automotive assembly. The microstructural features, mechanical properties and material flow in the stir zone have been investigated during friction stir spot welding of Mg and Al-alloys used in automotive industry.At first, the friction stir spot welding of similar Mg-alloys has been studied. In AZ31 spot welds, the tip of hook region curves backward from the keyhole periphery, while in AM60 spot welds, the tip of the hook region curves inward toward the keyhole periphery. The mechanical properties of AZ31 and AM60 friction stir spot welds are determined by the bonded width, the curvature of the hook region and the distance from the tip of the hook region to the nearest free surface, which is the distance from the tip of the hook to the upper surface in AZ31 spot welds and is the distance from the tip of the hook to the keyhole periphery in AM60 spot welds.Secondly, friction stir spot welding of dissimilar Mg-alloys has been investigated. There are many cracks in the thermal-mechanically affected zone induced by the formation of low temperature eutectics and there is no mixing in the stir zone of AZ31/AZ91 spot welds when AZ31 is the upper sheet. However, the stir zone of AZ91/AZ31 friction stir spot welds contain dissimilar intermingled lamellae, which results from the shear mixing when the upper and lower sheet materials move into the stir zone. The mechanical properties of AZ91/AZ31 spot welds are determined by the distance from the tip of the hook region to the keyhole periphery.Thirdly, the friction stir spot welding of dissimilar Ag-alloys has been studied. The bonded widths of Al5754/Al6111 friction stir spot welds are linearly related with the tool penetration depth into the lower sheet. Dissimilar Al5754/Al6111 intermingled lamellae are found in stir zones. When the pin length is 2.2mm and a three-flat/no thread tool is employed, the mechanical properties of Al5754/Al6111 spot welds are highest due to the highly serrated hook regions and high ( h d) ratios. Finally, the selection of tracer material and material flow in the stir zone has been investigated. The effectiveness of tracer materials can be evaluated by the comparision of weld microstructures, measured temperature and torque values in friction stir spot welds made with or without tracers. Based on the study of material flow in the stir zones, it has been proposed that the flow pattern in the stir zone is helical vertical rotational. The formation of dissimilar intermingled lamellae starts from the top of the thread on the rotating pin when a simple threaded tool is used, while the formation of dissimilar intermingled lamellae starts from the extremity of the tip of the rotating pin when a three-flat/no thread tool is employed.