Effect of tool geometry on microstructure and mechanical properties of friction stir lap welded magnesium alloy and steel

The effect of tool geometry on microstructure and mechanical properties of friction stir lap welded AZ31 Mg alloy (top sheet) and steel (zinc coated steel and brushed finish steel) sheets was studied. Tools with two different probe lengths were used in this study. The microstructure at the joining interface, the failure loads and the fracture locations of the joints varied significantly with the probe length. For zinc coated steel joints, the short probe contributed to defect-free joints and high-strength joints; for brushed finish steel joints, long probe improved significantly the failure loads of the joints. The joints welded using a long probe fractured at the stir zone of magnesium alloy side while those welded using short probe fractured at the joining interface.