Effect of a novel pin tool with large diameter disc on the microstructure and mechanical properties of Al/Mg dissimilar friction stir lap joint

In this paper, a novel friction stir welding tool with large diameter disc at the tip of the pin was developed to solve the problems in aluminum/magnesium friction stir lap welding. Based on the structural design of the shoulder at the tip of the pin, the temperature difference in the thickness direction of the joint was coordinated, and the aluminum-magnesium dissimilar lap joint was successfully prepared in two kinds of lap configuration. Taking AZ31B magnesium alloy and 6061 aluminum alloy as the research object, the defects and microstructure of weld joint were observed and analyzed by optical microscope and scanning electron microscope. Additionally, their mechanical properties were also evaluated. The results show that an excellent metallurgical bonding was formed between aluminum and magnesium. With the increase of the effective plate thickness and the decrease of the effective lap width, the unique hook defect shape of lap welding joint was optimized, which also promoted the improvement of the mechanical properties of the weld. In the tensile-shear test, it was found that the tensile shear strength was improved, and the joint fractured in a mixed mode of toughness and brittleness due to the change of the shape of the hook defect, and it is a mixed fracture of toughness and brittleness.