Effect of welding position and direction on the interfacial microstructure and mechanical properties of double-pass friction stir lap welded A6061/Q235 joints

The mechanical properties of a conventional single-pass friction stir lap welding (FSLW) aluminum/steel joint were difficult to meet the ever-changing industrial requirements. In this work, double-pass dissimilar FSLW was adopted to improve the mechanical properties of the joints between A6061 aluminum alloy and Q235 steel plate. After the first pass of friction stir welding, the welding position and welding direction were altered during the second pass, resulting in the formation of four double-pass weld joints with various plate configurations. A double-pass joint enlarged the interfacial bonding area, which significantly improved the mechanical properties of the joints. Specifically, the ultimate tensile load of the joints was increased by 48.8 % when the second pass was located on the retreating side of the first pass in comparison with single-pass joints. In addition, the fracture mode of the double-pass joint shifted from the interface fracture of the single-pass joint to the internal fracture of the heat-affected zone.