Effect of Welding Current on Microstructure and Mechanical Properties of Resistance Spot Welded Joint of 6061-T6 Aluminum Alloy Non-equal Thickness Three-Layer Plate

Resistance spot welding of 6061-T6 aluminum alloy non-equal thickness three-layer plate (upper, middle and lower plate thickness was 2, 2, 4 mm successively) was carried out at different welding currents (15, 17, 19, 21 kA). The effect of welding current on microstructure and mechanical properties of spot welded joints was studied. The results show that the joints at different welding currents were composed of base metal, weld nugget zone, and heat affected zone, and no obvious macroscopic defects were found. With the increase of welding current, the width of the columnar crystal zone in weld nuggest zone increased but that of the equiaxial crystal zone decreased; the grains in nuggest zone and heat-affected zone were coarsened; the number and size of second phases increased. The shear force of the joints first increased and then decreased with increasing welding current, reaching the largest values at welding current of 19 kA. The joints at different welding currents all showed button fracture features. The hardness of the base metal of the joint was the highest, followed by that of the heat affected zone, and the hardness of the weld nugget zone was the lowest. With the increase of welding current, the lowest hardness of the nugget zone increased.