Development of weld lobe for resistance spot-welded TRIP800 steel and evaluation of fracture mode of its weldment

The weld lobe in spot welding provides an indication of good quality joining and the tolerance of the weld schedule in production stage. In this study, TRIP800 steel was used for the experiments, and welding times of 5, 10, 15, 20, and 25 cycles were selected with welding currents ranging from 1 to 7 kA with an interval of 2 kA and from 7 to 10 kA with an interval of 1 kA. The effect of heat input associated with welding current and welding time on nugget geometry, such as diameter, height, nugget size ratio, and electrode indentation, was determined by optical microscope. The welded specimens were exposed to tensile shear tests. Tensile shear strength and failure mode associated with nugget geometry and electrode indentations were also evaluated and weld lobe was drawn accordingly. It was found that the nugget diameter and nugget size ratio of TRIP800 steel spot welds should be at least 4.5aet and 0.15-0.30, respectively, for pullout failure mode, acceptable shear strength, and surface quality.