Effect of welding current on microstructure and mechanical properties of 15% deformed TWIP steel joined with electrical resistance spot welding

The effect of current on properties of 15% deformed TWIP sheets joined with spot welding was investigated in this study. Optical microscopy, Scanning Electron Microscopy (SEM), SEM/Energy Dispersive Spectroscopy (SEM-EDS) and SEM/Electron Backscatter Diffraction (SEM-EBSD) techniques were used in microstructure characterization. Microhardness and tensile tests were performed to determine mechanical properties. The nugget size, indentation depth and heat affected zone (HAZ) width increased almost linearly with increased current, while the weld cavities decreased with increased current. Large annealing twins have appeared in HAZ, while deformation twins have disappeared. Grain coarsening and twin thicknesses in HAZ increased with increased current. However, the welding didn't cause any phase transformation. Therefore, hardness in weld zone was considerably lower than base metal hardness. In general, the lowest hardness was observed in HAZ. There was no correlation between current and hardness in weld zone. Fracture load increased with increased current: The highest fracture load was obtained at 10 kA. Button pullout fractures have emerged with higher currents, whereas interfacial fractures have occurred with lower currents. Fracture characteristics are generally a mixture of brittle-ductile. In weld samples with higher strength, the fracture characteristics of ductile in brittle-ductile zone and the trans-granular in brittle zone have increased.