Effect of short-time high-current post-heating on nugget microstructures and fracture mechanisms of spot welding in advanced high strength steel

Improvement of the joint strength of spot welds is crucial for reliability of light-weight car body applying of advanced high strength steel (AHSS). We have proposed a new post-heating pattern consisting main welding and two short-time high-current, called pulsed current pattern, to improve cross tension strength (CTS) even under nugget variation. The microstructures in nuggets are dominant factors for fracture pattern and the CTS, but the detailed microstructural phenomena and their effects on local fracture mechanisms have been unclear yet. This study focuses on the investigation of detailed microstructures and local deformation in spot welds of AHSS with different thermal history. Different welds were prepared with different welding conditions to differentiate thermal history. It is clarified that a pulsed current pattern with short holding time prevents fracture through nugget and resulted in high CTS. Electron back scattered diffraction (EBSD) techniques revealed that all samples had elongated grains towards solidification direction and the (101) plane was observed along the longitudinal direction of prior austenite grains. This suggests that one of the most influencing factors on the selection of martensite block is prior austenite grain boundary. EBSD evaluation also shows that the fracture path in CTS test propagated across the prior austenite grain and the propagation is prevented in a joint with pulsed current pattern. Tensile tests using miniatured sample in nugget reveals initial fracture selectively occurs either intergranular or grain boundary along weaker pass. These results suggest that slight difference of prior austenite grain formation was observed in these thermal history, but the crack sites, for example prosperous segregation, were changed through thermal history and the prevention of fracture initiation leads to high joint strength.