Cold Spot Joining of 980 MPa-class Ultra-high Strength Steel

Cold spot joining (CSJ), a new joining method using resistance heating, can join metals at solid-state low temperatures without melting, unlike conventional resistance spot welding (RSW), and therefore can solve the problems associated with fusion welding. In this study, 980 MPa-class ultra-high strength dual-phase steel sheets were joined to clarify the difference between CSJ and RSW joints. Since CSJ uses plastic flow to expand the joining diameter, it was confirmed that a larger joining diameter can be obtained and that the joint strength is also higher than that of RSW. Furthermore, since CSJ can be joined at low temperatures, it is possible to avoid the generation of spatter and to produce sound joints that prevent the reduction in joint volume and void defects caused by spatter. In addition, the distribution of HAZ softened regions in joints fabricated by CSJ was investigated, and the effect of the crack propagation direction during cross tensile tests on the fracture morphology was discussed.