Study of the brittle fracture mechanism at the softening zone near the fusion line in resistance spot welds

Recently, the application of high-strength steels to automobile bodies has been expanded to improve their collision safety and fuel efficiency. However, for high-strength steels with strengths of at least 780 MPa, the cross-tension strength of resistance spot-weld joints decreases as the steel strength increases. A new failure mode has been reported, apart from the three common failure types (plug, partial plug, and interfacial failures), wherein the cleavage occurs along the fusion line. This may be caused by the formation of a softening zone called the "halo ring" along the fusion line. The fracture surface along the softening zone near the fusion line is brittle; however, the reason for this is unclear. Therefore, this study reproduced and observed the failure along the fusion line in 1.5-GPa-class hot-stamped steel. The reason for the brittleness of the fracture surface along the softening zone near the fusion line is discussed. Moreover, the softening zone near the formed fusion line was analyzed using Thermo-calc and SORPAS. The fracture location near the fusion line generally coincided with the location of decrease in Vickers hardness. Mn was distributed near the fusion line, and the Mn-enriched area became harder. Subsequently, a cross-tension test was performed, and the results showed that Mn-enriched area was cleavage fractured. Mn was distributed because it spent a long time in the two-phase region of 6-ferrite and liquid.