Effect of intergranular boron segregation on cross-tension strength in medium-Mn TRIP and dual-phase steel dissimilar resistance spot welds

Resistance spot weld (RSW) joints between medium-Mn transformation-induced plasticity (TRIP) and dual-phase (DP) steels often exhibit premature interfacial fracture due to the brittle heat-affected zone (HAZ). Although boron (B) segregation is well known to influence the microstructure of press-hardened steels, its direct role in controlling the local ductility and cross-tension strength (CTS) of dissimilar TRIP/DP RSWs remains unclear. This study aims to elucidate how boron grain-boundary segregation controls the ductility and fracture mode of the coarse-grained heat-affected zone (CGHAZ), and consequently the joint-level CTS, thereby providing a quantitative basis for composition-controlled interfacial behavior. To isolate local properties, CGHAZ specimens were produced by Gleeble thermo-mechanical simulation and tested in tension. Boron was added up to 50 wt. ppm. CTS increased with boron and reached a maximum at ~20 wt. ppm. The improvement correlated with higher ductility of the CGHAZ and a transition from intergranular brittle fracture to mixed ductile fracture. Atomistic- to microscale characterization indicated that boron segregated to prior-austenite grain boundaries, which reduced boundary decohesion. At ≥30 wt. ppm, the benefit diminished. The reduction in CTS is attributed to boundary liquation, accelerated grain coarsening, and excessive segregation that promotes intergranular cracking. These results establish a compositional window in which boundary segregation is toughening, while over-addition reverses the effect. The combined use of dissimilar RSW testing and simulated CGHAZ tensile testing provides a mechanistic link between segregation, local tensile behavior, and joint-level CTS.