Susceptibility to weld solidification cracking in transformation-induced plasticity steel sheets was studied using a modified standard hot cracking test used in the automotive industry. To vary the amount of self-restraint, bead-on-plate laser welding was carried out on a single-sided clamped specimen at increasing distances from the free edge. Solidification cracking was observed when welding was carried out close to the free edge. With increasing amount of restraint, the crack length showed a decreasing trend, and at a certain distance, no cracking was observed. With the aid of a finite element-based model, dynamic thermal and mechanical conditions that prevail along the transverse direction of the mushy zone are used to explain the cracking susceptibility obtained experimentally. The results indicate that the transverse strain close to the fusion boundary can be used as a criterion to predict the cracking behavior. The outcome of the study shows that optimum processing parameters can be used to weld steels closer to the free edge without solidification cracking.

Hot cracking during laser welding of advanced high-strength steels is reported to be a serious problem by automotive manufacturers. In this work, hot cracking susceptibilities of transformation-induced plasticity (TRIP) and dual-phase (DP) steels are studied based on a multi-scale modelling approach. Transient temperatures measured from welding experiments are used to validate a finite element (FE) model. The temperature, thermal gradient and cooling rate in the weld fusion zone are extracted from the FE model and pre-defined as boundary conditions to a phase field model. The welding-induced microstructural evolution is simulated considering thermodynamic and mobility data. Results show that, compared to the DP steel, the TRIP steel has a broader solidification range, a greater pressure drop at the inter-dendritic regions, and an increased phosphorus segregation at the grain boundaries; all these make this steel more susceptible for hot cracking.