Development of Fracture Model for Laser Screw Welding

This paper describes the development of a fracture finite element (FE) model for laser screw welding (LSW) and validation of the model with experimental results. LSW was developed and introduced to production vehicles by Toyota Motor Corporation in 2013. LSW offers superb advantages such as increased productivity and short pitch welding. Although the authors had previously developed fracture FE models for conventional resistance spot welding (RSW), a fracture model for LSW has not been developed. To develop this fracture model, many comprehensive experiments were conducted. The results revealed that LSW had twice as many variations in fracture modes compared to RSW. Moreover, fracture mode bifurcations were also found to result from differences in clearance between welded plates. In order to analyze LSW fracture phenomena, detailed FE models using fine hexahedral elements were developed. The analytical results revealed that the fracture modes were determined not only by equivalent plastic strain on the element but also by the stress triaxiality state of the element. The model with new fracture criteria allowed for the fracture mode bifurcation to be recreated in different clearance cases, as observed in the experiments. Based on the above results, a simplified new efficient fracture model for LSW was developed for implementation into large-scale vehicle FE models. The new model consists of hexahedral elements for the nugget part and shell elements for the heat affected zone (HAZ). The new LSW fracture model correlated well with experimental results with respect to fracture mode bifurcation.