Characterization and modelling of fracture in press-hardened Ductibor® 1000-AS:Usibor® 1500-AS laser-welded blanks

The application of multi-material laser-welded blanks (LWBs) in automotive bodies has become a useful strategy to improve the crashworthiness of vehicles. Hot-stamped Ductibor® 1000-AS:Usibor® 1500-AS LWBs are potential candidates for application in energy-absorbing structural components of automobiles. The present work focuses on the fracture response of such weldments in both mono- and multi-gauge forms. The gradient in the material properties within the weld zones and parent metals was evaluated through microstructure and microhardness characterization. The weld regions possessed a fully martensitic microstructure with a monotonic decrease in hardness from the stronger (martensitic) Usibor® 1500-AS to the weaker (martensitic) Ductibor® 1000-AS. The fracture behavior was examined by means of uniaxial tension, Nakazima, and tight-radius V-bend tests for various orientations of the weld line relative to the principal loading direction. Fracture occurred in the Usibor® 1500-AS parent metal during most of the 0° tensile and 45° and 90° V-bend tests, while fracture was initiated within the Ductibor® 1000-AS parent metal in all of the 90° tensile and Nakazima tests. A few cases of weld failure were observed in the 0° tensile and 45° and 90° V-bend tests. The 0° V-bend test was the only case where fracture happened within the weld in all of the specimens. Finite-element simulations of the uniaxial tension and Nakazima tests were performed using LS-DYNA, in which the constitutive and fracture properties of the parent metals and weld zone were mapped based upon the measured microhardness. Using this approach, the predicted load-displacement and fracture response of the weldments agreed well with the experimental data. The close correspondence of the model and experiments demonstrated the capability of the hardness-mapped models to predict the mechanical behavior of such hot-stamped LWBs with martensitic microstructures.