Fracture Response in Hot-Stamped Tailor-Welded Blanks of Ductibor® 500-AS and Usibor® 1500-AS: Experiments and Modelling

Abstract

Hot-stamped tailor-welded blanks (TWBs) comprised of steel sheets with different chemical compositions are of interest within the automotive sector to enhance the crash performance of vehicles via imparting local ductile regions to ultra-high strength structural components. This study examines the fracture behavior of hot-stamped TWBs composed of mono- and multi-gauge laser-welded sheets of Ductibor® 500-AS and Usibor® 1500-AS, which are under consideration for use in crash-safety members. Microstructural and microhardness investigations were conducted across the weld line of the TWBs to address the variations of mechanical properties. A fracture characterization campaign via a series of coupon tests, including uniaxial tension, equibiaxial tension Nakazima dome, and V-bend, was conducted on specimens with a range of orientations of the weld line with respect to the loading direction. Numerical models of the tensile and Nakazima dome tests with and without considering the weld region in the TWBs were developed using the LS-DYNA finite-element method software based on mapping constitutive properties and fracture-limit curves across the weld and in parent metals as functions of local hardness. The fracture characterization tests revealed that weld fracture occurred only in the V-bend tests for which the weld line was parallel to the punch (0° V-bend), while in all of the other mechanical tests considered, fracture was initiated in one of the parent metals. The Ductibor® 500-AS region was the site of fracture initiation in the transversely-welded tensile and centrally-welded Nakazima dome samples, whereas fracture initiation occurred in the Usibor® 1500-AS region for the longitudinally-welded tensile and 45° and 90° V-bend samples. The predictions revealed that the inclusion of the narrow weld region of the laser-welded blanks within the hardness-mapped TWB models resulted in an improvement of the predictions in terms of the load–displacement response and fracture locations compared to the “without weld” scenario. Therefore, the correlation of variations in the flow and fracture response of the hot-stamped TWBs of Ductibor® 500-AS and Usibor® 1500-AS with the trend of changes in hardness across the weld can be a more suitable strategy for modelling the mechanical behavior of such TWBs in crash-safety simulations.