Effect of microstructure on tensile and fatigue properties of laser-welded DP780 and DP980 high-strength steel joints

Abstract

Abstract The aim of this study is to report the impact of microstructure on the tensile and fatigue properties of laser-welded DP780 and DP980 high-strength steel joints. The grain size at the fusion zone of the DP780/DP980 welded joint (WJ) was larger, but the microstructures at the sub-critical zone, inter-critical zone, fine-grained zone and coarse-grained zone at the DP780 and DP980 sides of DP780/DP980 WJ were not significantly different from those of DP780/DP780 WJ and DP980/DP980 WJ, respectively. The evolution trend of microhardness for the three types of WJs was consistent, featuring valleys in the softening zone and peaks in the hardening zone. The yield strength of DP780/DP980 WJ was 1.15 and 1.17 times greater than that of DP780/DP780 WJ and DP780 BM, respectively. The elongation of DP780/DP980 WJ was 6.49%, lower than the 9.22% of DP780/DP780 WJ but closer to the 6.19% of DP980/DP980 WJ. The yield-to-tensile ratio of DP780/DP980 WJ was 1.15 times that of DP780/DP780 WJ and 92.9% of DP980/DP980 WJ. The fatigue strength of DP780/DP980 WJ was 1.36 times that of DP780/DP780 WJ and 80.9% of DP980/DP980 WJ. The fatigue cracks of DP780 BM and DP980 BM propagated along the two-phase interface or inside the ferrite, and the secondary crack mainly occurred at the martensite and ferrite junction. However, the main fatigue cracks for all the WJs showed noticeable transgranular fracture, and the origin of the secondary crack was located at the grain boundary or inner grain. Fatigue crack initiations for all samples were located at the surface. The fatigue crack initiations for DP780 and DP980 BMs were mainly caused by the dislocation movement hindered by the harder martensite and impurities in the samples, while, the fatigue crack initiations of all the WJs were mainly due to oxide inclusions on the fusion zone surface.