Microstructure and fatigue properties of fiber laser welded dissimilar joints between high strength low alloy and dual-phase steels

The aim of this study was to evaluation the microstructure and fatigue properties of welded joints made with fiber laser welding (FLW) on a high strength low alloy (HSLA) and dual-phase (DP980, UTS ⩾ 980 MPa) steel in similar and dissimilar material combinations. The fusion zone (FZ) consisted of martensite, and the heat affected zone (HAZ) contained some newly formed martensite and partially tempered martensite on the DP980 steel side. A characteristic asymmetric hardness profile across the dissimilar HSLA–DP980 welded joint was observed. While a soft zone occurred on the DP980 side, it was absent on the HSLA side. Inside the FZ two hardness sub-regions were observed due to the difference in the alloying elements between two steels along with the fast cooling during FLW. The presence of soft zone on the DP980 side had no effect on the tensile properties, since the lowest hardness value in the soft zone was still higher than that of the HSLA base metal (BM). A joint efficiency of 97–100% was achieved with respect to the HSLA. The strain to failure of the dissimilar HSLA–DP980 welded joints was significantly (∼threefold) higher than that of the similar DP980–DP980 welded joints. Although the fatigue strength of the dissimilar HSLA–DP980 welded joints was lower than that of DP980–DP980 welded joints, it was equivalent to that of HSLA–HSLA welded joints. Failure occurred in the BM on the HSLA side in the tensile tests and fatigue tests at high cyclic stress levels, where yielding occurred. At the intermediate and lower cyclic stress levels, fatigue failure occurred in the weld area due to the higher sensitivity to the weld concavity.