Influence of hot forming on microstructure and mechanical properties of laser tailor-welded dissimilar ultra-high-strength steels

Hot-formed performances of laser tailor-welded blanks fabricated with 22MnB5 (one hot-formed steel) and DP980 (one dual-phase steel) are investigated. Tensile tests, microscopic analysis, and hardness tests are used to evaluate the thermoforming properties after laser stitching. The results show that the tensile strength of the thermoformed samples was increased by 75%, and the elongation at break was decreased by 50%. The sample without thermoforming fractured on the 22MnB5 side, while the thermoformed sample fractured on the DP980 side. Moreover, the thermoformed DP980 side showed the disappearance of the sub-critical HAZ (heat-affected zone) with an average hardness increase of 42%, and the inter-critical HAZ still presents. The microstructure of the subcritical HAZ and the critical HAZ on the DP980 side transformed into martensite, bainite, ferrite, and less residual austenite. Therefore, the tissue is still softened. In contrast, the microstructure of the fine-grained HAZ and coarse-grained HAZ on the DP980 side consists of hardened zones of slate-martensite. Thanks to the gradual transformation of the microstructure of 22MnB5 from pearlite and ferrite to slate-martensite, the microhardness of the hot-formed steel reach 2.6 times that of the unhot-formed sample. This study provides a way to make layered composites suitable for crash-resistant and energy-absorbing components for lightweight automotive.