Research on microstructure and properties of boron/Q235 steel laser welded dissimilar joints under synchronous thermal field

The mechanical mismatch effect frequently occurs in the dissimilar materials welded joints, thus leading to plastic gradient at the interface between the weld and heat-affected zone (HAZ). In this work, the boron steel and Q235 steel were selected for laser tailor welding, which obtained boron/Q235 steel tailor-welded blanks (TWBs). The method of welding with synchronous thermal field (WSTF) was utilized to eliminate the mismatch effects in TWBs. The WSTF was employed to adjust cooling rates of welded joints, thereby intervening in the solidification behaviors and phase transition of the molten pool. Boron/Q235 steel was welded by laser under conventional and WSTF (300−600 ℃) conditions, respectively. The results show that the microstructure of weld and HAZ (boron) was adequately transitioned to ferrites and pearlites instead of abundant martensite by WSTF. Meanwhile, the discrepancy of microhardness and yield strength between various regions of welded joints was greatly reduced, and the overall plasticity of welded joints was enhanced by WSTF. It is indicated that WSTF can effectively contribute to reducing plastic gradient and achieving mechanical congruity in welded joints by restraining the generation of hard-brittle phase, which could significantly improve the formability of TWBs in subsequent hot stamping.