Effect of welding speed on microstructure and mechanical behavior of laser welded Al-Si coated 22MnB5 steel

Abstract

22MnB5 steel is a high-strength steel which can realize weight reduction and safety of car body. Al-Si coating provides excellent barrier protection for 22MnB5 steel against oxidation and decarburization during hot-stamping, but it deteriorates the weldability of the steel plate. In this paper, the effect of Al-Si coating on the weld bead formation, aluminum (Al) element distribution, the microstructure and mechanical properties of laser welded 22MnB5 steel joint was studied under various welding speed. When the welding speed was increased from 2.4 m/min to 4.8 m/min, the concavity depth of the weld surface was decreased for a given laser power of 2000 W. A flat weld bead was obtained at the welding of 4.2 m/min. During laser welding and hot stamping process, the Al element in the fusion zone (FZ) was not inhomogeneously distributed in a form of Al segregation along the fusion line. After hot stamping, the location of α-ferrite in the FZ corresponded to the Al element distribution location, and long strip α-ferrite was formed at the Al segregation. As a result, the weld was soften and the average hardness value in the FZ was only 420HV, which was 83.6% of the base metal after hot stamping (HSBM). The stress concentration at the long strip α-ferrite was the source of crack initiation and the α-ferrite in the FZ further accelerated the crack propagation along the fusion line. The tensile strength and elongation of the welded joint under the optimized parameters were 1459.65 MPa and 2.0% respectively, which were only 91.0% and 29.0% of the HSBM (1603 MPa, 6.9%).