Fiber laser welding of hot stamping steel: effect of in situ annealing on the microstructure and mechanical properties

Abstract

Advanced high-strength steel (AHSS) is a class of promising materials for safe and lightweight structural applications in the transportation industry due to its high energy absorption capacity during severe plastic deformation. However, some types of AHSS tend to become brittle after welding and fail during forming operations. This paper proposes a novel method of laser welding AHSS class of 22MnB5 steel by induction-heating the samples in the bainitic range, which was identified via a theoretical analysis using finite element and physical metallurgy modeling. The calculated annealing temperature was 485 °C, and the subsequent laser parameters were set at a power of 800 W and a welding speed of 2 m/min. The welds performed on such pre-heated samples showed a bainitic microstructure consisting of bainitic ferrite along with uniformly distributed ultrafine carbide precipitates, in contrast to an almost fully martensitic microstructure conducted at room temperature. The average value of microhardness in the heat-affected zone thus decreased from ~ 670 HV without pre-heating to ~ 280 HV with pre-heating. The laser welds with pre-heating exhibited not only a slightly higher yield strength and elongation, despite a slightly lower ultimate tensile strength, but also a superior drawability with an Erichsen index increase by 28% compared with the welds without pre-heating.