Investigation of microstructure and stress in laser-MIG hybrid welded S355 steel plates

12 mm thick S355 steel plates were butt welded using laser-metal inert gas (MIG) hybrid welding with 9 kW laser power. Microstructure and hardness distribution in hybrid welded joints were analyzed. A heat source model combined with upper double ellipsoid heat source and lower three dimensional conical heat source was proposed to represent the energy distribution of the combined arc-laser heat input. Sequential coupled thermal-mechanical analysis under welding speed of 1.0, 1.5 and 2.0 m/min was performed using SYSWELD finite element software. Influence of welding speed on temperature field and residual stress field was investigated. Microstructure in coarse grained heat affected zone (CGHAZ) under three welding speeds mainly consists of martensite which causes maximum hardness levels above 350HV in three welded joints. Peak welding temperature decreases and postweld cooling speed increases with increasing welding speed. Level of Von Mises equivalent stress is high with stress concentration occurs in HAZ. As welding speed increases, the magnitude of Von Mises equivalent stress, longitudinal stress, transverse stress and through-thickness stress increase. However, as the welding speed was increased from 1.5 m/min to 2.0 m/min, tensile stress magnitude of the three stress components increases slightly while compressive stress magnitude of the three stress components increases remarkably.