Formation mechanism of undercut in laser-MAG hybrid horizontal welding: Effects of heat source arrangement

Laser-arc hybrid welding provides a feasible solution for high efficiency, high-quality joining of medium-thick high-strength steel plates in construction machinery applications. However, practical industrial applications face challenges such as undercut defects. This study systematically investigates the effects of heat source arrangement in laser-MAG hybrid horizontal welding, including the leading mode, laser-arc distance (DLA), and wire feeding angle on droplet transfer, molten pool behavior, and undercut formation mechanisms. Using high-speed imaging technology and experimental characterization analysis, this research reveals the mechanisms by which different heat source arrangements influence undercut formation. The results demonstrate significant differences in the process windows between different leading modes. In the laser-leading mode, weld formation is highly sensitive to the wire feeding angle: higher or lower angles lead to undercut at the upper or lower edges, respectively. In contrast, the arc-leading mode is more susceptible to the influence of the DLA. Furthermore, undercuts are primarily driven by two mechanisms: insufficient filling due to poor force balance in the molten pool and localized inadequate filling caused by molten pool dynamic instability. Through optimization of the heat source arrangement, the welding stability and weld quality of laser-MAG hybrid horizontal welding can be significantly improved, thereby preventing the formation of undercuts. This study presents an in-depth analysis of the formation mechanism of undercuts in laser-MAG hybrid horizontal welding, providing theoretical foundation and technical support for reliable, high-quality welding of medium-thick high-strength steel plates.