Keyhole development and the effects on weld formation and mechanical properties of Laser-MIG hybrid welded bottom-locking joints with assembly gaps

The bottom-locking joints are widely applied in high-speed trains. A numerical simulation model is developed to analyze the metal flow and keyhole dynamics in laser-MIG hybrid welding processes with double-direction assembly gaps. An innovative treatment method for surface tension is proposed to address the issue of spontaneous gap closure. The experimental tests are also conducted to investigate the effects of double-direction gaps. The weld morphology, mechanical properties, metal flows and keyhole dynamics are compared with different gap conditions. The results indicated that the penetration depth increased with the increase of the vertical gap size. The downward development of the laser keyhole was enhanced when the vertical gap enlarged. However, the weld depth decreased with the increase of horizontal gap size. The molten metal flowed into the horizontal gap, and the downward keyhole development was hindered. When the vertical gap or horizontal gap exceeded 0.6 mm, the collapse or incomplete fusion defects were formed, respectively. These defects negatively affected the tensile properties of welded joints. The weld formation was discussed when the two kinds of gaps existed simultaneously. This study provided a comprehensive understanding of metal flow behavior and will offer valuable guidance on welding CFD analysis with double-direction gap conditions.