## 4 Results and Discussion
At this time, with the increase of keyhole depth, resistance such as surface tension gradually increases, and the growth of keyhole depth slows down. After 30 ms, the recoil pressure and resistance reach a dynamic equilibrium, and the keyhole enters a stable oscillation state. In this stage, the keyhole depth varies regularly within a certain range. Analyzing the keyhole depth variation curve under different laser powers, it is found that increasing the laser power can shorten the time for the keyhole to enter the oscillation state. In addition, as the laser power increases, the keyhole depth also increases. Increasing the laser power can effectively increase the energy in the keyhole and increase the recoil  
Figure 12: Keyhole evolution at laser power 7.5 kW. pressure. The increased recoil pressure increases the keyhole depth and the increasing trend of the keyhole depth. Figure 14 shows the variation of the molten pool depth below the keyhole with time for three laser powers. The results show that the depth of the molten pool increases rapidly and linearly from 0 ms to 8 ms. At 8-30 ms, the molten pool growth rate gradually decreases, and it is inclined growth at this time. After 30 ms, the molten pool depth no longer changes and enters a state of equilibrium. The changes of the molten pool depth in each stage correspond to the changes of the keyhole depth in each stage. As the laser power increases, the depth of the molten pool under the keyhole increases. The multiple reflections of the laser in the keyhole promote the absorption of the laser by the keyhole. With the increase of laser power, the depth of the keyhole increases, the laser energy is delivered to the deeper layer of the workpiece, and the melt pool depth increases [29]. Figure 14: Depth of the melt pool below the keyhole at different laser powers. Figure 4:0 Keyhole depth for different laser powers. Figure 15 characterizes melt flow in a longitudinal section of a hybrid weld using various laser powers. The molten pool below the keyhole did not completely penetrate the workpiece at three laser powers, but the molten pool behind the keyhole did, which is consistent with the representation in Figure 4:1 As shown in Figure 15, the molten metal on the surface of the molten pool flows from the center of the arc to the rear of the molten pool, creating backflow under the influence of surface tension. Due to the impact of the molten droplet, the center of the arc produces an obvious depression zone. When the laser power is 7 kW, the molten metal in the depression area flows downward and produces backflow in the necking area. When the laser power is increased to 8 kW, the backflow gradually disappears, and the molten metal flows partly to the rear of the melt pool and partly to the melt pool below the keyhole. The width of the arc zone increases, and the depth of the laser zone increases. In the lower laser zone of the molten pool, the melt flows downward in the melt pool below the keyhole and backward and downward at the bottom of the molten pool. This action transfers energy deeper into the unmelted region, and eventually, the bottom metal melts. Combining the temperature profile and velocity profile of the \(\mathrm{D_{L}}\) line, it is found that as the laser power increases, the temperature at the bottom of the molten pool increases, and the melt downward flow rate increases, prompting the mixed melt pool to completely penetrate the workpiece. Figure 16 shows the measurement position of the microstructure. Four regions, D1, D2, D3, and D4, are selected on the cross section of the weld. Figure 17 shows the microstructure of the welded joint under the laser power of 7.5 kW. It can be seen from Figure 17a,c that the weld structure is a columnar solidified structure. In addition, the weld is mainly composed of bainite and ferrite. During laser-MIG hybrid welding of AH36 steel, the temperature of the molten pool rises rapidly and exceeds the melting point in a short time.