## 4 Results and Discussion
Figure 10 shows the temperature versus velocity variation at the widest part of the molten pool surface from the center to the edge, perpendicular to the welding direction, at the simulated 35 ms. As shown in Figure 10a, the molten pool temperature gradually  
Figure 8: (**a**) Laser power share, (**b**) laser zone area share. Figure 9: Simulation of melt pool at 35 ms.  
decreases from the center of the arc to the boundary. When the laser power is 7.5 kW, the molten pool surface temperature is higher than when it is 7 kW and 8 kW. As shown in Figure 10b, the melt flow rate on the molten pool surface decreases from the center of the molten pool to the edge of the melt pool. At laser power of 7.5 kW, the molten pool surface flow rate is higher than that at 7 kW and 8 kW. The flow rate below the arc is maximum under the effect of molten droplet impact and surface tension. In general, as the laser power increases, the molten pool absorbs more energy, the temperature increases, and the Marangoni force in the molten pool increases [19], leading to a more pronounced Marangoni effect and an increase in the melt flow rate. In this experiment, when the laser power is increased to 8 kW, the workpiece is collapsed, and some energy is shifted downward to the collapse. The molten pool energy is shifted downward, the energy in the upper arc region of the melt pool is reduced, the surface temperature of the molten pool decreases, and the molten pool rate decreases. Figure 11 shows the temperature versus velocity variation of the molten pool directly below the keyhole along the depth direction at the simulated 35 ms. As shown in Figure 11a, the molten pool temperature below the keyhole gradually decreases as the depth increases. The molten pool temperature at laser power 7.5 kW is higher than that at 7 kW and 8 kW. As shown in Figure 11b, the melt at the edge of the keyhole zigzagged downward, and the melt below the keyhole flowed straight downward. The high-energy-density laser light is refracted in the keyhole several times, which promotes the absorption of laser energy by the keyhole. As the laser power increases to 7.5 kW, the melt flow rate increases below the keyhole, the melt pool energy shifts downward, and the temperature increases at the same depth. As the laser power increases to 8 kW, the workpiece is collapsed, the melt pool energy continues to move down to the collapsed area, and the loss of laser energy causes the melt pool temperature to decrease. Figure 10: At simulated 35 ms: (**a**) Temperature of WA line, (**b**) speed of WA line. Since the main driving force for keyhole formation is the recoil pressure of the metal vapor generated by the high-energy-density laser, the laser power is the key factor affecting the lock hole depth [29]. Figure 12 shows the evolution of the keyhole at a laser power of 7.5 kW. At T = 2 ms, the laser acts on the workpiece, and the temperature in the laser radiation zone increases rapidly. At T = 3 ms, the temperature of the laser radiation area increased to 3100 K, and the metal vapor was generated. Due to the generation of metal vapor, the metal surface is recessed, and the recess becomes deeper and deeper under the action of recoil pressure, eventually forming a keyhole. As the welding process progresses, the keyhole size increases. The keyhole depth grows more pronounced than width. After the size of the keyhole increases to a certain extent, it will no longer increase and enter the oscillation stage. As shown in Figure 13, the transient evolution of the keyhole depth at three laser powers was obtained by simulation. It can be found that the keyhole depth increases rapidly and linearly from 0 ms to 8 ms. At this time, the recoil pressure is much higher than the surface tension, buoyancy, and other resistances that prevent the formation of the keyhole. At 8-30 ms, the growth trend of keyhole depth decreased, showing an oscillating growth state.