## 4 Results and Discussion  
Figure 4 shows the weld pool morphology and melt flow of welded joints under different laser-arc spacing. It can be seen that when other welding parameters are the same (laser power is 7.5 kW) and the laser-arc spacing is in the range of 1-3 mm, the laser-arc spacing is increased, and the composite pool length is increased, too. When the laser-arc spacing is 1 mm, the maximum velocity below the keyhole is 2.2 m/s. When the laser-arc spacing is 2 mm, the maximum velocity below the keyhole is 1.4 m/s. When the laser-arc  
Figure 3: Thermophysical properties of AH36 steel: (**a**) density, (**b**) heat capacity, (**c**) thermal conductivity, (**d**) viscosity. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Property** & **Symbol** & **Unit** & **Value** \\ \hline Solid density & \(\rho\)s & kg\(\cdot\)m\({}^{-3}\) & 7300 \\ Liquid density & \(\rho\)l & kg\(\cdot\)m\({}^{-3}\) & 6960 \\ Solid temperature & Ts & K & 1670 \\ Liquid temperature & T1 & K & 1780 \\ Vaporization temperature & \(Tg\) & K & 3100 \\ Heat conductivity & \(\lambda\) & W/(m\(\cdot\)K) & 44 \\ Heat capacity & \(Cp\) & J/(kg\(\cdot\)K) & 630 \\ Dynamic viscosity & \(\eta\) & kg/(m\(\cdot\)s) & 0.0064 \\ Latent heat of fusion & \(Hf\) & Hf & 2.26 \(\times\) 10\({}^{5}\) \\ Ambient temperature & \(Tref\) & K & 300 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Material properties of AH36 steel. spacing is 3 mm, the maximum velocity below the keyhole is 1.4 m/s. Thus, the smaller the laser-arc spacing, the greater the downward melt flow rate. According to Figure 4, when the laser-arc spacing is 1 mm, the melt flow rate between the two heat sources is about 1.8 m/s. Therefore, the laser weld pool and the arc weld pool can be joined together earlier to form a composite weld pool. However, the laser has a great influence on the arc and reduces the stability of the droplet transfer. In addition, the arc interferes with the keyhole and decreased keyhole stability. When the laser-arc spacing is 3 mm, the melt flow rate between the two heat sources is about 0.5 m/s. Because the distance between the arc and the laser is too far to couple well together, they almost form two molten pools. However, when the laser-arc spacing is 2 mm, the melt flow rate between the two heat sources is about 1 m/s. The composite molten pool can be connected together quickly, the droplet transition is stable, and the keyhole is stable. The laser beam was in front of the welding torch in the laser-MIG compound welding experiment. First, the laser melted the metal surface in the welding direction, and the molten droplets were transferred directly to the molten metal. During the welding process, the arc remained stable, and the spatter, which was caused by the molten drop transition, was reduced. Figure 5 shows the weld morphology of AH36 steel under different laser powers. It can be seen that when the laser power is 7 kW, the weld bottom is narrow, the fusion line is discontinuous, the weld depth is insufficient, and the formability is poor. When the laser power is 7.5 kW, the welded joint has better effect. When the laser power is 8 kW, there is an obvious edge-biting phenomenon at the top of the weld, the bottom of the weld reinforcement is too high, and there is an obvious collapse phenomenon. The results show that the welding joint bottom forming is best when the laser power is 7.5 kW under three laser powers. Figure 4: Morphology and flow rate of molten pool with different laser –arc spacing: (**a**) 1 mm, (**b**) 2 mm, (**c**) 3 mm. Figure 5: The shape of weld seam with different laser powers. To ensure the accuracy of the simulation results, the simulated and experimental results with different laser powers were compared. Figure 6a-c shows the simulated weld cross-sectional profiles with experimental results for laser powers of 7 kW, 7.5 kW, and 8 kW, respectively. \(W_{\text{A1}}\), \(W_{\text{A2}}\), \(W_{\text{A3}}\), \(W_{\text{a1}}\), \(W_{\text{a2}}\), and \(W_{\text{a3}}\) denote the widths of the upper surface of the melt pool.