Ultrahigh-power laser welding is an important development direction for plates with medium-thickness welding. The laser-arc hybrid welding method has obvious advantages in improving the appearance, quality, and efficiency of the weld. Therefore, the 10 kW level high power laser-arc hybrid welding technology has developed rapidly. However, when the laser power reaches more than 10 kW, the vaporization behavior of the materials, the interaction between the laser beam and plasma, the stable state of the molten pool flow, the mechanism of heat transmission, and the metallurgical behavior of the weld all change to different degrees, which will affect the stability of the welding process, leading to a poor appearance of the weld and generation of weld defects, and seriously limiting the popularization and application of 10 kW laser welding. The variation in the plasma morphology during the welding process indirectly reflects the stability of the welding process. In this study, the characteristic parameters are collected, which reflect the plasma morphology and appearance of welds of three different hybrid welding methods with different laser powers: laser-MAG single-wire hybrid welding, laser-MAG single-wire hybrid welding with filler wire, and laser-MAG double-wire hybrid welding, to seek the characteristic parameters for predicting the quality of welds and providing reference values for ultra-high-power laser-arc hybrid welding with different heat sources.

laser-MAG single-wire hybrid welding, laser-MAG single-wire hybrid welding with filler wire, and laser-MAG double-wire hybrid welding. The weld width and penetration were extracted when the laser power increased from 5 kW to 30 kW. Then, the plum and spatter, which were produced in the welding process and investigated by a high-speed camera, the plasma diffusion height, area, and plasma splash area with different laser powers were extracted for the three welding methods. The goal is to explore the relationship between the size of the weld and the morphological characteristics of the welding plasma for different welding methods and laser energy, which lays the foundation for 10 kW high power laser-arc hybrid welding.

As shown in Figure 4, the weld face of the three welding methods becomes worse with the increase in laser power, especially when the laser power is 20 kW. The appearance of the weld changes differently, and the differences among the three welding methods are gradually highlighted. The increase in the feature size of the weld is proportional to the increase in the laser power, but the relationship is not linear. Before and after the laser power reaches 20 kW, the increase in the weld feature size decreases slightly, and concave-convex points appear in the size curve; when the power is the same, the penetration of the laser-MAG single-wire hybrid welding is small, while that of the laser-MAG single-wire hybrid with filler wire is large. The former increases slightly with an increase in laser power, whereas the latter increases significantly. The variation law of the weld width with laser power is similar to that of penetration, and the weld size curve of the laser-MAG double-wire hybrid welding method is always in the middle position, as shown in Figure 6. For the three welding methods, the plasma area and the fluctuation increase with an increase in the laser power, and the variation trend of plasma fluctuation is the same as the fluctuation of penetration and the fluctuation of plasma spatter, but the fluctuation of weld width is smaller, as shown in Figures 9 and 11.

Three different welding methods were used to explore the regular appearance of the weld and plasma morphology with different laser powers. The results showed that when the power was increased, the plasma area and fluctuation of the three welding methods increased, and the weld width, penetration, and fluctuation values increased. When the power was increased to 20 kW, the increment in the plasma area and fluctuation decreased, the increment in the weld size decreased, the maximum increment of weld penetration for laser-MAG single-wire hybrid welding decreased by 71.64% compared with the other two welding methods, and the appearance of the weld worsened. In addition, when the power was constant, compared with laser-MAG single-wire hybrid welding, the plasma area and standard deviation increased, the penetration depth decreased, and the appearance of the weld deteriorated. When laser-MAG double-wire hybrid welding was adopted, the changes in the plasma morphology and appearance were not obvious. When the power was increased to 20 kW, the increment in the amplitude of the variation decreased. In addition, there is a correlation between the appearance of the weld and plasma morphology. The plasma morphology is related to the laser power and wire feeding mode: when the laser power increases or the filler wire is added, the plasma concentration in the incident direction of the laser increases, the stability worsens, and the attenuation and interference of the laser enhance, which leads to a decrease in penetration and an increase in the spatter. Therefore, the change in plasma shape can be used as a reference to predict the appearance quality of the weld.