Laser-arc hybrid welding combines a low-cost process-stable arc heat source with a highly efficient laser heat source, reducing the material demand for laser power and enhancing the material absorption rate of the laser. This increases the depth of fusion, enhances the welding speed, and improves the quality of the welding. However, due to the instability of the welding process, excessive spatter, humping, and porosity can occur, which seriously limit further development of this technology. The existence of pores inside the weld necessitates the use of instruments for detection, which increases the difficulty of detection and weakens the effective working section of the weld; this negatively impacts the strength and toughness of the joint. Studying the influence of porosity in composite welding is important for further understanding the physical process of laser-arc composite welding and the optimization of composite welding process parameters. In this study, the influence of laser-to-steam on composite welding keyhole porosity was investigated by linearly varying the laser power during the welding process. Subsequently, in combination with high-speed camera observation of the composite welding plasma morphology, the impact of the arc on the keyhole porosity was studied. Finally, considering the weld depth, weld width, weld formation and variation rules of porosity in the weld, the formation rules and influencing factors of keyhole porosity in the weld during the fiber laser-TIG hybrid welding were analyzed to establish the theoretical foundation for optimizing the laser-arc hybrid welding technology and understanding the energy coupling mechanism in laser-arc hybrid welding process.

The distribution of internal pores in the weld was first observed at different arc currents using a linearly varying laser power composite welding method. Then, the distribution of internal pores in the weld using fixed laser power composite welding and linearly varying laser power composite welding was compared, proving that the linearly varying laser power approach is feasible. Subsequently, the plasma morphology, surface morphology and internal porosity distribution of the weld were analyzed using a high-speed camera and an ultra-deep field microscope under different arc currents. Finally, the weld formation, weld depth, and weld width at different currents were compared by linearly varying the laser power.

Almost no pores are observed in the weld during fiber laser welding; there is only the effect of laser-induced steam on the molten pool of the keyhole wall in the deep penetrating keyhole. It is more conducive to improving the stability or maintenance of the keyhole than the existence of an arc. The keyhole does not collapse; therefore, the more laser-induced steam in the hole, the fewer pores in the weld (Fig. 2). A comparative experiment was conducted, and the results showed that the distribution trend of the pores in the weld was the same when the linearly varying laser power composite welding and the fixed laser power were used (Fig. 3). The greater the force of the arc on the molten pool (i.e., the greater the shielding gas flow rate), the more likely the deep penetration holes will be unstable and collapsed, and the more pores are in the weld (Figs. 6, 7). In the fiber laser-TIG hybrid welding, the penetration width and penetration depth are significantly improved compared with single fiber laser welding, and the surface forming quality of the weld and the spatter effect are significantly improved, as shown in Figs. 8, 9. When the laser power is between 0.8 and 2 kW, the laser power is roughly equivalent to the arc power, that is, the hybrid welding mode at this stage changes from the laser-assisted type to the laser-based type.At this time, the force of the arc acting on the convex liquid column on the rear wall of the small orifice cannot be ignored compared with the eruption of the steam caused by the laser in the hole. The force will press the convex liquid column behind the small orifice to the small orifice when the arc acts on the small hole and the force along the convex liquid column is too large (Fig. 11).

In the composite welding process, we obtained the variation law of small pores in the weld with the laser power, when the laser power increases linearly from 0 to 3 kW within 2 s. No pores were observed inside the weld when the laser power was less than 0.8 kW and more than 2.2 kW. When the laser power was approximately 0.8-2.2 kW or the shielding gas flow rate was more than 15 L/min, more pores were seen inside the weld. These pores were distributed in the middle of the weld. The introduction of the TIG arc significantly improved the surface formation of the weld seam in fiber laser welding and substantially increased the melt depth and melt width. When the laser power was 2 kW, the TIG arc at 150 A increased the depth width by 94% and the melt depth by 35%. In fiber laser-TIG arc hybrid welding, the formation of small hole-type pores in the weld is caused by the arc acting on the small orifice, which results in instability and collapse of the small hole. Increasing the shielding gas flow can significantly increase the porosity in the weld. The change in laser-induced evaporation in the hole caused by the change in laser power is the main factor that causes the change in the number of pores in the hybrid welding seam.