With the development of high-power fiber lasers, laser energy has increased from 100 W a few years ago to 10000 W or even 100000 W. Bottlenecks such as high cost, low efficiency, and limited penetration that previously restricted the development of laser-arc hybrid welding are expected to be overcome. Compared with traditional welding, laser-arc hybrid welding has the advantages of high efficiency, good weld quality, high degree of digitization, and environmental friendliness. However, the welding mechanism associated with thick plates is relatively complex, and welds are prone to defects such as splashes, pits, and pores, which degrade the welding quality significantly. Therefore, research on the suppression of defects during the welding process is particularly important. In this study, laser-arc hybrid welding experiments under different arc powers were conducted to clarify the mechanism(s) causing welding spatter, and the effect of arc power on the droplet transfer behavior and welding spatter was studied using high-speed camera technology.
Laser-arc hybrid welding was performed on a 10 mm Q345 steel plate using a welding system consisting of a 12 kW fiber laser and SKS welding systems equipment. A high-speed camera was used to observe the welding process. The laser wavelength was (1080±10) nm with a nominal focusing spot of 0.2 mm. The angle between the laser beam and the electrode axis was 45°, and the arc torch was applied in a tilted leading position. The laser power and defocus distance were 7.5 kW and 0 mm, respectively. The shielding gas was 90% Ar+10% CO2, which was injected at a flow rate of 20 L/min. The droplet transfer mode and number of droplet transfers within 500 ms under each parameter were counted to calculate the corresponding droplet transition frequency within 1 s. The images were binarized and filtered to obtain statistics regarding the diameter and number of splashes within 100 ms for each experimental condition. The metallographic samples were prepared by cutting, grinding, polishing, and etching the Q345 plate with 4% nitric acid alcohol. Finally, a visual microscope was used to observe the cross-sectional morphology of each weld.
Arc power significantly affects the weld morphology and droplet transfer behavior in laser-arc hybrid welding (Table 3). When the arc power was 4096 W and 7986 W, a deep depression was apparent on the weld surface (Fig. 2), whereas the weld surfaces were sound and smooth without undercut or underfill defects when the arc power was 6860 W. The form of the droplet transfer mode changes as the arc power increased (Fig. 3). With an increase in arc power, the droplet transfer mode gradually changed from the hybrid transfer mode with short-circuiting transfer to the single spray transfer mode. The former includes a hybrid transfer mode consisting of globular transfer, projected transfer, spray transfer, and short-circuiting transfer (Fig. 4). When the arc power is low, the droplet is separated from the welding wire by gravity, and the transfer frequency of the droplet is low. With an increase in arc power, the heat at the tip of the welding wire increases, the surface tension of the droplet decreases, and the droplet is gradually refined. Moreover, the electromagnetic and plasma flow forces promote droplet transfer, and the droplet transfer frequency increases significantly. Furthermore, the short-circuiting transfer in the overall hybrid transfer process is the main cause of spatter, and the amount of spatter during the welding process increases with an increase in the droplet transfer frequency. At low arc power, the attraction of the droplet by the laser can promote globular transfer (Fig. 6). In the projected transfer mode, the vapor plume at the keyhole promotes the occurrence of a short-circuit transition (Fig. 9). In the spray transfer mode, the vapor plume at the keyhole changes the flight trajectory of the droplet, resulting in the generation of large particle splashes with diameters close to the droplet diameter (Fig. 14).
In this study, the effect of arc power on weld morphology, droplet transfer behavior, and welding spatter in high-power laser-metal active gas (MAG) hybrid welding was investigated. The results revealed that oversized or undersized arc power could lead to a deep depression on the weld surface, whereas the weld surfaces were sound and smooth without undercut and underfill defects when the arc power was 6860 W. When the arc power was lower than 6860 W, the droplet transfer mode was a hybrid transfer mode, and the corresponding short-circuiting transfer caused the generation of spatter. With an increase in arc power, the droplet transfer frequency increased, leading to a larger number of spatters. In the single spray transfer mode, the vapor plume that erupted at the keyhole may promote the generation of spatter.
