Compared with laser welding and arc welding, laser-arc hybrid welding not only inherits the advantages of laser welding and arc welding but also makes up for respective shortcomings. Thus, it is an advanced welding process method with great application prospects. With the continuous development of laser technologies, laser power has exceeded 10 kW or even higher. Therefore, in order to make the development of lasers well meet the need of actual industrial production, the basic theoretical research on high-power laser-arc hybrid welding has been a hot spot in the academic community in recent years. Researchers have carried out a lot of research on the interaction mechanism between laser and arc. However, the laser power involved was mostly below 5 kW. There are few reports on the mechanism regarding the effect of a high-power (higher than 5 kW) laser on the droplet transfer in laser-arc hybrid welding. Therefore, in this study, a high-power (7.5 kW) laser is introduced into the different modes of arc [standard metal active-gas(MAG), cold metal transfer (CMT), and pulsed arc] welding process, and its effects on droplet transition, weld forming and welding efficiency are compared and studied by using high-speed camera, optical microscope, etc.
In this study, a high-power laser-arc hybrid welding platform was built, which mainly consisted of a continuous fiber laser, a welding system, a manipulator arm, and a high-speed camera system. The high-power laser-arc hybrid welding experiments were carried out on 10 mm thick Q345 steel, and the laser used in the test was a fiber laser (maximum output power of 12 kW), with an output laser wavelength of (1080±10) nm and a focused spot diameter of 0.2 mm. Before the welding test, an angle grinder was first used to grind the surface to be welded, and then the ground surface was cleaned with alcohol. The arc-guided laser-arc welding was chosen for obtaining a stable droplet transition process. In order to further understand the influence of a high-power laser on droplet transition in different modes of arc welding, the laser was coupled with three different arc modes (standard MAG, CMT and pulsed arc). The welding shielding gas used in the welding process was the Ar and CO2 mixture with a flow rate of 20 L/min. During the welding process, a high-speed camera was used to track and monitor the droplet transition behavior with a frame rate of 10000 frame/s. In order to obtain a clear droplet transition image, an infrared filter was added to the camera lens before the experiment began. Image pro plus software was used to process the pictures taken by the high-speed camera, and the droplet transition mode and the number of droplet transitions within 500 ms under each parameter were counted, so as to calculate the droplet transition frequency within 1 s. After welding, the forward and cross-sectional morphologies of the weld were observed by optical microscope.
The high-power laser has a significant effect on the droplet transition mode of arc welding in different arc modes. During standard MAG welding, the high-power laser attracts and compresses the arc, resulting in a significant reduction in arc length. Meanwhile, metal vapor and plasma ejected from the keyhole reduce the droplet transition frequency (Figs. 6 and 7). In the case of CMT welding, the high-power laser extends the single short-circuit transition period, and the resulting molten pool oscillation reduces the stability of the short-circuit transition (Fig. 8). Regarding the pulsed arc welding process, the high-power laser increases the melting rate of the welding wire. In the meantime, the droplet transition mode changes from the droplet transition to the jet transition, and the droplet transition frequency is significantly increased. The air flow at the key hole hinders the droplet transition, so that the droplet transits to the side of the molten pool (Figs. 11 and 12). Compared with that during arc welding, the weld melting width increases during laser-standard MAG and laser-pulsed arc hybrid welding, while no obvious change in weld width is observed in the case of laser-CMT hybrid welding. The residual height of welds in laser-standard MAG and laser-CMT hybrid welding decreases significantly, while the residual height of welds in laser-pulsed arc hybrid welding increases slightly. This is attributed to different degrees of influence of the laser on the droplet diameter and transition frequency in three different modes of arc welding. Furthermore, the melting energy increment value (ψ) of laser-arc interaction varies under different hybrid welding conditions, among which laser-pulse arc welding has the highest ψ value (36%), followed by laser- standard MAG welding (19%), while laser-CMT welding has the smallest ψ value (-12%).
In this study, the effects of laser (7.5 kW power) on droplet transition and weld formation in different modes of arc welding were investigated. The results reveal that the addition of laser has a significant influence on the droplet transition in standard MAG, CMT and pulsed arc welding processes. During standard MAG welding, the high-power laser attracts and compresses the arc, resulting in a significant reduction in arc length, and the metal vapor and plasma ejected from the keyhole reduce the droplet transition frequency. In the CMT welding process, the laser extends the single short-circuit transition cycle, and the melt pool oscillation caused by the high-power laser reduces the stability of the short-circuit transition. Regarding the pulsed arc welding process, the addition of a high-power laser increases the melting rate of welding wires. The droplet transition mode changes from the droplet transition to the jet transition, and the droplet transition frequency increases. Meanwhile, the air flow at the key hole hinders the droplet transition, so that the droplet transits to the side of the molten pool. Compared with arc welding, the weld melting width increases during laser-standard MAG and laser-pulsed arc welding, while no obvious changes in weld width are observed in the case of laser-CMT hybrid welding. The residual height of the welds in laser-standard MAG and laser-CMT hybrid welding decreases significantly, while the residual height of welds in laser-pulsed arc hybrid welding increases slightly. The melting energy increment values of the interaction between laser and arc under three arc modes are: laser-pulsed arc hybrid welding (36%), laser-standard MAG hybrid welding (19%), and laser-CMT hybrid welding (-12%).
