Stability Study Of The Dynamic Process Of Laser-arc Composite Welding Based On The Transition Behavior Of The Melt Droplet

Laser-Arc Composite Welding
Professor Peilei Zhang of the School of Materials Science and Engineering at the University of Shanghai for Engineering and Technology (USET), together with scholars from the University of Warwick, Shanghai Jiao Tong University and Jiangsu University of Science and Technology (JUST), have published a paper titled "Research Status of Stability in Dynamic Process of Laser-Arc Hybrid Welding" in the journal Coatings. Laser-Arc Hybrid Welding based on Droplet Transfer Behavior: A Review".
01 Introduction
Laser-Arc Hybrid Welding utilizes both a laser heat source and an electric arc heat source in the same region, and the synergistic effect of the two heat sources in the same molten pool results in an increase in weld speed and depth of fusion, as well as an increase in gap bridging capability and process stability. This article describes the current research status of laser-arc composite welding technology in terms of droplet transition behavior, droplet transition mode and droplet force analysis. By systematically sorting out the research papers and engineering applications, the working principle, technical advantages, engineering applications, and welding dynamic process stability studies of laser-arc composite welding are systematically outlined. Finally, the problems facing the future of laser-arc hybrid welding technology are summarized.
Laser-arc Hybrid Welding
02 Overview
This paper reviews the basic concepts and characteristics of the transition behavior of laser-arc composite welding droplets, including the droplet transition mode and droplet force analysis. Emphasis is placed on the mutual physical interaction between the laser and the arc and the effect of the combined laser-arc heat source on the weld stability. The melt drop transition behavior provides information on the stability of the welding process, arc behavior characteristics, melting efficiency, process characteristics such as welding fume and spatter, and welding metallurgical characteristics, etc., which is characterized by intuitiveness and visibility, and becomes an irreplaceable source and way of information acquisition in welding information technology. In the laser - arc composite welding process, the metal droplets to the molten pool transition mode, droplet size, transition frequency and stability depends on the welding material properties, welding parameters, shielding gas, laser energy, light wire spacing and other factors, and ultimately subject to a variety of forces such as gravity, electromagnetic force, plasma flow force, surface tension, the metal vapor force and other integrated role, as shown in Figure 1.

Figure 1 composite welding droplet force analysis schematic diagram
Laser-arc Hybrid Welding
03 Graphical Analysis
The filament spacing is a key factor in determining whether the laser and arc heat source are optimally coupled. The filament spacing has a significant effect on the depth of fusion, the transition mode of the droplet and the stability of the welding process. Researchers have found that when the filament spacing is small, the arc interferes with the stability of the keyhole, and the stability of the drop transition is greatly affected by the laser. The laser beam irradiation on the splattered droplets impedes the energy of the laser beam, resulting in a shallow depth of weld as shown in Fig. 2. In addition, the increase in filament spacing resulted in irregular molten metal movement leading to keyhole collapse, as well as previously solidified metal preventing the center molten metal from filling the weld toe area, ultimately resulting in a nibbling defect.
The relative positions of the laser and arc heat sources in the welding direction have a crucial effect on laser-arc composite welding. Some researchers and scholars believe that the laser-guided mode is superior to the arc-guided mode. They believe that the laser-guided mode results in a more stable welding process, better weld formation, fewer porosity and spatter defects, and better penetration and stronger welds. However, other researchers believe that the arc-guided mode is superior to the laser-guided mode. They believe that compared with the UHP laser-guided mode, the UHP arc-guided mode creates stable arc characteristics and melt pool flow, and the angle between the radius of the molten droplets and the weld surface creates a greater driving force, which promotes the separation of the molten droplets and improves the stability of the welding process, with less weld spatter and more stable weld shaping.
The importance of shielding gas should be considered for both single laser welding and electric arc welding. In laser welding, shielding gas is an effective means of eliminating plasma shielding effects, improving the stability of the welding process, and realizing deep fusion welding. In arc welding, shielding gas is the key factor to realize stable combustion of the arc and determine the distribution of the arc heat column and the transition mode of the molten droplet. Researchers believe that the addition of 30% He improves the compound effect of laser and arc, and the melt drop transition mode changes from unstable jet transition to stable jet transition, and improves the matching degree of rotating jet transition and arc pulse cycle, with less fluctuation of arc waveform, better weld seam shaping, and fewer welding defects. In addition, researchers believe that the He volume percentage to improve the weld depth of fusion and inhibit porosity defects should be 50%. The effective laser power density increases with the increase of He volume percentage, which contributes to the increase of weld depth of fusion. Weld porosity defects were effectively suppressed because the stability of small holes was improved while using Ar-He mixture.
Since the addition of laser to arc welding causes changes in arc morphology and molten pool morphology, leading to changes in arc force, electromagnetic field, and surface tension of the molten pool, changes in these factors will directly lead to changes in the transition characteristics of the molten droplet. Combining the advantages of deep melting of laser welding and bridging performance of arc welding, many researchers and scholars have paid attention to the transition behavior of molten droplets in laser-arc composite welding. They believe that the addition of laser has both promoting and inhibiting effects on the droplet transition. In the short-circuit and drop transition modes, the laser promotes the drop transition, while the laser hinders the drop transition in the jet transition mode. The magnitude and direction of the electromagnetic and plasma forces acting on the droplets are critical in influencing the droplet transition behavior. The magnitude and direction of the electromagnetic and plasma forces are transformed due to the change of the current distribution in the molten droplet, which is caused by the laser-induced plasma with low ionization potential.

04 Conclusion and Outlook
Laser-arc composite welding is a new type of welding processing method, which combines two heat sources with completely different energy transfer mechanisms and physical properties, and at the same time acts in the processing position, the interaction between different heat sources and the interaction between the heat source and the workpiece to generate enough heat to complete the welding process. As a new type of efficient welding heat source it can give full play to the respective advantages of the two heat sources, but also to make up for their shortcomings. Laser and melting electrode inert gas / active gas arc welding (MIG / MAG) is the most promising composite welding mode, there is an urgent need to further study the physical mechanism of the composite heat source. Meanwhile, the melt drop transition behavior is also very important in the composite welding process. The droplet transition behavior can provide accurate information for the welding process, and effectively determine the stability of the welding process.
With the continuous development of construction machinery, the thickness of plate is also increasing. In order to meet the stability of thick plate welding, beveling of thick plates is essential. Due to the complexity of the thick plate bevel, the stability of the arc in the welding process is also affected to a certain extent, resulting in the generation of welding defects. At the same time, the generation of defects is closely related to the transition behavior of the melt droplet. In high-power laser-arc composite welding, the generation of welding defects is inevitable. The optimization and progress of numerical simulation technology breaks through the limitations of defect analysis and provides a solid theoretical basis for the further development of innovative processes. Due to the large number of process parameters in laser-arc composite welding, the process parameter window is constantly narrowed in order to obtain the best weld formation, and the variation of the process parameters also has a great impact on the transition characteristics of the molten droplet. Therefore, the continuous exploration of process parameters is of great significance to the laser-arc composite welding melt drop transition mode.