Welding heat source factor
From the above research, we can see that the current arc heat sources used in WAAM can be divided into three basic categories: GMAW, tungsten gas shielded welding (GTAW) and PAW. In particular, GMAW-based cold metal transition technology (CMT), because of its ultra-low heat input, droplet transfer without splashing, and stable arc, is currently a widely used heat source. The difference in the output characteristics of the arc heat source leads to a large difference in the forming size and efficiency of WAAM [30].
No matter what kind of arc heat source, the initial positioning of the design is for welding rather than WAAM. In view of the "complex trajectory, dynamic thermal diffusion, weakly constrained molten pool" and other disturbance conditions in WAAM, the arc heat source is required to dynamically and freely match heat and mass transfer, which is difficult to achieve with traditional arc heat sources. In recent years, some multi-electrode arc heat sources have been proposed one after another, such as double-electrode gas shielded welding (DE-GMAW) combining the bypass non-melting electrode arc and the main circuit arc electrode, and the bypass melting electrode arc and the main circuit non-melting Polar arc phase combined bypass arc coupled plasma welding Arcing-wire PAW / Variable Polarity Plasma Welding (VPPA) / GTAW, etc. By reasonably distributing the main and bypass currents, the substrate heat input and welding wire melting Decoupling control. In addition, the dual-wire dynamic three-arc welding method proposed by Geng Zheng et al. Of Harbin Institute of Technology and the cross-coupled arc welding method proposed by Chen et al. of Beijing University of Technology can achieve thermal and mass decoupling control.
Such multi-electrode arc heat sources have shown great application potential in the field of additive manufacturing. Harbin Institute of Technology Yang et al. Applied DE-GMAW to additive manufacturing. Without changing the mass transfer, by changing the bypass current, it is possible to effectively control the heat input of the formed parts, and thus achieve a large range of forming morphology Regulate and obtain single-wall structures with different widths. Huang Jianjian, Lanzhou University of Technology, etc. applied bypass coupled micro-beam plasma arc welding (DE-MPAW) to additive manufacturing. By increasing the bypass current, the heat input to the molded part is reduced without reducing the forming efficiency , Which is more conducive to arc additive manufacturing. It can be seen that the applicability of the multi-electrode arc heat source in the field of additive manufacturing has been initially verified, demonstrating that the thermal and mass decoupling control has a strong control effect on the forming morphology, but how to base it on WAAM in complex disturbance conditions There is still a lack of relevant theoretical research on thermal and mass decoupling control under the forming stability requirements.
Droplet transition factor control
The droplet formed by welding wire melting is the basic unit in the additive manufacturing process, and its size and transition form directly affect the accuracy and manufacturing process of the component. The droplet transition behavior of the traditional arc heat source is essentially the breakage of the liquid droplet under the action of plasma flow force, electromagnetic contraction force and its own gravity to overcome its surface tension. Therefore, the control of the droplet transition can only be performed by improving the energy accumulation of the droplet itself and introducing an external force to force the transition. Table 2 briefly summarizes the representative achievements of droplet transfer control in the arc heat source welding process at home and abroad.
It can be seen from Table 2 that by modulating the current of a special waveform, the droplet transfer characteristics can be improved. Although it contributes to reducing heat input and suppressing spatter, on the other hand, the coupling between the droplet transfer and the welding current is also enhanced To a certain extent, the mass transfer in the welding process will be limited to the requirements of heat transfer. And only relying on the energy accumulation of the droplet itself to change the transition mechanism has a certain contribution to the regulation of the energy distribution of the substrate and the filling material, but in principle, it is difficult to conduct the movement process of the droplet by simply relying on the resultant force of the free arc acting on the droplet Effective control, while mechanical force, external magnetic field, ultrasonic and laser enhancement, etc., it is difficult to provide sufficient additional force and directivity to achieve quantitative directional transition of droplets. For the droplet transfer of additive manufacturing, the biggest difference between it and the welding process is that the position of the droplet drop point and the temperature field of additive manufacturing are strict to ensure the accuracy and performance of additive manufacturing. The droplet size and its consistency, the temperature matching of the droplet and the substrate, and the directivity and stability of the droplet transition put forward higher requirements. These indicators are difficult to meet due to the current droplet control methods in the welding process.
The force of plasma arc on the droplet is the plasma flow force, and its physical process can be described as the melting and detachment of the wire material under the high-speed erosion of the plasma jet, and the transportation process of the plasma jet to the metal droplet. Directivity. Therefore, the plasma arc has sufficient strength and is more advantageous in controlling the flight path of the droplet, which is more suitable for wire arc additive manufacturing. The cross-arc composite heat source proposed by the researchers is a general model, which includes all multi-electrode coupled arc working modes. By changing the number of electrodes and the polarity distribution, different arc heat sources can be constructed as needed. The multi-electrode coupled heat source can adjust the combination of thermal mass and force according to actual needs to achieve precise control of the droplet size, momentum and energy, completely decouple the wire melting process and the decoupling process, and lay the foundation for the accurate manufacture of complex structural parts. Make new structures that were difficult to manufacture in the past possible.
In summary, domestic and foreign scholars have carried out a lot of experimental work on additive manufacturing technology that uses arc as a heat source. In the existing arc welding process, additive manufacturing capability evaluation, complex structure additive planning, organization evolution rules and performance optimization Research results have been achieved, but the above research is based on the existing welding arc heat source, and simply transfers the welding process to additive manufacturing. It does not address the special environmental thermal requirements, droplet requirements, droplet transmission methods, The physical processes of additive manufacturing, such as weakly constrained solidification of the droplets, metallurgical bonding between droplets / layers, etc., have been systematically studied, ignoring the precise control of the momentum when the droplets are detached and the ambient temperature when contacting the substrate, which is precisely The key to improving the accuracy of arc additive manufacturing.