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1.INTRODUCTIONLaser-MIG hybrid welding is a fusion of laser welding and electric arc welding, with the advantages of energy-saving, high welding efficiency, and good weld seam formability. In the welding process, laser-arc hybrid welding uses two different welding heat sources, combining the advantages of both heat sources. Compared to the previous single heat source, laser-arc hybrid welding has the advantages of large melting depth, high stability, and high laser utilization rate, and is being used more and more widely in the aviation, marine industry, automotive, and other fields. At present, the laser-MIG hybrid welding technology for 304 stainless steel has been more extensive research, but the main direction of research is still focused on the relationship between the welding process, organization, and mechanical properties, for the melt pool numerical simulation of less research. Most are recorded through high-speed cameras welding process melt pool morphology and dynamic changes, the use of algorithms to extract the morphological characteristics of the melt pool, real-time monitoring of the melt pool1-2, or through algorithms to analyze the melt pool image plasmas to improve the accuracy of weld tracking3. Magneto-optical imaging-based inspection methods are also used to check the weld position, reduce the error in the use of optical flow methods through particle filtering, and accurately track as well as identify difficult-to-observe weld positions4; the literature simulated a weld cross-sectional shape consistent with the applied weld head by combining a Gaussian planar heat source, a double ellipsoidal heat source, and a Gaussian rotating body heat source with the aid of finite element analysis software5. Zhao Xin studied the effect of fillet spacing on laser hybrid welding of 6082 aluminum alloy, and the results showed that either too large or too small a fillet spacing would increase the number of pores in the weld6. In addition, the proportion of shielding gas will also affect the porosity of the weld to some extent7. A three-dimensional numerical model based on the laser beam transmission and the dynamics of the keyhole and melt pool was developed in the literature8. It is therefore indispensable to explore suitable process parameters through numerical simulations to reduce the amount of experimentation. This paper selects 4mm thick 304 stainless steel sheets for laser-MIG overlay welding test, studies the effect of laser power and arc voltage on weld formation and the influence of melt pool flow, through determining the optimal welding parameters and establishing a corresponding numerical model of laser-MIG welding9, that provide a theoretical basis for the application of laser-MIG composite welding technology. 2.TEST MATERIALS AND METHODSIn this paper, the laser-MIG composite welding test of 304 stainless steel is adopted. The welding process adopts the guiding mode of arc in the front and laser in the back10, and the oxide film on the workpiece surface is removed by mechanical grinding before welding. The experimental parameters are shown in Table 1. Table 1.Laser-MIG hybrid welding process parameters.
3.EXPERIMENTAL RESULTS AND ANALYSIS3.1Analysis of weld seam conditionThe experiments were carried out by using the single variable method to study the effect of laser power and arc current on the weld seam formation. The macroscopic morphological characteristics of the weld seam were observed after the welding was completed to analyze the effect of laser power and arc current on the weld seam formation. The weld seam shape of the different welding parameters is shown in Figure 1. Figure 1 shows that at a laser power of 2.5kW and an arc current of 100A, the weld surface is rough, with coarse fish scale patterns and uneven weld edges. This is primarily because the arc current is too low and the arc is unstable, which leads to poor stability of the weld formation. The plate is in a moderate state of melting, and the laser energy is just enough to penetrate it. The energy of the laser and the arc causes the plate to periodically melt through, at which point the molten pool falls due to gravity and forms a periodic weld tumor. When the laser power is 2.5 kW, the frontal weld formation in (b) and (c) is great, the weld surface finish improves, the fish scale pattern is dense, and the edge of the welding channel is straight, primarily because the temperature of the arc rises with the increase in arc power, the degree of gas ionization and thermal emission of the arc action area is enhanced, and the arc fully burned to increase the stability of the weld. The back side of figure (b) shows a lack of penetration, mainly due to the low power of the laser, which does not produce enough heat to penetrate the plate. Figure (c) shows a clear match between laser and arc energy, with both the front and back of the weld well formed and without obvious defects. 3.2Fluent-based simulation validation3.2.1Morphological Evolution of the Melt Pool and Keyhole.After the experiment can be seen that the parameters c (P = 2500W, U = 180V, I = 20A) have a better weld morphology, this paper discusses the simulation results through numerical simulation software, laser-MIG hybrid welding numerical simulation as shown in Figure 2. Figure 2a can be seen in the hybrid welding process under the irradiation of laser energy, the thin plate radiation zone at 7ms reached the boiling point of 304 stainless steel, the value of more than 3000K, the base material in the role of vapor pressure to overcome the surface tension of the keyhole wall and hydrostatic pressure, resulting in a large recoil pressure, the liquid metal is extruded to the area below the thin plate to promote the formation of the keyhole. In the initial stage of the welding process, the recoil pressure is much greater than the surface pressure and hydrostatic pressure. Under the action of the recoil pressure, the depth of the small hole increases rapidly, as shown in Figure 2. 3.2.2Effect of Droplet Transition on the Thermal-Hydraulic Coupling Field Distribution in the Melt Pool.Figure 3 shows the evolution of the flow and temperature fields in the longitudinal section of the melt pool corresponding to parameter c. The temperature cloud is shown on the left, with the red area representing the melt pool; the velocity cloud is shown on the right, with the size and color of the arrow representing the flow rate inside the melt pool. Comparing the flow field of the melt pool before and after the drop, the following flow patterns can be seen in the longitudinal section of the melt pool: (1) counterclockwise circulation formed by the upward impact of the rear wall of the melt pool with the tail of the melt pool by Marangoni; (2) counterclockwise circulation generated by the negative surface tension coefficient at the front of the melt pool; (3) flow from the bottom of the small hole towards the front along the bottom of the melt pool. Figure 3.Longitudinal section of the molten pool before and after the drop of the laser-MIG hybrid weld.  Under the laser energy, the depth of the small hole is deepened by the recoil pressure until a steady-state equilibrium is reached. The arc heat source and molten droplets melt the material behind the small hole and the molten metal stacks together and solidify to form the residual height of the weld. The melt pool flow field under the action of steam recoil pressure, the liquid metal behind the wall of the small hole downward flow, part of the momentum formed along the bottom of the melt pool to the front of the flow, another part of the bottom of the melt pool reflected to form a counterclockwise circulation. This forward flow transports the heat and momentum from the rear of the bath to the front, increasing the flow velocity of the bath. The tail of the melt pool forms a flow from the tail to the wall of the small hole under the action of the Marangoni, and together with the counterclockwise circulation generated in the middle of the melt pool, it impinges on the middle and upper part of the rear wall of the small hole and forms a gradually growing tab at this position, when the tab comes into contact with the front wall of the small hole, the small hole breaks and the gas generated in the small hole enters the melt pool to form a bubble, as shown in Figure 3d. Repeatedly generated and collapsed phenomenon. It can be seen in the laser arc hybrid welding process, that the molten pool of small holes in the flow of behavior patterns will have a great impact on the temperature field of the molten pool and the flow field, and the location of the middle of the small hole is more likely to collapse, thus producing bubbles affect the forming process of the molten pool, if the bubble does not escape from the molten pool in time under the action of the laser, it will produce small hole-type porosity defects, affecting the quality of the weld forming. When the melt droplet is incorporated into the melt pool, the liquid metal below flows rapidly towards the bottom and the edge of the melt pool, as shown in Figure 3c, the maximum velocity of the melt pool appears in the area below the melt droplet, which is about 1.56m/s. The melt droplet has an impact on the surface of the melt pool, making the melt pool The surface of the melt pool is concave in the area of action, the melt pool is shaken violently and the convection characteristics due to surface tension and Marangoni forces almost disappear. Subsequently, the liquid metal, which has just been squeezed to the edge of the molten pool by the droplet, flows back under the action of surface tension and gravity. It can be seen that in the laser arc hybrid welding process, the flow state of the fluid inside the melt pool is mainly influenced by the droplet drop, and the droplet impact is the main driving force of the melt pool fluid, and the melt pool flow behavior is dominated by surface tension or droplets during a dropped cycle. 3.2.3Simulation Results and Validation of Experimental Results.The simulation results of the welded front and back melt width and the actual measurement results are shown in Figure 4, the actual measurement of the front weld size and back weld size of 6.21mm and 2.03mm, respectively, after numerical simulation of the welded front and back size of 6.12mm and 1.83mm, respectively, after the calculation of the front and back weld deviation in 1.4% and 9.8%. It can be seen that the Fluent-based simulation analysis results are more accurate and close to the experimental results, proving the reliability of the simulation results and confirming the influence of welding parameters on the weld formation and melt pool flow characteristics. 4.CONCLUSIONComparison of 304 stainless steel in different welding parameters under the conditions of the weld seam morphology, and for a set of experimental parameters for the numerical simulation, the establishment of the laser-MIG hybrid welding for the “rotating Gaussian heat source model + double ellipsoidal heat source” of the combined body heat source, according to the results of experiments and numerical simulations, the following conclusions.
ACKNOWLEDGMENTSWe are grateful to Mr Gao, Mr Pan and all members of the laboratory for enlightening discussions. This project was funded by the Jiangxi Postgraduate Innovation Special Fund Project 2021 (YC2021-S576). REFERENCESGao, X. D., Zhang, Y. X.,
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