Effect of Droplet Transition on the Dynamic Behavior of the Keyhole during 6061 Aluminum Alloy Laser-MIG Hybrid Welding

The simulation method in laser-MIG hybrid welding, which involves two heat sources and multiple welding parameters, is beneficial to reveal the complex physical phenomena and dynamic behavior of molten pool keyhole during welding process. In this investigation, laser-MIG hybrid welding for 6-mm-thick 6061 aluminum alloy was performed under different heat input by the high-power disc laser, MIG welding system and KUKA Robot. The high-speed camera system was used to observe the droplet transition phenomenon in the welding process. Besides, a thermal-fluid coupling model was established to simulate the temperature field and flow field, which were changed by the droplet transfer during laser-MIG hybrid welding. The experimental and simulated results showed that the droplet transition behavior affected the formation of the keyhole. The keyhole was the smallest when the droplet contacted the molten pool. In addition, the droplet transition brought external momentum and energy to the molten pool, which was conducive to the increase of the flow rate of the molten pool.

advantages of obvious weight loss, slight deformation and high welding efficiency [8,9]. However, laser welding exists the weaknesses of poor bridging ability and strict requirement of the joint gap [10]. As an applicative and emerging deep penetration connection technology, laser-arc hybrid welding technology, which developed in the 1990s, has been widely applied in civil passenger aircraft, automobiles, construction machinery, rail transportation and equipment manufacturing fields [11,12]. In the laser-arc hybrid welding process, the dual heat sources synergy and interact effect between laser and arc produces narrow and deep welds, thereby greatly improving production efficiency [13]. Laser-arc hybrid welding with the characteristics of high welding energy, strong gap bridging ability and high joint quality [14], makes up for the deficiencies in the single processing heat source, meanwhile, effectively combines the advantages of the two welding methods [15].
Many researches have been carried out to observe the morphology of the molten pool during the laser-MIG hybrid welding process by high-speed cameras. Faraji et al. [16] used laser-MIG hybrid to fabricate 3-mm-thick 2198 aluminum alloy, and the mechanical property of the welded joint was compared and analyzed. It was found that comparing with a single laser heat source, the keyhole with the deep penetration was easy to obtained even at higher welding speed. Moreover, the micro-hardness and microstructure of high laser power with low arc power were better. Liu et al. [17] observed the droplet transfer and the keyhole in the molten pool during laser-arc hybrid welding by high speed camera. It revealed that the laser had a compressive effect on the arc pressure with the surface of the molten pool. Zhang et al. [18] performed laser-MAG hybrid welding of large thickness steel and investigated the influence of the ratio of laser power to arc power on the weld morphology.
What is more, the image of the molten droplet and arc morphology were observed with the help of a high-speed camera system. It was revealed that as the ratio of laser to arc energy increases, the aspect ratio of the weld exhibits a linear increase.
In recent years, the research method of simulation was used to explain complex physical phenomena, which promoted the research on the mechanism level. Lu et al. [19] proposed a three-dimensional transient model coupled with fluid flow, bubble motion and solidification to study the process of keyhole-induced porosity. The simulation results showed that the number of porosity was mainly determined by the frequency of keyhole collapse. As the laser power increased, as well as the welding speed and spot diameter decreased, the keyhole tended to collapse more easily. Guo et al. [20] performed laser-GMAW hybrid welding of 6 mm aluminum alloy, and simulated the formation of keyhole-induced pores with the help of two-dimensional flow field calculations. It was found that under the action of the arc force, relatively stable counterclockwise vortices appeared in the middle and rear of the composite molten pool, which tended to prevent the bubbles from moving to the low temperature area at the rear of the molten pool, thus helping the bubbles to float upward.
However, the research on the droplet transfer behavior in the laser-MIG hybrid welding process is still insufficient. It is not enough to study on the effect of droplet transition on the temperature field distribution of the molten pool. In this paper, with the help of high-speed camera and the establishment of a thermal-fluid coupling model, the droplet transition behavior is studied.

Experimental material and setup
The base metal used in this paper is 6061-T6 aluminum alloy, its chemical composition is shown in Table 1. The size of aluminum alloy plate is 150 mm×30 mm×6 mm, with a Y-shaped groove, as shown in Fig. 2(a). The selected filler wire is ER4047 with a diameter of 1.2 mm whose main chemical composition is shown in Table 1. The laser-MIG hybrid welding experiment equipment is adopted TruDisk-12003 disc laser with a maximum laser output of 12000 W, KR60HA robot, TPS 5000 Fronius welding machine and self-designed installation fixture. Besides, CP70 high-speed camera is performed to take pictures of the molten pool morphology and droplet transition during the welding process. The experimental equipment is shown in Fig. 1.

Experimental process
During the laser-MIG hybrid welding of 6061 aluminum alloy, the aluminum alloy plate is connected to the negative electrode effectively, considering the dense oxide film on the surface of aluminum alloy. The cathode crushing effect in arc welding is instrumental to remove the oxide film and facilitate the droplet transition. The schematic diagram of laser-MIG hybrid welding is as shown in Fig. 2. In order to prevent the damage of laser head by the vertical laser irradiation, the angle between the laser beam and the workpiece is 86.5°. At the same time, the angle between the MIG welding torch and the aluminum alloy plate is 45° considering the interference of the experimental equipment. Argon with purity of 99% is used as welding shielding gas and its flow rate is 15 L/min.
The welding parameters adopted in this experiment is as shown in Table 2.

Governing equations
The flow of the molten pool and the behavior of metal vapor during the laser-MIG hybrid welding process are both complex physical phenomena [21]. It is followed the law of conservation of mass, energy and momentum, which govern the thermodynamics and kinetics of the molten pool owing to the fluid characteristics at the welding process [22]. The formulas of the three laws are as follows.
Continuity equation Energy equation where H is the mixing enthalpy, T is the temperature, k is the heat conductive coefficient. qnet is the heat input during laser-MIG hybrid welding process.
The heat balance equation is: where qA is the heat input from the electric arc; qL is the heat input from the laser; qD is the heat input from droplet; qE is the evaporation heat loss from the molten metal; qR is the heat loss by radiation; qC is the heat loss by convective heat exchange with the environment. The unite of these parameters is J/m 2 /s.

Momentum equation
where u=(u, v, w), Sm is the source term for momentum conservation.

Driving forces
The vapor recoil pressure (PR) of the gas is an important reason for the keyhole, which is mainly driven by the plasma generated at the bottom of the keyhole, and the liquid metal is expelled to both sides of the molten pool [23,24]. The equation of recoil pressure is expressed as follows: where P0 is the ambient air pressure, Lv is latent heat of vaporization, Tb is the boiling temperature, and R is the gas constant value.
The surface tension is one of the driving forces to increase the width of the molten pool. The temperature coefficient of the surface tension with the aluminum alloy liquid metal is negative [25].
The molten metal on the surface of the molten pool flows around the keyhole, resulting in a wide and shallow cross-section of the molten pool.
Marangoni force has great effect on the width of molten pool. Especially, when the temperature gradient coefficient of surface tension is negative, the surface tension of the liquid metal increases as the temperature decreases. Thus, the surface tension is low at the center of the molten pool, and it is high near the molten pool edge, which contributes to an outward flow and consequently a wider and shallower molten pool. The equation of Marangoni force is expressed as follows: where τ is the Marangoni stress, dγ/dT is temperature coefficient of surface tension and dT/dy is the temperature gradient.
The buoyancy force is caused by the density variations. The variation of density in the molten pool induces the fluid rising in the hotter and less-dense region. At the same time, the fluid sinks in the cooler and denser region. The force is expressed by the following equation: where ρ is the density of liquid metal, β is the thermal expansion coefficient, and Tref is arbitrarily selected reference temperature.
Electromagnetic force is named as Lorentz force, which effects on the heat and mass transfer in the molten pool and contributes to smaller width and deeper penetration. The equation is expressed as: where J is the current density and B is the Magnetic field.
Arc pressure contributes to smaller width and larger penetration. On the contrary, the plasma flow force leads to larger width and shallower penetration. The scheme diagram of driving force in the molten pool is as shown in Fig. 3.  Table 3and Table 4, respectively.

Heat source
Considering the laser and MIG heat sources during the welding process, a hybrid heat source model of a Gaussian rotating body heat source combination and a double ellipsoid heat source are selected for simulation, as shown in Fig. 5. The heating range of the MIG heat source is wider and the width of the weld seam is larger, thus the double ellipsoid heat source model is better reflect the shape of the molten pool [26]. The laser heat source acts on the surface of the workpiece and produces a keyhole effect, which plays a leading role in the welding process [27].
where qs and qv are the heat flux distributions of Gaussian surface heat source and bulk heat source respectively, b is the heat flux concentration coefficient, Qs and Qv represent the power of surface heat source and bulk heat source, rs and R0 are the effective radius of action of the surface heat source and bulk heat source, respectively , H(t) is the effective depth of action at which the body heat source changes over time. And the relationship between Qs and Qv is expressed as: where P is the laser power and ηL is the thermal efficiency of the laser.
The double ellipsoid heat source model is described as: In the formula, qr and qf are the heat source densities of the two ellipsoids in the double ellipsoid heat source model, ff and fr are the heat distribution coefficients of the two ellipsoids, and I, U and v0 are the arc current, voltage and Welding speed, af, ar, b and c are heat source model parameters. ηA is the arc thermal efficiency.

Experiment validation
According to the heat-fluid coupling solution process of the laser-MIG hybrid welding process, the numerical simulation calculation is carried out. After the relevant parameters of the model are corrected, the results of heat source verification result for 1# is shown in Fig. 6. Comparing the weld cross section with the weld simulation results, it is seen that the macroscopic appearances of 1# is basically symmetrical, so the modified hybrid heat source model can be used for the subsequent simulation calculation of the temperature field and flow field of laser-MIG hybrid welding.   Compare with added droplet transfer, the surface morphology of the back of the molten pool without droplet is smoother, as shown in Fig. 10. It is found that the depth of the keyhole is the same, but the back wall of the keyhole changes greatly. The increase in the volume of the rear part causes a slight depression in the lower part of the rear wall of the keyhole. Because the molten droplet brings external momentum to the molten pool, the flow field distribution of the molten pool also changes. The simulation results of the flow field are shown in Fig.   11 and Fig. 12, respectively.
It is seen from Fig. 11  Meanwhile, the droplet completely fell into the molten pool, the impact force brought by the droplet was relatively large, and the maximum velocity reached 2.05 m/s.
For the flow field distribution of the molten pool without added droplet transition, the flow field distribution is basically the same as that of the added droplet transition, but its peak velocity at different times is less than the added droplet transition, mainly because the droplet transition brings about External momentum [28]. The change of the shape of the keyhole by the droplet transition is mainly achieved by promoting the downward flow of the fluid at the back of the molten pool to fill the depression below the back wall of the keyhole, thus squeezing the back wall of the keyhole.
When convex shape is appeared on the back wall of the keyhole, which affects the heating effect of the laser on the molten pool, the back wall of the keyhole will adjust the angle under the thermal capillary force to maintain the absorption of the laser by the molten pool.

Conclusions
In this investigation, high-speed camera was used to observe the droplet transition of laser-MIG hybrid welding for aluminum alloy. The thermal-fluid coupling model was established to study the effect of droplet transfer on the shape of the molten pool. Based on the experimental and simulation results, the conclusions are drawn as follows: (1) The process of the droplet falling into the molten pool hinders the formation of the keyhole, and the keyhole opening is the smallest when the droplets are in contact with the molten pool.
(2) The mass, energy and momentum brought by the droplet transition behavior, which changes the temperature field and flow field of the molten pool. In addition, the maximum velocity of molten pool is bigger than that without droplet.  The schematic diagram of computational domain High-speed camera photos in the droplet transition process