Effect of electromagnetic stirring force on molten pool under different welding processes

Abstract: This paper compares the current density and magnetic field strength of a workpiece under three different welding processes : conventional single-side plasma arc welding, PA-GTA double-sided arc welding and PA-GTA double-sided arc welding. And electromagnetic force distribution and electromagnetic stirring force on the molten pool fluid flow. The results show that in the double-sided arc welding process, the arc energy density is significantly increased due to the higher degree of arc compression, which increases the electromagnetic stirring force in the molten pool, in order to increase the weld penetration and improve the weld microstructure. Performance provides the conditions.
Keywords: plasma arc double-sided arc welding current density electromagnetic stirring force

Foreword

Single-power double-sided arc welding process is a new and efficient welding process [1]. In this process, the workpiece is not connected, but the two torches are connected to the two poles of the same welding power source, so that the two arcs work in series. Preliminary tests show that this process has the advantages of increased penetration, reduced post-weld workpiece thermal deformation, etc., and is particularly suitable for welding plate. Gao Hongming numerically simulated the current density distribution and electromagnetic force distribution in the double-power and single-power double-sided GTA welding workpieces, respectively, and found that the distribution of the latter is more concentrated [2]. In the process of double-sided arc welding, the arcs generated by the two welding torches are compressed to varying degrees, and the resulting electromagnetic stirring force will theoretically produce a large change. For the comparison of the electromagnetic force in the two cases of conventional tungsten-arc welding (GTAW) and double-sided tungsten-arc welding, Gao Hongming has made a detailed analysis in [2].
This paper will compare the current density and magnetic field strength of the workpiece under the three welding processes of conventional single-side plasma arc welding (PA), PA-GTA double-sided arc welding and PA-GTA double-sided arc welding. The distribution of electromagnetic force. Because the welding speed used in the test is small, the arc does not have obvious deformation in the walking direction of the workpiece during the welding process. Therefore, the 2D axial symmetry mathematical model can be established for the resulting current continuity equation. In addition, due to the symmetry distribution of the electrical parameters along the center line of the weld seam, only half of the workpiece area can be considered in the numerical calculation.

1 Potential Mathematical Model

The control equations of the potential mathematical models established under the three welding processes are the same, except that the boundary conditions are different. The calculation region of the current continuity equation is shown in Figure 1.

Current continuity equation

Fig. 1 Calculation area of ​​the current continuity equation

According to Ohm's law, the current density can be obtained

According to Ampere's law, self-induced magnetic field strength can be obtained

Electromagnetic force

The potential boundary conditions corresponding to the three welding processes are shown in the following table.
In the above table, for the PA-GTA double-sided arc welding, although the conductivity of the medium and the molten pool metal in the small hole is different, the electromagnetic force generated in the small hole does not affect the molten pool metal. Therefore, the conductivity of the gas in the small hole can be ignored, so that the potential boundary conditions at the small hole do not need to be separately processed, and the unified calculation can be performed directly.

Table The potential boundary conditions under the three welding processes

Because of the three welding processes, even if the same welding current is used, the resulting arc patterns are not the same. Among them, the arc of the double-sided arc welding is concentrated compared to the arc of the conventional plasma arc welding, and the double-sided arc is small. The arc welding of the hole is more concentrated than the arc of the double-sided arc welding. Therefore, the corresponding arc effective radius will also be different. This will directly affect the variation of the arc energy density, and thus affect the shape and quality of the weld.
In this paper, the effective radius of the current density under the three welding processes is as follows: rc=4mm, rc1=2mm, rc2=4mm, rc3=1.5mm, rc4=3mm.

2 solution method

This paper uses commercial software PHOENICS to solve the above-mentioned established model. The software uses finite volume method. The detailed introduction can refer to the literature [3].
The general formula for the control equation that PHOENICS software can solve is:

In the formula - the variable to be solved;
t - time;
U - speed vector;
Γψ - the diffusion coefficient of the ψ variable;
Xi - coordinate direction;
Sψ - source term.
The variables that can be solved can be pressure, temperature, enthalpy, velocity, or potential, etc. All the boundary conditions and the source terms in the system of equations are on the right side of the above formula. When using PHOENICS software, the geometry, meshing, thermal property parameters, boundary condition area definition, and solution method selection of the solution area are adjusted and set by the Q1 file for each complex source term and boundary condition. The implementation requires the user to write a program in the Ground.for file to complete. The general principle is that the Q1 file and the function module of Ground.for correspond one to one. The Q1 file calls the corresponding program in the Ground.for file according to the user's settings to achieve the realization of each function.

3 Comparison of Results Under Three Processes

Based on the mathematical model established above, this paper compares the potential distribution and the electromagnetic force on the welding test specimen under the three welding processes. For the sake of convenience, the coordinate axis is rotated 90° counterclockwise. Figure 2 to Figure 4 show the potential distribution. From the results it can be observed that the potential distribution on the workpiece gradually concentrates from single-sided plasma arc welding to double-sided arc welding. Among them, the maximum absolute value of potential under single-sided plasma arc welding is 0.0154V, and the maximum absolute value of potential under double-sided arc welding is 0.0167V, while the maximum absolute value of potential under double-sided arc welding is 0.033V, especially when the pinhole effect is generated, the maximum potential value in the workpiece increases to 2 times the original, and thus the effect of pinhole effect is significant. Then, the subsequent potential derivation—current intensity, magnetic field strength, and electromagnetic force—can be imagined. In order to explain the problem intuitively, this paper presents the comparison results of electromagnetic force on the workpiece under three welding processes. Fig. 5 to Fig. 10 compare the distribution of the axial and radial components of the electromagnetic force at different positions on the workpiece in detail. It can be clearly seen from the figure that the changes in the electromagnetic force under different welding processes are also illustrated on the two sides of the PA-GTA. In the arc welding process, electromagnetic force has a greater influence on the liquid metal flow in the bath than in the conventional PAW state.

Fig. 2 The potential distribution on the workpiece under single-sided plasma arc welding

Fig. 3 The distribution of the electric potential on the workpiece under the double-sided arc welding

Fig. 4 The distribution of electric potential on the workpiece under double-sided arc welding

Fig. 5 shows the distribution of the radial component of the electromagnetic force along the thickness of the test piece under the three welding processes of conventional single-sided plasma arc welding, double-sided arc welding and double-sided arc welding. Figure 6 The distribution of the axial component of the electromagnetic force in the width direction of the test piece under the three processes is shown. From the results, it can be seen that the absolute value of the radial component of the electromagnetic force on the PA side of the test piece under the PA-GTA double-side arc welding process It is 1.3 times larger than the conventional single-sided plasma arc welding process, while the absolute value of the axial component of the PA side electromagnetic force under double-sided arc small-hole welding process is 2.5 times larger than that of double-sided arc welding. On the side of the GTA, the electromagnetic force under the conventional plasma arc welding process is negligible, while the absolute value of the electromagnetic force under the double-sided arc small-hole welding process is 4.8 times larger than that of the double-sided arc welding.
Figures 7 and 8 show the distribution of the radial and axial components of the electromagnetic force along the width of the test piece on the PA side, respectively, for the three welding processes. It can be seen from the figure that the absolute value of the axial component of the electromagnetic force in the molten pool under the double-sided arc welding is more than 2 times higher than that of the conventional single-sided plasma arc welding, and the double-sided arc welding hole is welded in the molten pool. The absolute value of the axial component of electromagnetic force is more than 2 times higher than that of double-sided arc welding.

Fig. 5 Comparison of the radial component of the electromagnetic force in the direction of the workpiece thickness at the center line of the weld

Fig. 6 Comparison of the axial component of the electromagnetic force in the direction of the workpiece thickness at the weld centerline

Figure 7 Comparison of the radial component of the electromagnetic force along the width of the workpiece on the PA side

Fig. 8 Comparison of the axial component of the electromagnetic force along the workpiece width direction on the PA side

Due to the difference of arc contraction effect, the radial component of the electromagnetic force under the double-faced arc welding hole is the largest near the center of the weld pool, while the radial component of the electromagnetic force under the conventional plasma arc welding is the largest around the weld pool.
Figures 9 and 10 show the distribution of the radial and axial components of the electromagnetic force along the width of the test piece on the GTA side, respectively, for the three welding processes. It can be seen from the figure that due to the negligible electromagnetic force on the back of the workpiece under the conventional single-plane plasma arc welding process, the axial component of the electromagnetic force in the molten pool under double-sided arc welding is higher than that of the double-sided arc welding. About 5 times, while the radial component of the electromagnetic force in the molten pool under double-sided arc pinhole welding is about 4 times higher than that under double-sided arc melting.

Figure 9 Comparison of radial components of electromagnetic force along the width of the workpiece on the GTA side

1

Figure 10 Comparison of the axial component of the electromagnetic force along the width of the workpiece on the GTA side

From the above comparison results, it can be seen that the distribution of the electromagnetic force formed after the arc compression has undergone a significant change. The more the arc is compressed, the greater the influence of the generated electromagnetic force on the molten pool fluid flow.
From the above figure, it can be seen that the welding current flowing through the welded test piece is mainly concentrated near the weld pool on the surface of the workpiece, which is consistent with the results of other researchers [2, 4].

4 Conclusion

Through the numerical calculation of the electromagnetic force distribution on the workpiece during the conventional single-sided plasma arc welding, PA-GTA double-sided arc welding and PA-GTA double-sided arc welding, it was found that compared with conventional single-sided plasma arc welding, In the process of double-sided arc welding, the absolute value of the potential on the workpiece increases by a factor of 2, while the absolute value of the potential on the workpiece during double-sided arc welding increases by a factor of 2, and the corresponding electromagnetic force also increases exponentially. It can be seen that the degree of arc compression during the double-sided arc small-hole welding process is higher, thereby further increasing the energy density of the arc, increasing the electromagnetic stirring force in the molten pool, increasing the weld penetration and improving the weld microstructure. Provided the conditions.