Effects of Longitudinal External Magnetic Field on Metal Transfer Behavior and Spatter Formation in CO2 Arc Welding

Abstract

Excessive spatter formation in conventional CO₂ arc welding significantly diminishes welding quality and efficiency, posing a critical challenge for industrial applications. To address this issue, this study investigated the mechanisms of metal transfer behavior and spatter formation under the influence of a longitudinal magnetic field (LMF) using a shadow-graph technique with high-speed imaging and back-laser illumination, also coupled with Computational Fluid Dynamics (CFD)-based arc-droplet numerical simulations. The results show that increasing the magnetic flux density (MFD) from 0 to 2 mT shifted the transfer mode from the repelled transfer to the globular transfer, while higher MFDs (3-4 mT) induced rotating repelled transfer. The globular transfer at 2 mT was considered to be primarily produced by the centrifugal effect due to the rotational motion of the molten metal inside the droplet, which was caused by the Lorentz force affected by LMF. The higher droplet temperature in this condition also contributed to forming this transfer mode, preventing the formation of repelled transfer through a decrease in the arc pressure. On the contrary, in the higher MFDs, the droplet temperature decreased to increase the arc pressure, lifting the droplet up. Furthermore, the very strong centrifugal effect rotated the molten metal column around the wire axis to induce the rotating repelled transfer. The spatter formation was found to occur with the two-stage motion of the curved long tail without LMF and at 4 mT, and also with the exploding molten metal column at 4 mT, due to an imbalance of the Lorentz force acting on the molten metal. On the other hand, the neck formation facilitated smooth droplet detachment without forming the curved long tail at 2 mT, reducing spatter significantly. These findings offer valuable insights for optimizing welding quality and efficiency by stabilizing globular transfer under an optimal LMF.

Keywords: CO2 arc welding; longitudinal magnetic field; repelled transfer; spatter formation.

Figure 1

Schematic illustration of metal transfer behavior observation.

Figure 2

Typical images of metal transfer behavior under different MFDs.

Figure 3

Time-sequential images of a droplet in one cycle of metal transfer in conventional CO₂ arc welding.

Figure 4

Time-sequential images of a droplet in one cycle of metal transfer in CO₂ arc welding at MFD of 2 mT.

Figure 5

Time-sequential images of a droplet in one cycle of metal transfer in CO₂ arc welding at MFD of 4 mT.

Figure 6

Typical images of spatter formation in CO₂ arc welding under different MFDs.

Figure 7

Time-sequential images of spatter formation in conventional CO₂ arc welding.

Figure 8

Time-sequential images of spatter formation at MFD of 2 mT.

Figure 9

Time-sequential images of spatter formation at MFD of 4 mT.

Figure 10

Schematic diagram of the 3D calculation domain.

Figure 11

Arc temperature and velocity fields for (a) conventional CO₂ arc welding and for MFDs of (b) 2 mT and (c) 4 mT in gas phase immediately before detachment.

Figure 12

Temperature, velocity, and Lorentz force fields in metal phase immediately before droplet detachment for conventional CO₂ arc welding.

Figure 13

Molten metal velocity and spatter formation for conventional CO₂ arc welding right after detachment.

Figure 14

Temperature, velocity, and Lorentz force fields in metal phase immediately before droplet detachment for an MDF of 2 mT.

Figure 15

Horizontal velocity vector field on cross sections at z = 0.002 mm, z = 0.003 mm, and z = 0.004 mm for MFD of 2 mT at immediately before detachment.

Figure 16

Temperature, velocity, and Lorentz force fields in metal phase immediately before droplet detachment for an MDF of 4 mT.

Figure 17

Spatter formation right after detachment for MFD of 4 mT.

Figure 18

(a) Arc shape and (b) molten metal velocity field during the middle stage of the metal transfer process in conventional CO₂ arc welding.

Figure 19

Schematic of spatter formation in conventional CO₂ arc welding: (a) moment of detachment; (b) first stage of spatter formation; and (c) second stage of spatter formation.

Figure 20

Schematic of forces acting on a droplet just before detachment and spatter generation under magnetic flux densities of (a) 2 mT and (b) 4 mT.