| When welding speed is beyond a critical value, humping bead will occur in conventional gas metal arc welding (GMAW). Previous studies showed that the high momentum of backward flow jet of molten metal inside weld pool is responsible for the formation of humping bead during high-speed GMAW. Therefore, in this study, by applying an external magnetic field into the weld pool, an additional forward electromagnetic force is generated by external transverse magnetic field perpendicular to the unidirectional current, which can reduce the momentum of the backward flow of molten metal and change the distribution of fluid velocity in weld pool. Consequently, the humping bead can be suppressed.In order to investigate the control mechanisms of external magnetic field on high-speed GMAW arc and weld pool behaviors, this study focuses on numerical simulation of the distribution of a steady external magnetic field and the Lorentz force induced in weld pool. Firstly, a three-dimensional numerical model is established to calculate the distribution of external magnetic field on work-piece before welding, which is based on the excitation device arrangement applied in welding experiment. The calculated results with different excitation conditions are obtained, which are verified by measured data. Then, a thermal-magnetic coupled analysis method is presented to accurately calculate the distribution of magnetic field in weld pool, in which the influence of high temperature during welding process on the permeability of ferromagnetic work-piece (Q235b) is considered. Simulation results show that the applied magnetic field is transverse in both arc and weld pool regions. The magnetic flux density is very low on the work-piece area which has directly heated by welding arc, while on the work-piece area located in front side of weld pool, the magnetic flux density is much higher. Therefore, the magnet-arc distance as a critical parameter to adjust the distribution of external magnetic field is considered, which ensures that the value of magnetic flux density in weld pool can satisfy the requirement of adjusting backward flow jet.Based on the images of arc region, a deflection model of current conducting liquid stream under the action of external magnetic field is employed to analyze the mechanism of welding arc and droplet transfer behaviors. Through combining the distribution of external magnetic field on arc area, the offset distances between arc center and wire center with different excitation conditions are calculated. The measured data are also acquired by processing digital images of liquid stream, which indicate that calculated results are in good agreement with measured data.Through analyzing electromagnetic forces induced into GMAW weld pool with external magnetic field, computational model of electromagnetic force is developed, which includes external electromagnetic force and Hartmann electromagnetic force. Based on thermal-magnetic coupling, these electromagnetic forces are numerically simulated, in which the influence of the welding arc deflection is considered. Calculation results show that Hartmann electromagnetic force can be ignored in adjusting the backward flow jet in weld pool, since the value of additional electromagnetic force is far greater than that of Hartmann electromagnetic force.To verify the mechanism of mitigating backward flow jet using additional electromagnetic force into weld pool, high-speed GMAW experiment with external magnetic field is carried out, and GMAW experiment in downhill position with the same welding condition is also conducted. The additional electromagnetic force with excitation current of 8.7 A can mitigate backward flow jet in weld pool and suppress humping bead when welding speed is 2.37 m/min, and the gravity component along welding direction in 15°downhill experiment can play a similar role in suppressing humping bead. However, downhill position is an intentionally set welding condition, which is not practical during industrial production. Apparently, it is an easy and convenient method for external magnetic field to adjust weld pool flow, which has a broader applicability. |
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