The Heat Transfer And Fluid Flow Characteristics Of Welding Arc And Weld Pool For Magnesium Alloy Under Applied Longitudinal Magnetic Field

Magnetron control welding technology plays an extremely important role in the improvement of welds quality,which can optimize the welding process by applying an external magnetic field to the arc plasma and weld pool to change the energy distribution of the arc as well as fluid flow and heat transfer of weld pool.Therefore,the mathematical models for welding arc and molten pool need to be established in a scientific way.In order to realize the intelligence of welding process,the profile and energy distribution for the arc and the heat transfer and fluid flow characteristics for the molten pool under the electromagnetic stirring effect should be carefully analyzed.In addition,the prediction of the formation of resultant welds influenced by magnetic field is of great theoretical significance and may provide guidence for realizing the intelligence of welding process.Based on the basic principle of energy conservations,the fluid dynamic equations and Maxwell's equations,this theis established a 2D symmetric model for steady state GTAW arc and a 3D transient model for molten pool.The heat transfer and fluid flow characteristics of arc plasma and molten pool for magnesium alloy under the longitudinal magnetic field are simulated,in addition,the relative strength of the driving forces such as buoyancy,self-induced electromagnetic force,surface tension and applied electromagnetic force in the molten pool was analyzed by dimensionless analysis method.And the effect of driving force to the molten pool by individual was also systematically studied.In order to accomplish a reasonable results,computational mesh and boundary conditions must be taken care of,source items and conservation equations were solved by SIMPLE series algorithm.Finally,the models were validated via experiments and published literatures.The simulation results shown that the temperature of arc core and the plasma velocity decreases as long as the longitudinal magnetic field was applied.When the magnetic field strength reaches 0.03 T,the high-speed rotating arc starts to expand radially and forms a central hollow in its root,resulting in a positive pressure distribution along the axis.The so called positive pressure gradient generates a sucking force to the weld pool and makes the cathodic arc root to compress and expanding the anode attachment.Meanwhile,the negative pressure appearing in the center of the arc converges the heat from the molten pool to the arc core,and results in a decrease in the welding thermal efficiency by means of changing the heat flux and current density distribution at the interface of the molten pool from the conventional Gaussian distribution to the bimodal mode.The electromagnetic stirring effect caused by the longitudinal magnetic field in the molten pool makes the divergent flow dominated by Marangoni convection to a directional rotating flow,reduces the temperature gradient of the pool and makes the temperature distribution more broader and shallower.With the increases of the magnetic induction intensity,the flow velocity in the molten pool increases,increasing the flushing effect on the melting front.The dimensionless analysis shown that the relative strength of the driving forces in the molten pool were as follows: surface tension,self-induced electromagnetic force,external electromagnetic force and buoyancy force.The external electromagnetic force acts on the front of the molten pool mainly,hence,the convection flow of the molten pool is totally enhanced.