Numerical Simulation Of Plasma Behaviors In Laser-arc-magnetic Field Hybrid Welding

Welding of thick plates is one of the key technologies in high-level manufacturing such as aerospace,energy power and marine equipment.It is difficult to ensure the stability of hybrid plasma in laser-arc multi-layer welding,and thus it is easy to appear incomplete fusion at the side walls and between layers.Applying external magnetic field to laser-arc hybrid welding,which can control the plasma precisely and then ensure the stability of the welding process,is a new multi-energy field hybrid welding technology and has broad development potentials.However,plasma behaviors are very complex under the combined effects of laser,arc and magnetic field,and so it is difficult to reveal the essence of welding process only by experimental method.Therefore,it has important significances to develop a numerical simulation model to study the plasma behaviors and physical characteristics in laser-arc-magnetic field hybrid welding for studying the synergetic mechanism and improving the weld quality.By numerical simulations and experiments,plasma behaviors and characteristics were analyzed in magnetic field controlled MIG welding,laser-MIG hybrid welding and laser-MIG-magnetic field hybrid welding.The main work is listed as follows:(1)Theoretical basis related to simulation analysis of plasma were analyzed.Experimental platform for hybrid welding integrating high-speed imaging system and spectrum analysis system was set up,which could perform observations of plasma behaviors in magnetic field controlled MIG welding,laser-MIG hybrid welding and laser-MIG-magnetic field hybrid welding.(2)A numerical simulation model of magnetic field controlled arc plasma in MIG welding was established for the arc behaviors under magnetic field when the wire filler was inclined.In the model,“LTE-diffusion approximation”approach was adopted to deal with electrode sheaths.The effects of external longitudinal magnetic field with different magnetic induction strengths and directions were studied.The simulation results agreed with the experimental results by comparing plasma shapes obtained by the high-speed camera and the eventual weld appearances.The phenomena of arc plasma spreading out with a limited extent(the spread was the largest when the magnetic induction strength was-16mT)and deflecting to one side were found in magnetic field controlled MIG welding with inclined wire filler,and mechanisms of the two phenomena were analyzed.(3)For the behaviors of hybrid plasma in laser-leaded arc hybrid welding,a numerical simulation model of hybrid plasma in laser-MIG hybrid welding was developed.In the model,metal vapor ejecting from the keyhole was considered and diffusion coefficient was calculated by viscosity approximation approach.The effects of metal vapor ejecting parameters,welding current,distance between laser and arc,physical property on hybrid plasma characteristics were analyzed.The simulation results of plasma shapes and temperatures were consistent with the experimental results obtained from high-speed images and spectrum analysis.The metal vapor could decrease plasma temperature within its affecting zone(temperature drop could reach 5000K),thus could decrease the local heat input to the base metal(from 1.0E7W/m~2 to 5.0E5W/m~2).The current density in the metal vapor ejecting zone increased(from 5.5E6A/m~2 to 8.0E6A/m~2),which could make the arc more stable in the welding process.Within appropriate welding parameters,laser had higher stability ability for arc with lower welding current,and the stability ability strengthened with increased laser power.(4)A numerical simulation model of hybrid plasma in laser-MIG-magnetic field hybrid welding was developed,in which combined diffusion coefficients related to mole fraction gradient,pressure gradient,electrical field and temperature gradient were adopted.Numerical simulation of hybrid plasma behaviors in laser-MIG-magnetic field hybrid welding was conducted at the first time,and the simulation results were in good agreement with experimental results.Effects of longitudinal magnetic field,welding current and distance between laser and arc on characteristics of hybrid plasma were analyzed.The findings were as below:The bottom of the hybrid plasma spread out with the largest extent in weld width direction when magnetic induction strength was-16mT,and with the largest extent in welding direction when magnetic induction strength was-24mT.Constant longitudinal magnetic field made the hybrid plasma deflect to one side in weld width direction,resulting in asymmetry of the weld bead.The stability ability of laser for arc weakened when the distance between them was larger than 3mm,and the optimum distance was 2mm.With appropriate welding process parameters,laser-MIG-magnetic field hybrid welding can increase the spread of hybrid plasma to improve the gap-bridging ability,increase the velocity of hybrid plasma on the workpiece surface to enhance stirring effect for the molten pool,decrease the arc temperature to reduce the local heat input to the workpiece,and increase the current density near the vapor ejecting location to make the welding process more stable.