Research On Dynamic Process Of Pilot-main Arc Coulpling In Micro-plasma Arc Welding With Pulsed Current

Micro-plasma arc welding(MPAW)is a hybrid plasma arc which includes transferred plasma arc and non-transferred plasma arc.The power of both arc are supplied by two sources respectively.Arc caused by MPAW appears the small noodle,concentrated arc energy,high straighten,low heat input.When the current is lower than 0.1 A,micro plasma arc also can keep steady combustion because of the existence of constrained effect from nozzle and non-transferred plasma arc which can explained the application for welding ultra-sheet metal.The physics field of micro plasma arc welding has been numerically simulated by some scholars,but the effect of pilot arc is not considered which is existed in the actual welding process.Therefore,the research for coupling of main arc and pilot arc in MPAW is more correspond to actual welding process.It is essential to load pilot arc between tungsten rod and nozzle for further study to reveal the physical nature of coupling arc of main arc and pilot arc which can provide theoretical basis for control and application in actual welding process.In this study,a three-dimensional numerical analysis model of non-transferred arc in micro-plasma arc welding arc was established.Physical quantity of non-transferred arc in micro-plasma arc welding under direct current was calculated based on each model.Based on analysis of problems in each model,non-transferred arc model was improved to make the simulation results more accurate.Based on improved non-transferred arc model,distribution of temperature field,flow field,current density,electromagnetic force and arc pressure of non-transferred arc in micro-plasma arc welding were recalculated..Results show that: non-transferred arc temperature is the highest at 0.25 mm away from tungsten end,and current density and electromagnetic force is also the highest here.Velocity of non-transferred arc becomes stable after 0.6 s.The highest arc pressure is on the middle of section of arc.Based on the calculation of physical process of non-transferred arc,physical environment coupling method was applied to couple non-transferred arc and transferred arc.Calculation results of physical fields of non-transferred arc were transferred to transferred arc as environment load.Distribution of temperature field,flow field,electric field intensity,magnetic induction intensity,electromagnetic force,and arc pressure were studied based on non-transferred arc coupled with transferred arc model and was compared with distribution of temperature field,flow field,electromagnetic force and arc pressure of transferred arc.Results show that: temperature of non-transferred arc coupled with transferred arc is the highest at 0.25 mm away from tungsten end,and current density,magnetic induction intensity and electromagnetic force is also the highest here.Velocity of non-transferred arc coupled with transferred arc becomes stable at about 0.8 s.It is different from physical fields of transferred arc.Temperature of transferred arc at outlet of nozzle is lower than that of non-transferred arc coupled with transferred arc.Temperature distribution of transferred arc coupling is mainly concentrated on the area between tungsten end and workpiece,while temperature of non-transferred arc coupled with transferred arc is not only distributed between tungsten end and workpiece but also distributed between the tungsten bar and the nozzle.Axial velocity of non-transferred arc coupled with transferred arc within nozzle was bigger than that of transferred arc.Axial velocity of non-transferred arc coupled with transferred arc outside nozzle near nozzle exit was lower than that of transferred arc.Axial velocity of non-transferred arc coupled with transferred arc outside of nozzle is consistent with that of transferred arc.Electromagnetic force of non-transferred arc coupled with transferred arc within nozzle was more concentrated along arc axial center and was bigger than that of transferred arc.Arc pressure of non-transferred arc coupled with transferred arc was bigger than that of transferred arc.On the basis of analysis of the physical process of non-transferred arc coupled with transferred arc in micro-plasma arc welding with direct current,dynamic distribution of temperature field,flow field,electromagnetic force and arc pressure of non-transferred arc coupled with transferred arc in micro-plasma arc welding with pulsed current was further studied,and was compared with dynamic distribution of temperature field,flow field,electromagnetic force and arc pressure of transferred arc with pulsed current.Results show that: physical processes of non-transferred arc coupled with transferred arc in micro-plasma arc welding changed dynamically with welding current alternately changing between base current and peak current.Hysteresis and inertia existed during the dynamic process.Dynamic distribution of each physical field of non-transferred arc coupled with transferred arc was different from that of transferred arc.Finally,temperature distribution characteristics of visible non-transferred arc coupled with transferred arc(the arc outside of nozzle)in micro-plasma arc welding had more high agreement with distribution results by using spectrum detection and image processing of high-speed photography than temperature distribution characteristics of visible transferred arc,respectively.