Study On Numerical Simulation Of Arc Plasma Behavior In DC TIG And AC TIG Welding

As a commonly used welding method,TIG welding has been widely applied in production.Because of its high welding quality and good weld quality,it is often used in welding with high quality requirements.In the welding of Mg,Al and its alloys,they are easy to be oxidized due to their own properties.During the welding process,the oxides on the base metal influence the welding quality and weld formation seriously.For welding of magnesium,aluminum and its alloys,Direct current electrode positive(DCEP)or alternating current TIG welding are often used.When the DCEP TIG welding is used,the base metal is the cathode,the masses of the cations in the arc flow towards the anode,which has a great impact on the surface of the anode.The oxide film produced on the surface of the base metal can be cleaned to improve the welding efficiency and welding quality.But in DCEP TIG welding process,the tungsten electrode acts as the anode,and electrons flow to the anode,which makes the anode temperature rise,and finally causes the burn loss of the tungsten electrode.In the AC TIG welding process,when the current is in the negative half cycle,it is equivalent to Direct current electrode negative(DCEN)TIG welding,and the temperature of the tungsten is low,which can be used to cool the overheated tungsten electrode after the positive half cycle.The plasma characteristics of different welding methods are different.In this paper,the motion state of plasma in DC and AC welding is studied by means of numerical simulation,and the arc characteristics of DCEP,DCEN and AC TIG welding will be analyzed.Firstly,the welding conditions for TIG welding are analyzed,and two dimensional axisymmetric model is established.A mathematical model based on magnetohydrodynamics equations and Maxwell equations is established for welding process.The Fluent software is used to select the solver of the pressure gauge,and the Simply algorithm is used to calculate the steady state of the DCEP and DCEN TIG welding,and the transient calculation is carried out for the AC TIG welding.Secondly,the numerical simulation results of DCEP and DCEN TIG welding are analyzed and compared,and the effects of different current on the characteristics of arc plasma are calculated and analyzed.The results show that the temperature,potential,pressure,current density and speed of the plasma are increased when the current rises.In DCEP welding process,plasma has a certain degree of contraction,and the energy is more concentrated.Compared with DCEN welding process,the temperature,potential,pressure,current density and speed of arc are higher.Compared with the DCEN welding process,the temperature of the tungsten electrode can reach above the tungsten melting point when the DCEP welding occurs,which makes the tungsten electrode burn down.Under the same current,the pressure,current density and heat flux of the base metal of the DCEP welding are all higher than those of the DCEN welding.The arc contraction effect is more obvious during the DCEP welding process,and the high temperature region of the arc is closer to the tungsten electrode.The temperature gradient in the arc column area is larger than that in the pressure gradient.Finally,during the AC TIG welding process,the evolution process of plasma characteristics with time is studied,and the influence of arc current variation on it is analyzed.The results show that the plasma characteristics of AC square wave TIG arc periodically change.The temperature,electric potential,pressure and velocity of the plasma are far less than those of the steady current when the current goes through zero.The negative polarity during a period is higher than that of straight polarity,and the high temperature area of arc column is closer to tungsten electrode.When the current is zero,the temperature,potential,pressure and velocity of the rising edge of the current are all lower than the current falling edge.With the increase of current,the temperature,potential,pressure and velocity of the plasma increase.In the process of reaching the stable state after the current is zero,the change of plasma characteristics has a certain delay relative to the current change,and the greater the current of the arc,the more obvious the delay.