Numerical Simulation Of Non-equilibrium Characteristic Of Double Shielded TIG Welding

The welding arc, as a thermal plasma, is regarded to be in local thermodynamic equilibrium(LTE). However, large deviations from LTE appear in the electrodes and the fringe of arc column, duing to the low exchange rate of translational energy between electrons and heavy particles. It is essential to study the non-equilibrium characteristic of arc plasma by employing two-temperature model. A two-dimensional axisymmetric mathematical model for TIG and double shielded TIG welding arc plasma is established based on the assumption of two-temperature. The non-equilibrium characteristics of plasma are analysised by using FLUENT to solve the conservation equations of mass, momentum, electron energy and heavy particle energy, as well as Maxwell equations.Firstly, the distributions of electron temperature, heavy particle temperature, pressure, velocity and potential for TIG arc are obtained, the computing results are compared with those of Hsu’s along with his experimental results. The plasma shows a near-LTE behaviour along the arc axis and in a region surrounding the axis, but a large deviation from LTE is observed in the near-electrode regions and outside the fringe of arc column. Comparisions between the calculated results and those of Hsu’s show good agreement,and the distribution of temperature is in good agreement with experiment.Secondly, the fluences of gas flow rate and welding current on arc characteristics of TIG welding are studied numerically. It is shown that the arc shrinks and the area of non-equilibrium expands with the increase of the flow rate. Arc pressure and current density at the anode increase and the temperature rises when the welding current is higher.Thirdly, the distributions of electron temperature, heavy particle temperature, pressure, velocity and potential, especially the non-equilibrium parameter for double shielded TIG welding arc are obtained based on 2T assumption. The fluences of gas flow rate, welding current, coupling degree, arc length and nozzle configuration on plasma non-equilibrium characteristics of double shielded TIG welding are studied numerically. It is shown that the non-equilibrium phenomenon is caused by the low exchange rate of translational energy between electrons and heavy particles, high temperature region shows a near-LTE behaviour because of the large electron number density and high collision frequency. Increasing outer layer gas flow rate and inner layer gas flow rate both enhance the non-equilibrium characteristic of arc plasma. The area of non-equilibrium shrinks with the increase of the welding current as a result of the expansion of high temperature region. The non-equilibrium phenomenon of arc plasma weakens in radial direction with the increase of arc length. With the increase of coupling degree, the non-equilibrium effect becomes weakly. The reduce of the angle between the outer gas channel and the level results in the enhancement of cooling, which strengthens the non-equilibrium phenomenon.Finally, the inelastic collisions in plasma is discussed. It is found that the plasma temperature is affected by the energy interchange in inelastic collisions. The temperature drops to near the results calculated using the assumption of LTE. The inelastic collisions can be ignored in order to simply the calculation.