The methods used to model thermal plasmas, including treatments of diffusion in arcs in gas mixtures, are reviewed. The influence of thermophysical properties on the parameters of tungsten–inert-gas (TIG) welding arcs, particularly those that affect the weld pool, is investigated using a two-dimensional model in which the arc, anode and cathode are included self-consistently. The effect of changing each of six thermophysical properties on the characteristics of an argon TIG arc is assessed. The influence of the product of specific heat and mass density is found to be particularly important in determining the arc constriction. By examining the influence of the different properties on the heat flux density, current density and shear stress at the anode, it is concluded that the weld pool depth can be increased by using shielding gases with high specific heat, thermal conductivity and viscosity. The effect of metal vapour on the arc and weld pool properties is assessed. The most important effect of the metal vapour is found to be the increased electrical conductivity at low temperatures, which leads to lower heat flux density and current density at the weld pool, implying a shallower weld pool.
Tungsten–inert-gas welding arcs are modelled using a two-dimensional axisymmetric computational code. Both electrodes (the tungsten cathode and the metal anode workpiece) and the arc plasma are included self-consistently in the computational domain. The influence of adding helium, hydrogen and nitrogen to the argon shielding gas is investigated. It is found that addition of any of the gases increases the heat flow to and the current density at the anode. The shear stress and the arc pressure at the anode surface are increased by adding hydrogen or nitrogen or up to about 50 mol% helium, but decrease when more helium is added. It is predicted that the effect of adding any of the gases is to increase the depth of the weld pool, in agreement with the experimental evidence. The results are explained by referring to the thermodynamic and transport properties of the gas mixtures.
The influence of shielding gas composition on arc properties including temperature, voltage, heat flux and shear stress at the anode and also the weld depth as indicated by the maximum temperature of a water cooled anode is investigated. It is found that the additions of helium, hydrogen and nitrogen to argon all increase the arc and anode temperature. For helium, this is due to the lower electrical conductivity; in the other cases, it is due to the higher specific heat.
A gas tungsten arc in helium and argon was modelled taking into account the contamination of the plasma by metal vapour from the weld pool. The whole region of gas tungsten arc atmosphere including the tungsten cathode, arc plasma and weld pool was treated using a unified numerical model. A viscosity approximation was used to express the diffusion coefficient in terms of viscosity of shielding gas and metal vapour. The transient two-dimensional distributions of the temperature, velocity of plasma flow and iron vapour concentration were predicted, together with the weld penetration as a function of time for a 150 A arc current at the atmospheric pressure, both for helium and argon welding gases.It was shown that the thermal plasma in gas tungsten arcs is influenced by iron vapour from the weld pool surface and that the concentration of iron vapour in plasma is dependent on the temperature of the weld pool.
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