A MATHEMATICAL-MODEL OF THE ARC IN ELECTRIC-ARC WELDING INCLUDING SHIELDING GAS-FLOW AND CATHODE SPOT LOCATION

A mathematical model of a free-burning TIG electric are with non-consu mable electrodes using a flat anode and argon shielding gas is present ed. Differential equations describing the conservation of mass, moment um and energy are solved together with Maxwell's equations describing the electromagnetic field. The dependence of transport coefficients on temperature is taken into account. The gas flow is assumed to be lami nar and the partially ionized plasma is assumed to be in local thermod ynamic equilibrium. The mathematical model can cope with a broad range of operating conditions. The model is used to demonstrate the strong influence that the velocity and temperature of the flow of gas enterin g the top of the electric are in the region of the cathode can have on the are column. In particular, it is shown that cathode flows of stre ngth sufficient to produce a significant constriction of the electric are need to be assumed in order to account for experimentally measured electric fields in the are column as well as the total voltage drop f or 10 mm arcs. The use of this model also shows the part played by the cathode spot and its location in the nature of the electric are colum n. In particular, two complementary techniques for studying the are co lumn are highlighted. In the first, a strictly stable static are is ne eded in order to employ the spectroscopic technique of temperature mea surement. In contrast, when the position of the are is less stable, a rapid measurement with an electric probe is indicated in order to meas ure the electric field. The are model described by Ducharme et al (199 3) will then yield the temperature field. The model presented here pro duces the results for the temperature distribution and for the electri c field based on the use of appropriate boundary conditions.