Two-temperature modelling and optical emission spectroscopy of a constant current plasma arc welding process

Abstract

Summary form only given. Plasma arc welding (PAW) is a process where an electric arc is created between a pointed thoriated tungsten electrode and a workpiece to melt it. The electrode is positioned inside the torch body and plasma gas is fed around it. The arc is constricted through a copper nozzle in order to increase the plasma velocity and temperature. A sheath gas is injected axially (usually with a swirl component) and concentrically around the arc, allowing the protection of the weld pool from contamination of the surrounding oxidant atmosphere. The combination of constriction and convection stabilization provides a high speed focused plasma jet with deeper weld penetration and higher energy concentration and resistance to perturbations than the gas tungsten arc welding (GTAW) process.In this paper the Authors compare the results obtained by means of both LTE and non-LTE two-temperature (2T) numerical modelling with the results of OES diagnostics, proposing a method to extend the Boltzmann plot technique to regions where lines s/n is poor and discussing its validity in case of thermal non-equilibrium conditions; also demonstrating how this approach can be effectively used to characterize a plasma source of industrial interest. For this reason modelling and diagnostics activities have been performed on a commercial plasma source torch. A PAW process with constant current in the range 25-70 A operating in pure Ar has been characterized by means of both thermo-fluid-dynamic modelling under the assumption of local thermodynamic equilibrium (LTE) and two-temperature thermal non-equilibrium modelling (2T), allowing a comparison of the LTE temperature fields with electron and heavy particle temperature fields: thermal non-equilibrium is strongest in the fringes of the arc and upstream the plasma flow even though a temperature difference between electrons and heavy particles has been found also in the arc core in the nozzle orifice, due to the high velocity of the gas. Also, excitation temperature of Ar atoms has been obtained from optical emission spectroscopy measurements using a new method (called hybrid method) that extends the usability of the Boltzmann plot method to spatial regions where the signal to noise ratio of the spectral lines adopted in the calculation is poor. A good agreement has been obtained between the modelling predicted electron temperature and the measured excitation temperature in the whole investigated spatial region.