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

In this work, a plasma arc welding process with constant current in the range 25-70 A operating in pure Ar is 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 is also found in the arc core in the nozzle orifice, due to the high velocity of the gas. Also, excitation temperature of Ar atoms is 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. Good agreement is obtained between the modelling predicted electron temperature and the measured excitation temperature in the whole investigated spatial region.