Investigations of GMAW plasma by optical emission spectroscopy

We report on investigations of gas metal arc welding plasma operated in pure argon and in a mixture of argon and CO2 at a dc current of 326 A. The spatially resolved electron densities and temperatures were directly obtained by measuring the Stark widths of the Ar I 695.5 nm and Fe I 538.3 nm spectral lines. Our experimental results show a reduction of the plasma conductivity and transfer from spray arc to globular arc operation with increasing CO2 concentration. Although the electron density ne increases while approaching the core of the plasma in the spray-arc mode, a drop in the electron temperature T-e is observed. Moreover, the maximum T-e that we measure is about 13 000 K. Our experimental results differ from the Haidar model where T-e is always maximum on the arc axis and its values exceed 20 000 K. These discrepancies can be explained as a result of underestimation of the amount of metal vapours in the plasma core and of the assumption of local thermal equilibrium plasma in the model.