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 CO 2 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 CO 2 concentration. Although the electron density n e 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.