Temperature profiles of welding arcs and its interpretation

Summary form only given. Pulsed gas metal arc welding is a widespread and established joining technology. Improvements of efficiency and join quality can be obtained by reducing and focusing the heat input into the workpiece. The energy transfer takes place mostly during the high-current arc phase. Its spatial distribution depends on the arc property profiles and those of the arc root at the weld pool surface. Radial temperature profiles of the arc at different distances from the weld pool in the high-current phase of several pulsed gas metal arc welding processes under different shielding gases have been determined by optical emission spectroscopy. In addition, the plasma composition and, in particular, the density profiles of metal vapour in the arc have been deduced. Therefore, the absolutely calibrated spectral radiances of atom lines of metal and shielding gas species have been recorded and analysed over the arc cross section. The arc shape, assumed as nearly rotational symmetric, and its dynamic behaviour during the spectroscopic recording have been controlled by high-speed imaging. As examples of recent studies, results for the arc in a onedrop-per-pulse process and in a short-circuit process will be discussed. A high content of metal vapour, in some cases more than 50% mass fraction, has been observed in the arc centre resulting from the strong evaporation of the molten wire tip. This metal content causes plasma temperatures in the arc centre lower than in outer regions, hence, a temperature minimum in the arc axis under certain conditions. Here, the choice of the shielding gas is an important parameter. The use of either the rare gas argon or larger additions or even pure molecular gases like CO2 results in a different arc structure and weld pool behaviour. It can be used to influence the arc profile and, hence, to focus the energy transfer to central regions of the weld pool. The interaction of arc regions with low and high content of metal vapour can be interpreted by a simple power balance taking into account the leading terms of Ohmic heating and radiation. Considering the temperature dependence of the electrical conductivity and the net emission for different gas mixtures, a qualitative picture of the arc structure is deduced. Effects like the temperature minimum in the arc axis find a clear interpretation.