Metal vapour in atmospheric-pressure arcs
Authors: Murphy Anthony B 
Published: October 2010

Metal vapour has a significant, and in some cases dominant, influence in many applications of atmospheric-pressure plasmas, including arc welding, circuit interruption and mineral processing. While the influence of metal vapour has long been recognized, it is only recently that diagnostic and computational tools have been sufficiently well-developed to allow this influence to be more thoroughly examined and understood. Some unexpected findings have resulted: for example, that the presence of metal vapour in gas–metal arc welding leads to local minima in the temperature and current density in the centre of the arc. It has become clear that the presence of metal vapour, as well as having intrinsic scientific interest, plays an important role in determining the values of critical parameters in industrial applications, such as the weld penetration in arc welding and the extinction time in circuit breakers.
In gas–tungsten arc welding, metal vapour concentrations are formed by evaporation of the weld pool, and are relatively low, typically at most a few per cent. Moreover, the convective flow of the plasma near the weld pool tends to direct the metal vapour plume radially outwards. In gas–metal arc welding, in contrast, metal vapour concentrations can reach over 50%. In this case, the metal vapour is produced mainly by evaporation of the wire electrode, and the strong downwards convective flow below the electrode concentrates the metal vapour in the central region of the arc. The very different metal concentrations and distributions in the two welding processes mean that the metal vapour has markedly different influences on the arc. In gas–tungsten arc welding, the current density distribution is broadened near the weld pool by the influence of the metal vapour on the electrical conductivity of the plasma, and the arc voltage is decreased. In contrast, in gas–metal arc welding, the arc centre is cooled by increased radiative emission and the arc voltage is increased.
In low-voltage circuit breakers, metal vapour is formed by evaporation of the electrodes (runners) and the splitter plates, and can have a major influence on the dynamics of arc motion.
While the influence of metal vapour on arcs is now understood in general terms, there are many unresolved questions. Areas in which improvements and new insights are required include:

diagnostic techniques for measurements of arc properties in the presence of metal vapour, and understanding of the possible deviations from local thermodynamic equilibrium and their influence on such measurements;

measurements of the influence of metal vapour in circuit breakers, in which the arc occurs within a solid enclosure, and in gas–metal arc welding, in which the formation of metal droplets and arc instabilities can disrupt standard techniques;

determination of the concentration of metal vapour species in different types of arcs;

understanding of the relative importance of the different effects of metal vapour (such as increased radiation and electrical conductivity, and the rapid influx of relatively cold gas) on the arc for different configurations;

the influence of metal vapour on the electrode boundary and sheath regions;

the treatment of radiative and mass transport in computational models;

understanding and treatment of the vaporization, condensation and nucleation of metal species, and methods of incorporation of these processes in computational models.

In this cluster issue, many of these and related issues are addressed. The twelve contributions cover gas–metal arc welding, gas–tungsten arc welding and low-voltage circuit breakers, and include both experimental and computational studies, in some cases with striking results. A review of the influence of metal vapour in welding arcs is followed by three accounts of spectroscopic measurements of gas–metal arc welding, which are difficult to perform and until recently have rarely been attempted. The application of spectroscopic techniques to determine Stark widths of spectral lines is discussed in a further contribution. Two papers address the calculation of important plasma data sets, in particular net radiative emission coefficients and diffusion coefficients, which are vital input for computational models. Four sophisticated computational modelling studies of the influence of metal vapour on gas–metal arc welding, gas–tungsten arc welding, and arc splitting in low-voltage circuit breakers are then presented. The final contribution describes the application of a multiscale computational model to investigate the important occupational health problem of the production of fume from the metal vapour produced in welding arcs.
Overall, the papers presented give an overview of the state of the art of research into metal vapour in atmospheric-pressure arcs, and at the same time constitute real progress in this topical and important field.