Investigation of transient metal vapour transport processes in helium arc welding by imaging spectroscopy

In order to understand the transient transport mechanisms of metal vapour in an arc plasma, imaging spectroscopy was performed for helium arc plasmas burning on pure metals. The measured images were converted into the spectral intensity distributions at the central cross-section of the arc plasma by image processing and Abel inversion. The transient transport process of metal vapours inside the arc plasma are discussed in terms of the diffusion coefficients of metal vapour. The metal vapour generated from the weld pool surface was transported into the arc plasma and to the region near the tungsten electrode within a few seconds. As time passed, the metal vapour spectral intensities gradually increased near the weld pool surface, near the electrode and in the intermediate region. In contrast, the He I spectral intensity was weak and the He I distribution shrunk toward the centre of the arc. This is because the helium arc plasma was cooled by the strong radiation from metal vapour. For an arc burning on pure chromium, chromium ions reached to centre of the arc after about 40 s. In contrast, on pure iron, iron ions did not reach to the centre of the arc. This is because the helium arc plasma on pure chromium was cooled to a temperature at which the electric field and ordinary diffusion coefficients of chromium are largest, whereas the temperature of the plasma on pure iron remained high. Two factors contributed to this; the higher boiling point of iron, which reduces the iron vapour concentration, and the lower radiative emission coefficient of iron.