Metal Vapour in MIG Arcs Can Cause (1) Minima in Central Arc Temperatures and (2) Increased Arc Voltages

Abstract

Radiation emission coefficients of plasmas from high temperature metal iron vapour are about a factor of a hundred larger than emission coefficients of high temperature argon plasmas as occur in argon welding arcs. Iron vapour can be easily introduced into the central region of such a welding arc due to melting of an electrode containing iron. The high radiation emission can produce several effects contrary to the usual behaviour of arcs in pure argon. Firstly, a central cylindrical core of iron vapour will be highly radiating and calculations show that the central core can consequently be at a lower temperature than the surrounding argon plasma. Because this central core has higher radiation emission it can give the appearance of having a higher central temperature. Secondly, it is predicted that radiation cooling of the arc due to iron vapour reduces the arc temperature and has the effect of increasing the arc voltage. The effect of increased radiation in the main arc column outweighs the effect of iron vapour increasing the electrical conductivity of the plasma at low temperatures which has the effect of reducing the arc voltage. This latter effect is limited to a small region near the electrodes.