Mechanisms giving increased weld depth due to a flux

Abstract

It is known that for ‘tungsten inert gas’ welding of stainless steel, the weld depth can be increased by a factor of two or more if a thin layer of flux powder is initially coated on the steel. Numerical predictions from a unified electrode-arc-weldpool model are made elucidating the possible role of three different mechanisms previously proposed to explain this effect, and we also examine a new fourth possible mechanism, namely the effect that the flux is an electrical insulator. (1) Calculations support the suggestion that if the dissolved flux in the weld pool changes the temperature dependence of the surface tension to increase rather than decrease with temperature, the direction of convective circulation within the weld pool can change from radially outwards to radially inwards, leading to an increased weld depth of the order of 2 mm. (2) The effect of electron attachment to flux vapour in the arc, if the vapour contains an attaching gas such as oxygen, is found to have a negligible effect in producing arc constriction and thus a higher current density to increase weld depth. (3) Our treatment assumes that the surface of the weld pool is undepressed by the arc, so that we have not examined the suggestion that increased weld depth is caused by the flux lowering surface tension and thus increasing depression of the weld pool surface. But experimental observations indicate that such depression is negligible for currents of less than 200 A. (4) If the flux produces an insulating layer on the metal surface, calculations indicate that there can be a marked increase in weld depth of the order of 5 mm or more due to the flux blocking the current in the outer regions of the arc thus producing a higher current density at the arc centre. Furthermore these calculations indicate that the flux causes an arc spot to form at the anode, in agreement with similar experimental observations of an anode spot in the presence of a flux.