An Improved Theoretical Model for A-TIG Welding Based on Surface Phase Transition and Reversed Marangoni Flow

It is experimentally shown that a thin layer of silica flux leads to an increased depth of weld penetration during activated TIG (=A-TIG) welding of Armco iron. The oxygen-content is found higher in the solidified weld metal and it is linked to the increased depth of penetration through the reversed Marangoni convection. It is theoretically shown for the first time that the basic reason of the reversed Marangoni convection is the phenomenon called “surface phase transition” (SPT), leading to the formation of a nano-thin FeO layer on the surface of liquid iron. It is shown that the ratio of dissolved oxygen in liquid iron to the O-content of the silica flux is determined by the wettability of silica particles by liquid iron. It is theoretically shown that when the silica flux surface density is higher than 15 µg/mm2, reversed Marangoni flow will take place along more than 50 pct of the melted surface. Comparing the SPT line with the dissociation curves of a number of oxides, they can be positioned in the following order of their ability to serve as a flux for A-TIG welding of steel: anatase-TiO2 (best)-rutile-TiO2 (very good)-silica-SiO2 (good)-alumina-Al2O3 (does not work). Anatase (and partly rutile) are self-regulating fluxes, as they provide at any temperature just as much dissolved oxygen as needed for the reversed Marangoni convection, and not more. On the other hand, oxygen can be over-dosed if silica, and other, less stable oxides (such as iron oxides) are used.

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