An Improved Theoretical Model for A-TIG Welding Based on Surface Phase Transition and Reversed Marangoni Flow
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joining processes, such as transient liquid phase bonding,[1] brazing,[2,3] friction stir welding,[4–15] laser welding[16–21] and gas tungsten arc (GTA) or tungsten inert gas (TIG) welding[22,23] are in the centre of metallurgical and materials engineering. The success of these processes is dependent on interfacial phenomena. In this paper the A-TIG (=activated TIG) welding process will be particularly discussed. This process was developed by the Ukrainian Paton Institute in 1964 for titanium welding.[24] Later the process was applied for welding of stainless steels.[25–28] The essence of the process is the application of an activating flux, applied on the surface prior to welding, which provides 2–3 times deeper penetration compared to the conventional TIG welding. Although the A-TIG welding process is empirically well known, there is still a controversy in the literature regarding its physical background and theoretical explanation. The following four main theoretical models exist: T. SA´NDOR, Global Segment Manager, is with Esab Kft., Tere´z krt. 55-57, Budapest, 1062 Hungary, and also with Research Group for Metals Technology of the Hungarian Academy of Sciences, Bertalan Lajos utca 7, Budapest, 1111, Hungary. C. MEKLER, Head of Laboratory of the Department of Nanomaterials, is with BAY ZOLTAN Nonprofit Ltd., Igloi 2, Miskolc, 3519 Hungary. J. DOBRA´NSZKY, Senior Fellow, is with Research Group for Metals Technology of the Hungarian Academy of Sciences. G. KAPTAY, Professor, is with BAY ZOLTAN Nonprofit Ltd., and also with the University of Miskolc, Egyetemvaros, E/7, 606, Miskolc, 3515, Hungary. Contact e-mail: [email protected] Manuscript submitted November 29, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
(a) The Simonik model[29] is based on giving the major role to vaporized ions formed from the flux, generated by the heat of the arc. The ions at the edge of the arc are assumed to have lower mobility than in the centre of the arc, leading to an increased current density at the centre of the arc, causing deeper penetration. If the electric conductivity of the arc is due mainly to the presence of ions in the arc, this mechanism is indeed feasible. However, it is known[30] that in fact electrons (and not ions) dominate the electrical conductivity of the arc above 4500 K (4223 ºC). Thus, the theory of[29] probably does not describe the major factor why A-TIG welding provides such a different result from TIG welding. (b) The model of Savitskii and Leskov[31] is based on the reduction of the surface tension due to the flux. The decreased surface tension allows the arc pressure to create a deeper depression in the weld pool which consequently enhances the penetration depth. As will be shown below, the flux indeed leads to the reduction in the value of the surface tension. Consequently, it indeed allows deeper depression of the weld pool. However, it is not the major factor to make a difference between TIG welding and A-TIG welding, as was shown in our previous welding experiments, performed on a vertical plate, using h
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