Marangoni convection in weld pool in CO 2 -Ar-shielded gas thermal arc welding
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8/12/04
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Marangoni Convection in Weld Pool in CO2-Ar–Shielded Gas Thermal Arc Welding SHANPING LU, HIDETOSHI FUJII, and KIYOSHI NOGI Small CO2 additions of 0.092 to 10 vol pct to the Ar shielding gas dramatically change the weld shape and penetration from a shallow flat-bottomed shape, to a deep cylindrical shape, to a shallow concavebottomed shape, and back to the shallow flat-bottomed shape again with increasing CO2 additions in gas thermal arc (GTA) welding of a SUS304 plate. Oxygen from the decomposition of CO2 transfers and becomes an active solute element in the weld pool and reverses the Marangoni convection mode. An inward Marangoni convection in the weld pool occurs when the oxygen content in the weld pool is over 80 ppm. Lower than 80 ppm, flow will change to the outward direction. An oxide layer forms on the weld pool in the welding process. The heavy oxide layer on the liquid-pool surface will inhibit the inward fluid flow under it and also affects the oxygen transfer to the liquid pool. A model is proposed to illustrate the interaction between the CO2 gas and the molten pool in the welding process.
I. INTRODUCTION
THE shape and penetration of gas thermal arc (GTA) welds have long been of concern, because the GTA welding process is often applied in modern industry, where high quality and precision welds are required. Surface-active elements, such as oxygen, sulfur, and selenium of group VIA, can significantly change the weld penetration in GTA welding when their concentration in the weld pool is sufficient. In order to increase weld production, the demand for automatic and precise control of the weld with deep penetration is steadily increasing. Furthermore, for welds made by automatic equipment, it is difficult to compensate for variability in the weld-pool geometry once the welding parameters have been set. Understanding and precisely controlling the effect of minor elements on the weld shape are critical for generating a satisfactory weld joint. After decades of development, there are several ways available to add surface-active elements to the weld pool. These active elements can be contained in the material to be welded,[1–6] supplied to the welding pool by a preplaced flux layer on the substrate,[7–20] or adjusted by active gaseous addition to the argon shielding gas.[21,22,23] The intentional or unintentional addition of a small amount of minor elements to the base material significantly changes the weld penetration. However, the addition of the active elements to the substrate sometimes deteriorates its mechanical properties.[22] By smearing a layer of halides or oxides on the plate surface, deep weld penetration was first obtained (active flux TIG welding or ATIG) in the 1960s at the Paton Electric Welding Institute in Ukraine (Kiev).[24] Former research results[25,26] showed that the effect of oxide fluxes on the weld-pool oxygen content and weld shape is sensitive to the flux quantity for SUS304 stainless steel containing a low oxygen content (16 ppm). The flux smearing
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