Marangoni Convection in Weld Pool in CO_2-Ar-Shielded Gas Thermal Arc Welding

摘要

Small CO_2 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 concave-bottomed shape, and back to the shallow flat-bottomed shape again with increasing CO_2 additions in gas thermal arc (GTA) welding of a SUS304 plate. Oxygen from the decomposition of CO_2 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 CO_2 gas and the molten pool in the welding process.