Mechanism and optimization of oxide fluxes for deep penetration in gas tungsten arc welding

Five single oxide fluxes-Cu2O, NiO, SiO2, CaO, and Al2O3-were used to investigate the effect of active flux on the depth/width ratio in SUS304 stainless steel. The flux quantity, stability, and particle size effect on the weld-pool shape and oxygen content in the weld after welding was studied systematically. The results showed that the weld depth/width ratio initially increased, followed by a decrease with the increasing flux quantity of the Cu2O, NiO, and SiO2 fluxes. The depth/width ratio is not sensitive to the CaO flux when the quantity is over 80 X 10-5 mol on the 5 X 0.1 X 50 mm slot. The Al2O3 flux has no effect on the penetration. The oxygen content dissolved in the weld plays an important role in altering the liquid-pool surface-tension gradient and the weld penetration. The effective range of oxygen in the weld is between 70 and 300 ppm. A too-high or too-low oxygen content in the weld pool does not increase the depth/width ratio. The decomposition of the flux significantly depends on the flux stability and the particle size. Cu2O has a narrow effective flux-quantity range for the deep penetration, while the Al2O3 flux has no effect. The SiO2 flux with a small particle size (0.8 or 4 mum) is a highly recommended active flux for deep penetration in actual gas tungsten arc welding (GTAW) applications.