Marangoni convection and weld shape variations in Ar-O2 and Ar-CO2 shielded GTA welding

Increasing the oxygen or the carbon dioxide concentration in the argon-based shielding gas leads to an increase in the weld metal oxygen content when the oxygen or carbon dioxide concentration is to be lower than 0.6 vol. % in the shielding gas. However, when the O-2 or CO2 concentration is higher than 0.6vol.% in the Ar-based shielding gas, the weld metal oxygen is maintained around 200ppm-250ppm. An inward Marangoni convection mode in the weld pool occurs when the weld metal oxygen content is more than 100 ppm. When it is lower than 100 ppm, the Marangoni convection would change to the outward direction and the weld shape varies from a deep narrow to a shallow wide shape. The effective ranges of O-2 and CO2 concentrations for deep penetration are same. A heavy layer of oxides is formed when the O-2 or CO2 concentration in the shielding gas is more than 0.6 vol.%. Based on the thermodynamic calculation of the equilibrium reactions of Fe, Si, Cr and Mn with oxygen in liquid iron for the oxide products, FeO, SiO2, Cr2O3 and MnO and the experimental oxygen content in the weld metal, Cr2O3 and SiO2 oxides are possibly formed at the periphery area of the liquid pool surface under the arc column during the welding process. One model is proposed to illustrate the role of the oxide layer on the Marangoni convection on the pool surface at elevated temperature. The heavy oxide layer inhibited the fluid flow induced by the Marangoni convection and also became a barrier for the oxygen absorption into the molten weld pool. (C) 2004 Elsevier B.V. All rights reserved.