Numerical modelling of fluid flow and weld penetration in activated TIG welding

Abstract

This research work emphasis on understanding the weld pool development while carrying out Activated Tungsten Inert Gas (A-TIG) welding by employing Computational Fluid Dynamics (CFD). The influence of temperature coefficient of surface tension (∂γ/∂T) and the concentration of oxygen, the main surface active component on weld bead geometry is studied. A three-dimensional (3D) numerical model has been developed for A-TIG welding process with varying oxygen levels employing ANSYS Fluent for 316LN stainless steel. The general conservation equations consisting mass, energy and momentum were solved in order to acquire the temperature and velocity fields with oxygen levels varying from 50 to 300 ppm in weld pool. The impact of driving forces particularly buoyancy and Marangoni on the weld pool convection were examined. The simulation confirmed that the weld bead shape was influenced mainly by Marangoni convection consecutively dependent on the oxygen content in the weld metal in the course of A-TIG welding. The addition in oxygen content changes the surface tension temperature coefficient ∂γ/∂T from negative to positive gradient causing inward flow. It was found that minor oxygen content in the molten pool until 150 ppm exhibited outward flow and with higher oxygen content beyond 150 ppm exhibited an inward flow. The oxygen content in the weld bead without flux and with flux was found to be 69 and 336 ppm respectively for the type 316LN stainless steel plates welded by A-TIG welding. The simulated weld bead profile was validated by comparing with the experimentally obtained weld bead profile. There was excellent agreement between the two weld bead profiles.