The effect of multi-coupled driving forces on molten pool behavior and defect formation in high-current and high-speed gas tungsten arc welding is investigated quantitatively and systematically through combination of sensitivity analysis and experimentally validated numerical modelling. The arc shear stress is the dominant driving force to facilitate free surface deformation of molten pool and backward flow of liquid metal, which masks the effect of Marangoni force. The capillary pressure plays a dominant effect on suppressing free surface deformation. Three dimensionless groups with explicit physical implications are derived to evaluate tendency of high-speed weld bead defect formation using Buckingham it-theorem. The relevant sensitivity analysis shows that arc shear stress and capillary pressure are the most promoting and suppressive factors for defect formation, respectively. High active element concentration induces undercut and humping defects by decreasing capillary pressure. (C) 2016 Elsevier Ltd. All rights reserved.