Effect of molten pool behaviors on welding defects in tandem NG-GMAW based on CFD simulation

Abstract

The simulation of tandem narrow-gap GMAW based on computational fluid dynamics (CFD) was carried out to establish the relationship between the molten pool behaviors and weld bead formation under different wire feed rates (vf) and welding speed (vw). Inclined arcs were considered in the model by a novel rotated coordinate system. The variation of effective arc radius with welding current was also taken into consideration in the model. The experimental results illustrate that small vfor large vw easily leads to lack-of-fusion (LOF) defect. However, large vf also causes finger penetration. The simulation results indicate that a clockwise vortex in the right sidewall and an anticlockwise vertex in the left sidewall enhance the transverse transmission of fluid and heat, which is conducive to forming a concave weld bead. When vf is small or vw is large, due to low arc heat input, the groove sidewall cannot be melted and the transverse flow in the molten pool is weak. Furthermore, owing to the small flow velocity and large solidification rate of molten metal, the preferentially solidified metal further hinders the spreading of liquid metal to the sidewalls. As a result of high vf, the droplet that has high transition frequency and velocity transfers the arc heat and the high momentum to the molten pool bottom. The downward flow in the molten pool center is enhanced and its velocity is also increased, resulting in finger penetration.