Numerical analysis of molten metal behavior and undercut formation in high-speed GMAW

[Display omitted] •A self-adaptive integrated arc heat-force distribution model is developed for GMAW.•Backward flow, gouging region and thin metal layer are processes initiating undercut.•Two dimensionless numbers are developed to predict the growth rate of undercut. A computational fluid dynamics model was established to investigate the undercut formation in high speed gas metal arc welding (GMAW). A double-ellipse integrated "arc current density-arc pressure-electromagnetic force-arc heat" distribution model, which could be self-adaptive to weld pool surface evolution, was developed. The multi-coupling transport phenomena in weld pool were simulated, and the effect of driving forces on the behaviors of molten metal and the formation of undercut was analyzed quantitatively. The results show that the high-velocity backward molten metal flow, large-size gouging region and prematurely solidified thin metal layer at weld toe were typical processes that initiated undercut. The evolution of undercut morphology was determined by the characteristics of inertia force, hydrostatic pressure and arc pressure in the different stages. Based on Buckingham-π theorem, the dimensionless growth rate of undercut area was a parabolic function of a dimensionless group including characteristics of stress state, molten metal flow and morphology of weld pool. This study clarified the physical origin of undercut defect in GMAW and might also provide some basic guidelines for its suppression.