Influence of molten metal flow on undercutting formation in GMAW

Molten metal flow on weld pool surface in gas metal arc welding process is investigated using a vision-based sensing system and an interpolation algorithm. Bead formation is investigated by analysing flow patterns and its driving forces of weld pool under different welding speed, welding current and shielding gas. Results show that if longitudinal to transverse velocity ratio exceeds 2.0 in the front of weld pool, outward molten metal mainly driven by arc force cannot reach the widest section of the weld pool. Meanwhile, the transverse spreading of molten metal is still hindered in the middle of weld pool as it turns to be inward flow dominated by Marangoni force. These phenomena impede molten metal supply to weld toes which causes undercutting defect. Scaling analysis shows that the predicted undercutting defect agrees well with that resulted from experiments.