Effect of process parameters on root pass welding and analysis of microstructure in V-groove pulsed gas metal arc welding for mild steel

This study performed root pass welding with a V-groove using a pulsed gas metal arc welding process for mild steel. Welding process parameters such as weaving, root gap size, and travel speed were major factors in the formation of the back-bead. A high-speed camera and a synchronized data acquisition system captured dynamic molten pool images and welding signals (current and voltage) simultaneously. The back-bead shape differed depending on the root gap opening. Without a root gap, the accumulated molten pool created a cushion effect in the arc center which reduced the momentum of downward flow. In contrast, the back-bead depth could be formed with a root gap opening (1 mm, 2 mm), but the back-bead shapes were different from each other at different travel speeds. The back-bead shapes also varied depending on the weaving conditions. Despite the same amount of heat input, the microstructures also varied depending on the weaving. The microstructures in the heat-affected zone with weaving were mixed with ferrite and pearlite. However, the microstructures in the heat-affected zone without weaving mainly consisted of coarse bainite. Acicular ferrite with some amount of grain boundary ferrite dominated in the weld metal in all cases.