Effects of processing parameters on molten behavior, arc stability, and defect forming in swing-arc narrow gap GMA vertical up welding

Swing-arc narrow gap GMA vertical-up welding is a high-efficiency technique. However, typical lack-of-fusion defect could still exist. We tried to clarify the forming mechanism of the typical defects. Experiments of EH40 grade steels under various processing parameters were carried out in this study. Processual phenomena of molten behavior and arc stability were described and analyzed by synchronously collecting and counting the high-speed graphic and electronic features of the welding procedure. Effects of traveling speed, swing amplitude (α), sidewall staying time (ts), and the peak pulsed voltage (Up) on the metal transfer, statistical characteristics of the max pulsed current (Ip) and the wire tip position, and weld formation were investigated. The tendency and the forming mechanism of the lack-of-fusion defect were investigated. It is realized that a relatively fast traveling speed would increase the tendency of a convex bead and make the arc more divergent in the swinging period, which would increase the trend of interlayer lack-of-fusion defect. The change of any parameter among α, ts, and Up which makes the Ip-value distribution much above the threshold globular-spray transfer Ip-value range would lead to a deviation from the stable pattern of the wire-tip trajectory and a severe climbing on a certain sidewall. The sidewall lack-of-fusion defect usually occurs on the other side after the severe climbing on the previous side. The continuous arc premature climbing onto the sidewall during the swinging period causes the sidewall lack-of-fusion defect.