Double stage plasma arc pipe welding process

Plasma arc welding (PAW) keyhole mode uses highly penetrating plasma jets to produce narrow inner diameter (ID) beads but causes bead irregularity, undesirable wetting angles, and spatter. For the melt-in mode, plasma jets are less penetrating and greater heat inputs are needed to achieve the needed penetration. As a result, wider weld beads are produced similarly as gas tungsten arc welding (GTAW). For orbital pipe welding, reduced heat inputs and weld pools are needed to reduce the ID convexity around 12 o'clock and achieve the needed convexity around 6 o'clock. To this end, a novel PAW mode, namely the double-stage PAW, is proposed that uses a keyhole stage to build a partially penetrated keyhole and then a melt-in stage to finally reach complete joint penetration. While the keyhole stage helps reduce the heat inputs and weld pools, the melt-in stage finishes the complete penetration at reduced impacts from the plasma jets producing the desired weld bead geometry and regularity. A closed-loop control system has been developed to adaptively determine the duration of the melt-in stage using arc signals to assure the complete joint penetration. Extensive experiments have been conducted on stainless steel pipes around the full circumferance producing smooth weld beads of moderate width with no spatter. In comparison with keyhole PAW, bead geometry and regularity are significantly improved with slightly increased net heat inputs. In comparison with melt-in PAW and GTAW, the net heat input is reduced approximately 40% resulting in 40% reduction in ID weld bead width.