TWI Industrial Member Report Summary 804/2004
Stephen Mulligan researches AC pulsed MAG and powder plasma arc welding of positional unbacked root passes.
Two recent developments in arc welding technology offer potential for improved productivity in root pass welding without backing. The alternating current (AC) pulsed Metal Active Gas (MAG) and Powder Plasma Arc Welding (PPAW) processes both offer the capability to adjust the arc energy independently of filler metal deposition rate. In AC pulsed MAG, the wire is operated for a proportion of the pulse cycle on electrode negative polarity. This produces an increase in deposition rate and a reduction in heat input to the base material compared with conventional positive polarity direct current (DC) pulsed transfer. The wire feed speed can be varied independently of mean welding current by altering the proportion of electrode negative polarity in the pulse waveform.
PPAW uses a powder filler metal fed concentrically through the shielding gas nozzle. The process can be operated in keyhole mode giving deep penetration and the potential to use a larger root face. Powder is added to fill the joint and control the weld composition. The effect of the powder addition is to cool the weld pool and alter the weld pool fluidity.
A feasibility study has been carried out to determine whether the degree of independent control over heat input and metal deposition rate offered by the PPAW and AC pulsed MAG processes is sufficient to make positional unbacked root welds on 7.5-15mm thick carbon-manganese steel material.
- To review the application of PPAW and AC pulsed MAG for unbacked root welds.
- To assess the feasibility of positional welding of unbacked root passes using the AC pulsed MAG process.
- To assess the feasibility of positional keyhole welding of unbacked root passes using the PPAW process.