Arc initiation in gas metal arc welding

Are initiation with gas metal are welding (GMAW) was studied. Are starts were observed experimentally using a high-speed video camera and computer data acquisition of are current, voltage and wire feed speed waveforms. Tests were performed at short-circuiting and spray transfer machine settings and with clipped and unclipped wires. The experimental data gathered during this work revealed that the initial melting of the wire extension occurred either at the base metal contact point or somewhere along the length of the extension. In the former cases, the are tended to evolve smoothly to steady-state conditions. In the latter case, the are either extinguished subsequent to initiation or was sustained and evolved to a steady-state condition. Extinguishment was more often observed at short-circuit conditions, while sustained arcs were more often observed under spray conditions. Initiation of the are at the point of contact with the base metal was more likely to occur if the wire end was freshly clipped and/or if the wire feed speed was relatively low when contacting the base metal. However, this was not always the case. Thermal-electrical simulations of the wire extension during the early stages of are initiation were performed. These simulations indicated that a finely pointed wire end should first melt at the point of contact with the base metal, while blunt wires should first melt at mid-extension. However, as noted above, the experimental results did not always conform to this prediction. A possible explanation for the differences between experimental and simulation results is unmodeled variations in wire-to-base metal contact resistance and geometry.