ARC INITIATION IN GAS METAL ARC-WELDING

Are initiation with gas metal are welding (GMAW) was studied. Are star ts were observed experimentally using a high-speed video camera and co mputer data acquisition of are current, voltage and wire feed speed wa veforms. Tests were performed at short-circuiting and spray transfer m achine settings and with clipped and unclipped wires. The experimental data gathered during this work revealed that the initial melting of t he wire extension occurred either at the base metal contact point or s omewhere along the length of the extension. In the former cases, the a re tended to evolve smoothly to steady-state conditions. In the latter case, the are either extinguished subsequent to initiation or was sus tained and evolved to a steady-state condition. Extinguishment was mor e often observed at short-circuit conditions, while sustained arcs wer e 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 rel atively low when contacting the base metal. However, this was not alwa ys the case. Thermal-electrical simulations of the wire extension duri ng the early stages of are initiation were performed. These simulation s indicated that a finely pointed wire end should first melt at the po int of contact with the base metal, while blunt wires should first mel t 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 unmodel ed variations in wire-to-base metal contact resistance and geometry.