Investigation of metal transfer in gas-metal arc welding.

Abstract

Metal transfer in gas metal arc welding was investigated with various welding parameters such as welding current, electrode extension, and shielding gas. Drop diameters were calculated using the pinch instability theory and the static force balance method. The dispersion equation was obtained in terms of arc current density using perturbation theory. The drop diameters which were determined by considering arc effect were found to be greater than those drop diameters which were obtained without the arc effect. When compared with experimental results, the pinch instability method was found to be useful for high currents in argon shielding gas, while the static force balance method was dominant in the low current range. The transition current was obtained in terms of the wire diameter, assuming that the transition occurred when the arc covered an entire droplet. To observe the metal transfer phenomenon, a high speed motion analyzer (1000 frames/sec) and an arc shadow-graphing system based on a laser and related optical equipment were used. The drop frequency increased when the current was increased, and transition from globular to spray transfer occurred in argon or argon-CO$\sb2$ gas mixtures. As the carbon dioxide content to argon increased, the transition current increased. However, transition occurred at a lower current in 95% argon-5% CO$\sb2$ than in any other shielding gas. As the electrode extension increased, the drop frequency increased and the transition current decreased. In helium or carbon dioxide, transition did not occur even at high currents. As the content of carbon dioxide increased, the wetting characteristics improved while spatter increased. A drop is accelerated by the gas plasma during transfer. The measured drop acceleration was compared with that calculated by drag theory. The melting rate was measured for different shielding gases and electrode extensions. The melting rate was found to be highest in carbon dioxide, followed by the melting rate in helium, and then the melting rate in argon, for the normal welding current range. Joule heating did not vary much when the shielding gas was changed. However, as the electrode extension increased, the melting rate due to Joule heating increased. The longer electrode extension resulted in a high slope of increasing rate of melting rate.