MONOPULSE INVESTIGATION OF DROP DETACHMENT IN PULSED GAS METAL ARC-WELDING

The drop detachment process in pulsed welding was investigated by a novel, non-continuous method. A pendent drop was formed on a fixed electrode by a low-current arc. The drop was then detached by a single high-current pulse. The detachment process was studied by high-speed filming and by current and voltage measurements. The shielding gas was Ar with various amounts of CO2, O2, H-2 and He. Comparative experiments with ordinary pulsed welding were also done. We found ample evidence for an upward force on the pendent drop, most likely caused by the recoil force of evaporating metal. The force increases with CO2 concentration, base current and base current duration. The recoil force gives rise to a critical concentration of the molecular additive, above which it is impossible to detach the drop almost regardless of pulse width. This critical concentration decreases with increasing base current and base current duration. It also seems to decrease with increasing dissociation energy of the molecular additive. The drop velocity decreases with increasing CO2 concentration. For 95Ar5CO2 we also present results on the effect of drop mass and pulse current on drop velocity, minimum pulse width for detachment, and shortest detachment time (t(dmin)). We find evidence for a minimum detachable drop mass being smallest for large pulse currents. In the pulse current range 210-350 A we observe l 1.7/p t(dmin) = 43, and in the range 350-600 A, l 1.2/p t(dmin) = 2.4 (basic SI-units).