Observations on Droplet and Arc Behaviour during Pulsed GMAW

The influence of the oscillation behaviour of the weld pool and the pendant droplet on the electric arc in pulsed GMA welding has been studied. High speed video images are related to measurements of the voltage. It is found that the weld pool is triggered into motion by the impact of the transferred liquid droplet. The increase of the current in the peak pulse period hardly affects the weld pool. The remaining liquid at the electrode tip may start to oscillate by the action of surface tension and gravity after the droplet is detached. It appears that the oscillation pendant droplet changes the structure of the arc. A conceptual model is derived to explain the observed behaviour: visual observations and voltage measurements. Two different regions can be distinguished in the arc, a bright region and a normal region. The bright arc region surrounds the droplet and shrinks and expands with the up and down movement of the droplet. The behaviour of the bright arc region can be correlated with the measured arc voltage. The shape of the droplet has a negligible effect on the measured voltage, it only initiates the changes occurring in the arc. The periodical release of additional metal vapour in the arc affects the arc voltage considerably. The results of the experimental work presented here cannot yet explain the mechanism behind the alteration of the arc structure and further research is required to elucidate these phenomena. The results presented in this paper focus on bead on plate-pulsed GMA welding in partial penetration conditions.