Observations of solidification and surface flow on autogenous gas tungsten arc weld pools

This work investigated Gas Tungsten Arc Weld (GTAW) solidification and weld pool surface flow through direct observations and measurements using high speed computerized frame grabber system. Quantitative information was obtained about weld pool flow and also the formation of weld bead ripples during solidification.; Convection flow patterns were observed, by tracking the motion of normally occurring oxide particles floating on the weld pool surface. Particle velocity measurements revealed fluctuations in surface flow. The flow Reynolds number was approximately 107, average velocity 25.4 mm/s, and bead width of 2.5mm. Assuming turbulent flow, the turbulent intensity was found to be 22%. The flow was rotational based on a non-zero graphical evaluation of the vorticity vector at the weld pool surface.; Current weld solidification theory indicates that the solidification front advances at a rate less than or equal to the weld travel speed, directed towards the center of the weld pool. Analysis of the high-speed images of the weld pool liquid-solid interface during weld surface ripple formation revealed the presence of a fluctuating velocity component acting tangent to the solid-liquid interface. The magnitude of this component was found to vary between a minimum of approximately zero to a maximum of 43 mm/sec for a weld travel speed of 2.4mm/sec.; When arc parameters which optimized ripple formation were held constant and travel speed varied, ripple spacing showed a near linear relationship between ripple spacing and travel speed. Ripple spacing decreased with increased travel speed. Rapidly occurring melting events of newly formed solid were also observed. Re-melting of the solid, when it occurred, was rapid, on the order of 300 mm/s. As weld pool size increased the solidification mode was seen to transition from predominantly ripple formation to a continuous mode in keeping with current weld pool solidification theory.