A commonly observed welding defect that characteristically occurs at high welding speeds is the periodic undulation of the weld bead profile, also known as humping. The occurrence of humping limits the range of usable welding speeds in most fusion welding processes and prevents further increases in productivity in a welding operation. At the present time, the physical mechanisms responsible for humping are not well understood. Thus, it is difficult to know how to suppress humping in order to achieve higher welding speeds. The objectives of this study were to identify and experimentally validate the physical mechanisms responsible for the humping phenomenon during high speed gas metal arc (GMA) welding of plain carbon steel. A LaserStrobe video imaging system was used to obtain video images of typical sequences of events during the formation of a hump. Based on these recorded video images, the strong momentum of the backward flow of molten metal in the weld pool that typically occurred during high speed welding was identified as the major factor responsible for the initiation of humping. Experiments with different process variables affecting the backward flow of molten weld metal were used to validate this hypothesis. These process variables included welding speed, welding position and shielding gas composition. The use of downhill welding positions and reactive shielding gases was found to suppress humping and to allow higher welding speeds by reducing the momentum of the backward flow of molten metal in the weld pool. This would suggest that any process variables or welding techniques that can dissipate or reduce the momentum of the backward flow of molten metal in the weld pool will facilitate higher welding speeds and productivity.

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The effects of turbulence in stationary gas tungsten arc (GTA) welds in AISI 304 stainless steel were examined using a finite element thermofluids model. The model includes buoyancy, Lorentz and Marangoni driven fluid flow, a large deformation model of the free surface, and a k–∊ turbulence model. To facilitate implementation of the wall function boundary conditions for the k–∊ turbulence model, a dynamic numerical grid remapping technique was used to clearly separate elements in the liquid from those in the solid. The influences of sulphur content of AISI 304 stainless steels on the turbulent viscosity, fluid flow, and weld pool dimensions were simulated. Good correlation between experimentally observed and predicted weld pool shapes and dimensions was obtained. Also, the effect of sulphur concentration on AISI 304 weld pool dimensions was correctly predicted. The simulations indicate that the flow in such stationary GTA weld pools in AISI 304 stainless steel is not laminar and that quantitatively accurate predictions of weld pool fluid flow and weld shapes and dimensions will only be possible if the effects of turbulence in GTA weld pools are modelled correctly.

A comprehensive survey of high speed weld bead defects is presented with strong emphasis on the formation of humping and undercutting in autogenous and non-autogenous fusion welding processes. Blowhole and overlap weld defects are also discussed. Although experimental results from previous studies are informative, they do not always reveal the physical mechanisms responsible for the formation of these high speed weld bead defects. In addition, these experimental results do not reveal the complex relationships between welding process parameters and the onset of high speed weld bead defects. Various phenomenological models of humping and undercutting have been proposed that were based on observations of events in different regions within the weld pool or the final weld bead profile. The ability of these models to predict the onset of humping or undercutting has not been satisfactorily demonstrated. Furthermore, the proposed formation mechanisms of these high speed weld bead defects are still being questioned. Recent welding techniques and processes have, however, been shown to be very effective in suppressing humping and undercutting by slowing the backward flow of molten metal in the weld pool. This backward flow of molten weld metal may be the principal physical phenomenon responsible for the formation of humping and undercutting during high speed fusion welding.

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