The humping phenomenon during high speed gas metal arc welding

Abstract

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.

Understanding Bead Hump Formation in Gas Metal Arc Welding Using a Numerical Simulation

“…This prediction corresponds to the observations of Nguyen et al, [10] who mentioned that accumulation of liquid at the back to the weld pool initiated hump formation. Due to the subsurface flow pattern of the longitudinal circulation, the relatively hot molten metal from the front of the weld pool advects a large amount of thermal energy from direct arc heat input and molten droplets along the solid/liquid interface (the bottom fusion line).…”

“…Mendez and Eagar [9] proposed that strong arc pressure pushes the molten metal back to form humps. More recently, based on inferences from high speed video images, Nguyen et al [10] proposed a backward flow stream that originates from the bottom of the weld pool crater and sweeps backward along the centerline solidification boundary to the trailing edge of the pool. Based on video images, they propose that two other rearward-directed streams flow further up on either side of the crater and merge with the centerline flow somewhere between the crater and the trailing edge of the weld pool to become the strong backward fluid flow, referred by these authors as a ''wall jet.''…”

Effect of external magnetic field on weld pool flow conditions in high-speed gas metal arc welding

“…6,7 The high momentum of the backward flow of molten metal results in the formation of humping bead. High-speed GMAW usually uses high level of welding current to maintain the required heat input.…”

“…1,5 However, the weld bead quality is worsened, and severe geometric defects such as humping bead begin to occur if the welding speed is increased to some extent. 6,7 That is to say, there exists a critical welding speed beyond which humping bead will deteriorate the weld quality in gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW). The welds with humps cannot satisfy the requirements of manufacturing industry, and effective measures must be taken to avoid or suppress the occurrence of humping bead.…”

“…After solidification, humped bead forms. [5][6][7]10 To suppress humping bead, it is crucial to reduce the momentum of the backward molten metal flow in the weld pool. Some modified GMAW used two wires to expand the action area of the arc force so that the backward molten metal flow in the weld pool is weakened.…”