The discontinuous weld bead defect in high-speed gas metal arc welds

A common weld bead defect that occurs at high fusion welding speeds is the periodic undulation of the weld bead profile, also known as humping. In the present study, when using argon shielding gas, 0.9-mm-diameter ER70S-6 and ER70S-3 electrode wires, and welding powers between 9 and 12 kW during high-speed gas metal arc welding of SAE-AISI 1018 cold rolled steel plate, swinging spray metal transfer was observed and the welding speed was found to be limited to 15 mm/s by the onset of the periodic humping phenomenon. However, rotational metal transfer was observed when using reactive shielding gases at these powers and the welding speed was limited to 22 mm/s by the onset of a new, as yet unreported, weld defect that was distinctly different from humping. This new high-speed defect is referred to as the discontinuous weld bead defect, since the defective weld bead is broken up into several good bead segments by aperiodic or irregularly spaced valleys or depressions where melting of the base metal occurred but no filler metal was deposited. The results also indicated that nominal electrode wire composition did not appear to play a significant role in the formation of the humping or the discontinuous weld bead defects. A Laserstrobe™ video imaging system was used to obtain video images of typical sequences of events during the formation of the humping and discontinuous weld bead defects. From these images, the discontinuous weld bead defect was found to be caused by the inconsistent, aperiodic deposition of molten filler metal during rotational filler metal transfer mode when using reactive shielding gases. The long molten filler metal string on the end of the electrode wire was erratically fragmented and required time to re-form prior to the resumption of the transfer of filler metal. The temporary disruption of filler metal deposition created a filler-metal-free depression that broke up the otherwise good weld bead, thereby forming the discontinuous weld bead defect. The irregular fragmentation of the molten filler metal string during rotational transfer and subsequent formation of the aperiodic discontinuous weld bead defect are phenomena that have not previously been observed or reported in the open literature.