2012 Volume 30 Issue 1 Pages 77-85
It is recognized that the fatigue properties of weld joint is successfully enhanced by mitigating the stress concentration at weld toe with smotherning the profile, as well as managing the residual stress at the weld toe. To attain the desireble weld toe profile without deteriorating the productivity of manufacturing processes, the development of welding process consisting of the CO2 gas shielded arc welding as the leading electrode and plasma arc welding as trailing electrode was studied. Through the obsevations of welding arc and weld pool of the bead-on-plate welding experiments using high speed video, it was found that the polarities of the leading CO2 gas shielded arc welding and trailing plasma arc welding electrodes take an important role on the management of weld toe profile. When the leading CO2 gas shielded arc welding and the trailing plasma arc welding are both electrode negative (EN), the arcs of those attracted each other and the molten metal was dammed up between those arcs, promoting the sideward flow, which resulted in a wide and low profile of the weld bead with the obtuse adge of weld toe. Moreover, offsetting the trailing plasma arc welidng was effective on the smoothening the weld toe profile on the offset side, deliverately limiting the flow of molten metal on the offset side. The effect was verified with the lap-fillet welding of 3.2 mm thick 780N/mm2 grade steel sheet. The developed welding process, plasma-arc hybrid welding, exhibited the improved weld toe profile, in terms of the carvature radius and frank angle, and the fatigue strength approximately twice as much as that of conventional CO2 gas shielded arc welding.