Gap bridging for two modes of laser arc hybrid welding

The topology of welds is an important quality criterion for which the mechanisms behind are only partially understood. The impact of gap width on the resulting weld shape produced by laser-arc hybrid welding was studied for two arc modes: pulsed arc and cold metal transfer. The weld bead and root topology were scanned and analysed. For widening gaps below a limiting width, top reinforcement turns into underfill. Above a certain gap width (0.8 mm in this case), a bridge of melt is created across the weld zone. Although high speed imaging shows very different drop transfer and weld pool formation for the two arc modes, the bridging behaviour shows the same trend for both modes. This bridging effect is important from an industrial point of view because, when viewed from the top, it gives the misleading impression that a good weld has been produced. For even wider gaps the melt collapses into the gap. These different regimes are analysed and discussed for both modes.