A validated CFD model coupling the multiple phases and multiple physics was developed for the dynamic
simulation of pulse wave laser welding (PWLBW) process of 1.2 mm-thick Ti-6Al-4V alloy sheets assembled in
butt joint with a reserved air gap. A laser spot diameter of 700 μm was used to compensate for the energy loss
with the clearance applied at 0.2 mm and therefore improve the gap bridging ability. The distributions and
evolutions of temperature, velocity, phase interface and weld pool dimensions were characterized and discussed
to reveal the joining mechanism and the heat and flow behaviors during welding. The results show that the liquid
bridges firstly occur in the middle height of sheets as the welding begins, and rapidly extend to the whole
thickness with the increasing heat input. The reduction on laser power causes the rebounding of keyhole surface
and the backfilling of molten metal. The keyhole closes at root surface and rebounds completely in 2 microseconds,
leading to the merging of liquid bridges, bottom-up moving of high temperature area, and temporarily
enlargement on weld pool size at root surface. During the pulse interval, the weld pool volume shrinks significantly
and the molten metal slows down from Marangoni vortexes to thermal buoyance flows. The proposed
process parameters allow an air gap accounting for 16.67% of specimen thickness and result in a well-formed and
defect-free weld bead.