Simulations and experiments of laser butt welding are presented in the paper with the laser beam
travelling along an inclined line across the gap. Full penetration was observed when the laser beam
focused on the gap, while partial penetration was achieved when the laser beam deviated from the
gap. A three-dimensional numerical model coupled with a ray tracing algorithm was established to
investigate and compare the transient dynamics of the keyhole, molten pool and laser induced plume.
The shape of the simulation cross-sections of the welds show good agreement with the experimental
results. The gap contributed to transmission of laser beam energy deep into the inner part of the workpiece. The absorbed energy by the workpiece decreased when the laser beam was focused on
the gap as some rays escaped through the opening gap. The decrease in absorbed energy resulted in
a lower velocity of the molten pool and lower average pressure on the free surface. The velocity and
volume of the plume on the top side of the workpiece also decreased. Flow patterns in the molten
pool were also identified and discussed.