Laser weld keyhole dynamics

The dynamics of keyholes formed by high energy density laser-material interaction were studied by x-ray videography experiments and by simulation. High-speed video x-ray images showed that stationary (non-traveling) laser weld keyholes were unstable. The molten metal lining the sides of the keyhole periodically collapsed due to the surface tension forces, trapping a void at the bottom of the keyhole. As the laser radiation continued to impinge on the material, the keyhole was subsequently re-opened and recollapsed in a cyclical fashion. To gain insight into the mechanisms of keyhole instability, keyholes were simulated using the Volume of Fluid (VOF) computational technique. Heating and melting of a steel surface by a Gaussian laser beam focus spot was modeled. The evaporation recoil pressure, calculated from the thermodynamic relations, was the driving input that caused motion of the molten material and keyhole formation. The propagation of the Gaussian laser beam within the keyhole was calculated using a ray-tracing technique. The simulated laser-induced keyholes underwent cyclical collapse due to surface tension forces, in qualitative agreement with the experiments.