Keyhole modeling during laser welding

A model, which generalizes the concept of keyhole formation due to sideways melt displacement, allows us to study the keyhole geometry as a function of the main operating parameters such as welding speed, laser incident intensity, or sample material. It is based on a drilling velocity whose combination with the welding velocity causes the inclination of the front keyhole wall. This front inclination is shown to be stationary and stable all along the front side. The penetration depth results from the product of this drilling velocity and a characteristic time defined as the beam diameter divided by the welding speed. By using a ray-tracing procedure, the dynamics and the complete keyhole geometry can be determined by taking into account the possible multiple reflections inside the keyhole and a simplified description of the closure process of the rear keyhole wall. It is shown that for usual conditions of laser irradiation, this rear keyhole wall cannot be stationary all along its surface and only an adequate laser intensity distribution can make it stationary. The interest of elongated focal spots or twin spots is then demonstrated. At high welding velocity the front wall is inclined and is composed of several layers resulting from the successive reflections. The rear wall fluctuates around an apparent equilibrium, and corresponding fluctuations occur at maximum penetration depth. At low welding speeds, the keyhole appears to be more symmetric as a consequence of the multiple reflections between the rear and the front keyhole walls.