Dynamical description of the keyhole in deep penetration laser welding

We study the keyhole geometry as a function of the main operating parameters such as welding speed, laser incident intensity, or sample material. This model is based on a drilling velocity whose combination with the welding velocity causes the inclination of the front keyhole wall (FKW). This front inclination is shown to be stationary and stable all along the FKW. 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 multiple reflections inside the keyhole and a description of the closure process of the rear keyhole wall (RKW). We show that this RKW 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. (C) 2000 Laser Institute of America. [S1042-346X(00)00404-6].