A study of the effect of multiple reflections on the shape of the keyhole in the laser processing of materials

Abstract

In the deep penetration laser welding of materials as well as in other laser material processing it is usual for the laser to generate a keyhole in the material. One of the principal mechanisms of energy absorption in the keyhole is the Fresnel absorption process on the keyhole wall. In order to take account of the Fresnel absorption process properly it is necessary to include the effect of multiple reflections in the keyhole. With this end in view an axisymmetrical numerical model is presented in which the keyhole geometry is not fixed at the outset, but instead the keyhole wall is considered to be a free boundary whose shape changes after each iteration in the numerical process. In this connection each complete set of reflections constitutes a step in the iteration process. Convergence is reached for a reasonably wide set of initial geometries and discretization steps. The model is used to analyse keyhole profiles and intensity distributions with depth for the cases of a Gaussian and of a uniform top-hat distribution in the laser beam. The model allows the effect of inverse bremsstrahlung absorption in the keyhole to be taken into account, this being the other principal mechanism for the absorption of laser light in the keyhole.