Multiple reflection and its influence on keyhole evolution

In laser drilling and keyhole welding, multiple reflection phenomena determine how the energy is transferred from the laser beam to the workpiece, and, most importantly, all other physics such as fluid flow, heat transfer, and the cavity shape itself depend on these phenomena. In this study, a multiple reflection model inside a self-consistent (or self-evolving) cavity has been developed based on the level set method and ray tracing technique. In the case of drilling, it is observed that the laser energy tends to concentrate near the center, where the effective intensity reaches a value two orders of magnitude higher than the original distribution. In keyhole welding, however, the maximum laser intensity is only around five times higher than the original during the entire process. Combined with the strong keyhole fluctuation, the redistributed intensity patterns are very dynamic. The intensity fluctuation drives the keyhole fluctuation, and the keyhole fluctuation, in turn, affects the intensity fluctuation. This study demonstrates that drill holes are highly efficient surfaces to focus a large amount of energy in a tiny area while keyholes have the capacity to evenly distribute the energy in a large area. It is also shown that multiple reflection phenomena are highly geometry dependent and a preassumed hole shape as adopted in many prior studies may lead to an inappropriate result.