Study of keyhole behaviour for full penetration Nd-Yag CW laser welding

The understanding of keyhole behaviour is still a topic of great importance. The keyhole dynamics can be rather easily analysed in the case of full penetration laser welding experiments. Moreover, this configuration can allow an easy access to the complete keyhole geometry, if thin metallic sheets are welded. Indeed, in this case, only one laser beam reflection occurs on the front keyhole wall (KW). We present the results of the study of these welding configurations, where a Nd: Yag laser with a top-hat intensity distribution is considered. Several diagnostics have been used. For different experimental conditions, the dynamics of the keyhole and its complete geometry (front wall inclination, top and bottom apertures) have been analysed by using on axis visualizations through the top and the bottom of the keyhole with a 4 kHz high speed video camera. The measurement of the transmitted laser power also gives access to the front KW reflectivity. These results are compared and are used to validate our dynamic keyhole modelling, where ray tracing is included. Finally, for these conditions, an analytical model can be proposed that relates the laser parameters (incident laser power, spot diameter) with the processing parameters (material, sheet thickness and welding velocity) for Nd-Yag laser welding.