Study of keyhole behavior for full penetration Nd-YAG cw laser welding

The understanding of keyhole behavior is still a topic of great importance. The keyhole dynamics can be rather easily analyzed in 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. In that case, one or two laser beam reflections occur on the front keyhole wall. We will present the results of the study of these welding configurations. 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 analyzed 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 gives also an access to the front keyhole wall reflectivity. These results are compared and are used to validate our dynamic keyhole modeling, 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.