2005 Conference on Lasers and Electro-Optics Europe

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

R. Fabbro, F. Coste, LALP (CNRS)/ GIP GERAILP, 16 bis, Av. Prieur de la Cote d'Or 94114 ARCUEIL Cedex FranceS. Slimani. F Briand, AIR LIQUIDE- CTAS, 13, Rue d'Epluches 95319 St OUENL'AUAfONE France

The understanding of keyhole behavior is always a topic of great importance where many questions are still of concern.We have analyzed the keyhole dynamics in the case of a very simple configuration where we have used full penetrationlaser welding conditions. Moreover, this configuration can allow an easy access to the complete keyhole geometry, if athin metallic sheet is welded. In that case, the operating parameters are selected in order that only one laser beamreflection occurs on the front keyhole wall. We will present the results of the study of these welding configurations,where experimentally a 4 kW Nd:Yag laser has been used, characterized with a 0.4 mm spot diameter and a "top-hat"intensity distribution.

Several diagnostics have been used: For different experimental conditions, the dynamics of the keyhole and its completegeometry (that are the front wall inclination and the top and bottom dimension apertures) have been analyzed by usingon axis visualizations of the top and the bottom of the keyhole through the focusing optical head, by using a 4 kHz highspeed video camera. The front keyhole wall inclination has been determined as a function of the welding speed or theincident intensity. The measurement of the transmitted laser power by using an integrating sphere, combined with theanalysis of the involved reflecting keyhole surfaces, gives also access for the first time to the measurement of the frontkeyhole wall laser reflectivity, for these conditions. It appears that the front keyhole reflectivity is rather low, typicallyin the 20 to 30% range. From these experiments, the rear keyhole wall geometry can also be inferred and its behaviorcan be completely understood by taking into account the laser reflection on the front keyhole wall. These differentresults are compared and are used to validate our 2-D dynamic keyhole modeling ("KEYHOLE" code), where raytracing with possible multi-reflections and dynamics of the keyhole walls are taken into account.

Finally, for these conditions, an analytical model 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, with a "top-hat" intensity distribution, can be easily proposed.

0-7803-8974-3/05/$20.00 ©2005 IEEE 659