Influence of the wrinkle surface structures on the vapor flow and keyhole stability in 20 kW high power laser welding

Abstract

The stability of the extremely narrow keyhole is still the biggest challenge for the 20 kW or higher-level laser welding. The underlying causes of keyhole instability, including the wrinkle structures-induced thermal instability and vapor-induced dynamic instability, are studied in this work. A multiphase flow model integrated with a high-efficient free surface reconstruction algorithm and a high-accuracy wrinkle surface structures observation platform are built to study the morphology evolution and thermodynamic behaviors on the extremely narrow keyhole. It is found that the wrinkle structures (humps) that exist on the front keyhole wall could significantly regulate the energy distribution, and change the pressure condition in keyhole civility. The excessive energy accumulation, which is aroused by the abnormal hump distribution, is the reason for the unstable keyhole wall fluctuation. By increasing the welding speed, the intermittent-contact behavior between the laser beam and the keyhole opening is suppressed, which helps reduce the hump generation. By increasing the laser power, the drilling effect on the solid-liquid interface could be avoided, which helps to improve the downward flow efficiency of the melting layer on the front keyhole wall. An optimizing strategy based on the welding speed and laser power adjusting is offered to regulate wrinkle morphology. The experiment results show that keyhole stability is significantly improved under the optimized wrinkle surface structures.