The Simulation Research On The Keyhole Evolution In Laser Welding Based On Dynamic Mesh Model

During the laser beam welding, a highly concentrated light energy is converted into thermal energy, so the weld seam has high depth-to-width ratio,and the process splash ,and the heat affect zone is small, as well as the weld form is nice. The laser beam welding is used more and more widely in the welding field. When the laser beam density reaches to the order of 101 0 W m2, strong evaporation occurs on the melt surface. The recoil pressure associated with energetic evaporation is sufficient to produce a deep, narrow depression in the molten material,which is called keyhole.After the keyhole is formed, the energy-absorb-mechanism is changed qualitatively. Before the keyhole is formed, the laser beam energy is absorbed by the thermal conduction;after its formation,the energy absorption is changed into the laser beam's multiple reflection and Fresnel absorption and inverse Bremsstranhlung absorption of the plasma in the keyhole. So, the study of the evolution of the keyhole in peneration laser welding has great theoretical and practical significance,and it is one of the most important research directions in the field of laser beam welding simulation at the present stage.In this study, a computional simulation of the evolution of the laser welding keyhole is done, based on the software FLUENT's dynamic mesh model.First of all, the heat transfer characteristics are studied in the actual process of laser beam welding, and a wide range of heat source models are introduced. The effect of the multiple reflection and Fresnel absorption to the energy transfer is also studied.Then, according to geometrical optics theory, by laser tracing technology,the distribution of energy density in the keyhole is simulated. And the energy density distribution in cone-shaped and parabolic keyholes is calculated.Secondly, according to the theoretical model of the evolution of keyhole, with the pure-evaporation-mechanism, the velocity of the keyhole surface is calculated. Then,with the dynamic mesh model and remeshing function in FLUENT software,the temperature distribution and transient evolution of keyhole are simulated .In the end ,based on the technologies of laser tracing and dynamic mesh, laser welding keyhole's evolution process of ZL114 and TC4 are simulated ,under the situations that the laser beam is parallel and the beam energy is flollow the Gaussian distribution function, as well as the similarities and differences between these two alloys'keyholes are compared and analysed.Based on FLUENT, the study achieves the simulation of the evolution of keyhole by using the moving-mesh-interface-tracking technology, which provides a sound theoretical basis and implementation of keyhole evolution for the development of laser welding simulation system.