Modeling of keyhole mode laser welding

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High intensity keyhole mode laser welding, in which the laser makes a deep key-shape hole from the surface of the melting pool, is especially used to make thick welded joints. This capability of the laser welding is almost unique in this field. It makes laser welding very attractive. However, this laser welding mode is typically associated with keyhole instability that often creates imperfections. The main objective of this research program is to develop numerical tools for the simulation of the laser welding in keyhole mode, especially the behavior of the keyhole. It includes the modeling of multiphysics phenomena occurring in the laser welding process, the application of advanced numerical methods and the development of the in-house object oriented code in C++. The research program includes also a significant amount of experimental investigation to characterize the behavior of laser sources, the behavior of materials and the heat transfer during process. To reach the main objective, the research program is split into three following steps. Firstly, the whole system of equations for the multiphysics modeling will be developed and implemented in COMSOL. It allows evaluating the capability and the limits of different available approaches for keyhole laser welding process simulation. In this stage, a series of laser welding tests on aluminium will be performed to characterize the laser heat source. It allows understanding the manner the laser hits and transfers the energy to materials. Also, a series of tests will be carried out to characterize the surface tension of aluminium liquid using pyrometer and high speed camera. Secondly, different approaches for modeling the interaction of laser heat source and the molten pool will be developed. This step is very important for simulating the behavior of the keyhole. Experimental results of pulsed laser will be used to validate these approaches. Finally, based on the knowledge from the two first steps, the proposed multiphysics modeling of the keyhole laser welding will be developed and implemented in the in-house code using advanced numerical methods. Experimental results of pulsed laser welding will be used to validate laser welding simulations in terms of temperature, keyhole behavior, weld cross-section and porosity. At the end of the research program, a better understanding of the keyhole formation in laser welding through experimental and modeling investigations will allow improving process control and development, avoiding defects and increasing weld quality. This knowledge will open the door for the industry to be able to transfer from conventional welding to laser welding which makes an important gain in productivity. Moreover, this research framework is generic and could be adapted for other material manufacturing processes such as cutting, forming and particularly additive manufacturing processes. Keywords: laser welding, keyhole, instability, defects, experimentation, modeling, simulation