4 research outputs found

    Transient dynamics and stability of keyhole at threshold in laser powder bed fusion regime investigated by finite element modeling

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    A Finite element model is developed with a commercial code to investigate the keyhole dynamics and stability at keyhole threshold, a fusion regime characteristic to laser microwelding or to Laser Powder Bed Fusion. The model includes relevant physics to treat the hydrodynamic problems - surface tension, Marangoni stress, and recoil pressure - as well as a self-consistent ray-tracing algorithm to account for the "beam-trapping"effect. Implemented in both static and scanning laser configurations, the model successfully reproduces some key features that most recent x-ray images have exhibited. The dynamics of the liquid/gas interface is analyzed, in line with the distribution of the absorbed intensity as well as with the increase of the keyhole energy coupling. Based on these results, new elements are provided to discuss our current understanding of the keyhole formation and stability at threshold.The authors are grateful to Anthony D. Rollett and Tao Sun for helpful discussion on their x-ray experiments. This work has been supported by Safran Additive Manufacturing and Association Nationale de la Recherche et de la Technology (ANRT)

    Physical mechanisms of conduction-to-keyhole transition in laser welding and additive manufacturing processes

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    Thermo-hydrodynamic phenomena which take place during laser welding or additive manufacturing processes as laser powder bed fusion, have been investigated for years, but recent advances in X-ray images and in situ analysis have highlighted new findings that are still under debate. Conduction-to-keyhole transition, and more broadly, keyhole dynamics, are typical cases, where complex coupling between hydrodynamic and optical problems are involved. In this paper, a keyhole and melt pool model is developed with the software COMSOL Multiphysics®, where laser energy deposition is computed self-consistently thanks to a ray tracing algorithm. The model successfully reproduces experimental findings published in the literature and helps to analyze accurately the role played by the beam trapping phenomenon during the conduction-to-keyhole transition, in both spot welding (i.e., stationary laser illumination) and welding configurations (i.e., with scanning speed). In particular, it is shown that depending on the welding speed, multiple reflections might be either a stabilizing or a destabilizing factor. Understanding these mechanisms is thus a prerequisite for controlling the stability of the melt pools during the joining or the additive manufacturing processes

    A mesoscopic approach for modelling laser beam melting (LBM)

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    Laser Beam Melting (LBM) is currently garnering industrial attention and many numerical researches have been carried out in order to understand the physics behind the process. However, due to the gap between the grain scale (micrometres) and the bead scale (millimetres), current state-of-the-art multi-physical models are computationally expensive as each powder grain is individually represented. Hence, simulating more than a single LBM track in a reasonable computational time is a challenging task. To overcome this limitation, a new mesoscopic approach is proposed, which intends to bridge the fine thermo-hydrodynamic representation and the macroscopic thermal models. The powder bed is represented by a homogeneous medium with both equivalent thermal and fluid properties. A bulk heat source is considered when the laser heats the powder bed whereas a surface heat flux is imposed on the melted powder bed surface. Apparent viscosity and surface tension are attributed to the homogenized medium so that modelling powder densification, melting and spheroidization of the melt pool is made possible by solving compressible Navier-Stokes equations. In addition, thermocapillary effects as well as vaporisation-induced recoil pressure are implemented, so that realistic thermo-hydrodynamic phenomena are successfully taken into account
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