Study and Modelling of the Melt Pool Dynamics during Selective Laser Sintering and Melting

Selective laser sintering (SLS) and Selective Laser Melting (SLM) are parent layer manufacturing processes that allow generating complex 3D parts by consolidating layers of powder material on top of each other. Consolidation is obtained by processing the selected areas using the thermal energy supplied by a focused laser beam. In SLS partial fusion of powder particles takes place, followed by a solidification of the created liquid. SLM is essentially the same process as SLS, with the difference that the particles are completely molten under the laser beam. This development is driven by the need to produce near full dense objects, with mechanical properties comparable to those of bulk materials and by the desire to avoid lengthy post processing cycles. Identification of the optimal process conditions (so-called process window) is a crucial task for industrial application of SLS/SLM processes. Operating parameters of the process are adjusted in correspondence with optical and thermal properties of the processed material. Nowadays in SLS/SLM there is a tendency to increase the speed of the fabrication as a consequence of the available higher laser powers. It leads to increase of laser scanning speeds. In these circumstances, to rely only on experimental investigations in order to adjust process and material parameters is time-consuming and ineffective. Simulation tools are strongly needed for the visualization and analysis of SLS/SLM processes. In SLM the powder grains under the laser are completely molten and form a liquid domain called melt pool. Evolution of the melt pool during the process, its interaction with the laser, the substrate and the surrounding non-molten powder strongly affect the quality of the final part. The goal of this work is to study the melt pool dynamics by means of the finite-element simulation software, built specially for SLS/SLM. The numerical model is based on the homogeneous medium hypothesis. It considers the interaction between the laser and the powder material, the phase transformations and the evolution of the material properties during the process. We also study the influence of the phase change on the process efficiency. The macroscopic model is completed by the sub-models, which allow to study at microscopic level the processes taking place in the powder bed during its laser heating and melting. Melting of separate powder particles during laser irradiation is studied by means of the improved Single Grain Model. The capillary phenomena taking place in the powder bed during SLS/SLM are also studied. The interconnection of powder grains during their melting is approached by the mechanism of liquid drops coalescence. According to the obtained results, the depth-dependent sintering threshold for powder materials is proposed.

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