Selective laser melting (SLM) is a popular metal additive manufacturing technique that allows the development of new metal matrix nanocomposites by fusing metallic powders with nanoparticles. In the current study, a novel numerical model was proposed to simulate microstructure formation considering local nanoparticle distribution during the SLM process. The proposed model formulates a three-dimensional computational fluid dynamics (CFD) model with Lagrangian particle tracking to simulate a single-track, single-layer SLM process of aluminum alloy reinforced with TiB2 nanoparticles in ANSYS FLUENT. A very low weight fraction (0.0009%) of nanoparticles was considered due to the computational limitations of the software package. The temperature distribution and particle distribution results calculated by the 3D CFD model were one-way coupled with a 2D Cellular Automata (CA) model to predict the solidified microstructure using MATLAB. The coupled CFD-CA model and Lagrangian particle tracking were separately validated in this study. The result showed that the nanoparticles move along the recirculation zones generated by the Marangoni and natural convection in the molten pool fluid. The microstructure predicted by this model showed that the introduction of the nanoparticles increased bulk nucleation during solidification. It disrupted columnar grain growth by promoting small, randomly oriented equiaxed grains. The average grain diameter decreased by 40% when nanoparticles were present compared to microstructures without nanoparticles.

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