| Summary: | Direct energy deposition (DED) is a promising additive manufacturing technology for large-scale fabrications of high-value components. Grain structure control is challenging but meaningful for achieving desirable mechanical properties. A multi-scale three-dimensional (3D) Finite Volume Method-Cellular Automaton (CA-FVM) model is developed. The grain structure evolution in the transition-mode melting is investigated, and the simulated grain structures show fairly good qualitative and quantitative agreement with the experimental results. The influences of laser power and scanning speed on the formed grain structure are examined. A progressive columnar-to-equiaxed transition (CET) is found. The elongated grain is the primary grain morphology, even with the CET. The effects of high temperature gradient on the development of columnar structure are difficult to overcome. Moreover, nanoparticle reinforcement is numerically investigated as a promising technique to realize the site-specific grain structure control by interfering with the columnar growth. We expect this study to provide a deeper understanding of the DED-produced grain structure and improve confidence in the site-specific structure control.
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