In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (for example, 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67°rotation between layers.

a b s t r a c tThe CoCrFeMnNi high-entropy alloy is a promising candidate for metal additive manufacturing. In this study, single-layer and multi-layer builds were produced by laser powder bed fusion to study microstructure formation in rapid cooling and its evolution during repeated metal deposition. CoCrFeMnNi showed good printability with high consolidation and uniform high hardness. It is shown that microstructure in the printed alloy is governed by epitaxial growth and competitive grain growth. As a consequence, a bidirectional scanning pattern without rotation in subsequent layers generates a dominant alternating sequence of two crystal orientations.

In laser powder bed fusion (LPBF) the surface layer temperature is continually changing throughout the build process. Variations in part geometry, scanned cross-section and number of parts all inuence the thermal eld within a build. Process parameters do not take these variations into account and this can result in increased porosity and dierences in local microstructure and mechanical properties, undermining condence in the structural integrity of a part. In this paper a wide-eld in-situ infra-red imaging system is developed and calibrated to enable measurement of both solid and powder surface temperatures across the full powder bed. The inuence of inter-layer cooling time is investigated using a build scenario with cylindrical components of diering heights. In-situ surface temperature data are acquired throughout the build process and are compared to results from porosity, microstructure and mechanical property investigations. Changes in surface temperature of up to 200°C are attributed to variation in inter-layer cooling time and this is found to correlate with density and grain structure changes in the part. This work shows that these changes are signicant and must be accounted for to improve the consistency and structural integrity of LPBF components.

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