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Influence of geometry and microstructure on the failure of additively manufactured millimeter-scale metal features


Millimeter-scale components produced by metal additive manufacturing (AM) expand the design possibilities in myriad applications from aerospace engines to biomedical bone implants. A major obstacle to commercialization of this versatile technology is the inconsistent failure behavior of such parts, attributable primarily to surface defects, geometric irregularities, and microstructural heterogeneities. There is a gap in process-structure-property knowledge for millimeter-scale metal AM because at small scales, additive melt pool sizes approach to part dimensions, and this limits control of effective mechanical properties. The in-situ scan strategy and post-process machining options employed to produce desirable properties in larger parts become unfeasible at small length scales, often leaving mm-scale parts with as-printed microstructures and surface roughness.

In this work, experimental and computational approaches are used to explore the implications of mm-scale component geometry for the mechanical performance of high and low hardening structural alloys. Two types of approaches are taken: bottom-up and top-down. In the bottom-up studies, (a) surface roughness and microstructure are determined to have a statistical influence on the mechanical performance variation in mm-scale AM Ti-6Al-4V struts; and (b) defect-driven mechanical property debits are quantified by finite element analysis (FEA) for a thin strut intersection topology with two deformation modes, as a function of defect location, spacing, and depth, as well as material hardening rate. The material scope of the latter investigation is intentionally broad and dimensionless so that the results may be generalized to multiple structural alloys such as Ti-64, steels, and Inconel alloys. In the top-down studies, (a) FEA is used to solve the inverse problem and determine the effective properties of AM Inconel 625 thin strut T-intersections to reveal the influence of critical part dimension on the properties of high hardening materials; (b) the value of the proxy geometry method for predicting the mechanical performance of AM parts with complex geometries is investigated, and the possibility of locally varying and anisotropic properties from printing-induced microstructure variation is examined by FEA; and (c) assuming limited property optimization can be employed in mm-scale parts, an extensive FEA parameter study over the reported property range for AM IN718 bounds the impact of AM-induced material property variations on the mechanical response of a bend-dominated thin strut intersection.

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