Fatigue behavior of IN718 microtrusses produced via additive manufacturing

• Inconel 718 microtrusses are designed with a fill ratio of 18.1% and fabricated via direct metal laser sintering.
• A combined approach uses digital image correlation, fractography, microstructure characterization, and structural analysis.
• The microtruss failed one ligament at a time, resulting in local strain accumulation in the surrounding ligaments.
• Interior ligaments failed first, due to increased surface roughness resulting from the inability to shot blast.
• Microtruss ligaments display a large variability in grain sizes, with small grains around the perimeter.

Advances in additive manufacturing are enabling the design and production of part geometries that have previously been unrealistic and/or economically infeasible. Direct metal laser sintering (DMLS), in particular, is an additive manufacturing method that is capable of producing highly complex parts. In this study, IN718 cellular microlattices or microtrusses are fabricated by DMLS. The microtrusses were tested under fatigue loading and with a range of stress amplitudes and compared to IN718 dogbone specimens produced under the same conditions. The failure of the microtrusses was observed to occur at the intersection between the nodes and ligaments of the microtruss, which is in concurrence with the highest stress concentration by finite element analysis. Initial ligament failure occurred in the interior of the microtruss structure, where a direct path for shot blasting could not be established, resulting in rougher surfaces. Grain analysis, through electron backscatter diffraction, displayed variability in the grain size distribution within the ligaments; many large grains were present in the interior of the nodes and ligaments, while small grains, comprised of partially sintered powder, were present at the perimeter.

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