Modeling Dual Scale Porosity Effects in Additively-Manufactured MetalsR. Muro-Barrios, J. Lambros, H.B. ChewDepartment of Aerospace Engineering, University of Illinois at Urbana-Champaign, IL 61801, USA&nbsp;Localized microstructural defects often leading to unpredictable fracture behavior have limited the wide adoption of additively manufactured (AM) alloys in mass production and critical components. In addition to the background porosity arising from nucleated &nbsp;particles and inclusions (2-10 μm) responsible for ductile failure in conventional alloys, defects resulting from the AM process can include large pre-existing voids (~30 μm) in AM alloys resulting in a dual-scale porosity failure process. As an AM alloy is loaded, voids at both scales grow, interact, and coalesce with nearby voids, ultimately forming micro-cracks and leading to failure. The effect of these microstructural defects on the fracture behavior of AM Direct Metal Laser Melted (DMLM) Ti-6Al-4V samples has been previously observed and captured through SEM imaging and DIC analysis. It is suggested that these large manufacturing voids lead to the premature failure of fatigue specimens.In the present work, a numerical approach was taken to further explore the dual-scale void and crack interaction processes in both 2D and 3D. A small-scale yielding, modified boundary layer model with monotonic KI-T displacement loading was used to study crack propagation through a material with local distribution of dual-scale pre-existing voids resembling AM Ti-6Al-4V. The Gurson ductile damage model was implemented to model typical background pores and their interactions, while the larger AM defects were explicitly represented in a finite element mesh. Micrographs were taken of actual AM Ti-6Al-4V specimen cross-sections to determine the size and expected frequency of the larger AM defects. Fracture resistance curves were numerically generated for randomly generated AM void distributions with increasing levels of AM defects. Individual material samples with off nominal fracture resistance, both over and underperforming, were analyzed in more detail through observation of the 3D void interactions. It is shown that AM defects activate a larger fracture process zone ahead of the crack tip, promote crack tortuosity, and at times appear to have increased local material toughness over a conventional alloy. Inclusion of the AM defects also generates more opportunities for localized plastic work dissipation; an energy analysis suggests increasing AM defects results in greater plastic work performed relative to the total energy applied to the material, hinting at the potential for material design using strategic void placements, which must be balanced against the void-induced material strength and modulus reduction.

Modeling Dual Scale Porosity Effects in Additively-Manufactured Metals

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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