Analyses of Compositionally-Graded Co-Cr-Mo Processed by Directed Energy Deposition

Abstract

The overarching goal of this work is to investigate the processing, structure and properties of compositionally-graded Co-Cr-ΔMo bulk structures fabricated via Directed Energy Deposition (DED) additive manufacturing. Co-Cr-Mo is a bio-compatible alloy commonly used in hip/knee implants. A bio-inspired strategy to improve surface wear resistance while maintaining the toughness of the bulk structure (e.g., like in mammalian teeth) is to 'tune' the depth-dependent hardness and modulus of the structure via compositional alterations at specific resolutions – elucidating the processing-structure-property framework for realizing this is the motivation for this undertaking. Specifically, this study strives to understand the fundamental influences of laser-based manufacturing process conditions on the resulting macro- and micro-structures, the ensuing material property distributions, and local/global mechanical performance as a function of changing %Mo within the layer-wise structure. For this, experimental designs were deployed to elucidate the effects of process parameters along with compositional differences on the resulting microstructures and mechanical properties of Co-Cr-ΔMo. These helped lay the foundation to understand composition-based laser energy requirements for obtaining structurally-integral parts, the property resolutions achievable, and the resulting mechanical performance implications. With a changing %Mo content, specific types/amounts elemental migrations toward/away from cell boundaries were observed, along with certain intermetallics and carbides being formed at different locations; such spatial composition differences along with the extent of carbide networks, manifested as sharp hardness transitions. Altogether, this inquiry helped to understand the DED of compositionally-altered structures, the resulting micro/macro structures, and their consequent impact on mechanical properties, thus laying the foundation for the design and manufacture of compositional-graded components.