Study of mechanical characteristics of additively manufactured Co-Cr-Mo-2/4/6Ti alloys for knee implant material

Abstract

This paper presents study of microstructure, aspect ratio (i.e. ratio of deposition width to deposition height), tensile and compression testing results along with their fractography, and abrasion resistance of the optimized depositions of new materials for knee implant developed by adding 2, 4, and 6 wt% of Ti to Co-Cr-Mo alloy by μ-plasma based additive manufacturing process. It is complemented with finite element analysis of tibial tray made of the developed alloys. Minimum aspect ratio of 1.11 and continuous deposition was obtained for 264 W μ-plasma power, 2.5 g/min deposition material powder mass flow rate, and 50 mm/min deposition head travel speed. Hence it was used for manufacturing multi-layer multi-track depositions of powder of Co-Cr-Mo-xTi alloys. Co-Cr-Mo-xTi alloys showed high % of Co-rich matrix consisting of α-Co phase formed to minor alloying elements Ni and Fe and ε-Co phase stabilized by molybdenum, ß-Ti phase, inter-metallic phase CoTi2, and lamellar chromium carbides Cr7C3 and Cr23C6 on grain boundaries which will impart more wear and corrosion resistance to Co-Cr-xTi alloys. Their samples fractured within their gauge length with slight localized necking indicating strain localization in a very small region before their ductile fracture during their tensile testing. Optimized deposition of Cr-Co-Mo-4Ti alloy has more uniform distribution of porosity and absence of thermal crack which is desirable for knee implant material as it allows better osseointegration with the human bones and tissues. It has higher tensile and compressive yield strength, ultimate tensile and compressive strength, and ductility indicated by higher % elongation and % increase in cross-section area due to more uniform porous structure, finer grain size, absence of thermal cracks, and presence of chromium carbide phases which obstruct movement of dislocations. It also has minimum value of avg. scratch track width and coefficient of friction, and maximum value of scratch hardness number imparting it better abrasion resistance due to presence of fine lamellar carbide phases. Finite element analysis of tibial tray revealed that equivalent Von-Mises stress and total deformation increase with increase in wt% of Ti in Co-Cr-Mo alloy. This study found Co-Cr-Mo-4Ti alloy as a better knee implant material due to its uniform porosity without thermal cracks, finer grain size, higher yield compressive and tensile strength, higher ductility, and abrasion resistance. © 2022 Elsevier Ltd