Effects of Section Size, Surface Cooling Conditions, and Crucible Material on Microstructure and As-Cast Properties of Investment Cast Co-Cr Biomedical Alloy

ASTM F75, a biomedical grade cobalt alloy, is commonly used for orthopedic implants manufactured using investment casting methods. The quality of the cast implants is paramount. Defects such as porosity, inclusions, or misruns are not tolerated and the mechanical properties have to conform to predetermined standards. Generally, castings are subjected to post-casting thermo-mechanical treatments to homogenize structure and improve mechanical properties. However, casting parameters have a significant effect on the as-cast properties of materials, and so if solidification and secondary phase precipitation can be controlled at this stage to limit defects and optimize properties, then the need for such expensive additional treatments may be reduced. The initial objective of this work was to determine the effect of section size and cooling rate on the microstructural characteristics and resultant mechanical properties of the investigated alloy. This was done by conducting a series of instrumented control experiments involving solidification of the alloy in typical investment shell molds, characterization of the as-cast micro/macrostructure, and a program of mechanical testing. In the process of conducting these experiments, unexpected microstructural features were observed that could not be found in the current literature. Through a series of tests conducted to ascertain the origin of this alternative microstructure, it was found that the crucible material used for melting the alloy was highly influential in defining the as-cast properties.