EFFECTS OF SECTION SIZE AND COOLING RATE ON MICROSTRUCTURE AND AS-CAST PROPERTIES OF INVESTMENT CAST CO-CR BIOMEDICAL ALLOY

摘要

ASTM F75, a biomedical grade cobalt alloy, is often used in orthopaedic implants manufactured using investment casting methods. Quality of the cast medical implants is paramount, defects such as porosity, inclusions or misrun are not tolerated and similarly the mechanical properties have to conform to predetermined standards. Generally castings are subjected to thermomechanical treatments post-casting to improve required properties. It is recognised that casting conditions have an effect on the as-cast properties of materials and that a highly controlled and optimised process can help limit defects and optimise properties thereby reducing the need for such additional treatments. The 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 alloy investigated. This was done by conducting a series of instrumented control experiments involving solidification of the alloy in typical investment shell moulds, characterisation of the as-cast micro/macro-structure, and a programme of mechanical testing. Any defects observed were also noted.