Digital fingerprints of microstructural variation in titanium alloy hip joint forgings via machining force feedback analysis

Considering actual trends regarding life expectancy, it is forecast that the total number of hip replacements will increase in the next few decades due to an increasingly ageing population. This situation will require an increase in component supply and experience of titanium alloy closed-die forging experience in the manufacturing supply chain. However, the scrutiny of microstructural evolution during processing for orthopaedic implants is not at the same level compared to the aerospace industry. This study is focused on understanding the current microstructure quality in the international forging supply chain by applying standard characterisation techniques, currently used in the aerospace industry for safety critical titanium alloy components, as well as a Sheffield developed machining force feedback technique. Three different manufacturers whose near net shape hip joint forgings all successfully pass the industry standards were studied. Through digital fingerprint reconstructions, the microstructural variation has been shown to be linked to the forging process parameters which could help to determine the individual components' fatigue performance. Furthermore, significant variations have been identified in microstructure quality that could compromise the part performance. The study demonstrates that the materials standards for orthopaedic implants could be more stringent to avoid such large microstructural variations in the supply chain.