Effects of Superplastic Forming on Modification of Surface Properties of Ti Alloys for Biomedical Applications

In the present work, both the surface chemical contamination and the mechanical alteration of Ti-6Al-4V (Ti64) and Ti-6Al-4V extra low interstitial (Ti64ELI) titanium alloys subjected to superplastic forming (SPF) for the manufacturing of highly customized biomedical prostheses have been investigated. As case study, a cranial implant was considered. The design of the manufacturing process was assisted by a numerical model calibrated on free inflation experimental tests. Glow discharge optical emission spectrometry (GDOES) analyses, nanoindentation tests, and metallographic analyses allowed to relate the mechanical alteration to the oxygen enrichment due to the environmental exposition during processing. While similar diffusion kinetics were found, different oxidation rates were measured in the two investigated alloys. The hardness variation was strictly related to the oxygen content. In order to verify the material biocompatibility, cytotoxicity tests were conducted on the most oxidized part. Results highlighted that the oxygen enrichment due to the manufacturing process did not significantly affect the cells viability.