Fretting wear behaviour of biomedical grade Ti6Al4V produced by electron beam powder bed fusion

Abstract

In this work, the fretting wear of Ti6Al4V Grade 23 obtained by electron beam powder bed fusion (PBF-EB) with different surface conditions, as-built and mechanically machined, has been studied. The results are compared with those obtained for the same alloy produced by conventional hot rolling. The tests were carried out in air at room temperature for up to 106 cycles according to the ASTM G204 friction standard using a contact load of 10 N and 50 μm imposed displacement amplitude. A 90◦ cross-cylinder configuration was used. Scanning electron microscopy and light microscopy were employed in the wear damage characterization. Topography and wear volume were calculated using optical profilometry. The morphology, size and crystal structure of the debris detached during the test were determined by transmission electron microscopy and energy dispersive X-ray spectroscopy. The coefficient of friction, calculated as the ratio between frictional and contact load, for the machined surface PBF-EB is similar to that of the rolled material, close to 1, while that of the unmachined surface PBF-EB is double that of the rolled material. The energy dissipated by friction was greater for the PBF-EB material in both surface conditions compared to the rolled case. The coefficient of friction of the pairs was more affected by surface roughness, while the microstructure of the materials dominates the frictional dissipated energy. The debris removed consisted of oxide particles between 0.1 and 10 μm in size and additional metallic flakes several tens of micrometres in width. The largest debris were formed by an agglomeration of nano sized particles exhibiting the TiO2 rutile phase. Considering the coefficient of friction and the Archard-based wear coefficient, PBF-EB with machined surface and the hot rolled material exhibits similar tribological behaviour. As-built surfaces act as a debris sink, reducing fretting wear and resulting in shallower scars.