Parametric optimization of deep cryogenic treatment for the wear response of implant material UNS R56700: Taguchi's approach

Using the Taguchi's robust design of experiments methodology, this article presents the systematic identification and optimization of most influential parameters of deep cryogenic treatment process to minimize the specific wear rate of UNS R56700 (Ti6Al7Nb). In addition to the different soaking durations (0-96 h) at 77 K and different tempering temperatures (room temperature, 403-523 K), three commonly used variables of pin-on-disk test, namely, sliding speed (1.047-2.723 ms^-1), contact pressure (0.641-1.282 MPa), and sliding time (600-2280 s) were chosen to conduct the tests. During dry sliding conditions, pin-on-disk tribo-tests were performed to slide Ti6Al7Nb on the surface of UNS 52986 (En31) material as per standardized ASTM G99 guidelines. Experimentally measured wear rate values were converted to signal-noise ratio to statistically analyze the influence of five control variables using pooled analysis of variance and F-test. Statistically found influential control variables are confirmed experimentally. The results show that sliding speed, contact pressure and soaking duration are the most significant factors influencing the wear rate. In contrast, the parameters, that is, tempering temperature and sliding time, exhibit a lower level of influence. Microstructural characterizations done using optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction techniques suggested that deep cryogenic treatment favors the refinement of grain size of present phases with reduction in β-stabilization (β-phase) in Ti6Al7Nb. The possible reasons for the improvement in wear rate of Ti6Al7Nb underlying the morphological alterations have been explained.