Machinability Enhancement of Ti6Al7Nb Biomedical Alloy Through MWCNT-Nanofluid MQL and Vortex Tube-Assisted Side...

Ti6Al7Nb biomedical alloy is difficult to machine because its low thermal conductivity and high chemical reactivity promote heat accumulation, high cutting loads, and poor surface quality. This study evaluated the side milling performance of Ti6Al7Nb under dry machining, vortex tube cooling, and MWCNT-assisted nanoparticle minimum quantity lubrication (NMQL) to identify a more sustainable and effective machining strategy. Experiments were conducted using two cutting speeds and three feed rates, and machinability was assessed in terms of cutting temperature, resultant cutting force, surface roughness, Tol wear and tool life, chip morphology, and multi-criteria ranking. Compared with dry machining, vortex tube cooling provided the strongest thermal control, reducing cutting temperature by 25-36% compared with dry conditions, owing to the cold air stream generated by the Ranque-Hilsch effect. MWCNT-NMQL produced the greatest reductions in cutting force and surface roughness, with improvements of 7-28% and 10-20%, respectively, compared with dry conditions, due to improved lubrication, reduced adhesion, possible tribofilm formation, rolling/sliding effects of MWCNTs, and enhanced heat transfer. Chip morphology observations confirmed that both assisted environments improved chip formation compared with dry machining. The overall ranking identified vortex tube cooling at Vc = 30 m/min and f = 0.08 mm/rev as the best overall condition, while NMQL was more favorable for force reduction and surface finish improvement. The findings of this study provide practical guidance for the selection of sustainable and effective cutting strategies in the precision machining of biomedical titanium alloys.