Investigation of the role of tribolayer formation in improving drilling performance of Ti-6Al-4V using minimum quantity of lubrication

Relationships between tribological properties of Ti-6Al-4V and the machining behaviour of the same alloy drilled by a WC-Co cutting tool using minimum quantity lubrication (MQL) were studied. The tribological behaviour of Ti-6Al-4V sliding against WC-Co was evaluated by determining the changes in the coefficient of friction (COF) using cutting fluids employed in MQL and flooded drilling tests. When pin-on-disk tests were carried out under the boundary lubricated condition, the MQL fluid (MQLF) provided a low COF of 0.16 at 25 ℃ and 0.24 at 250 ℃. The observed low COFs were attributed to formation of a tribolayer on the WC-Co counterfaces. A high COF of 0.45 observed at 300 ℃ indicated that the tribolayer was no longer stable. Application of conventional water-based cutting fluid (CF) also resulted in a low COF of 0.26 at 25 ℃ but the COF rose to 0.32 at 100 ℃. During drilling tests, MQLF was supplied at a rate of 80 ml/h using an external MQL system. The average drilling torque was reduced by 35% compared to CF as a result of the formation of tribolayer on the cutting edge of the tool as well as on the drilled hole surfaces, as the cutting temperature remained <250 ℃. According to Raman and X-ray photoelectron spectroscopy (XPS) analyses, the tribolayer on the drilled hole surface was formed as a result of degradation of additives in the MQL. The presence of phosphate- and sulfate-based anti-wear components in the tribolayer proved to be beneficial in preventing metal-metal contacts during drilling and provided a low COF. The low COF between tool-workpiece contact resulted in a low and uniform torque response throughout the drilling cycles and prevention of built-up edge (BUE) formation, leading to an improved surface finish.