Evaluation of lubrication mechanism of hybrid nanolubricants in turning hardened AISI D6 tool steel

Turning of hardened tool steels has posed a significant challenge for machining professionals. Usually, hardened tool steels are machined under dry with ceramic or PCBN inserts. However, dry machining imposes a condition where high cutting temperatures are developed, which becomes prohibitive when the part's geometric and dimensional accuracy is required. On the other hand, using mineral oil-based cutting fluid has encountered increasing restrictions because of its unsustainable nature. In this context, developing new sustainable lubri-cooling techniques, such as using vegetable oils blended with nanoparticles , could be an appropriate alternative. Thus, the main objective of this study was to investigate the performance of three different vegetable oil-based nanolubricants blended with three different nanoparticles (CuO, a-C:H, and CuO + a-C:H), applied under MQL when machining a quenched and tempered AISI D6 tool steel. For this purpose, facing turning trials were performed using solid PCBN inserts with the following cutting parameters: V c = 100 m/min, a p = 0.3 mm, and f = 0.1 mm/rev. For comparison, facing turning tests were performed under dry and pure vegetable oil (without nanoparticles) and applied under MQL . Output variables included average surface roughness (R a ), flank wear (VB C ), wear mechanisms of the cutting edge, chip shape, and chip compression ratio (R c ). The results showed that the vegetable oil-based nanolubricants applied under MQL improved the tribological conditions in the chip-tool and workpiece-tool interfaces, mainly in the case of CuO + a-C:H nanolubricant. In this case, it enhanced the lubricating action of the vegetable oil, decreasing cutting tool wear probably because of the combination of rolling mechanism, - provided by the CuO nanoparticles, and the formation of a protective film, supplied by the a-C:H nanoparticles. • Hybrid nanofluid from aC:H and CuO can enhance the cutting fluid performance. • The synergy of different lubrication mechanisms, the rolling mechanism of CuO, and protective film formation from a-C:H gives the best performance of the hybrid nanofluid. • Only CuO nanoparticles in nanofluid decreased a little the tool's life, probably because they act as a third body, promoting abrasive wear.