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.