Evaluation of Inconel 718 machinability under the hybrid Al2O3 and ZnO nanoparticles suspended in coconut oil-based lubricants

Inconel 718, a precipitation-hardened nickel-based superalloy, is widely utilised in aerospace applications due to its excellent mechanical strength, corrosion resistance, and high-temperature stability. However, its toughness and work-hardening behaviour pose substantial machining challenges, resulting in rapid tool wear, elevated cutting forces, and increased energy consumption. This study investigates the performance of a bio-based hybrid nanolubricant composed of aluminium oxide (Al2O3) and zinc oxide (ZnO) nanoparticles dispersed in coconut oil, applied under minimum quantity lubrication (MQL) conditions during turning operations. Machining trials were conducted at cutting speeds of 25, 50, and 100 m/min using two nanoparticle concentrations (1 wt% and 2 wt%), with results benchmarked against dry cutting. The HNCO 2% exhibited the greatest improvement in tool wear resistance and extended tool life by 55.73% compared to dry cutting, while HNCO 1% resulted in a 35.96% improvement. These enhancements were attributed to increased viscosity and the formation of a stable tribo-film at the tool–workpiece interface. Adhesive wear mechanisms, including built-up edge (BUE) formation and chipping, were predominant, while EDX analysis confirmed oxidation-induced tribo-film formation. Cutting forces decreased with increasing speed due to reduced friction and chip thickness, with reductions of 3.9% and 10.19% observed for HNCO 1% and HNCO 2%, respectively. HNCO 2% also yielded the lowest surface roughness (Ra = 1.03 µm) at 100 m/min, with Ra values improving by 14.07% and 15.31% for 1% and 2% concentrations, respectively, over dry cutting. These findings underscore the potential of bio-based hybrid nanolubricants for enhancing machining performance, promoting tool longevity, and reducing environmental impact in the processing of difficult-to-machine materials such as Inconel 718.