An Analysis on the Cutting Force Coefficient in Al2O3/MoS2 Hybrid Nanofluid MQCL Hard Milling of Hardox 500 Steel

Hardox 500 is a wear-resistant steel with a nominal hardness of 500 HBW and is widely recognized for its high strength, superior abrasion and impact resistance, and good toughness. These properties make it suitable for demanding applications such as heavy machinery and construction equipment. However, the elevated cutting temperatures, high cutting forces, and accelerated tool wear associated with this material present significant challenges when using conventional machining methods. Consequently, the development of sustainable cooling and lubricating techniques has become increasingly important. This study evaluates the effects of an Al₂O₃/MoS₂ hybrid nanofluid under a Minimum Quantity Cooling Lubrication (MQCL) environment on the cutting force coefficient during the hard milling of Hardox 500 steel. A Box–Behnken experimental design was employed to investigate the influence of Nanoparticle Concentration (NC), cutting speed (v), and feed rate (f) on the cutting force coefficient Fx/Fz. The results demonstrated a notable improvement in the hard milling performance of Hardox 500, with NC identified as the most influential parameter affecting Fx/Fz. Optimal ranges for the tested variables were determined as NC = 1%–1.5%, v = 80m/min –85 m/min, and f = 0.09 mm/month–0.15 mm/tooth, which collectively contributed to a reduction in Fx/Fz. The optimal combination, NC = 1.5%, v = 80 m/min, and f = 0.13 mm/tooth, yielded the lowest cutting force coefficient, Fx/Fz = 0.2269. These results were validated by confirmation experiments. The Al₂O₃/MoS₂ hybrid nanofluid MQCL condition delivered superior performance in both Fy/Fz and surface roughness (Ra) compared to dry machining, flood coolant, pure MQCL, and single-nanofluid MQCL (Al₂O₃ or MoS₂). Relative to dry, flood, and pure MQCL conditions, Fy/Fz decreased by approximately 34%, 28.8%, and 27.5%, respectively, while Ra was reduced by 37.8%, 24.6%, and 20.2%. These findings confirm that the Al₂O₃/MoS₂ hybrid nanofluid provides enhanced cooling and lubricating capability compared to individual nanofluids. Furthermore, integrating vortex-tube MQCL with the Al₂O₃/MoS₂ hybrid nanofluid for machining Hardox 500 is a key advancement and the principal contribution of this study.