Hybrid cryo-nano MQL strategy for sustainable and high-performance machining of Monel 400

This study investigates the combined influence of advanced lubrication and cooling techniques on enhancing the machinability of Monel 400, a nickel-based superalloy widely employed in marine, chemical, and aerospace applications due to its exceptional corrosion resistance and mechanical strength. Four machining conditions-dry cutting, Minimum Quantity Lubrication (MQL), nano-enhanced MQL (NMQL), and a hybrid approach integrating cryogenic liquid nitrogen with NMQL (Cryo + NMQL)-were systematically compared. The experiments were performed using fixed machining parameters: a cutting speed of 80 m/min, feed rate of 0.15 mm/tooth, radial depth of cut of 6 mm, and axial depth of cut of 0.8 mm. Among these, Cryo + NMQL, featuring carbon nanotube (CNT)-enriched sunflower oil nanofluid combined with liquid nitrogen (LN₂) as the cryogenic medium, achieved superior outcomes, reducing cutting force, cutting temperature, surface roughness, and tool wear by 23.99%, 24.66%, 44.58%, and 54.85%, respectively, compared to dry cutting. SEM analysis confirmed adhesion and abrasion as dominant wear mechanisms, which were significantly mitigated under Cryo + NMQL. Multi-Objective Response Surface Methodology (MORSM) optimized the machining parameters to achieve an ideal balance of performance metrics, with a composite desirability index of 0.956. This study shows that combining LN₂ cryogenic cooling with CNT-based NMQL provides superior machinability of Monel 400-offering lower forces, temperatures, surface roughness, and tool wear-while contributing new insights for an alloy rarely examined under hybrid cooling environments. The findings validate Cryo + NMQL as an eco-benign, high-performance machining strategy, offering enhanced tool life, improved surface finish, and sustainable outcomes.