Tool life and surface quality in GTD-450 milling under dry, MQL, and nanofluid-MQL strategies

The machining of iron-nickel-based alloys, such as GTD-450, presents substantial tribological challenges arising from their high-temperature strength and low thermal conductivity. These material characteristics induce rapid tool wear and compromised surface integrity across a range of manufacturing operations. Consequently, improving the machinability of GTD-450 constitutes a critical research imperative, particularly given the limited literature on this alloy despite its widespread industrial applications. These applications span power generation-including steam and gas turbine components such as compressor blades-as well as aerospace and industrial machinery sectors, where GTD-450 is employed in landing gear, high-pressure valves, and pump shafts. One approach to addressing these challenges involves selecting appropriate lubrication strategies, lubricant types, and associated parameters-namely, flow rate, additive components (including reinforcing nanoparticles), and surfactants. To enhance the machinability of GTD-450, the present study systematically investigates and compares the performance of five distinct lubrication-cooling strategies: dry machining, water-immersion cooling, conventional minimum quantity lubrication (MQL), MQL enriched with Al₂O₃ nanoparticles, and MQL enriched with ZnO nanoparticles. The primary response variables are tool flank wear (VB) and workpiece surface roughness (R_a), with feed rate, cutting speed, and depth of cut examined as additional machining parameters. To date, no published work has reported on the use of Al₂O₃ or ZnO nanoparticles to enrich MQL strategies for machining GTD-450. Statistical analysis using analysis of variance (ANOVA) revealed that the most critical factors influencing VB and R_a are, respectively, feed rate and cutting speed. Among the five lubrication strategies, nanofluid MQL (NMQL) outperformed dry, water immersion, and conventional MQL methods. Specifically, NMQL with ZnO nanoparticles achieved the greatest reduction in tool wear, up to 10% relative to conventional MQL and up to 35% relative to dry machining. Surface roughness (R_a) was also improved, with reductions of approximately 11% relative to MQL and 32% relative to dry machining. Scanning electron microscopy (SEM) analysis of worn tools indicated that this enhanced performance is attributable to the in-situ formation of a stable triboprotective layer at the tool-chip interface. This layer effectively mitigates severe wear mechanisms, including adhesion and abrasion. The findings confirm that NMQL-particularly with ZnO nanoparticles-represents a highly effective and sustainable strategy for improving the machinability of hard-to-cut superalloys such as GTD-450.