Spectral analysis and power spectral density evaluation in Al2O3 nanofluid minimum quantity lubrication milling of 45 steel

Nanofluid minimum quantity lubrication (NMQL) is one of the main modes of sustainable manufacturing. It is an environment-friendly, energy-saving, and highly efficient lubrication method. With the use of nanoparticles, the tribological properties of debris-tool and workpiece-tool interfaces will change. However, spectrum analyses of force and power spectral density (PSD) of surface microstructures are limited. In the present work, the milling force, friction coefficient, specific energy, surface roughness, and surface microstructure of debris were evaluated in milling of 45 steel for different lubrication conditions, namely, dry, flood, minimum quantity lubrication, and Al2O3 NMQL. Results demonstrated that compared with other lubrication conditions, NMQL achieves minimum milling force peak (F (x) = 270 N, F (y) = 160 N, F (z) = 50 N), friction coefficient (mu = 1.039), specific energy (U = 65.5 J/mm(3)), and surface roughness value (R (a) = 2.254 mu m, RSm = 0.0562 mm). Furthermore, a spectrum analysis of the milling force and PSD of the surface microstructure was conducted for validation. The spectral analysis of milling force revealed that NMQL obtained the lowest milling force and amplitude in the middle-frequency region, thereby indicating the minimum abrasion loss of the tool. Meanwhile, the PSD analysis indicated that NMQL had the lowest proportional coefficient in the low-frequency region (0.4766) and the highest proportional coefficient in the high-frequency region (0.0569). These results revealed that the workpiece surface gained by Al2O3 NMQL obtained higher wave fineness than other working conditions. By combining with the lowest R (a), NMQL contributes the best workpiece surface quality. Therefore, machining experiments using NMQL showed the best lubrication performance.