Surface quality evaluation of cold plasma and NMQL multi-field coupling eco-friendly micro-milling 7075-T6 aluminum alloy

Micromilling has been extensively employed in different fields such as aerospace, energy, automobiles, and healthcare because of its efficiency, flexibility, and versatility in materials and structures. Recently, nanofluid minimum quantity lubrication (NMQL) has been proposed as a green and economical cooling and lubrication method to assist the micromilling process; however, its effect is limited because high-speed rotating tools disturb the surrounding air and impede the entrance of the nanofluid. Cold plasma can effectively enhance the wettability of lubricating droplets on the workpiece surface and promote the plastic fracture of materials. Therefore, the multifield coupling of cold plasma and NMQL may provide new insights to overcome this bottleneck. In this study, experiments on cold plasma + NMQL multifield coupling-assisted micromilling of a 7075-T6 aluminum alloy were conducted to analyze the three-dimensional (3D) surface roughness (S_a), surface micromorphology, burrs of the workpiece, and milling force at different micromilling depths. The results indicated that, under cold plasma + NMQL, the workpiece surface micromorphology was smooth with fewer burrs. In comparison with dry, N₂, cold plasma, and NMQL, the S_a values at different cutting depths (5, 10, 15, 20 and 30 μm) were relatively smaller under cold plasma + NMQL with 0.035, 0.036, 0.041, 0.043 and 0.046 μm, which were respectively reduced by 38.9%, 45.7%, 45.9%, 47% and 48.9% when compared to the dry. The effect of cold plasma + NMQL multifield coupling-assisted micromilling on enhancing the workpiece surface quality was analyzed using mechanical analysis of tensile experiments, surface wettability, and X-ray photoelectron spectroscopy (XPS).