Cooling/Lubrication Performance of Dry and Supercritical CO2-Based Minimum Quantity Lubrication in Peripheral Milling Ti-6Al-4V

Cryogenic environments are often adopted in machining difficult-to-cut materials such as Ti-6Al-4V alloy to control its cutting heat and improve machinability. This paper aims to study the machinability of Ti-6Al-4V in finish milling process under four green cutting environments (CEs), i.e. dry, supercritical carbon dioxide (scCO2), supercritical CO2-based minimum quantity lubrication with water-based cutting fluid (scCO2-WMQL), supercritical CO2-based minimum quantity lubrication with oil-on-water droplets cutting fluid (scCO2-OoWMQL). Peripheral finish-milling of Ti-6Al-4V was conducted under various cutting speeds, feed rates, radial depth of cut and CEs. The cutting force, cutting temperature, surface morphology and surface roughness were analysed. The experiment results show that the minimum cutting force and its coefficients, cutting temperature and surface roughness are obtained in scCO2-OoWMQL environment because of its excellent cooling/lubrication and chip evacuation performance, whereas the worst friction in the tool-workpiece interface leads to the worst performance under scCO2 CE. Additionally, the machined surface profile, cutting force and their spectrums under four CEs were studied. The fundamental frequency of machined surface profile is equal to that of cutting force envelope under dry and scCO2 CEs, which is also called beat frequency, therefore scCO2-WMQL and scCO2-OoWMQL CEs can improve the milling stability. The results indicate that scCO2-OoWMQL provides the best performance with regard to cutting force, cutting temperature, surface finish and clean production.