Influence of biodegradable lubricant on the ecology, lubrication, and machining process in minimum quantity lubrication

In minimum quantity lubrication (MQL) processing, lubricating oil is passed through the air flow field in the form of oil droplets. The oil droplets penetrate the field and are adsorbed by it to form a film in the capillary tube at the workpiece–tool interface. Therefore, the type, atomization, penetration, and adsorption of the lubricating oil have crucial influences on the lubrication quality. Three types of lubricant—trimethylolpropane trioleate, vegetable oil, and polyethylene glycol—were used in this research. To protect the environment and meet application requirements, the biodegradability, oxidation stability, and placement stability of the aforementioned three oils were tested in accordance with the relevant national standard. The biodegradation rate and storage stability of vegetable oil 6000 were high; however, its oxidation stability was poor. A three-dimensional particle dynamic analyzer was used to measure the atomization of oil droplets, and the amount of oil penetrating the tool–workpiece contact area was quantitatively evaluated using a specially designed penetration sampling device. The contact angles of the droplets on AISI 1045 carbon steel and TiN coating were measured, and experiments were performed on the MQL machining of AISI 1045 carbon steel. According to the experimental results, the main cutting force, coefficient of friction, surface morphology, and roughness obtained using vegetable oil 6000 were superior to those obtained using polyethylene glycol 400 and trimethylolpropane trioleate, because a more focused oil drop distribution, higher amount of penetrated oil, and higher wettability were obtained with vegetable oil 6000.