Feasibility of Metalworking Fluids Delivered in Supercritical Carbon Dioxide

This paper presents a new method to lubricate, cool, and evacuate chips in metalworking operations using supercritical carbon dioxide (scCO2). Water-based and straight oil metalworking fluids (MWFs) are currently being used to perform these functions even though they are characterized by high economic, occupational health, and environmental costs. Carbon dioxide above its critical temperature and pressure is a finely tunable solvent that dissolves certain oils, creating the possibility of using scCO2 to carry lubricants to the cutting zone in minimal and precise quantities, while significantly reducing the occupational health and environmental risks associated with MWF systems. In the proposed process, an oil-in-CO2 dispersion is sprayed out of a nozzle at high speed and pressure to deliver oil and form dry ice near to the cutting zone. The rapid expansion of the CO2 leads to cooling at cryogenic temperatures, and the combination of high pressure and low surface tension provides access to interstitial spaces that are inaccessible to conventional MWF oil and water jets. Research with the tapping torque test shows that soybean oil dissolved in scCO2 performs significantly better than straight soybean oil, even when less oil is applied-meaning that scCO2 provides additional benefit to the tapping process. This soybean oil-in-scCO2 MWF also performed better than straight petroleum mineral oil and emulsions of soybean oil or petroleum oil. Scanning electron microscopy images of the chip surfaces produced by the tapping experiments illustrate that higher tapping torque efficiency correlates well with less frictional wear on chip surfaces.