Carbon Dioxide Based Metal Working Fluids.

Abstract

Metalworking fluids (MWFs) are necessary for most machining operations to ensure proper cooling and lubrication. Conventional mixtures of water, emulsified oils and additives can lead to significant environmental impacts and worker health risks. Delivery of MWFs in minimum quantities using compressed air reduces many of these impacts but the cooling capacity of these sprays is lower than aqueous MWFs. In this research, a novel class of MWFs delivered in carbon dioxide was developed that is environmentally preferable and capable of providing cooling on par with conventional fluids. Above its critical point, CO2 dissolves many common MWF lubricants and can produce a spray of frozen lubricant and dry ice when delivered through a nozzle. The goal of this research was to evaluate the feasibility of supercritical CO2 (scCO2)-based MWFs for manufacturing applications. A prototype for delivery of scCO2-based MWF was developed and proved capable of reducing friction significantly relative to conventional MWFs. Research on the delivery of scCO2–based MWFs indicated that the lubricating potential is easily “tunable” to meet the needs of different machining operations. To test the cooling capacity of scCO2-based MWFs, heat-induced diffusive tool wear was evaluated in high-speed cutting of hard metals. The results yielded compelling reductions in tool flank wear provided by scCO2-based sprays when delivered directly to the flank surface. The results also illustrated the importance of nozzle geometry, proximity of tool and workpiece, and gas pressure for effective heat dissipation. To evaluate the environmental impacts associated with switching to scCO2-based MWFs, a life cycle analysis (LCA) was performed to compare them with water- and air-based systems. It was found that dramatic reductions are possible in aquatic toxicity, water use, and solid waste when switching from water to scCO2-based MWFs. The switch results in increased greenhouse gas emissions but the magnitude of the increase is small compared to other factory operations and can vary depending on allocation strategies. Taken together, the research reveals that scCO2-based MWFs have great potential to improve manufacturing process performance, while reducing tooling costs and the most prominent worker health risks and environmental impacts associated with MWFs today.