Machining of metals continues to be very challenging with a large number of variables needing to be considered for parts to be successfully processed. Variables include the machining operation, the type of metal alloy and the specific process such as drilling a hole to a specified dimension.

To successfully machine a part and meet every tightening specifications, end-users will typically apply metalworking fluid using a flooding technique. With this technique, the metalworking fluid provides lubrication, cooling, transport of chips and corrosion protection.

The challenge for the metalworking fluid industry is that each machining operation can be unique requiring a specific product. As a consequence, metalworking fluids consist of complex formulations that include up to 13 different types of additives.¹ Adding to the complexity is that four different fluid types (straight oils, emulsifiable oils, semisynthetic fluids and synthetic fluids) are available for use in machining. Each of these fluid types has specific advantages and disadvantages that formulators and end-users must keep in mind when recommending one fluid for a particular application.

Use of flooding metalworking fluids can have some disadvantages related to issues such as microbial contamination, foaming, hard water instability and corrosion. Concern about health and safety issues related to specific additives used in formulating metalworking fluids also has led to end-users looking for other techniques to machine metal parts.

Methods such as dry machining and minimum quantity lubrication (MQL) are currently in use but do have their own drawbacks. The importance of keeping the machining temperature down to optimize tool life also led to the evaluation of liquid nitrogen and liquid carbon dioxide as alternative methods. Both approaches have proven to exhibit poor lubrication, have environmental health and safety issues and difficulty being integrated into conventional machine tools.

Over 10 years ago, researchers started to evaluate the potential for using supercritical carbon dioxide as a medium to facilitate machining. Steven Skerlos, Arthur F. Thurnau professor of mechanical engineering at the University of Michigan in Ann Arbor, Mich., and founder and chief technology officer of Fusion Coolant Systems in Canton, Mich., says, “Supercritical carbon dioxide is a state that is neither a gas nor a liquid but exhibits properties of both. The supercritical state of carbon dioxide is produced by compressing it to a pressure greater than 74 Bar at a temperature above 31 C. Carbon dioxide completely transforms itself into a magic substance, acting simultaneously as a solvent in the manner of a liquid but also displaying the mobility of a gas with a density 80% that of water.”

These unique properties led Skerlos to examine how supercritical carbon dioxide can be used as a medium to facilitate machining.

Renewable approach
Skerlos and his colleagues found that food-grade carbon dioxide in the liquid state can be pressurized in a single pump cabinet and then moved to the machine tool where it is converted to the supercritical state through application of heat, injected with lubricant and is then delivered to where the tool is interacting with the workpiece. He says, “Storage of carbon dioxide is easy, and the additional equipment required to transport supercritical carbon dioxide to the machining location is already commonplace in manufacturing environments.”

Machine tools need to be retrofitted to handle supercritical carbon dioxide, but the only issue that requires significant attention is the need to replace the rotary joint. Skerlos says, “For common metalworking fluid applications through the tool, rotary joints are needed at the top of the machine tool spindle. Most rotary joints do not leak water and do not need fine specifications. We replace rotary joints on retrofits since supercritical carbon dioxide is highly mobile and will leak through any gap present in a rotary joint.”

Supercritical carbon dioxide can be applied through the cutting tool or delivered externally in a similar manner to a metalworking fluid. Figure 3 shows a stream of supercritical carbon dioxide emerging from a cutting tool.


Figure 3. Supercritical carbon dioxide can be applied through a cutting tool to provide beneficial performance characteristics in large chip machining operations. Figure courtesy of the University of Michigan.

Upon application, supercritical carbon dioxide cools upon expanding, while delivering high velocity cold oil droplets, leading to an increase in tool life. Large chip machining operations conducted on a variety of metals such as drilling, milling and turning have led to superior results versus semisynthetic metalworking fluids. Skerlos says, “We found that tool life will increase four times compared to emulsion-based metalworking fluids in drilling operations—under conditions where we increase material removal rates by two times.”

For operations requiring more lubricity, an oil can be added to the supercritical carbon dioxide. This can result in a significant improvement in surface finish and machining speeds. Skerlos says, “We found that adding one drop of an oil every two minutes leads to excellent performance. The oil is dispersed at a particle size in the nanosize range and acts as microball bearings.”

Vegetable oils and synthetic esters work very well with supercritical carbon dioxide, according to Skerlos. In some cases, the best approach is to add droplets of the original metalworking fluid concentrate.

An added benefit of using supercritical carbon dioxide is that this provides a good carbon life cycle footprint. Skerlos says, “The carbon dioxide we use is obtained from waste emissions nearby and is reused in metal cutting before it reaches the atmosphere. The technology reduces tool consumption and machine energy consumption while eliminating water pollution and harmful aerosols from cutting fluid breathed by machinists.” For this potential benefit, Skerlos received the 2020 EPA Green Chemistry Academic Award.

Machinist also have a favorable view of using supercritical carbon dioxide. Skerlos says, “Operators like supercritical carbon dioxide because of its cooling benefit and their ability to actually see the actual machining process.”

Skerlos points out that the technique requires the use of common mist collectors when oil is used. All of the main metals used in metalworking (ferrous alloys, aluminum, magnesium and titanium) can be machined with supercritical carbon dioxide. Depending upon the metal alloy and the application, some oil might need to be used. He adds, “Even hard to machine alloys such as Inconel have worked well with supercritical carbon dioxide metalworking fluids.”

Future work will involve evaluating supercritical carbon dioxide on small chip operations such as grinding and in metalforming operations. Additional information can be found by contacting Skerlos at skerlos@umich.edu or by reviewing information at the Fusion Coolant Systems website, www.fusioncoolant.com.

REFERENCE
1. Canter, N. (2018), “The Chemistry of Metalworking Fluids,” in Metalworking Fluids, Third Edition, edited by Byers, J., CRC Press, pp. 143-169.