**1. Introduction**

In order to accommodate the demands of the current technological landscape, engineers have developed novel materials that meet an increasingly expansive range of lofty performance indices. One pertinent example is that of an aero-engine disk. In such an application, the subject material must exhibit high flexural rigidity, fatigue strength and creep resistance, all whilst being exposed to operating temperatures in excess of 800 ◦C [1]. In this case, it crucial that the material chosen exhibits both high shear strength and low thermal conductivity. Whilst these properties serve a clear purpose in an aeroengine, they generate a challenging machining environment, which must be met with a robust metalworking fluid (MWF) strategy. Unfortunately, however, whilst they remain an operational necessity, the use of conventional MWF strategies is accompanied by a series of negative environmental, social and ergonomic implications. To elaborate, their disposal is not only unsustainable, but both extremely costly and time consuming; they pose a health risk to machine operators and their maintenance requires a significant time commitment that could be allocated to more productive tasks. In contrast, whilst cryogenic and CO2 cutting fluids have their own limitations, they offer markedly improved sustainability (Section 4); they have been shown (situationally) to produce significantly improved machinability outcomes (Section 5), and by virtue of their return to the atmosphere post machining cycle, they remove the burden of fluid maintenance and disposal.

Despite the prospective benefits of cryogenic and CO2 MWF strategies, the use of CO2 as an MWF remains confined to niche applications. This is due to the fact that whilst LN2

**Citation:** Proud, L.; Tapoglou, N.; Slatter, T. A Review of CO2 Coolants for Sustainable Machining. *Metals* **2022**, *12*, 283. https://doi.org/ 10.3390/met12020283

Academic Editor: Francisco J. G. Silva

Received: 22 December 2021 Accepted: 25 January 2022 Published: 5 February 2022

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is, at this point, extensively researched, the available literature on CO2 is (comparatively) very much in its infancy. As such, if CO2 MWF strategies are to become commonplace, it is crucial that the body of literature is compiled in a means that is both accessible and categorised according to research interest. With this in mind, the proceeding review looks to outline the scenarios in which CO2 MWF strategies have, at this stage, been employed, and further, serves to identify avenues for future research.

## **2. Review Structure**

Given the extensive research that focuses upon the use of liquid nitrogen (LN2) MWFs, the proceeding text is primarily focused upon the emerging use of CO2 coolants, with only an ancillary consideration of LN2. The review first provides a background (Section 3) on the subject matter of cryogenic and CO2 MWFs, focusing foremost upon the historical use of cryogenic MWF's (Section 3.1) followed by their motivation for use (Section 3.2) and the mechanism of action for scCO2 coolants specifically (Section 3.3). Thereafter, the document considers the sustainability of CO2 coolants in lieu of conventional MWFs (Section 4), giving primary consideration to the environmental, social and ergonomic implications of the widespread disruptive use of CO2 coolants. The review next focuses upon the current state-of-the-art by way of outlining the available literature on CO2 MWFs, categorizing said literature foremost according to the workpiece material considered (Section 5) and then according to a range of other operational variables (Section 6), i.e., feeds and speeds, tool design, etc. Finally, the document is brought to a conclusion (Section 7). When locating research for this review article, the following search terms were used: "CO2", "Machining", "Cryogenic", "MQL", "Milling" and "Turning".
