*5.1. CO2 Machining of Steels*

For almost two centuries, steels have found a broad range of applications owing to their high strength and durability. For these reasons, it is intuitive that a significant portion of the cryogenic research landscape would be focused upon the machining of steel, and indeed this has historically proven to be the case. In recent times, these articles have generally focused upon the use of CO2 cooling to improve the machinability otherwise generated by conventional MWFs. Such research is often motivated by the early success of LN2 as an MWF for the machining of steel. Some of these trials are discussed in the following section, where they have been selected to provide a variety of steel species and to cover a range of research interests within the field of cryogenic machining.

In order to understand the fundamental physics of CO2 as an MWF, it is useful to adopt a reductionist strategy towards the employed machining model. This approach was recently undertaken by Rahim et al. [23], who examined the impact of a scCO2 MWF in the context of the orthogonal cutting of AISI 1045 medium carbon steel. The performance of said cryogenic coolant was thereafter measured across four primary performance indices (cutting force, chip thickness, tool-chip contact length and cutting temperature) and compared to an equivalent machining trial undertaken with MQL (where MQL was chosen as a competitive sustainable machining strategy). The paper notes that the scCO2 machining strategy leads to 5–14% lower cutting forces, 0.5–2.5% thinner chips, 1–10% shorter tool-chip contact length and 15–30% lower cutting temperature than the MQL condition. Each of these variables is heavily correlated to machinability. Foremost, lower cutting forces are beneficial for a

range of reasons including improving system dynamics and reducing tool holder deflection (thereby improving dimensional accuracy); thinner chips tend to be more breakable and, as such, pose a significant chip handling benefit. A shorter tool-chip contact length infers lower friction (and thus, superior lubricity). Finally, lower cutting temperature corresponds to the reduced severity of diffusion dominated wear mechanisms and allows the retention of a strong cutting edge. These findings make for a strong mechanistic rationale for the use of CO2 as a cutting fluid, and further, leave a strong foundation upon which research with a higher degree of specificity to real, practical machining set-ups can be built.

A preceding paper by the same authors [24] utilised a similar set-up to trial the effectiveness of scCO2 and scCO2 + MQL as an MWF strategy for the turning of, again, AISI 1045 medium carbon steel. In this trial, the performance of the coolant was quantified according to the cutting temperature, cutting force and the surface roughness. Again, performance was compared to MQL in isolation. The paper noted that the use of a scCO2 coolant was able to generate a reduction in cutting temperature of 6–30%, dependent upon presence (or lack thereof) of MQL, nozzle configuration and proximity to the cut, where the tool cooled with scCO2 in isolation generated a lower cutting temperature. Despite scCO2 in isolation generating the lowest temperature of the three coolant configurations trialled, the scCO2 + MQL strategy corresponded to the lowest cutting force and surface roughness of the three strategies trialled. These findings further build upon the prospective positive impact of the usage of scCO2 as an MWF, or in supplement to the use of MQL.

Whilst AISI 1045 steel is extremely well researched, other steel species are increasingly being considered, one such novel species being that of high thermal conductivity steel (HTCS). As part of their 2017 paper, Mulyana et al. [25] compared the performance of scCO2 to MQL and dry machining in the milling of HTCS. In accordance with the previous research undertaken by Rahim et al. [23], the authors considered the impact of cutting fluid on temperature, cutting force and tool wear in addition to considering the underlying wear mechanisms associated with the machining process (by the assigned MWF). The author noted that across all major performance metrics, MQL, scCO2 and scCO2 + MQL outperformed dry machining and, further, each cryogenic strategy outperformed MQL in isolation. With regard to both cutting force and tool life, the scCO2 + MQL strategy led to the best performance, generating 28–64% lower cutting forces and 60–85% longer tool life relative to MQL in isolation (Figure 2). Moreover, in each of the three performance indices considered in the paper (cutting force, tool life and cutting temperature), elevated scCO2 input chamber pressure corresponded to superior performance. The author reasons that this phenomenon is a consequence of the elevated density of scCO2 and higher volumetric flow rate of MQL.

**Figure 2.** Graphs to show the variation in: (**a**) cutting force, (**b**) cutting distance with coolant and scCO2 input chamber pressure. Reprinted with permission from reference [25], Copyright 2000 Elsevier.

In a 2014 conference proceeding, Cordes et al. [26] outlined a novel strategy for the internal, selective, multi-channel supply of LCO2, MQL or a combination of the two. As part of the proceedings, they trialled their tool holder configuration in the machining of 1.4962 stainless steel. The article employed a "radial multiaxial turn milling strategy" (as is common in the roughing of aerofoils), where the extent of tool wear was measured at the end of the roughing cycle of one blade and thereafter compared to dry machining. Ultimately the obtained tool life was inferred to be vastly increased by the application of a CO2 coolant. This inference came as a consequence of the cryogenic cooling strategy corresponding to a 63% lower flank wear (after one pass). Interestingly, the authors then went on to increase the cutting speed and feed rate of the cryogenic machining strategy to the point of which flank wear (after one pass) was equivalent to that which was observed in dry machining. They note that a 37.5% increase in feed rate and a 25% increase in cutting speed were required to elevate flank wear to that of the dry machining trial. In such a scenario, material removal rate (MRR) was increased by 72%, outlining the potential of CO2 cooling in increasing the productivity of the manufacturing sector.

Similarly, in 2019, Fernández et al. [20] published a paper considering the application of CO2 as an MWF in the cryogenic face milling of hard-to-cut materials. As part of their research, they undertook tool life testing on grade EA1N steel when machined with gaseous CO2, indexing its performance against the tool life obtained by a Hysol XF (6–8%) emulsion. The authors noted that whilst the underlying wear mechanism, which led to tool failure, was, in both cases, abrasion, the rate at which the tool became abrasively worn was significantly reduced in the cryogenic trial. Moreover, in accordance with ISO 8688-1:1989, the authors utilised a tool life criterion wherein tool failure was inferred by insert fracture. As such, the article notes that CO2 cooling is able to generate a 175% longer tool life than that obtained by a conventional emulsion strategy. This finding is of particular interest owing to the authors observing a favourable comparison between the performance of CO2 and emulsion (rather than dry machining), which is generally best practice for the machining of most materials.

As is outlined in the proceeding work, much of the research that has thus far been conducted has focused upon the machining of steels, and although there remains a great deal of novelty yet to be explored, the field is becoming increasingly well understood. Whilst this is of great utility in the sense that steel is an extremely popular engineering material, it fails to address the demands of many performance-driven engineering sectors, namely aerospace, motorsport and biomedical. In many of these scenarios, weight restrictions are implicated into the design process and, as such, specific, rather than generic, mechanical properties are of great importance. As these applications are (in general) driven by factors such as performance and lead time (as opposed to cost and durability), they are generally better positioned to become early adopters of technological advancements in the manufacturing sector or elsewhere. For this reason, it is intuitive that early research efforts focus more heavily upon high-performance materials such as titanium alloys, nickel-based superalloys, composites, etc. Consequently, the proceeding work examines CO2 as an MWF in the context of these aforementioned materials.
