*5.3. Additional Applications of CO2 MWFs*

Although steel and titanium alloys are the focus of a large portion of the current cryogenic machining literature, CO2 is speculated to function as a potentially efficacious MWF strategy for a range of other materials. As an example, in nickel-based superalloys, the combination of high hardness and low thermal conductivity leads to excessive heat forming in the cutting zone, and by proxy, the tool. This, in turn, leads to a litany of problems, not least of which are reduced tool life and thermally induced geometric distortion. Despite presenting a clear cause for concern, these challenges are certainly not unique to nickelbased superalloys, and in fact are commonplace in titanium alloy machining. Given this realisation, it is rational to extrapolate the potential benefits of CO2 in the machining of titanium alloys to a prospective application in the machining of nickel-based superalloys. Whilst this reasoning is undoubtedly useful in deciding upon the most likely avenues for future research, prior machining trials on mechanistically similar materials of course cannot be regarded as a direct citation of the efficacy of CO2 for new, unproven material species. For this reason, the use of CO2 as an MWF for the machining of nickel-based superalloys, whilst currently in its infancy, is proving to be a burgeoning field of cryogenic machining research.

**Figure 3.** Tool wear and burr formation during the cryogenic machining of Ti-10V-2Fe-3Al. (**a**) Emulsion; (**b**) CO2. Reprinted with permission from reference [35], Copyright 2011 Elsevier.

Whilst superalloys have, in recent times, been the subject of an increasing number of promising cryogenic machining trials (with LN2) [36], CO2 trials are, despite being supported by a strong mechanistic rationale, relatively unexplored. Of the limited research that is currently available, results are generally mixed. In 2016, Busch et al. [21] undertook turning trials with coated CNMG 120,408 cemented carbide inserts on Inconel 718. The authors went on to compare the performance of both CO2 in isolation, and ADL + CO2 to a high-pressure emulsion coolant strategy, as is regarded as the current best practice for the machining of this material. The performance of the three coolant strategies were thereafter assessed primarily according to tool life, specific energy consumption and general machined surface quality. Whilst the authors noted a generally invariable energy consumption across each of the trialled media, both the CO2 and ADL + CO2 strategies generated markedly lower tool life relative to the use of high-pressure coolant. In addition to the poor tool life, the authors observed no clear chip handling benefit of ADL + CO2 and further went on to note the presence of adhered material at the machined surface when CO2 cooling was applied in isolation. Whilst the article makes note of the operational convenience of retrofitting a CO2 coolant delivery system, and equally the benefits of oil residue free machining, the paper states that any scope for the future efficacy of these CO2 cooling strategies will be reliant upon future optimisation to the delivery of the cryogenic media (and the ADL).

In addition to these findings, Patil et al. [37] also conducted CO2-assisted machining trials on Inconel 718. In accordance with the work of Bush et al. [21], the authors employed an OD turning model utilising TiAlN-coated SNMG120408-cemented carbide inserts. Thereafter, the authors examined both cutting forces and surface phenomena over a range of feeds and speeds, ultimately comparing the performance of CO2 cooling to a dry machining condition. Whilst the authors observed an increase in both cutting and feed forces when CO2 assistance was applied, this effect was observed to be of a much smaller magnitude than the impact of cutting speed and feed rate on tool force. Moreover, when compared to dry cutting, the presence of a CO2 MWF led to a reduced surface roughness. This effect is visible across all feeds and speeds; however, it was most pronounced at 0.1 mm/rev, 100 m/min. When surface integrity was further analysed (by way of taking microhardness measurements of the machined surfaces), the CO2 assisted trials corresponded to an increase in surface hardness (relative to bulk), whilst the dry machined surfaces were of equal or lesser hardness to the as-supplied bulk. The authors note that this finding is likely a consequence of the cold work hardening of the workpiece in the presence of CO2, presumably in lieu of the elevated microstructural recovery, which would otherwise be observed at elevated temperatures (as in dry machining). This increase in hardness is regarded, by both the authors of the study and others [6], to be a desirable outcome for improved surface integrity. Further, where increased surface and subsurface hardness has previously been observed (in the LN2 machining of Inconel 718) [38], it has been accompanied by higher compressive residual stress, which makes positive implications for fatigue life (via the inhibition of crack propagation).

Given the research of Busch and Patil [21,37], the holistic impact of CO2 coolants in the machining of Inconel 718 remains uncertain. Clearly, future research is necessary to optimise the operational parameters used in the CO2 machining of each workpiece material. This will likely involve optimising variables such as nozzle position, coolant flow rate, feed rate, cutting speed, tool material, tool geometry, etc. over a range of machining operations (Section 5). Moreover, whilst material classifications are helpful when analysing the density of research in a given domain, there remains no guarantee that similar species of material will behave similarly in an equivalent cryogenic machining context. In the case of superalloys specifically, although iron-, nickel- and cobalt-based superalloys are often employed in similar roles, their machinability is likely to vary in accordance with their metallurgical differences. Whilst this variability is likely to be most noticeable amongst dissimilarly based alloys, it will undoubtedly persist within each species. As an example, nickel-based superalloys used in aero-engine disks (i.e., RR1000) may, and likely will, exhibit markedly different machinability to alloys used in aero-engine turbine blades (i.e., Inconel 738LC) owing to their variation in mechanical, and thermal properties. For this reason, whilst a CO2 MWF may not be regarded as efficacious for one alloy, it may be wholly suitable for another.

Whilst the cryogenic machining of superalloys is a burgeoning field of research, there remains a mechanistic rationale for many other materials. In fact, many of the most thoughtprovoking avenues for future cryogenic machining research will invariably focus upon materials that otherwise are ineffectively machined by way of their material properties; one pertinent example being that of viscoelastic polymers such as polydimethylsiloxane (PDMS) or ultra-high molecular weight polyethylene (UHMWPE). In general, the high elasticity and susceptibility to adhesion make for a material that is challenging to machine by conventional means. These properties correspond to a litany of adverse machining outcomes including geometric deformation of the workpiece material and undesirable

chip formation. Moreover, the propensity of polymeric material to adhere to the cutting tool often corresponds to an increase in effective edge radius, which in turn corresponds to a ploughed, rough surface (owing to the size effect). Despite this, with the rise of the burgeoning field of microfluidics, small scale, geometrically accurate polymeric chips are becoming increasingly necessitated, not least of which within the pharmaceutical industry, wherein they serve a range of roles including drug screening and metabolic research [39]. Currently, microfluidic chips are manufactured via micromolding techniques, whereby the micromolds are photolithographically formed in the polymeric substrate. Whilst this technique allows for accurate fabrication, the process is not easily customisable. As such, future iterations of a chip, or equally new chip geometries, generally necessitate the development of a new, bespoke mould. Given this reality, and with the rise of micromachining technology, cryogenic machining may prove to become a cost-effective strategy for the manufacture of viscoelastic polymers.

Where the cryogenic machining of polymers has been undertaken, the strategy generally involves either a workpiece pre-cooling strategy or, further, the submersion of the polymer during the machining process. This strategy takes advantage of the changed chip-formation mechanisms that occur below the glass transition temperature (Tg) of the polymer. In a 2015 paper by Aldwell et al. [40], UHMWPE was submerged in a vat of liquid nitrogen for a period of 24 h prior to turning. Thereafter, cutting force, surface roughness and chip morphology were measured in both the pre-cooled and room temperature billets. As a consequence of the elevated elastic modulus of the polymer (sub-Tg), cutting force was elevated when cryogenic-assisted machining was applied. Ordinarily, increased cutting force has negative implications for productivity; however, as tool life is of limited relevance in the small batch machining of polymers, surface quality is a much more vital parameter. In this regard, cryogenic pre-cooling was shown to be efficacious from a surface finish perspective, whereby mean surface roughness was reduced by more than 20% in the pre-cooled material. In addition, the authors generally observed a greater propensity to produce chips (rather than dust) when LN2 pre-cooling was employed. Although this effect was more apparent when worn tools were used, the finding is of potential utility to the manufacturing sector as, an inability to form chips is, in general, accompanied by an inability to dispel heat, which, particularly in the machining of polymers, can become extremely problematic.

In addition to the research of Aldwell and colleagues, Kakinuma et al. published an article employing the micro-end milling of cryogenically cooled PDMS [41]. The authors utilised a complete emersion strategy wherein the both the (single crystal diamond) milling cutter and PDMS workpiece were submerged in a vat of liquid nitrogen; thereafter, a ductile mode milling strategy was employed with a varying depth of cut. By employing such a strategy, the PDMS workpiece is held below the glass transition temperature such that it retains properties of low adhesion and elasticity. In abstract, this allows the thermal deformation (which is implicit during a machining operation) to be suppressed, and thus form inaccuracies to be minimised. When machining trials were conducted, the authors observed that the large quantity of heat generated in the cutting operation was sufficient to vaporise the proximal LN2 and, subsequently, that the generated surface transparency was unsatisfactory. In response to this realisation, Kakinuma and colleagues went on to additionally apply cryogen directly to the cutting zone by way of LN2 jet. In doing so, heat flux into the workpiece was minimised, and subsequently a high-quality surface with lower opacity and surface roughness was generated (Figure 4). Given this success, the authors went on to machine a series of microfluidic chips with the outlined approach, noting benefits over micromolding processes such as a comparative ease in forming stepped channels and a generally far reduced lead time.

**Figure 4.** A demonstration of machined surface quality in micro-milled PDMS. (**a**) LN2 immersion method; (**b**) LN2 immersion + external supply cooling method. Reprinted with permission from reference [41], Copyright 2012 Elsevier.

Ultimately, although cryogenic MWFs are mostly well researched in the context of steels and titanium alloys, it is apparent that their early success has birthed widespread interest. In addition to the strides that have been made in researching the cryogenic machining of superalloys and polymers, cryogenic methods have already been applied to magnesium alloys [42], cobalt-chromium alloys [43] and composites [44], garnering generally positive results. Moreover, with the current pace of research, it is likely that trials will expand further into various other niche materials and manufacturing processes. Fortunately, in this regard, various avenues of research still exist. As an example, one material that may be suitable for future machining trials is that of oxygen free copper (OFC). Whilst OFC parts are in demand as high-conductivity electronic components, they are often extremely difficult to machine owing to poor chip handling and subsequent surface integrity. Given the prior use of cryogenic-assisted machining for chip-breaking purposes in other materials, it is reasonable to foresee future machining trials on OFC. In addition to examining novel materials, it may be equally worthwhile to consider alternative manufacturing process (to those commonly employed in a cryogenic machining context) as an example, rather than turning and milling, broaching has the scope to be significantly augmented by the addition of cryogenic cooling. By way of an explanation, in many articles researching the effects of CO2 as an MWF, benefits (over flood coolant) are often relatively incremental. This incremental improvement in operational efficiency may be insufficient to justify the purchase of a CO2 delivery system to an SME manufacturing company with a turning or milling focus. In contrast, broaching operations generally feature extremely expensive tooling, and create a great deal of added value. As such, if incremental process improvements can be made (i.e., to tool life) by cryogenic assisted machining, ample financial motivation may be available for early adoption of the technology. To summarise, cryogenic-assisted machining is an extremely interesting field of research, which, given its current promise, has scope to become a disruptive technology in the near future.
