*6.1. Workpiece and Tool Material*

When reviewing cryogenic machining research, one of the most superficially apparent variables that dictates trial performance is the choice of workpiece material. In fact, it is reasonable to assume that the perception of success of a given research paper is, in absence of other compelling evidence, generally thought of as a consequence of the suitability, or lack thereof, of said workpiece material. Whilst this perception is somewhat short-sighted (given the extent to which cryogenic assisted machining trials can vary), the choice of machined material invariably plays a significant role in determining which MWF strategy is preferential for a given trial. Given this fact, it is unsurprising that this most fundamental dependency has been observed since the earliest iterations of cryogenic machining research. To be specific, the aforementioned work of Uehara and Kumagai [3,4] notes that a strategy of cryogenic pre and intra workpiece cooling lead to tool life improvements in both carbon steel and commercially pure titanium workpieces despite presenting no clear performance benefit in the context of stainless steel.

Needless to say, the trend of differing machining trial performance by workpiece material persists into the modern day and is in fact a topic of discussion in most, if not all, journal articles published in the field (not least of which this one). Despite this, it is also true that conclusions of workpiece suitability should not be equitably drawn from each cryogenic machining article on the basis of a lack of experimental control. In order to illustrate this point, take the example of two machining trials that both employ jet cooling, utilise equivalent feed rates, cutting speeds and depths of cut and consider two different workpiece materials (one per paper). In this example, it may seem appropriate to draw like for like comparisons between the two papers, particularly given their stark similarity. Whilst the results of these two papers would likely share some commonality, the problem with a direct comparison in this context is the invariable fact that across some parameters (i.e., tool nozzle diameter, or means/standards of measuring tool life), the employed experimental practice of the two papers will differ. Taking this into consideration, the most academically rigorous way to analyse the relative cryogenic machinability of different materials would involve the analysis of research papers that examine various material species in the one paper, or equivalently, those papers published as part of a research chronology by one set of authors (where DOE is unaltered). For this reason, Section 5, whilst citing papers utilising various materials, does not effectively serve to illustrate which materials are suited to cryogenic machining strategies. With this in mind, the following section employs such an approach with the outlook of understanding the conditionality with which different materials can be cryogenically machined.

One paper that considered various materials was the aforementioned work of Fernández et al. [20]. In their article, Fernández and colleagues undertook the cryogenic face milling of three challenging-to-machine materials: Grade EA1N steel, gamma Ti-Al and Inconel 718. Interestingly, the authors observed markedly different (tool life) performance across each trialled material. In the case of Inconel 718, both the CO2 and emulsion strategies lead to equivalent tool life, such that the tools in both trials failed in each case after two machining passes (and a tool life of 5.3 min). Moreover, in both trials, the tools failed via a similar cracking mechanism said to be a consequence to the combined effects of abrasion adhesion and microchipping (Figure 5). In contrast, during the gamma Ti-Al trials, the authors observed that conventional emulsion cooling led to tool failure after the first machining pass, whilst the CO2 cooled tools generally had not failed prior to undertaking the second machining pass and thus achieved a projected 100% increase in tool life. This performance was shown to extend to the EA1N steel trials, wherein the cryogenic cooling strategy led to a 175% increase in tool life relative to emulsion flood cooling. Moreover, despite this significantly reduced rate of tool wear, wear mechanisms were, in both cases, found to be predominantly abrasive despite being markedly more aggressive in the emulsion cooled trials. Ultimately the results of this paper would seem to suggest that both EA1N steel and gamma Ti-Al may be more suitable for CO2-assisted machining than Inconel 718; however, this should be developed upon with further research.

In addition to the work of Fernández and colleagues, an earlier paper by Wang and Rajurka [22] was published examining the cryogenic machinability of Ti-6Al-4V, Inconel 718, Reaction Bonded Silicon-Nitride (RBSN) and (commercially pure) Tantalum. The paper employed a simple turning model with two different tool materials, Cubic Boron Nitride and Cemented Tungsten Carbide (WC-8 wt%Co). In contrast to the more recent research of Fernández, the authors employed a LN2 remote-cooling strategy wherein LN2 was recirculated in close proximity to the cutting edge via a network of copper tubes in a proximal tool cover. As part of their research, the authors primarily focused upon tool life, in addition to cutting temperature (via a mounted thermocouple), surface roughness and cutting force.

**Figure 5.** Worn inserts (at tool failure) during the machining of Inconel 718. Reprinted from reference [20].

During the machining of RBSN, three different varieties of CBN tools were employed, in all cases showing prolonged tool life during LN2 remote cooling (relative to dry machining). Despite this commonality, not all tools were equally positively implicated by the LN2 cooling strategy (Figure 6). Specifically, despite wearing most aggressively during dry machining, the CBN50 tool (produced by Sandvik) was most drastically impacted by LN2 cooling, wherein after completing the 158 mm cutting length (which was allocated to each tool), a sub 0.5 mm flank wear was observed. These promising results equally persisted during the turning of Ti-6Al-4V (with H13A WC-8 wt%Co tools), whereby an over five times increased cutting length was required of the LN2 cooled inserts to generate the equivalent flank wear to an 'oil cooled' equivalent.

**Figure 6.** Wear progress curves during the machining of RBSN with three different CBN tools. Reprinted with permission from reference [22], Copyright 2000 Elsevier.

In addition to the trials conducted on RBSN and Ti-6Al-4V the author also considered the cryogenic machinability of Inconel 718 and Tantalum (with a H13A tool), again observing a markedly reduced rate of tool wear when LN2 cooling was employed (in lieu of dry machining). Despite this clear trend, it is difficult to accurately extrapolate tool life from the figures present in the report. In the case of Tantalum, at the points where the tools were equivalently worn, the magnitude of flank wear was extremely low, which of course raises questions as to the efficacy of any conclusions which are drawn. With

this limitation in mind, it was nonetheless observed that the LN2-cooled tool required an approximately 90 mm cutting length to reach a flank wear of approximately 0.15 mm, whilst the uncooled tool reached such a wear state after only 20 mm (with LN2 cooling). Unfortunately, however, when Inconel 718 is considered, the scope to predict the impact of cutting fluid on tool life is further hampered, wherein the available data are not at any point coincident and thus insufficient to make a sober estimate.

Whilst there are a limited number of papers to consider multiple workpiece materials, there are, of course, a few other examples of similarly inclined research, one such paper being that of Busch et al. [21], who conducted CO2-assisted OD turning of Inconel 718 and Ti-6Al-4V. In contrast to some of the earlier promising research, Busch and colleagues observed generally poor performance markers for CO2 cooling. The key observation made in this article (from a cryogenic machining perspective) was one of reduced tool life when CO2 cooling was used in lieu of high-pressure (HP) coolant. This phenomenon was somewhat more pronounced during their Inconel 718 trials. Specifically, the tool life obtained during the CO2 machining of Inconel 718 was almost 70% reduced relative to HP coolant, whilst the equivalent figure was closer to 60% in the Ti-6Al-4V trials. In this regard, the findings of Busch and colleagues are largely in keeping with the work of Wang and Rajurka, as both papers contribute to a future lack of candidacy of Inconel 718 for cryogenic machining research.

In summation, current research implies that Inconel 718 has, so far, not shown itself to be a particularly compelling candidate for future cryogenic machining research owing to generally poor susceptibility to cryogenic cooling, at least from a tool life perspective. Mechanistically, this may be a consequence of the markedly low thermal conductivity of Ni-based superalloys impeding the success with which CO2 is able to effectively cool the cutting zone; however, this of course must be supported by more substantive data. Importantly however, although Inconel 718 is not shown to be particularly promising in any of the previously considered research, the suitability or lack thereof of superalloys remains to be seen. In contrast, cryogenic machining strategies are, at this stage, shown to have some scope to improve the machinability of both titanium alloys and steels.

#### *6.2. Feeds and Speeds*

One of the most critical aspects in machining is the appropriate selection of cutting parameters in order to optimise the overall efficiency of a production process. This selection must take into consideration not only the material that is being processed, but also other factors including the cutting tool characteristics and more importantly the lubri-cooling technique employed and the delivery method. Different cooling techniques would result in different cutting temperatures and loads on the cutting tool; therefore, the cutting parameters would need to be adjusted to allow for optimal cutting conditions. Given this complexity, it is often desirable to encompass various feed rates and cutting speeds centred around estimated best practice data established in the literature, or via practical machining guidelines.

It is undeniable that the success (or lack thereof) a machining trial is highly contingent upon the operational parameters employed during said trial. It follows that the way in which cryogenic coolants are perceived in a trial could, and likely would, equally be dependent upon the use of appropriate feeds and speeds. It is also apparent that any feed rate and cutting speed data will be incredibly specific to the material being machined. As way of an example, one online machining data base [45] recommends a cutting speed of 175 m/min for the turning of austenitic stainless steel, whilst equivalently recommending speeds as high as 860 m/min for wrought aluminium alloys. Although this effect is quite clearly exaggerated by the choice of materials, similar implications would undoubtedly persist across more (mechanically) similar species, i.e., titanium and nickel alloys. Given this, it is thus crucial that an extensive dataset of cryogenic machining performance is developed for a range of feeds, speeds and materials. Unfortunately, such data are not currently available in the public domain, and as such, investment into CO2 cooling systems remains speculative until the research landscape is able to catch up.
