*5.2. CO2 Machining of Titanium Alloys*

As a consequence of their remarkable mechanical properties, corrosion resistance and ability to function effectively at elevated temperatures, titanium alloys have become commonplace at the high-performance end of the engineering sector. Although these exceptional properties allow titanium alloys to serve a broad range of demanding applications, they equally correspond to poor machinability. For this reason, it is intuitive that titanium alloys would be the focus of a great deal of cryogenic machining research, and indeed this has proven to be the case. Whilst the cryogenic machining of titanium is undoubtedly an area of significant research, many of the articles published focus specifically on Ti-6Al-4V [27–30]. Although this research direction is intuitive based upon Ti-6Al-4V being the most popular titanium alloy (accounting for 50% of titanium production worldwide as of

April 2020) [31], there remains a great degree of novelty to be explored in the machining of more exotic titanium alloys. The proceeding section is dedicated to the current portion of the research landscape focused upon the cryogenic machining of titanium alloys with CO2 MWF strategies.

In much of the cryogenic machining research currently available, there is significant variance in the relative performance of the coolant strategies employed. To give an example, in 2016, Sadik et al. [32] published an article examining the use of LCO2 as an MWF for the cryogenic face milling of Ti-6Al-4V. In the article, the authors note a tool life increase of 250–350% when LCO2 was applied in lieu of an emulsion flood cooling strategy. In contrast, to the strong performance of CO2 in the research of Sadik et al., the later work of Tapoglou and colleagues [7] noted that CO2 failed to compare favourably to conventional (emulsion based) MWF strategies. In their research, Tapoglou and colleagues undertook shoulder end-milling trials on a Starrag LX051, 5-axis horizontal milling centre employing a LCO2 cooling strategy. In contrast with the significant tool life improvements experienced by Sadik et al., Tapoglou observed markedly inferior tool life when utilizing both CO2 in isolation and CO2 + MQL strategies (in comparison to a both a flood and medium pressure through tool emulsion cooling strategy). Tapoglou did, however, note that the use of a CO2 + MQL MWF strategy corresponded to an increase in tool life relative to both MQL and CO2 in isolation.

Further to the variability of the data, An et al. [33] undertook side-milling trials on Ti-64 with three CO2 MWF strategies: scCO2 in isolation, scCO2 with water based MQL and scCO2 with vegetable oil-based MQL. As part of the trial, An and colleagues measured (and simulated) flank wear, cutting torque and surface morphology, indexing performance against dry cutting conditions. The authors noted that the scCO2 strategy generated the most flank wear, followed by dry, scCO2 with water-based MQL and, finally, scCO2 with vegetable oil-based MQL. In the paper, An et al. hypothesises that the elevated wear state experienced during scCO2-only machining strategy is likely a consequence of increased friction at the tool-workpiece interface, thermal cyclic fatigue and adhered chips accumulating on the tool. Interestingly, the extent to which flank wear was reduced by the scCO2 + MQL strategies, relative to dry machining, was extremely marginal, wherein the difference in flank wear between the three strategies fell within the variance of the data set.

It is clear that, whilst there are non-trivial differences in the experimental design of the three papers in question (differing cutting speeds, CO2 rather than LCO2, end-milling/sidemilling rather than face milling, etc.), their nonetheless exists a clear variance in the findings of Sadik, Tapoglou and An et al. It is thus the opinion of the author that this is illustrative of the conditionality of the machinability outcomes that are achieved when employing CO2 MWF strategies. Further to this point, it is not immediately obvious as to which variables are of the most importance in manipulating performance indices such as tool life. For this reason, a compelling case is made to work towards establishing an appropriate processing window for CO2 MWF strategies in a range of different machining context. In doing so, the processing parameters that allow the cryogenic media to offer comparative (or superior) tool life to conventional means would be outlined, thereby allowing manufacturers to employ the most appropriate MWF strategy for a given application.

One of the few articles to consider non Ti-6Al-4V titanium alloy (CO2) machining was published in 2018 by Kaynak and Gharibi [34]. The paper was published with the outlook of examining the impact of LN2 and CO2 as MWF's for the turning of Ti-5553 and as such, undertook trials with the following performance indices in mind: cutting temperature, tool wear and dimensional accuracy. Thereafter, the performance of each of the two media was compared against each other, in addition to a dry machining baseline. The authors observed that in both the CO2 and LN2 trials, the measured maximum temperature was shown to be significantly decreased relative to the dry machining trials, whilst LN2 cooling generally corresponded to a marginally lower maximum temperature than was observed in CO2 machining. Moreover, the authors observed a general trend in both cryogenic media reducing the extent of flank wear, noting that this effect was particularly exaggerated at

elevated cutting speeds, where, at the maximum trialled cutting speed, CO2 cooling lead to a 22% reduction in flank wear (relative to dry machining) and LN2 a 59% reduction in flank wear. In addition, although both of the cryogenic media trialled led to improvements in dimensional accuracy, LN2 cooling corresponded to a lower maximum deviation from the nominal diameter than the CO2 strategy. Whilst this article generally points to the superior performance of LN2 (relative to CO2) in this context, it is important to note that both cutting force and feed force were most optimally reduced by the CO2 cooling strategy, whereby a noteworthy reduction was observed across the majority of cutting speeds. This reduction in feed force provides further evidence of the excellent lubricity, which can be obtained by employing CO2 cooling strategy in a machining context.

In addition to the work of Kaynak and Gharibi, Machai and Biermann [35] undertook cryogenic OD turning trails on Ti-10V-2Fe-3Al. The authors compared the tool life performance of CO2 snow (as a coolant) to an emulsion, flood coolant strategy. Moreover, Machai and Biermann went on to outline the transient wear progression of the cutting insert when subject to each media, as well as discuss the inherent mechanisms associated with said wear progression. The authors observed that the CO2 cooling strategy corresponded to increased tool life at each of the cutting speeds trialled (Figure 3); moreover, whilst the emulsion-cooled tool was subject to significant notch wear, the CO2 cooled tool was entirely devoid of notching. Moreover, CO2 cooling additionally contributed to reduced feed force during later machining passes, despite initially generating elevated radial forces, findings that have been observed elsewhere in the literature. In addition to the positive tool wear implications of CO2 cooling, Machai and Biermann observed that, during the later passes with emulsion cooling, burrs formed at the tools' exit from the cutting zone; in contrast, no such burrs were observed during the CO2 machining trials (Figure 3). The authors went on to suggest that the presence of burrs (or lack thereof) was a consequence of the periodic impact of a worn, notched tool, and in this sense, the lack of burr formation in the CO2 trials is an intuitive finding.

In conclusion, the current landscape of the literature around titanium alloy machining remains, at this stage, inconclusive. Although some of the variability between the current research is undoubtedly a result of the differences in employed processing parameters (Section 6), the significant contrast between otherwise experimentally similar remains a challenge that must be addressed by the research community. Clearly, it is possible to establish a range of operating conditions with which CO2, or CO2 + MQL could be rendered the optimal MWF strategy for a given material; however, in the available research, the efficacy of CO2 cooling remains subjective. Further work should thus focus upon establishing a suitable operating range (for multiple titanium alloys), whereby CO2 is able to function as a suitable coolant. Moreover, should future research become more aligned with the findings of Tapoglou, it would undoubtedly be beneficial for researchers, who have an interest in the adoption of cryogenic machining technologies to further develop the consortium of literature studying the auxiliary benefits of CO2 as an MWF, rather than simply focusing upon tool life. Alternatively, it may also be beneficial to consider directing further research towards the cryogenic machining of alternative titanium alloys or even entirely new material species. With this recommendation in mind, the following section will consider current examples of CO2 usage as an MWF for the machining of various other material species.
