*3.1. History of Cryogenic MWFs*

In many regards, formal research on cryogenic cutting fluids began in 1967, when Okoshi [2] observed, and later went on to present the chip handling benefits of LN2 at the annual general meeting of the Japan Society of Precision Engineers (JSPE). Despite sparking interest, it was not until 1969 that the first journal articles were published in the field. In the original paper on cryogenic machining, Uehara and Kumagai [3] examined the variability of cutting temperature, chip formation and tool force with workpiece temperature for plain carbon steel, stainless steel and commercially pure titanium. In their research, Uehara and Kumagai employed a simple turning model where LN2 was applied in situ to the three workpiece materials. Thereafter, two separate thermocouples were employed to measure temperature (at the workpiece and chip-tool interface), and a dynamometer utilised to measure force components. The ultimate findings of the paper noted that whilst the cryogenic machining strategy led to an improved surface finish in the carbon steel workpiece, in the stainless steel and titanium samples, surface roughness was either comparable or worsened by the application of LN2. Moreover, the paper found that during the machining of titanium sample, workpiece temperature and cutting force followed a negative correlation. The authors went on to conclude that the variability with which the different materials responded to reduced workpiece temperature was largely due to the extent to which a given material exhibited low temperature brittleness. As the amount of shear flow stress implicit during low temperature machining increases, surface roughness is worsened and cutting force is generally increased (if not offset by increased brittleness).

In their following paper, Uehara and examined the tool life implications of a cryogenic, workpiece cooling strategy [4]. In this regard, the first truly promising tool life rationale for the use of cryogenic coolants was made, where tool wear was noted to decrease at reduced temperatures in both the carbon steel and titanium test pieces. Moreover, this paper went on to define the conditionality of cryogenic cooling strategies, noting that the extent of flank wear in the stainless steel workpiece followed the reverse trend to that of both carbon steel and titanium. As was the case in their original paper, this article again concludes by defining three variables responsible for the observed phenomenon: the temperature

dependency of tool wear, low temperature brittleness of the workpiece and the protective effect of built-up edge (BUE) against tool wear (the latter being heavily cited in the authors reasoning for the conditionality of recorded results). Interestingly, whilst both titanium and stainless steel failed to exhibit low temperature brittleness, the impact of workpiece temperature on tool life differed dramatically between the two materials. The authors went on to state that this discrepancy is likely a consequence of the protective effect of a BUE having a much more significant impact on tool life in the machining of stainless steel (then titanium), and as such, the extinguishment of the BUE at cryogenic temperatures was a more relevant phenomenon in the machining of stainless steel. Additionally, it can be argued that in the machining of titanium, temperature dependency of tool wear is higher based upon the low thermal conductivity of titanium and the known susceptibility of titanium and titanium alloys to diffusion dominated wear mechanisms such as crater wear.

Given the early research of Uehara and Kumagai, many authors have since followed suit in the study of cryogenic MWF's. Generally, much of this research has historically focused upon the use of LN2 coolants. Whilst these articles are the subject of a great deal of academic interest, much of their contents have already been previously considered in earlier review papers [5,6]. Additionally, it is the rationale of the author that the significant incentives offered by CO2 MWFs (Sections 3.2 and 4) make further LN2 focused reviews inconsistent with the current research trend of the cryogenic machining community. Given this statement, a case is thus made in support of the proceeding work's focus upon CO2 (rather than LN2) MWF's.
