1 Introduction

The word “Kryos” is a Greek word that means cold. Thus, cryogenics means working in the environment of low temperature. The first cryogenic laboratory was built by Heike Kamerlingh Onnes in the year 1882. The lab was set up at University of Leiden (Netherlands) which became one of the leading cryogenic center and remained to be so for a period of over five decades. Inconsistencies have been reported for identification of the beginning temperature of cryogenics. In accordance with most research and standard organizations, cryogenics begins at or below 123 K. National Institute of Standards and Technology has considered the temperature to be lower than 93.15 K as beginning point for cryogenics. Temperatures below 120 K have been regarded as cryogenics temperature by the Cryogenic Society of America. Various researchers consider temperatures below 0 °C as the cryogenic temperature [1,2,3,4,5,6].

Reitz [7] in the year 1919 reported the use of liquefied gases as cooling agent for carrying out the machining operations. Carbon dioxide gas was used by Reitz as a coolant in machining operations. The concept of cryogenic hardening was developed by scientists when they discovered that a greater wear resistance was exhibited by metals frozen to low temperatures. CryoTech Company of USA was the first to introduce the concept of cryogenic machining in the year 1966. Increase in metal tool life was reported by carrying out the machining process under cryogenic environment. Subsequently, in the year 1976, ASHARE [8] was credited to establishing of guidelines for applications in the domain of cryogenic applications.

With the gradual acceptance of application of cryogenics, Uehara and Kumagai [9] introduced cryogenic machining in the year 1968. Traditional oils was replaced as a coolant with the liquid gases such as helium, carbon dioxide, and nitrogen. Alteration in properties of workpiece, cutting tool, and the quantum of heat dissipated in the cutting zone has been reported by various researchers investigating cryogenic machining [10,11,12]. The potentiality of cryogenic machining to obtain surfaces with lesser damaged surfaces and with lower residual thermal stresses was henceforth proved after the investigations.

One of the well-known coolant for machining operation, i.e., water, has been evidenced for machining processes way back in sixteenth century [13]. With time, cutting fluids based on oil and water cutting fluids gained popularity in the twentieth century and have been used widely in machining of steel alloys. The main objective of employing cutting fluids is to enhance the rate of heat transfer from the cutting area and therefore to diminish the thermal stresses as well as the mechanism of chemical wear. In totality, the employability of cutting fluids improves the overall quality of the product [14,15,16,17,18]. However, the use of cutting fluids can result in human health complications such as respiratory diseases, dermatitis, and cancers [19,20,21].

The machining operations were made sustainable with the advent of coated cutting tools in the 1970s. Such machining processes were referred to as dry machining. However, due to the lack of availability of suitable coating materials, dry machining processes could not get the desired popularity. The applicability of the near dry machining processes was evidenced in the 1990s and with further progress became a more suitable sustainable machining alternative to dry machining. Employability of near dry machining processes resulted in reduction of amount of lubricants or the cutting fluids employed in carrying out various machining processes. Enhanced machining performances in terms of power consumption, surface roughness, and tool life have been reported with the near dry machining processes. Even after such advantages, the applicability of near dry machining processes has been limited. Further, the environmental and health implications of such machining processes requires further investigations. Investigations have been reported by various researchers on some of the challenges and limitations associated with the near dry machining processes [22,23,24].

Cryogenic machining process is yet another emerging sustainable machining which has done away with the conventional metalworking fluids. Liquefied gases such as nitrogen and the other non-toxic, odourless, and non-hazardous liquefied gases have replaced the different metalworking fluids. Further, the requirements for maintenance as well as post-machining disposal and cleaning processes are also minimized [25,26,27,28,29,30]. Further, higher rate of material removal and hence lower consumption of energy [31, 32] has been achieved owing to the increased hardness and toughness of the cutting tool material. Lower working environment is credited to increased tool material hardness and toughness. Components obtained using from cryogenic machining have been reported to show enhanced properties such as fatigue life, wear resistance, and corrosion resistance [31,32,33,34,35,36]. The conditions under which the coolants are used dictate the performance of the machining processes. The use of aforementioned cooling fluids is, however, limited owing to the constraint associated with usage in bulk for cooling.

Cryogenic machining therefore offers dual advantage as it combines the advantages of dry machining with that of the rapid rate of cooling. The two prominently used gases, i.e., CO2 and LN2 have got different requirements for their use. Further, they also distinguish from one another in the mechanism of generation of low temperatures. Technological advancements have led to the development of different strategies to aid in lubricating the cutting zone. As for instance, CO2 and LN2 have been combined with very minute quantities of oil particulates in MQL form.

With the today’s world being focussed on sustainability, the present work aims to summarize the recent aspects of cryogenic machining. With this major objective, thermal aspects related to the cryogenic-assisted machining processes have been discussed in Sect. 2 of the work. Section 3 highlights the major applications of the cryogenic-assisted machining processes. In the penultimate section of the work, i.e., Sect. 4 depicts the economic aspect of the cryogenics in machining processes. Section 5 highlights the sustainability issues of the cryogenics-assisted machining processes and the work finally concludes with the concluding remarks encompassing the future perspectives.

Figure 1 depicts the top journals that have contributed to the field of cryogenic machining.

Fig. 1
figure 1

The top 10 journals contributing toward the domain of cryogenic machining

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The top 10 countries to have contributed actively to the cryogenic manufacturing domain have been delineated in Fig. 2.

Fig. 2
figure 2

The top 10 countries contributing towards the domain of cryogenic machining

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2 Cryogenic machining of Al and Ti alloys

A network of capillaries aids the penetration of cryogenic fluid at the tool–chip interface. Diffusion is another penetration mechanism through which the cryogenic fluid enters the cutting area through the primary zone of deformation. The cutting fluid aids in reduction of the temperature by reducing the energy required for cutting and also by enhancing the potential ability to extract heat from the cutting zone. Reduced fracture strain of the workpiece material has been observed for cryogenic machining of ductile materials [37]. Enhanced energy of cutting was also observed for the same. Heat extraction is proportional to the difference between temperature of the coolant and the surface temperature of the body and also to the coefficient of heat transfer. Conventional cutting fluid have temperatures of around 25 °C, whereas liquefied nitrogen gas has temperature of around − 196 °C [39]. Therefore, a total advantage of 220 °C of the liquefied nitrogen gas over the other conventional fluids. As a result of heat transfer to the cryogenic fluid, gas cushion is formed at the tool–chip interface due to the evaporation of the cryogenic coolant [38].

To control the cutting temperature, it is quintessential to control the direction and manner in which the cryogenic cutting fluid is supplied to the cutting area. The cryogenic cutting fluids are applied over the chip flowing area, zone of primary deformation arising on chip and at the two interfaces, i.e., tool flank–workpiece interface and the other at tool–chip interface. Provision is also there to direct the cutting fluid on to the tool insert [40]. Therefore the tool and the workpiece material can be cooled separately. The flow of cryogenic fluid is known as confined flow if the direction of flow is from flowing over the chip through the tool–chip interface. The free flow condition is established if the fluid flows towards the tool flank–workpiece interface or if on to the tool insert. The distinction between the type of flow aids in establishing the dynamics of fluid flow and also the heat transfer mechanism.

Under the free flow condition, three regions can well be identified, i.e., free jet region, wall jet region and stagnation region. In the wall jet region, the flow follows outward radial direction. Further, the boundary layer starts to develop at the stagnation point having a very prominent role in the rate of heat transfer. A structured coherent vortices develop around the impinging jet owing to the entrapment of the surrounding fluid because of shear. The impinging jet in cryogenic-assisted machining process is in liquid phase at the nozzle exit. The state of impinging jet on injection is, however, at pressure and temperature, both at the subcritical state. Width of the impinging jet is very small when the ratio between the jet pressure and the critical pressure is around 0.03. Under such conditions, the impinging jet has appearance that of liquid ligaments and the ejection takes place in the form of drops [40, 41]. The heat transfer rate is enhanced with the dissolution of the gas in the impinging jet. The enhanced heat transfer rate is observed prominently in the sub-cooled boiling region [42].

The diameter of the jet is approximately ten times the diameter of the injector. This depends on the length of dense core and temperature of the impinging jet. The cooling capacity of the jet is maximum in the liquid jet core region and therefore the tool insert surface that has temperature lower to that of core region receives the major advantage of cooling from the impinging jet. Classical liquid breakup theory decides the behaviour of the impinging jet in conditions that are highly sub critical [52]. It is estimated that by the time the nitrogen gas reaches the target, it is around 50%in liquid phase given the fact that the hot material workpiece surface is at distance approximately 35 times the diameter of the impinging jet. The ensuing result is fluid expansion that has a positive effect on the chip clearance. This becomes beneficial for applications limiting the employability of forced air [43].

Baron and Stanely [44] have investigated the mechanism of film boiling occurring with the impinging cryogenic jet. They established the relationship for the average coefficient of heat transfer for jet impinging on a circular plate. The relationship between average Nusselt number, Prandtl number and the Reynold number was established. These were in turn revealed to be dependent on the nozzle exit diameter, thermal properties, physical properties and mass flow of the impinging jet.

Heat transfer mechanism in the cryogenic-assisted machining process has been investigated by Dreister [45]. The investigation was carried out for nozzles with different ranges of diameters, temperatures of target surface and flow velocities. It was observed that depending on the inclination angle of the surface and the average jet velocity, a layer of liquid boundary layer is formed over a zone that is approximately fivefold the diameter of the impinging jet. The coefficient of heat transfer was also found to be forty notch higher in comparison with that for natural convection.

Investigations on transient behaviour of impinging jet have been carried out by researchers [46, 47]. It was observed that the impinging jet has contact for a very short duration of time with the workpiece material. Further, a higher surface cooling rate of up to 8 °C/ms was also reported. Also the surface coefficient of heat transfer was revealed to be very small at the separation point between the tool and the chip [47].

The value of surface coefficient heat transfer was reported to reach the order of 104 w/m2k in the open region cloud. Flow analysis for multiphase flow was carried out for the liquefied nitrogen gas [48]. Flow rate was maintained at 1 L/min, nozzle diameter was kept at 2 mm, and the distance from the target surface was maintained at 15 mm during the CFD analysis. The cooling rate was estimated to be 2 °C/ms at the intersecting point of the axis of the impinging jet with that of the workpiece material. The coefficient of heat transfer at the same point was revealed to be of the order of 104 w/m2k. This decreases along the wall of the impinging jet. The rate of decrease was reported to be 600 w/m2k/mm.

Zhang et al. [49] investigated the behaviour of liquefied nitrogen gas jet being confined in a larger tube. The investigations were carried out for different surfaces of heat transfer under varied conditions. Rigorous disturbances were reported on to the heat transfer surfaces owing to the strong exchange of fluid momentum. The momentum exchange occurs when the flow fluid reverses its direction in the confined domain and that too in a very short duration of time. The flow penetrates into the boiling region with the gradual increase in the temperature of the hot surface. This is the point where the critical heat flux is approached. Further, I was reported that with the increasing jet velocity of liquefied nitrogen gas, the capability of heat transfer as well the critical heat flux increases. In the confined region, the convective evaporation process dictates the nucleate boiling mechanism.

However, the complexity is further increased with the presence of wedged opening [50]. The dominant behaviour of the conducting heat in the narrow gap was attributed to the reason for added complexity.

To judge the effectiveness of a cutting fluid, it is quintessential to quantify the heat transfer taking place between the cryogenic cutting fluid and the cutting tool and also between the cutting tool and the surface being machined. There are various modes of heat transfer mechanisms, the common being conduction, radiation and convection. However, the researchers have focussed primarily on the convection mechanism of heat transfer mechanism [51]. Cryogenic fluid in liquid phase has higher dynamic viscosity and specific heat in comparison with the gas and therefore has more heat transfer capability [52, 53]. However, a vapor cushion is formed owing to the evaporation of the liquid cryogenic fluid [54]. The vapor film restricts the heat transfer potentiality of the cryogenic fluid [55]. The boundary film can, however, be eliminated keeping in vey some of the critical parameters such as the exit velocity at the nozzle and also the pressure of the liquid cryogenic fluid [56]. Further, the orientation and positioning of the nozzle also effects the tool-wear [11, 30]. The modeling of the heat transfer process at the cutting zone is difficult as at the exit of the nozzle, there may be either liquid, gas or mix of two. Monitoring of cryogenic fluid has been carried by Pusavec et al. [57] by employing a phase sensor.

The quantification of the heat transfer mechanism is quite difficult using a direct approach. Therefore, inverse methodology has been often used by the researchers for determination of the heat transfer coefficient. This is done by comparison of the cutting temperatures obtained numerically and experimentally. Heat transfer coefficient as high as 50,000 w/m2k was reported by Rotella and Umbrello [58] when machining AA7075-T651 alloy. A value of 20,000 w/m2k for machining of Ti-6Al4V was reported by Rotella and Umbrello [59]. The value was reported for cases of cryogenic cooling. A value of 20 w/m2k was reported for free convection condition while machining Ti-6Al4V. Coefficient of heat transfer with value approximately 2000 w/m2k was reported for machining under dry compressed air [60].

There is, however, requirement to assess the literatures on forced convection boiling mainly with the sole objective to enhance the heat transfer process in cryogenic-assisted machining processes. Increased coefficient of heat transfer was reported by providing coating of a material possessing low thermal conductivity to the surface to be cooled [45]. Various alternatives to the liquefied nitrogen have been used for cooling purposes in various applications. However, their applicability for cryogenic machining process still needs to be explored. Usage of nitrogen in gas phase and combination of air with liquefied nitrogen gas could be explored for machining processes. In case of latter, the liquid particles possess greater inertia as gained from the air streamlines and results in the formation of a thin liquid film around the stagnation point [48]. Single and multiple impinging jets with turbulent conditions have resulted to provide significant heat transfer enhancement [61,62,63].

3 Cryogenic machining of PDMS

Machining of elastomers has one of the problems of providing suitable fixtures. Investigations and advent of cryogenic machining has aided in fixing the aforementioned problem. Further, it has also become possible to keep them lower than the glass transition temperature. An array of reports have been published on this aspects by varied researchers [64,65,66,67]. The condition to machine below the glass transition temperature is required so as to prevent the polymeric material to convert into rigid state. Under this condition of workpiece temperature, cutting forces can be applied and thereby justifying the feasibility of CNC machining. Specially designed fixture was employed by Kakinuma et al. [65] for carrying out cryogenic machining of polydimethylsiloxane (PDMS). It was reported that the machinability of PDMS was enhanced with the employability of cryogenic machining. However, geometrical inaccuracies were also reported within the workpiece material.

Cryogenic cooling was also employed for the milling operation [67]. Three nozzles were employed around the milling tool that aided in effective cooling of the workpiece material in all the directions. A noteworthy improvement in the workpiece material surface integrity was reported with the assistance of cryogenic cooling. Furthermore, a better chip flow characteristics as well as reduced workpiece deformation was obtained with the process. The cryogenic-assisted milling process was revealed to machine elastomeric structures with thickness as low as 0.5 mm.

One of the major applications of cryogenics in polymeric machining is that of freezing of the raw material. This in turn leads to effective machinability of the polymeric material. The effectiveness in machinability is achieved with the change of material state of polymeric material from being ductile to brittle. The transformation takes place at the glass transition temperature. Enhanced ductility as well as elongation is achieved for the polymeric material with the assistance of cryogenics. The improved properties aids the polymeric material to withstand the machining forces without much deformation.

Milling of ethylene–vinyl acetate to obtain shoe soles was successfully achieved by Dhokia et al. [67]. The milling process was done in assistance with the cryogenics, i.e., with the employability of liquefied nitrogen gas. The cryogenic-assisted milling process was put forth as an alternative to injection moulding which otherwise is expensive as well as time consuming. Kakinuma et al. [65] have designed and developed a fixture to freeze PDMS materials. This has been done for the microfluidic application. The micro-fluidic chips are manufactured generally by micro-moulding process or by photolithography. With the aid of cryogenics in freezing the workpiece material and in machining, micro-milling has been used for the production of micro-fluidic chips.

4 Cryogenic machining of Inconel and stainless steel

The cost associated with the flow rates of liquefied nitrogen and carbon dioxide gas was estimated by Pusavec et al. [68] and Klocke et al. [69]. The estimation of cost for carbon dioxide was done for five different nozzle, while only single nozzle was considered for liquefied nitrogen gas. The real cost of the process can be accessed by combining consumption of cooling lubrication with the productivity of tool life. Lower machining costs have been reported for cryogenic machining of Inconel 718 material [70]. Conventional machining is economical only at lower speeds of cutting which is, however, not optimal from production rate point of view. Therefore, for higher rate of production and higher efficiency, cryogenic-assisted machining must be opted over conventional machining.

Further investigations have been carried out and comparison has been made between cryogenic machining with high pressure liquefied nitrogen gas and the conventional machining with oil emulsions [71, 72]. A case of Austenitic stainless steel workpiece material was investigated under the study. It was reported that the tool life was enhanced by 100% for the case of cryogenics employing liquefied nitrogen gas. The tooling costs reported was much higher for conventional machining than the cryogenic-assisted machining. A direct relationship between the flow rate and the machining performance with the cost for the cryogenic machining was reported by Lu [47].

Further, the cost associated with cryogenic machining in terms of total life-cycle cost is justified with the enhanced functional performance of the manufactured components obtained using cryogenic associated machining processes.

5 Sustainability issues associated with cryogenic-assisted machining

Meeting the requirements of customer through the production of goods, systems and services is the primary reason of existence of machining processes. The term “sustainability” on the other hand can be explained as “meeting the needs of the present without jeopardizing the ability of future generations to meet their needs” [73]. As such in accordance to the Department of Commerce, USA, sustainable machining can be defined as the “creation of manufactured products through the processes that are non-polluting, energy conservative and nature conservative and also economically sound and safe for employees”.

The sustainability in machining has been adopted through the utilization of efficient resources, recyclable materials, reusing of machine-tools, minimization of waste, minimization of energy consumption, enhancement in tool life, expanding the managerial scope for the usage of metalworking fluids, lubricating oils, and adoption of life cycle assessment methods [74]. Cryogenic machining is a machining approach that complies with the three vital pillars of any sustainable machining: environmental, societal, and economic [75].

Employability of conventional coolants in machining processes results into numerous problems as such environmental pollution, biological problems, soil contamination, water pollution, and additional infra-requirements in the form of floor space and machining equipment [76]. The disposal of conventional coolants after their usage is limited by environmental laws and hence adds to the sustainability issues. Cryogens on the other hand are safe and evaporates after their usage and therefore leaves no residue. Also, cryogenic machining is economic as the cost is low to machine parts in comparison with the processes without cryogens. Some of the elements that make up the total cost of cryogenic machining are machining costs, cutting fluid cost, electrical cost, cleaning cost associated with the final part, cost associated with recycling and cost of cleaning the chips and other residual matter [77]. Sustainability in societal terms is also provided by the cryogenic machining which include labor rights, operational safety, personal health, and labor justice [77].

A case study on comparison of sustainability performance for the cryogenic machining and conventional flood cooling was conducted by Lu et al. [78]. The factors that were considered for the study were cost, productivity, energy consumption were evaluated. The case for which the study was made concerned the milling of Ti-6Al-4V blank that was employed for the fabrication of frame structure. Cryogenic machining process was able to machine at twice the rate in comparison with the conventional flood cooling process and therefore the same machining plant can produce twice the capacity without making investment on its infrastructure. The cryogenic machining consumed 22.8% lesser power in comparison with the conventional flood cooling process. The greenhouse gas emitted were almost 50% lesser for the cryogenic-assisted machining. Furthermore no liquid waste was generated by the cryogenic machining process because of the evaporation of LN into air that leaves no residue. There was a total cost reduction of 26% for machining of one piece of part using cryogenic-assisted machining process. Therefore through the case study, the potential benefits of the machining process in terms of economic and sustainability aspect was justified.

6 Technical and economic aspects of cryogenic-assisted machining

The cryogenic-assisted machining process has proved to be successful in removing the heat and therefore accelerating the chip fragmentation process. This in turn results into overall improvement in the tool life of the employed tools. The employability of liquid nitrogen is wider due to its potential ability to rapidly evaporate from the cutting zone. The rapid evaporation aids in hampering the tool softening process. Moreover, the evaporation of liquid nitrogen readily into the atmosphere and hence reduces the friction as well as the post-process cleaning operations [79]. The effect of the effect of cryogenic cooling onto the heat generation was simulated by Dinesh et al. [80] for different cutting speed levels. 15–20% lower temperature was achieved in temperature in comparison with the temperature generated during dry machining. Cryogenic machining has reported to enhance the machinability of hardened steels [81], titanium [82] and Inconel [83]. The positive effect of cryogenic-assisted machining was reported on the high-temperature alloys by Kaynak [84]. The effects were studied on the cutting temperature, crater wear, flank wear and the quality of the machined surface. The tool life of the tool was compared for the conventional and cryogenic-assisted machining processes [85]. At lower depth of cut, the machining processes with conventional cooling showed higher tool life. However, higher depth of cut was achievable with the cryogenic-assisted machining process with the same tool life.

There has been, however, an enormous lacuna to investigate the cost reduction with industrial outlook. It is the cost of the fluid that plays a crucial role in the cryogenic-assisted machining processes. The cost of fluid is supposed to vary the total machining cost from 15 to 17% [20]. The other prominent cost that the machining encounters is that associated with the tooling cost. The tool wear plays a prominent role in deciding the cost of machining titanium alloys [86]. Another major proportion of the machining cost is associated with the energy consumption of the machine that is being employed to carry out machining process [87]. 39% less machining cost has been reported in a study related with cryogenic-assisted machining [88].

7 Conclusion

The present work is a modest attempt to provide the readers with the different characteristics of the cryogenic-assisted machining process that has emerged as one of the most suitable sustainable processes. Thermal and economic aspects of the cryogenics-assisted machining process have been highlighted in the present work. Different findings as reported in the present work depicts the efficacy of cryogenics to aid in production of products that are far more superior in their functionality. Further, the cost effectiveness of the process over the conventional machining process has been revealed in the discussion on economic aspect. Cryogenics have made it easy the machining of polymeric materials. However, modeling of such processes is still a complex problem that remains to be suitably addressed by the research community working in this domain. Further, development of performance predictive models is still an area that provides for future research directions to the scientific community.

In the near future the possibility of cryogenic machining to be integrated with the non-conventional machining [89] needs to be reviewed. Moreover, the reviews must be made with regard to the integration with Industry 4.0 considering the behaviour of different materials [90, 91]. The micromachining processes may also be extended to incorporate cryogenics [92].