1 Introduction

The train wheel consists of an axle and two wheels pressed together. No relative displacement is allowed between the wheels and axles during the operation of the vehicle. The wheel set bears the entire weight of the vehicle and also carries various other forces from the car body and rails while traveling at high speed on the track. The materials of the shaft are mainly carbon steel and alloy steel. Among these, high-quality carbon structural steels such as carbon steel C35 (AISI 1035), C45 (A106 steel), and C50 (AISI 1050) are widely used due to their high comprehensive mechanical properties. The most commonly used is C45 steel, and it must be normalized or quenched and tempered to improve its mechanical properties. Carbon structural steels such as Q235 and Q275 are suitable for use in shafts with little or minimum stress. Alloy steel exhibits improved mechanical properties, but at a higher cost compared to other steel varieties. It is commonly employed in shafts that possess certain requirements. For example, low carbon alloy structural steels such as 20Cr and 20CrMnTi are commonly used in high-speed shafts that have basic bearings. After carburizing and quenching, the wear resistance of the journal can be increased, and the machine rotor shaft must have good wear resistance when working under heavy capacity circumstances. 40CrNi and 38CrMoAlA steels are often used for high temperature mechanical properties. Considering the mentioned application areas, it is seen that one of the most common production methods used in bringing these materials to their final dimensions is the machining method [1]. A decrease in machinability with an increase in favorable qualities creates high cutting zone temperature during machining of these alloys [2], the formation of cutting forces and low cutting tool life [3], which causes the inability to work at high cutting speeds. In addition, these effects occurring during the machining process include deformation in the microstructure of the produced workpiece [4], grain shrinkage [5], phase transformation [6], microhardness change [7], etc. cause microstructural transformations and are predicted to affect the mechanical properties, corrosion behavior and therefore fatigue life of the final product.

Sustainable manufacturing, often known as green or eco-friendly manufacturing, reduces environmental effect, conserves resources, and promotes social responsibility [8, 9]. Sustainable manufacturing aims to manufacture goods that are commercially viable, ecologically friendly, and socially responsible throughout their life cycle [10, 11]. The terms related with sustainable manufacturing are shown in Fig. 1a. Nowadays, sustainable machining processes are very popular to enhance the machining performance [12], and the advantages of sustainable machining processes are shown in Fig. 1b. Further, the cutting fluid [13], MQL [14], flood cooling [15], pressure cooling [16], air cooling [17], cryogenic method [18], etc. are widely used in different sectors to provide the cooling and lubrication effect at the main cutting zone [19, 20]. In this context, interest in machining applications carried out with cryogenic methods has been rapidly increasing [21]. Dry machining is chosen for several reasons, one of which is its considerable contribution to employee health by eliminating the use of cutting fluid [22]. Additionally, the absence of cutting fluid eliminates the need for its disposal after processing [23, 24]. Nevertheless, the rise in temperatures inside the cutting zone leads to an escalation in tool wear, thereby impacting crucial factors such as surface roughness and tool life in an adverse manner [25, 26]. MQL machining is the preferable method due to the absence of cutting fluid disposal, unlike dry machining. Moreover, the utilization of vegetable cutting fluids ensures that there are no adverse effects on employee health. Another contribution of the MQL method is that low surface roughness can be achieved with the lubrication feature it provides [27]. However, in this method, the tool life is not at the desired level because of the lower temperature compared to dry turning and incomparably higher temperature compared to the use of coolant. In the cryogenic cooling process, which is based on cooling the cutting area, different cryogenic gases are sent to the cutting area. The most important disadvantage of cryogenic cooling is that it has no lubricating properties [28]. In addition, it has been stated in many studies that it provides positive contributions to surface roughness by providing lubrication at the tool-chip interface [29, 30].

Fig. 1
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a Sustainable manufacturing terms generally used [31]. b Characteristics of sustainable machining processes [32]

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Using dry, flood, and cryogenic cooling techniques, Dhar et al. [33] conducted a turning experiment on C60 steel. The aim was to examine the impact of the cutting environment on tool wear, dimensional deviation, and surface finishing. The results demonstrate that the implementation of cryogenic cooling technique effectively mitigates tool wear, enhances tool life, minimizes dimensional deviation, and improves surface roughness. The authors Venugopal et al. [3] machined the challenging Ti6Al4V alloy using uncoated carbide tools. Strano et al. [34] compared the performance of cryogenic and flood cooling conditions in the machining of Ti6Al4V. The authors claim that the tool life was enhanced by 40% when using cryogenic cooling. Sartori et al. [35] used cryogenic chilling to improve machinability and reduce tool wear in Ti6Al4V superalloy machining. The study found that cryogenic cooling reduced cutting zone temperature by 58%, reducing crater formation. Dhar et al. [36] machined AISI 4037 steel under flood and cryogenic cooling conditions. The study showed that cryogenic cooling improved tool lifespan and surface quality as compared with other conditions. Usca et al. [37] studied the impact of dry, cryogenic, and MQL on surface roughness, cutting temperature, tool wear, and energy consumption in the machining of 5140 steel. The authors claim that the cryo machining performed good results as compared to other cutting conditions.

The literature review has demonstrated that the application of dry machining, MQL, cryogenic cooling, and high-pressure cooling techniques are used as alternatives to traditional cooling liquids. However, these methods have generally been used on familiar engineering materials and the studies on sustainable turning of EA1N railway axle steel represent a commendable effort in addressing the specific machining characteristics of this material. This targeted approach is essential for developing efficient and sustainable machining processes tailored to the unique properties of EA1N steel. For this reason, the machining of train axle steel was performed under dry, MQL, and cryogenic conditions and the comprehensive results were examined because the research on energy consumption and machining characteristics in sustainable turning has the potential for significant industry impact.

2 Materials and methods

2.1 Material, cutting inserts, and machine tool details

For turning operation with train wheel axle material, a Doosan-Puma-4100-XLM CNC machine was used. The representation of the machine tool, MQL and cryogenic cooling systems is shown in Fig. 2. The workpiece material (railway axle EA1N steel) is supplied in Ø200 × 2000 mm dimensions. The chemical composition of EA1N steel is C ~ 0.4%, Si ~ 0.5%, P ~ 0.020%, S ~ 0.015%, Cr ~ 0.30%, Mb ~ 0.08%, Ni ~ 0.30% and the balanced is Fe.

Fig. 2
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Experimental setup

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Machining experiments for railway train wheel axle steel was performed under dry, MQL, and LN2 cryogenic cutting environment. Figure 2 shows the experimental setup followed in current work. TiAl, TiN, and TiCN, which are generally used as coating materials in coated tools, and the high-temperature-related reaction that occurs during the machining process of different steels can cause diffusion wear in cutting tools [38]. For these reasons, CNMG 120408 M1 883 uncoated carbide inserts were used [39] within the scope of the cutting tests carried out using train axle steel materials [40].

2.2 Cooling conditions

Dry, MQL, and cryogenic cooling conditions are used in this work. The LN2 tank with 8 bar working pressure used in experimental studies. In order to ensure effective cooling in processes carried out with cryogenic cooling (LN2), operating below a certain pressure value due to the structure of the systems reduces the performance of the system. Appropriate operating pressure values required for the effective use of cryogenic systems were determined by taking into account LN2 phase diagrams and studies in the literature [41, 42]. Considering all these evaluations, it is possible to work in the range of 5–6 bar for LN2. In the experiments carried out with cutting fluid, semi-synthetic QualıChem Xtreme Cut290 oil approved by BOEING company was used in 5% emulsion. The cutting fluid output during cutting was measured as 5 bar. The schematic demonstration of MQL and cryo system during turning are shown in Fig. 3.

Fig. 3
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The schematic demonstration of MQL and cryo system during turning [43]

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2.3 Experimental procedure

Three ranges of cutting speed (200–300-400 m/min), depth of cut (1 mm), and feed rate (0.2 mm/rev) were chosen for the cutting tests. Then, the Mitutoyo SJ-210 surface roughness device with a sensitivity of 0.002 μm was used for surface roughness measurements. Based on JIS B0601 standards, 0.8 mm, which is determined for roughness values between 0.1 and 2.0 μm, was chosen as the scanning range, and average surface roughness values (Ra) were obtained by taking the average of four measurements taken from each sample. The power consumption during the turning of EA1N steel was measured by KAEL-network-analyzer and the values were converted to energy consumption by multiplying the power via machining time. The tool wear and chips were assessed by Huamoa branded optical microscope and detailly by the scanning electron microscope (SEM) in Karabük University Material Research Center.

3 Results and discussion

3.1 Energy consumption

Machining energy consumption is the total amount of energy used to cut, shape, and finish metal in a production setting. Precision components for a wide range of sectors rely on machining techniques like milling, turning, drilling, and grinding. Machining can use a lot of energy, therefore it's important to find ways to make it more efficient for both the economy and the environment. Machining energy consumption is affected by the following parameters [44]:

  • Material removal process

  • Machine tool efficiency

  • Coolant systems

  • Cutting tools and tool life

  • Monitoring and control systems

Thus, reduced machining energy usage saves producers cost and reduces energy’s environmental impact. Manufacturers pursue energy-efficient methods, innovative technology, and modern equipment to increase their sustainability and market competitiveness. Li et al. established a comprehensive modeling framework to characterize machining system energy usage [45]. Figure 4 hierarchically depicts the machining system’s energy flow by machine tool, task, and auxiliary production levels [45]. In Fig. 5, the average energy consumption at all speeds examined under dry conditions was observed as 238.66 kJ. When the cooling conditions are examined, the highest energy consumption occurs in a dry environment. When shifting from a dry environment to a MQL state, the amount of energy that is used drops by 49.21% at a cutting speed of 200 m/min, by 46.22% at a cutting speed of 300 m/min, and by 40.31% at a cutting speed of 400 m/min, respectively. When shifting from the dry environment to the cryogenic condition, there was a drop of as much as 62.36% at a cutting speed of 200m/min, 58.86% at a cutting speed of 300m/min, and 53.38% at a cutting speed of 400m/min, respectively.

Fig. 4
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Holistic energy flow in machining systems (modified from [45])

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Fig. 5
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Comparison of energy consumption values under different conditions

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In MQL, cooling liquid is pressure-fed to the cutting zone. Vaporized fog clouds can also absorb heat from the environment and the coolant also lubricates along with the reducing friction. Consequently, decreasing the contact pressure might indirectly lead to a decrease in power consumption. In arid environments characterized by elevated temperatures and high friction coefficients, there exists a potential for an increase in energy consumption. Extending the lifespan of the cutting tool and preventing the formation of BUE can be achieved by decreasing the temperature in the machining region. To enhance energy efficiency, one can minimize friction by implementing methods to prevent the generation of BUE. The combination of oil coatings and MQL fog droplets in the deformation zones leads to a decrease in cutting forces, hence resulting in a proportional reduction in energy consumption. Observations reveal that there is a direct correlation between the rise in cutting speed and the corresponding increase in energy demand. The energy extracted from the spindle motor also increases proportionally with the increase in cutting speed. Furthermore, high cutting speeds can potentially reduce the lifespan of tools when working with difficult-to-machine materials. Cryogenic temperatures increase power consumption due to the workpiece’s hardness. Higher cutting speeds and feed rates increased energy usage and this phenomena explains why better thermal conductivity materials generate more heat at higher cutting settings. The cryogenic liquid nitrogen can affect cutting process parameters and maintain cutting edge hardness. Dry machining uses more energy due to tool-chip friction without lubricant or coolant and it increases tool wear, which threatens system stability and causes vibrations, which increase energy consumption.

3.2 Average surface roughness (Ra)

Average surface roughness (Ra) represents a workpiece or material’s finely spaced surface imperfections. It measures the surface profile’s average deviation from a centerline. Ra is measured in μm or μin. Surface roughness can be measured with profilometers or contact and non-contact testers. The measurement method depends on the surface, precision, and material. Moreover, Dhananchezian et al. [46] also mentioned that surface parameters like Rz (average maximum height) and Rt (total height variation) provide extra information. When Fig. 6 is examined, the average surface roughness value in dry conditions is noticed as 3.79 µm. Changing from dry to MQL reduces surface roughness. Specifically, 200 m/min cutting reduces surface roughness by 2.90%. At 300 m/min, the reduction is 5.47%; at 400 m/min, 7.71%. Changing from dry to cryogenic reduced surface roughness. Cutting at 200 m/min reduced surface roughness by 10.17%. Cutting at 300 m/min reduced surface roughness by 9.58%. Finally, 400 m/min cutting reduced surface roughness by 14.01%. Dry machining without lubrication or cooling raises the Ra value. Insufficient lubrication during the cutting operation might cause elevated temperatures and friction (Fig. 7), resulting in diffusion from the insert, degradation and inadequate removal of chips. This is the underlying cause for this situation. These situations may lead to inconsistencies and imperfections on the machined surface, hence increasing the Ra values. Conversely, employing cryogenic machining, which involves operating at extremely low temperatures, has the capacity to enhance the surface roughness (Ra). Cryo-cooling is employed to minimize thermal loads and tool wear in the machining process by reducing heat generation and so the diffusion. When compared to dry machining, this method results in smoother machined surfaces with lower roughness values since there is less friction and wear. On the other hand, MQL is used to minimize resistance and enhance the dispersion of heat. In contrast, this approach improves the efficiency of the machining process, leading to a greater Ra value in comparison to cryo machining. This technique is the only ecologically beneficial way since it reduces the need for lubricants while simultaneously obtaining a good surface quality by decreasing friction between the tool and the workpiece [47].

Fig. 6
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The average surface roughness based on different lubrication/cooling

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Fig. 7
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Machined surface SEM analysis at the cutting speed of 400 m/min

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3.3 Tool wear

The term “tool wear” is used to describe the slow but steady erosion of a cutting tool’s surface when it is used for machining. This happens because of the many mechanical and thermal interactions between the cutting tool and the workpiece, which is an inherent and inevitable part of cutting metal [48]. The efficiency of the machining process, the quality of the machined parts, and the total cost of manufacturing can all be greatly affected by tool wear [49]. Different kinds of tool wear have different origins and consequences as also be declared by Fang et al. [50]:

  • Flank wear

  • Crater wear

  • Adhesive wear

  • High-temperature wear

  • Thermal cracking

  • Tool fracture, chipping, or breakage

Machining requires minimizing tool wear for consistent, high-quality production. Monitoring tool wear, choosing proper cutting settings, utilizing high-quality cutting tools, and applying effective cooling and lubrication solutions can reduce tool wear in machining processes. New tool materials, coatings, and cutting methods aim to increase tool life and boost machining efficiency [51]. When Fig. 8 is examined, the average tool wear value at all speeds in dry conditions is seen as 195 µm. From a dry environment to the MQL condition reduces tool wear by 25.61% at 200 m/min, 27.16% at 300 m/min, and 9.93% at 400 m/min. Tool wear decreased by 36.36% at 200 m/min, 35.18% at 300 m/min, and 17.88% at 400 m/min after switching from dry to cryogenic conditions.

Fig. 8
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The tool wear based on different lubrication/cooling

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Optimizing cutting conditions and implementing appropriate lubrication during the machining of railway steel can effectively minimize the occurrence of sticking and tool damage. Failure to exploit these circumstances will lead to the complete deterioration and catastrophic failure of the cutting tool. Consequently, the introduction of a little quantity of oil, such as MQL, effectively reduced adhesion and mitigated the influence of abrasive wear (Fig. 9). As a result, the friction between the contacting surfaces was minimized. Figure 9 illustrates that tool wear during the turning process is more pronounced when using MQL compared to dry cutting, occurring at a greater depth. This is an observable phenomenon. Nevertheless, it cannot be conclusively asserted that the utilization of MQL completely eradicates nose wear. Itoigawa et al. [52] state that MQL can provide enough lubrication if the appropriate lubricant is used. Nevertheless, it is unattainable to entirely prevent the tool’s potential damage or the buildup of particles on its surface. Figures 9 and 10 illustrate the impact of a cryogenic environment, especially the use of liquid nitrogen (LN2), on the wear of tools during the process of machining railway axle steel. This is in addition to the effects of dry turning and MQL turning conditions. Upon analyzing the tool wear, it is apparent that it is reduced as compared to both MQL and dry circumstances. In addition, tool wear is somewhat more tolerable in wet situations and when using MQL. Placing cryogens in close proximity to the contact surfaces can efficiently absorb the significant amount of heat energy found in the cutting region. The cryogenics efficiently absorb and disperse the thermal energy that is exchanged across surfaces due to their low boiling points. When liquid nitrogen (LN2) touches the material and cutting tool, it decreases the surface area of contact and lowers the temperature in the chip tool, hence reducing friction and BUE/BUL [53] as also shown in Fig. 10. Consequently, a higher level of uniformity and rationality in tool wear is attained.

Fig. 9
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Flank wear under different conditions

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Fig. 10
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Crater wear under different conditions

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Figure 11 displays the SEM images of flank wear, which are utilized to recognize the many factors contributing to its occurrence. Tool wear resulting from chip flow damage is observable in MQL, and cryogenic machining, although it is less frequently observed in the latter two methods. This is illustrated in Fig. 9, which depicts the phenomena. Due to the chip incorrect movement, the damage extends ahead of the cutting region and affects the primary cutting edge of the tool. The study found that the increased machining durations resulted in chip flow damage, which significantly impacted the overall tool wear. The depth of cut employed in this study is 1 mm, surpassing the radius of the tool handle by 0.8 mm. This is done to prevent the chip from becoming obstructed by the apertures of the cutting tool. Eventually, the chip will generate a combined flow that includes both an upward and a sideways spiral. With an increase in temperature, the material exhibits enhanced flexibility while experiencing a decrease in both its yield and final tensile strengths. The existence of BUE during dry machining is attributed to the factors depicted in Fig. 9. Due to the favorable circumstances, the material is directed regarding the notch area of the insert, resulting in the BUE formation from SEM image as shown in Fig. 9 [43]. During the process of cutting metal alloys such as steels, it is common for the workpiece to adhere to the cutting tool, as observed in multiple instances [54]. The BUE is reliable for the friction stress observed at the rake face [55]. This stress can be distinguished by its ability to induce fractures rather than transferring stress. According to Trent and Wright [56], the uninterrupted movement of the chip might lead to the deterioration of the BUE, resulting in the failure of larger parts from the rake face. As a result of this circumstance, the fracture that was seen when dry cutting was found to be quite near to the cutting edge. The SEM image obtained for the MQL condition shows distinct edges on the surface of the tool. Chetan et al. [57] state that the edge is created when fragmented carbide inserts are repeatedly rolled in the cutting area. Currently, the amount of cutting fluid needed in the MQL condition is significantly lower compared to typical cooling methods. Due to the absence of oil in the cutting region, the tool was more susceptible to the edge formation resulting from abrasion wear induced by both the subject material and the tool. The SEM micrograph captured for the MQL condition reveals visible ridges on the tool side face. Cemented carbide commonly exhibits abrasive wear in the linear pattern forms [58]. The reason for this is the exceptional hardness of tungsten carbide. Figure 10 depicts the occurrence of crater wear on the rake face following the utilization of dry, MQL, and cryogenic machining techniques for turning the railway axle steel material. Crater wear occurs partially due to the chip exerting pressure on the notch of the tool [59]. Friction occurs when the rake face and the chip come into contact and rub against each other. This results in a decrease in the durability of the cutting tool. Ultimately, this leads to the formation of crater wear, as depicted. According to Manoj et al. [60] the rise in the carbon content on the rake face is mostly attributed to elements diffusion resulting from elevated temperatures. The authors postulated that the augmentation in the quantity of carbon across the rake face is predominantly attributed to elemental diffusion induced by elevated temperatures. The burn mark on the rake’s surface likely resulted from a combination of strong oxidation and substantial elemental diffusion. Moreover, the EDX and MAP images in Fig. 11 show that Ti content decreases in dry condition due to high abrasive wear phenomenon since the cutting inserts used in this study have TiN coated. When analyzed MQL and cryo cooling, the content ratio of Ti increases (not so much effect on the inserts) by the help of lubrication and cooling. On the other hand, the content of Fe shown as the red points is increasing in dry condition due to high adhesion from the workpiece material of railway axle steel. However, its gradually decreasing when the material is machined under MQL and cryo condition.

Fig. 11
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The EDX-MAP analysis of the inserts under different conditions

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3.4 Chip morphology

In terms of machinability, the workpiece material that allows removing more chip volume per unit time is better in terms of machinability. In machining tests, the ease of formation of chips during operation and their breaking without damaging the tool, workpiece and operator are taken into account. It is possible to obtain information about material behavior through chip morphology. Easier chip formation is not definitive without data on lower cutting force or tool wear [61]. Lack of coolant or lubrication in the cutting zone has caused significant plastic deformation at the tool-chip contact. This is similar with the dry machining conditions [62]. The highly damaged chip was thus made for dry machining and not for the other cooling and lubricating methods shown in Fig. 12. Moreover, it has been found that cryogenic machining, followed by MQL machining, and dry machining, causes an increasing amount of damage at the chip contact. The suggestion for MQL machining is rooted in its capacity to reduce tool wear more effectively than cryo and dry machining, in that order. As has already been mentioned by Sap [63], the reduction in tool wear leads to less friction at the tool-chip interface, which in turn reduces chip damage. But, without lubrication, dry and cryo machining may produce coarse lamella. As previously shown in Fig. 12c, a rapid shift in the temperature of the cutting area under cryogenic cooling circumstances might cause heat impacted zones or thermal shock. Cryogenic machining, MQL, and dry machining all showed the same trends in the observed patterns of the cutting tool-chip friction coefficient values [64]. Every one of the three machining techniques suffered this same fate.

Fig. 12
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Chip morphology at the cutting speed of 400 m/min

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4 Conclusions

The study examined the machinability of train wheel axle steel using TiN-coated carbide cutting inserts, under three different cooling conditions: dry, MQL, and cryogenic cooling. An attempt was made to enhance the machinability criteria such as energy consumption, tool wear, surface quality, and chip morphology of train wheel axle steel under these cutting circumstances. The findings obtained as a result of the study are summarized as follows:

  • In order to improve energy efficiency, it is possible to reduce friction by employing techniques to avoid the production of BUE.

  • The utilization of oil coatings and MQL fog droplets in the areas of deformation causes a decline in cutting forces, hence leading to an appropriate decrease in energy consumption.

  • Cryo-cooling is utilized to decrease thermal loads and tool wear during the machining process by decreasing heat production and diffusion. Compared to dry machining, this process yields smoother machined surfaces with reduced roughness values due to less friction and wear. Conversely, MQL is employed to reduce resistance and improve heat dissipation. On the other hand, this method enhances the effectiveness of the machining process, resulting in a higher Ra value compared to cryo machining.

  • The existence of BUE during dry machining was caused by the interaction of friction and high temperatures. Because cryogens boil at very low temperatures, they were able to absorb a great deal of the heat energy that was present in the cutting region when placed on the contact surfaces. Cutting tools and materials are subjected to less contact and lower temperatures when LN2 is introduced to the chip tool. In addition to lowering the resistance, MQL made it easier to apply a thin coating between the chip and tool. Tool degradation is reduced and a higher level of uniformity is achieved.

  • Insufficient lubrication might cause rough lamella in chips during dry and cryo machining. In cryogenic cooling, sudden temperature changes in the cutting region might generate heat damaged zones or thermal shock. The cutting tool-chip friction coefficient was consistent during dry, MQL, and cryogenic machining.