The effect of addition of MWCNTs nanoparticles to CryoMQL conditions on tool wear patterns, tool life, roughness and temperature in turning of Ti-6Al-4V


 In recent years, advancements in the field of nanotechnology have actively been reflected in the manufacturing industry, particularly in the production of aviation components, ships, and medical devices. However, titanium alloys have low thermal conductivity, making them difficult to cut at high temperatures. The traditional processing method is not only inefficient, but can also cause serious harm to the environment and operators. In this paper, multi-walled carbon nanotubes were used as additives to disperse into degradable vegetable oil as a green cutting fluid. This green cutting fluid was cooled to analyze the processing of a titanium alloy (LN2 + Nano-MQL (0.6%)). Cutting experiments were carried out under dry, MQL, LN2, LN2 + MQL, and LN2 + Nano-MQL (0.6%) conditions. The temperature of the machining area, tool wear process, tool wear mechanism, surface roughness, and surface morphology were studied. The results show that the cutting-zone temperature is reduced by 65.1%, the tool life is prolonged by 30%, and the surface roughness is reduced by 48.1% under LN2 + Nano-MQL (0.6%) in comparison to the dry condition. The cutting environment supported by MQL is conducive to inhibiting the formation of craters. Element detection confirmed that the tool coating was well protected under the condition of LN2 + Nano-MQL (0.6%). In addition, the tool nose wear is small under the LN2 + MQL and LN2 + Nano-MQL (0.6%) conditions.

the turning process of AISI 9310 alloy steel. Compared with ooding and dry cutting, MQL has a lower cutting temperature, lower tool wear, and favorable chip characteristics.
Although MQL seems to provide enough lubrication, the cooling effect of the processing area is insu cient under heavy processing conditions. The high heat generated during chip removal cannot be controlled, which signi cantly reduces the processing e ciency. Low-temperature cooling is a preferred cooling method for thermal control. In general, cryogenic cooling involves transporting a refrigerant gas in the liquid form to the tool-workpiece interface using a nozzle [7] . LN 2 is one of the most popular coolants in machining operations. It is lighter than air and can diffuse into the surroundings after use. This reduces the need for maintenance, post-processing cleaning, and processing requirements [8] . Lowtemperature cooling has been considered to be a green production technology and is one of the leading cooling methods for sustainable manufacturing [9] . Islam et al. [10] compared dry cooling, ood cooling, and low-temperature cooling in terms of the processing of EN24 steel. The results show that lowtemperature cooling has good effects on the surface roughness, cutting force, and tool wear.
Dhananchezian et al. [11] compared low-temperature cooling with conventional cooling during the processing of Ti-6Al-4V alloys. The results show that low-temperature cooling reduces the cutting force by 35-42%. Further, the cutting temperature decreased by 61-66%, the surface roughness decreased by 36%, and the tool wear decreased by 27-39%. Previous studies have con rmed that, for titanium alloy processing, the MQL and low-temperature environments have exhibited better results than ood and dry cutting. However, the low-temperature environment only provides a cooling effect and has insu cient lubrication. In contrast, MQL mainly provides lubrication and is usually ineffective under heavy processing conditions, especially for titanium alloy processing. Therefore, a large number of researchers have explored mixed low-temperature and MQL conditions. Weinert et al. [12] proved that cryogenic minimum quantity lubrication (CMQL) is a safe and effective cooling and lubrication method that can effectively replace conventional MQL technologies. Shokrani et al. applied CMQL to milling Ti-6Al-4V.
They carried out milling experiments on a tool life model and compared the tool life under ood, MQL, low-temperature, and low-temperature micro-lubrication environments. The results show that, in a lowtemperature micro-lubrication environment, the tool life is 30 times longer than that of ood cooling [13] .
CMQL is a relatively new cooling lubrication method that has received signi cant research attention in recent years. It mainly provides lubrication and cooling for the processing area, improving its processing effect. This method employs two nozzles, one nozzle which sprays oil and the other sprays cryogenic liquid. However, during actual processing, the method of cooling by liquid nitrogen is the most effective. The minimum temperature of liquid nitrogen can reach −196°, which can be further improved by lubrication. Under heavy processing conditions, the continuous high temperature in the processing area leads to the instantaneous evaporation of vegetable oil, which will weaken the lubrication effect in the processing area. Similarly, the coolant may not fully penetrate the microstructure of the tool-workpiece. Nano uids have recently been used in MQL to increase the cooling and lubrication characteristics. In nano uid technology, when additives (called nanolubricants or nanoparticles) are added to the base uid, the physical, friction, and thermal properties of the cutting uid improve, depending on the properties of the solid particles [14] . The most commonly used additives are molybdenum dioxide, boron nitride, alumina, silica, carbon nanotubes, titanium dioxide, copper oxide, graphite, etc [15][16][17] . In mechanical processing, although nano uid-MQL has been studied for many years, the multi-walled carbon nanotube (MWCNTs) mixed nano uid-MQL, which is a novel concept, has not been su ciently researched.
In fact, few studies have focused on the surface roughness/surface morphology, tribological mechanism, tool wear, and wear mechanism of MWCNTs. In addition, the application of MWCNTs and vegetable oil combined with MQL is also rare. The main reasons for choosing MWCNTs as solid lubricants are their atomic structure and high speci c surface area, along with their excellent thermal conductivity (of about 3000-3500 W· m −1 · K −1 ). Owing to these advantages, it is important to study the interactions between nano-cutting uids and the tool and workpiece materials at low temperatures, and to establish a selection of sustainable processing titanium alloys (Ti-6Al-4V). Therefore, this study mainly analyzes the in uence of the addition of MWCNTs in the low-temperature trace wetting (CMQL) system on the cutting performance indicators, including the tool wear and life, wear mechanism, the machining surface state (surface roughness and surface morphology), and the peak temperature of the cutting zone when turning titanium alloys (Ti-6Al-4V) with a titanium nitride coated carbide cutting blade. Therefore, ve different experimental environments were designed to facilitate a comparison (Dry, MQL, LN 2 , LN 2 + MQL, and LN 2 + Nano-MQL (0.6%)). In these comparisons, the addition of 0.6% MWCNTs on the basis of lowtemperature minimum lubrication is analyzed.

Experimental material
In the experimental study, Ti-6Al-4V titanium alloy was used as the workpiece, with dimensions of 50 mm×200 mm. The chemical composition of Ti-6Al-4V for turning experiments is shown in Table 1. The coated cemented carbide tool DCMT11T304-SMIC907, produced by ISCAR, was used for cutting experiments. Canola oil was selected as the base oil, with superior lubricating ability. In addition, it is environmentally friendly, naturally biodegradable, and free of chlorine and heavy metals. MWCNTs are different types of carbon nanotubes formed when multiple carbon nanotubes intertwine with each other. Figure 1 shows the microstructure of MWCNTs at different magni cations under an electron microscope. Detailed information regarding MWCNTs is given in Table 2. Although the number of nanotubes in MWCNTs is at least three, it can also reach 20. Their inner and outer diameters are between 2 and 50 nm, respectively. Like single-walled nanotubes, they exhibit extraordinary electrical, thermal, and mechanical properties. For these reasons, functionalized MWCNTs were used as additives in this study. Before the preparation of nano uids, preliminary experiments were carried out to determine the optimal proportion of nanoparticles, which was found to be 0.6% MWCNTs. This amount was then added to the base cutting uid. A high-precision electronic balance (precision = 0.001 g) was used to weigh the canola oil and carbon nanotubes. The solution mixture was stirred for 90 min at a speed of 1500 r/min with the help of a SUNNE (made in China) magnetic stirrer. Finally, the solution was dispersed using ultrasound for two hours. Finally, as shown in Figure 1, MWCNTs-rich nano uids were obtained. In the mechanical processing experiment, to eliminate the potential deposition/agglomeration of MWCNTs, a fresh nano uid mixture was used. The process for preparing nano uids is shown in Figure 2.  [18] . Cutting temperatures were measured using an FLIR T630sc thermal infrared imager. The working temperature ranged from −40°C to 600°C and the image acquisition frequency from 50 Hz to 200 Hz. To guarantee the accuracy and reliability of the data, the temperature resolution of the equipment was less than 0.1°C. First, the infrared thermal imager was adjusted to about one meter from the lathe. Second, the infrared thermal imager screen was adjusted until the tool and workpiece contact area could be clearly seen on the screen. Finally, the computer identi ed the highest cutting temperature in the processing area. The TR240 surface roughness instrument was used to measure the average surface roughness under each cutting condition. By rotating the workpiece by 40°, nine measurement values under each condition were obtained, and then the arithmetic average of these values was taken. In addition, to obtain the three-dimensional surface morphology and two-dimensional surface image of the machined surface, three-dimensional optical pro ler equipment (Bruker Contour GTK 3D) was used. Finally, the tool wear was analyzed in detail through eld emission scanning electron microscopy (ZEISS). In addition, energy dispersive X-ray (EDX) analysis was carried out on the tool surface to accurately analyze the chemical composition of the cutting zone.

Experimental methodology
The cutting conditions of the turning experiment were a constant cutting speed (V) of 100 m/min, a feed speed (f) of 0.1 mm/rev, and a cutting depth (a p ) of 0.5 mm. The ve processing environments were dry, MQL, LN 2 , LN 2 + MQL, and LN 2 + Nano-MQL (0.6%). The pressure of the air compressor was 10 bar.
According to ISO 3685, the time when the ank wear value reaches 0.6 mm is the effective life of the tool [19] . In this study, the maximum wear VBmax=0.6 mm was used as the evaluation standard, and two different methods were used to evaluate tool wear. The rst method involved comparing the tool wear (VBmax) under all machining conditions after cutting for 8 min. The other method involved continuing the test on the basis of previous test results until the tool wear reached the wear standard value of 0.6 mm, and then comparing the processing time (tool life) under different conditions. Each experiment was carried out with new tools and was repeated twice. The wear of the cutting tools was measured using a VMX-2000C ultra-large depth-of-eld optical three-dimensional microscope. First, the photos of the tool ank were captured; then, two parallel lines were drawn to limit the maximum tool wear width, and the maximum tool wear was calculated. The cutting conditions are shown in Table 3, and the entire experimental device is shown in Figure 2.
Table3 Cutting conditions for turning operations.

Peak temperature in the cutting zone
During the cutting process, mechanical energy is mainly converted into thermal energy. The high temperature of the machining area has a direct impact on the dimensional accuracy, geometric accuracy, and surface integrity of the workpiece, especially the tool wear/life, which is crucial to the machinability [20] . Therefore, herein, the chip-tool interface temperature was measured by multiple sets of experiments, and the average temperature was calculated. The results are shown in Figure 3, which shows that the most effective cooling/lubrication environment for the cutting temperature is LN 2 + Nano-MQL (0.6%). The dry cutting temperature is the highest; in dry cutting, due to the poor thermal conductivity of titanium alloys, the heat in the contact area between the tool and workpiece is not effectively released. Birmingham et al. compared several different machining environments by turning Ti-6Al-4V, and found that the cutting temperature in the dry cutting environment was the highest and increased with the processing time [21] . It is further observed that MQL has a lower cutting temperature than that of dry cutting. In fact, MQL provides lubrication for the processing area, where a small amount of cutting uid (10-100 ml/h) is atomized with compressed air and sprayed into the cutting area as lubrication and cooling aerosol [22] . However, Li et al. [23] pointed out that most vegetable oils contain a large number of unsaturated bonds with ideal low temperatures and ideal viscosity-temperature characteristics. This characteristic is attributed to the fact that fatty acids in vegetable oils usually form lubricating lms through physical and chemical adsorption. Polar groups-such as COOH, COOR, and metal surface materials-are physically adsorbed by van der Waals (VDW) forces to form a physical adsorption lm, which plays a role in lubrication and friction reduction. In fact, it is normal to provide cooling for MQL to lower the temperature. This is related to the cooling characteristics of liquid nitrogen. Effective cooling provided by liquid nitrogen has a more positive effect on the tool-chip and workpiecetool interface, resulting in a better cooling effect at low temperatures. Liquid nitrogen can also absorb heat and evaporate rapidly, forming a uid gas protective layer between the chip and the blade, which acts as a lubricant [24] . However, it is worth noting that the temperature under LN 2 and MQL + LN 2 conditions is very similar. Previous studies have shown that the cryogenic environment has good cooling performance under high pressure and high-temperature processing conditions [25] . However, it is impossible to determine whether it has the same performance in terms of lubrication. In other words, in the low-temperature cooling process, the lubrication process is particularly weak for heavy processing conditions, so the e ciency is reduced. However, the temperature was further reduced by adding nanoparticles on the basis of LN 2 + MQL. We know that the thermal conductivity of the base uid increases when nanoparticles are added. Therefore, the atomic structure, high speci c surface area, and excellent thermal conductivity of MWCNTs (about 3000-3500 W·m −1 ·K −1 ) can help the cutting zone disperse more heat because it can improve the thermal conductivity and convective heat transfer coe cient of pure liquids [26,27] . Compared with the dry condition, the cutting temperature of LN 2 + Nano-MQL (0.6%) decreased by 65.1%. Similarly, compared with LN 2 + MQL (without nano additives), the cutting temperature of LN 2 + Nano-MQL (0.6%) reduced by 32.8%.

Flank wear
Tool wear refers to the material loss in the contact area between the tool and workpiece material, which is the most basic metric for tool life. The cutting performance indexes, such as the surface roughness, cutting temperature, and surface integrity, are often related to tool wear and are directly affected by it. The wear itself is determined by the tool material, coating material, coating performance, and other parameters [28] . It can be seen from Figure 4 that the tool wear under dry conditions is the largest (0.532 mm). The tool wear is signi cantly improved by providing lubrication for the machining area (0.476 mm). The chip removal amount in the cutting area and the effective spraying of oil mist droplets are possible through the nozzle and compression. They appear at the interface between the tool and the workpiece, forming a layer of processing uid, which mainly reduces the friction between the tool and the workpiece. However, it is worth noting that low-temperature cooling is superior to micro lubrication for tool wear (0.388 mm). This may be due to the cooling effect of liquid nitrogen. This result directly contradicts that reported by Yıldırım et al [29] , who found that MQL is more effective than low-temperature machining in terms of reducing tool wear. Low-temperature cooling only helps to reduce the temperature of the cutting zone through forced convection, while the effect of the MQL method is two-fold. First, MQL's lubricants contain a layer of oil in the cutting area, which helps reduce friction. Secondly, due to the evaporation of oil droplets, MQL facilitates heat transfer. The difference between their results and the ones obtained herein could be because of the signi cant differences in the workpiece material properties. In addition, other factors-including the cutting conditions (cutting parameters and tool geometry) and nozzle anglecan also be determined. For these reasons, it is di cult to conduct a scienti c and rigorous comparison between previous studies and this study. Although MQL provides lubrication for the contact area between the tool and the workpiece, effective heat dissipation is also very important for the machining area, which was con rmed by the ndings of Khanna et al. Although MQL has a good lubrication ability, its cooling characteristics have certain disadvantages [30] . However, after cooling for MQL (LN 2 + MQL), the tool wear was signi cantly reduced, to 0.374 mm. The effective cooling provided by liquid nitrogen has a more positive effect on the tool-chip and workpiece-tool interfaces, resulting in a better low-temperature cooling effect. In addition, MQL can promote chip removal from the cutting area and the low temperature can prevent the formation of built-up-edges on the tool, which also reduces tool wear [31] . However, the focus of this study is to improve the tool life by adding carbon nanotubes on the basis of lowtemperature micro lubrication (LN 2 + MQL). It can be seen from Figure 4 that the tool wear under LN 2 + Nano-MQL (0.6%) treatment was signi cantly lower than that under LN 2 + MQL. Titanium alloys have low thermal conductivities and heavy loads during processing, resulting in the rapid evaporation of vegetable oil. It has previously been mentioned that adding solid lubricants to MQL could signi cantly improve the process e ciency [32,33] .

Tool life
Extending the tool life is very important for improving process e ciency. However, in actual research, tool wear is not controlled, and it is di cult to estimate when a tool will reach a standard value of wear. To ensure that the VBmax values in each processing environment are consistent, the tool life is further explored on the basis of tests after turning for 8 min. A wear value is obtained at each turning, and then compared with the standard value. Turning is repeated to approach the standard value until VBmax = 0.6 mm, nally giving the tool life under different processing environments. It is worth noting that, to ensure the continuity of the machining process, new blades should be used after each turning. Similarly, to obtain the VB value accurately, the processing time is shortened when the VB value is close to 0.6 mm. VB = 0.6 mm cannot be accurately obtained during actual processing. Therefore, to reduce the test error, the VB value is controlled within 0.003 mm. A total of 41 cutting tools were used in this study. Table 4 shows the variation of tool wear VB with machining time under different machining conditions. It can be seen from Table 4 that the tool life in a dry cutting environment is the shortest (527 s), followed by that for MQL (560 s). However, an interesting phenomenon is that the tool life in LN 2 and LN 2 + MQL environments is very similar, at 600 s and 605 s, respectively. This result is consistent with that for the cutting-zone temperature (3.1). It is likely that, with increases in the processing time, the tool wear is severe and the mechanical load increases, resulting in the lubrication effect of vegetable oil becoming not very obvious. However, the longest tool life (685 s) was obtained by adding nanoparticles to lowtemperature micro lubrication (LN 2 + MQL). This indicates that the tool wear rate will decrease when using nano uids. In comparison to the dry and LN 2 + MQL conditions, the tool life under LN 2 + Nano-MQL (0.6%) conditions increased by 30% and 13.2%, respectively. Figure 5 shows the variation of tool wear with cutting time under ve cutting conditions when cutting Ti6Al4V at 100 m / min. It can be observed from the gure that the wear trends of tool anks in LN 2 and LN 2 + MQL environments are relatively similar, which is consistent with the relatively close tool life of the two. It is worth noting that the growth of tool wear under each processing condition is generally relatively stable.

Tool wear mechanism
To explore the main wear mechanism affecting tool wear, scanning electron microscopy (SEM) photographs of different regions of the worn tool were taken at the end of the wear process test. To accurately compare the wear mechanism under various cutting environments, the SEM images of tool wear were taken at the same cutting time, of 8 minutes, and at constant cutting parameters (i.e., a cutting speed of 100 m/min, feed rate of 0.1 mm/rev, and cutting depth of 0.5 mm).
Further, crater wear ( Figure 6) is the main form of tool failure. Kitagawa et al. [34] pointed out that the chemical sensitivity of titanium alloys to tool materials as a response to elevated temperature-through the activation diffusion wear mechanism-is the main cause for crater wear on the rake face. Owing to the introduction of a sustainable cooling/lubrication environment into the system, some damage has been reduced or eliminated, but other forms of damage still exist in the insert. Figure 6 shows that crater wear was the most obvious and accompanied by cracks in dry cutting environments (Figure 6a). In a recent survey, Liang et al. [35] also studied the same type of wear/damage for turning titanium alloys under dry conditions. Without lubrication and cooling, the cracks in the crater formed on the tool rake face are caused by stresses from the sudden increase in the chip load owing to the adhesion during cutting.
Similarly, a small portion of crater wear was found under LN 2 (Figure 6d). However, crater wear was not detected under the MQL, LN 2 + MQL, and LN 2 + Nano-MQL (0.6%) conditions, as shown in Figure 6 (b, c, and e). The results show that the cutting environments supported by MQL, such as MQL, LN 2 + MQL, and LN 2 + Nano-MQL, are conducive to inhibiting the formation of craters on the tool rake face. In other words, MQL is conducive to preventing crater wear on the tool rake face. However, it is worth noting that craters were found under LN 2 . This may be because LN 2 can only provide a cooling effect, but the lubrication effect on the tool rake face is missing. However, this is not consistent with the results of previous MQL conditions under high loads and high temperatures. However, in this study, MQL and LN 2 nozzles are always aligned to the tool rake face. Therefore, the sensitivity of the tool rake face to lubrication is much higher than that of the tool ank.
In addition, during the machining process, the excessive pressure on the cutting tool causes the workpiece material to attach or fuse with the cutting edge; the attached or fused materials then accumulate during processing. This phenomenon, called built-up edges (BUEs) in metal cutting operations, is often observed during titanium alloy processing. During the machining process of titanium alloys and other ductile materials, BUE can lead to changes in the cutting force and affect the surface roughness and tool wear process of the workpiece [36] . Unstable BUEs will exist on the cutting edge, changing the tool geometry and the optimal shear angle change during the cutting process, which reduces the cutting ability of the tool and increases the cutting force, vibration, and surface roughness.
However, for the processing of titanium alloys, cooling and lubrication methods cannot completely eliminate BUEs. In other words, cooling and lubrication alone are not su cient to eliminate the chemical activity of titanium alloys on tool surfaces. Further, in this study, BUEs are inevitable in all cutting environments. The interactions between BUEs and tools could be weakened-to some extent-by using lubrication and cooling methods; however, it cannot be completely eliminated. As can be seen from Figure   6, BUE formation under LN 2 + MQL and LN 2 + Nano-MQL (0.6%) systems is signi cantly lower than that in other cutting environments. This proves that the application of LN 2 + MQL and LN 2 + Nano-MQL (0.6%) reduces the temperature and provides convenient tool-chip interactions, thereby reducing the friction and BUE formation. As mentioned in section 3.1, the lowest temperature is obtained by providing cooling and lubrication for the processing area. In addition, SEM photographs also showed that tool nose wear occurred under the dry, MQL, and LN 2 conditions; tool nose wear is generally caused by abrasive wear mechanisms [37] . On the other hand, the strength of the cobalt phase in cemented carbide tools decreases at high temperatures, which also leads to plastic deformation of the tool nose. However, the tool noses under the LN 2 + MQL and LN 2 + Nano-MQL (0.6%) conditions remained basically intact. In addition, it can be seen from the above discussion that, compared with other environments, the nose wear, crater wear, and BUEs of the tools under LN 2 + MQL and LN 2 + Nano-MQL (0.6%) are signi cantly reduced, but the difference between them is not obvious. However, an interesting phenomenon can be observed. LN 2 + MQL with MWCNTs exhibited no damage to the tool coating. In comparison to other cutting environments, the protection of the tool coating under LN 2 + Nano-MQL (0.6%) conditions was the most effective. The boundary between the tool substrate and tool coating under dry, MQL, LN 2 , and LN 2 + MQL conditions can be clearly seen from the electron microscope images in Figure 6. These results can also be obtained from EDS element detection, shown in Figure 7. Element detection showed that important elements, such as Co and W (the left side of the electron microscope image), were found in the tool substrate under dry, MQL, LN 2 , and LN 2 + MQL conditions; however, the N in the tool coating material was detected on the other side of the boundary (the right side of the electron microscope image). For the LN 2 + Nano-MQL (0.6%) condition, the element detection range was expanded, and no Co and W were found in the tool substrate. This shows that MWCNTs are conducive to protecting the coating material of the tool. Under the action of compressed air, MWCNTs will be transported to the processing area with vegetable oil, forming lubrication, thereby protecting the coating of the tool (Figure 8).

Surface roughness
Surface roughness (Ra) is usually expressed as the average height change of peaks and valleys relative to the baseline. Many parameters affect the surface roughness; the lubrication/cooling environment is one of these. To compare the surface roughness values under different cutting environments, experiments must maintain constant cutting speeds, feed speeds, cutting depths, and nozzle ows. Figure 9 shows the average surface roughness in different cutting environments; when the cutting speed is 100 m/min and the feed rate is 0.  [38,39] . In addition, an interesting result is obtained from this study in that LN 2 is found to be superior to MQL in terms of reducing the surface roughness during titanium alloy turning. In addition, the surface roughness was not signi cantly improved after providing MQL support for LN 2 . These results are similar to those for tool wear discussed above. However, the surface roughness decreased signi cantly after the addition of MWCNTs under LN 2 + MQL conditions. According to the size, shape, hardness, and mechanical properties of nanoparticles in lubricating oil, some tribological enhancement mechanisms were reported in a previous study [40] . As shown in Figure 1, due to the tubular shape and atomic structure of MWCNTs, some mechanisms will play a more effective role in processing and provide a satisfactory effect on the interface when dispersed in the processing area [41] . As shown in Figure 10, the addition of MWCNTS under the condition of LN 2 + MQL will reduce the surface roughness, and its role can be summarized in terms of the following mechanisms [42] : 1. In the surface repair mechanism (Figure 10 (a)), nanoparticles can settle in the pits on the surface of the workpiece, reducing the contact between the insert and the workpiece material by lling and repairing, thereby reducing friction and wear. 2. Nanoparticles form a protective lm on the friction interface, as shown in Figure 10 (b).
3. In the rolling/sliding mechanism (Figure 10 (c)), nanoparticles can act as rotating bearings between the tool/workpiece interface to reduce contact, thereby reducing friction and wear.
4. In the polishing mechanism shown in Figure 10 (d), due to the high hardness of the nanoparticles, they can effectively remove residues and help reduce the surface roughness of the workpiece.

Surface topography
Since the surface morphology directly affects the friction, wear, fatigue, and sealing performance of the parts, it is very important to study the surface morphology [43] . Figure 11 shows the 2D and 3D morphology images of the Ti-6Al-4V titanium alloy processed under different cutting environments at a 100 m/min cutting speed and 0.1 mm/rev feed rate. It can be seen from the gure that the valleys and peaks formed on the machined surface are clearly visible. Due to the nature of the cutting process, it is expected that the surface morphology of the processed material is irregular along the feed line. On the other hand, a more regular surface morphology is expected along the cutting speed line than along the feed line. However, this situation can be prevented by various external factors, such as vibration, tool wear, chip adhesion, and deformation caused by thermal damage [44] . The red, blue, and green areas on the color map in Figure 11 show the height of the peak, the depth of the valley, and the nominal (favorable) position of the machined surface, respectively. Similarly, the maximum difference Rt between the peak and valley of the machined surface is shown at the rightmost red focus in Figure 11. By analyzing the 2D surface images in Figure 11, it can be seen that there are more red areas under dry and MQL conditions, and the machining marks (feed marks) are very obvious. When the 3D surface image is checked, it can be seen that the peak value under dry cutting is very high; under dry cutting conditions, the maximum difference between the peaks and valleys along the machined surface is greater than 17 µm. Under MQL, although the peaks gather more, the maximum difference between the peaks and valleys is 13.618 µm, which is smaller than that of dry cutting. It can also be seen in Figure 11 that the red area under LN 2 is signi cantly less than that under MQL and is replaced by a green area. The results obtained here are similar to their surface roughness results. When the existence of a nominal surface and the difference between hills and valleys were considered for evaluation, LN 2 + Nano-MQL (0.6%)-assisted machining achieved the best results in terms of the surface morphology and surface roughness. The maximum difference between the peak and valley was 3.33 µm, which is the smallest among all processing environments. This can be attributed to the fact that MWCNTs nanoparticles added in the cutting oil can signi cantly improve the friction and wear mechanism of the tool-chip interface (including the repair effect, protective lm effect, rolling effect, and polishing effect), so as to obtain a better surface morphology. It is believed that MWCNTs solid nanoparticles added to the cutting oil can also signi cantly reduce the thermal degradation of the surface with high thermal carrying capacity, further improving the surface morphology [45] . In fact, as mentioned above, the minimum temperature obtained by nano uid-MQL supports this result.

Conclusion
The cutting temperature, tool wear and life, tool wear mechanism, surface roughness, and surface   Effect of cooling and lubrication methods on temperature Optical image (a-e) and wear result (f) of the tool ank under different machining conditions.

Figure 5
Changes of ank wear with machining time  Effect of cooling and lubrication methods on the surface roughness Figure 10 Four tribological effects of multi-walled carbon nanotubes.

Figure 11
Machined surface morphology under different cooling and lubrication conditions