Performance Evaluation of MQCL Hard Milling of SKD 11 Tool Steel Using MoS 2 Nanoﬂuid

: The present work shows an experimental investigation on the e ﬀ ect of minimum quantity cooling lubrication (MQCL) during hard milling of SKD 11 tool steel (52–60HRC). The novelty here lies on the use of MQCL technique, which comprises the cooling strategy based on the principle of Ranque-Hilsch vortex tube and MQL method. Moreover, MoS 2 nanoparticles are suspended in MQCL based ﬂuid to improve the lubricating character. The response parameters, including surface roughness, surface microstructure, and surface proﬁle are studied. The obtained results show that MQCL using nanoﬂuid gives out better surface quality compared to dry, MQL, and MQCL with pure ﬂuid. Also, the di ﬀ erent concentrations of MoS 2 nanoparticles are investigated to ﬁnd out the optimized value as well as the interaction e ﬀ ect on machined surface.


Introduction
Hard cutting processes have been developed along with the growing demand of high productivity, easy adaptation to complex parts, the elimination of cutting fluids, good surface quality, and the reduction of manufacturing cost. Especially, for the growing concern with the climate change, they have gained much attention of the researchers and manufacturers around the world for the promising alternative solution for many of traditional finish grinding operations. These processes directly use the geometrically defined cutting edges for machining the workpieces, with the hardness value typically in the range of 45-70 HRC [1]. The selection of the proper cutting inserts always faces the challenge in order to ensure the good tool life and high precision of the machined parts. Furthermore, the thermal shock must be seriously considered to avoid the breakage of the inserts due to the use of cutting fluids. In the earliest type, dry hard machining, one of the environmental friendly processes, had been applied, gave out the obvious cost benefits, and contributed to overcome the climate change. However, the high-grade inserts, such as coated cemented carbide, ceramics, (P)CBN (Polycrystalline Cubic Boron Nitride), PCD (Cubic Boron Nitride) tools are always demanded for dry cutting due to the high hardness materials and the enormous amount of generated heat [2][3][4][5][6][7]. There have been many studies for developing the coating layers of carbide tools in order to reduce the contact length at tool chip interface at rake face [2][3][4][5][6]. The obtained results reveal the improvement of hard cutting performance with the reduction of friction coefficient, cutting forces, cutting temperature, and tool wear. On the other hand, the very high cutting temperature was noted during hard machining process [6], which would lead to thermal failure and the reduction of tool life of cutting inserts. In addition to that, the problems caused by thermal distortion of machined parts, handling, and testing process are necessarily considered. Thus, minimum quantity lubrication (MQL) has been developed and widely used to overcome the drawbacks of flood and dry machining. The high effectiveness of lubricating is achieved by the use of small amount of cutting fluid with oil mist form that is directly sprayed to contact zone, which leads to the decrease of friction coefficient, cutting forces, cutting temperature, and tool wear, as well as the improvement of surface quality and tool life [8][9][10][11][12][13]. Davim et al. [14][15][16][17] studied the effect of MQL during turning and drilling process and made the comparison to dry and flood conditions. From the obtained results, cutting power, specific cutting forces, and surface quality in both conditions were similar, but were better than those under dry cutting. Furthermore, the authors pointed out that, the lower the vibrations, the higher the surface roughness under MQL condition, while the opposite occurs when dry condition was used [18]. Accordingly, the dry and flood conditions can be successfully replaced by MQL to reduce the negative effects on environment and human health, which will be a step toward sustainable machining [19][20][21]. The limitation of the cooling effect makes MQL technology difficult to use for hard cutting processes and difficult-to-cut materials. Besides, the use of vegetable oils in order to retain the environmental friendly character of MQL technique faces the difficulty because of the low ignition temperature [22,23]. To develop MQL technique assisted for hard cutting, the new approaches including MQL using nanofluids and MQCL have been studied and prove the promising results.
The application of MQL using different types of nanofluids for difficult-to-cut materials has been an up-to-date research topic in recent years. The MQL base fluids enriched by nanoparticles, such as Al 2 O 3 , MoS 2 , SiO 2 , ZrO 2 , CNT, PCD, and so on, are proven to possess the enhancement of the tribological property and viscosity, which lead to improve the cutting performance significantly. Li et al. [24] studied the heat transfer performance of MQL grinding while using different nanofluids (MoS 2 , ZrO 2 , CNT, PCD, Al 2 O 3 , SiO 2 ) for Ni-based alloy. From the obtained results, the improvement of the viscosity and thermal conductivity of base fluids enriched by nanoparticles was observed. From that, the use of nanofluids contributed to reduce the cutting forces and cutting temperature during grinding process. Among these, CNT (Carbon nanotubes) nanofluid has the highest heat transfer coefficient. Lee et al. [25] studied MQL micro grinding using diamond and Al 2 O 3 nanofluids. Based on the experimental results, it clearly revealed that the grinding forces much reduced and the surface quality was significantly improved. The higher nanofluid concentration and smaller size of nanoparticle were more effective for decreasing the cutting forces. Ali et al. [26] investigated the tribological characteristics of Al 2 O 3 and TiO 2 nano-lubricant in automotive engines with different concentrations (0.05, 0.1, 0.25, and 0.5 wt %). The results revealed the reduction of friction coefficient, power consumption, and wear. The kinematic viscosity of nano-lubricants decreased slightly, but the viscosity index increased. The most outstanding contribution of this study is that the formation of self-laminating protective films of Al 2 O 3 nanoparticles made the worn surfaces smoother. The rolling effect created by nanoparticles was important to reduce the friction coefficient. Pashmforoush et al. [27] studied the influence of water-based copper nanofluid on the grinding of the Inconel 738 super alloy. The experimental results indicated the improvement of wheel loading and surface roughness were about 59.19% and 62.16% as compared to dry grinding, and 35.13% and 36.36% compared to conventional fluid grinding, respectively. Uysal et al. [28] evaluated the MQL milling performance of AISI 420 stainless steel using MoS 2 nanofluid. The obtained results also revealed the better lubricating effect of MoS 2 nanofluid, which led to the reduction in both tool wear and surface roughness. Zhang et al. [29] investigated the effect of nanoparticle concentration of MoS 2 and (carbon nanotubes) CNTs nanofluids for MQL grinding of Ni-based alloy. The results indicated that the use of hybrid MoS 2 -CNTs nanofluids gave out better surface quality than that of MoS 2 and CNTs nanofluids due to the lubrication effect and micromachining of nanoparticles. The nanoparticle concentration had a strong effect on surface quality, so it needed to optimize. Yıldırım et al. [30] newly studied MQL turning of Ni-based Inconel 625 using hBN (hexagonal boron nitride) nanofluid. From the results, the authors indicated the significant improvement of tool life and surface quality by using hBN nanofluid (0.5 wt %). They also showed that the cutting temperature and wear rate reduced by using MQL nanofluids when compared to dry machining. Garg et al. [31] studied the effect of nanofluid concentration of MQL micro-drilling process. Based on the obtained results, the authors showed that the nanofluid concentration had the strongest influence on the torque and power consumption. Lee et al. [32] studied the stable dispersion of diamond nanoparticles in oil-based fluid and their tribological properties as lubricant additives. The authors concluded that the diamond nanofluid (0.05 wt %) reduced the friction coefficient by 23% and provided the excellent anti-wear properties. The diamond nanoparticles exhibit the great potential application as a new lubricant possessing the excellent tribological performance. Wang et al. [33] conducted the experiments for evaluating the lubrication properties of different nanofluids in MQL grinding. Among the investigated fluids, the authors found that Al 2 O 3 nanofluid exhibited the good lubrication performance due to characteristics of high hardness and nearly spherical morphology. Luo et al. [34] also indicated that this type of nanofluid demonstrated good resistance to high temperature.
Recently, minimum quantity cooling lubrication (MQCL) has been considered the new solution to overcome the low cooling effect of MQL technique, which can bring out new alternative approaches for machining difficult-to-cut materials and enlarge the MQL applicability. Pervaiz et al. [35] investigated the influence of MQCL in turning Ti6Al4V. The results indicated that the values of surface roughness, cutting forces, and tool wear reduced due to the better cooling and lubricating effect when compared to dry and flood cutting. Maruda and his co-authors [36,37] studied MQCL parameters for hard turning AISI 1045 steel. The authors concluded that the formation of emulsion mist by using MQCL plays an important role in enhancing the cooling lubrication in cutting zone, which contributes to improve the machining performance. The smaller droplet diameters tend to form the lubricating films and penetrate into the contact zone. The authors made the study of the influence extreme pressure and anti-wear (EP/AW) additives on surface topographies during MQCL turning. The formation of tribofilm on the tool-chip interface contributed to reduce the friction and tool wear [38]. They also analyzed the chip formation zone in turning of austenitic stainless steel 316L under MQCL condition [39]. The favorable chip shape has been obtained and the values of the chip thickening coefficient reduced under MQCL condition, which prove the better cooling and lubricating performance. Furthermore, the obtained results indicated that the surface quality and surface topography improved when compared to dry machining. The main reason is that the formation of emulsion mist causes larger amounts of heat to discharge, from which the deformation of the machined surface reduced [40]. The droplet diameter is strongly affected by the distance of the nozzle, and the diameter and number of droplets can be controlled by the volumetric air flow and the nozzle distance from cutting zone. The most outstanding results are the possibility to choose the condition for mist generation, in which all droplets falling on the heated surface within 1 second are evaporated from this surface [41].
However, almost all the studies of MQCL application used the base fluid having the cooling and lubricating property like emulsion. On the other hand, the use of nanoparticle additive to the MQCL base fluid is a new topic and needs to study. Gutnichenko et al. [42] studied the influence of graphite nano additives to vegetable-based oil for MQCL assisted hard turning. The results indicated that the cutting performance significantly improved due to the effect of graphite nanoparticles in reducing the friction in combination with cooling characteristic of MQCL method. The cooling technique assisted cutting processes has been studied in recent years. Sartori et al. [43] investigated the temperature effects when using cooled gaseous nitrogen in semi finishing turning of Ti6Al4V. The authors pointed out that the significant reduction of rake and flank wear, as well as the improvement of surface integrity, was observed by using N 2 cooled at −150 • C. They also investigated the solid lubricant (SL) assisted MQC and MQL techniques for turning [44]. The SL-assisted MQC gave out the lowest wear and best surface quality when compared to dry, conventional wet, and pure MQL technique. Busch et al. [45] studied the effects of CO 2 and N 2 cooling strategies for turning process of difficult-to-cut materials. The extension of energy efficiency and enhancement of surface integrity were reported. Pereira et al. [46] used the hybrid CO 2 and MQL in milling Inconel 718. The experimental results revealed better cutting performance due to the superior cooling and lubricating compared to wet machining. Bagherzadeh et al. [47] studied the effectiveness of cryogenic CO 2 cooling and MQL for high-speed turning of difficult-to-cut materials. The results indicated that tool wear decreased, tool life increased, and surface quality improved. Similar observations were reported when using liquid nitrogen and CO 2 cooling [48][49][50].
From the literature review, it is well documented that almost all the studies used CO 2 or nitrogen for cooling, but the use of the principle of Ranque-Hilsch vortex tube [51], a mechanical device that separates a compressed gas into hot and cold streams from ordinary air, in combination with MQL method to form MQCL, is not reported. Hence, the authors are motivated to conduct MQCL hard milling experiments of SKD 11 tool steels (52-60 HRC). Moreover, the study also investigates the effects of MoS 2 nanofluid as the MQCL fluid on hard machining performance.

Experimental Set Up
The experimental set up is shown in Figure 1

The Preparation of MoS 2 Nanofluid
The non-uniform distribution of the nanoparticles in the based fluids will lead to failure in the use of nanofluid [23] and cause the waste. To ensure the uniform suspension of MoS 2 nanoparticles in emulsion-based fluids, the prepared nanofluids are kept in Ultrasons-HD ultrasonicator (JP SELECTA, Abrera, Spain), generating 600 W ultrasonic pulses at 40 kHz for 6 hours. In order to use the obtained nanofluids and avoid the precipitation of agglomerated nanoparticles during the long time of machining, the nanofluid was placed in the 3000868-Ultrasons-HD and directly used for MQCL system.

Experiment Design
Minitab 18.0 software (Minitab Inc., State College, PA, USA) is applied for the Box-Behnken experimental design with three control parameters and their values on three levels are listed in Table 2. Table A1 summarizes the design of experiment with test run order and output in term of surface roughness. The fixed parameters are the feed rate of 0.012 mm/tooth, depth of cut of 0.12 mm, air pressure of 6 Bar, flow rate of 30 mL/h; the room temperature 24-27 • C; the temperature of output cool air 4-8 • C. The experimental trials are repeated by three times under the same cutting parameters.

The Effects of Input Machining Parameters on Surface Roughness
The ANOVA analysis is carried out at a confidence level of 95% (i.e., 5% significance level). Table A2 shows the results of ANOVA analysis. The regression model of surface roughness R a is given below in Equation (1).
The Pareto chart of the standardized effects with α = 0.05 for the response parameter R a is shown in Figure 3. The nanoparticle concentration (x 1 ) has a strongest influence, followed by the hardness (x 3 ) and cutting speed (x 2 ). The effects of these input machining parameters are also reflected by the corresponding coefficients in Equation (1). The interaction effects CC (x 3 x 3 ), AA (x 1 x 1 ) reveal the significant influence on the investigated function, which is contrary to BB (x 2 x 2 ). The other interaction effects of x 1 x 2 , x 1 x 3 , x 2 x 3 have a very little influence and are not investigated in the model. From the analysis of the effects of three input machining parameters, the proper selection of nanoparticle concentration and the material hardness need to study in order to improve the surface roughness.  (Figure 5b), the effects of nanoparticle concentration (x 1 ) and hardness (x 3 ) on surface roughness R a indicate that the optimized value R a is about 0.12 µm, with x 1 = 0.5 wt % and x 3 = 55-58 HRC.
From the surface plot ( Figure 6a) and contour plot (Figure 6b), the effects of cutting speed (x 2 ) and hardness (x 3 ) on surface roughness R a indicate that the optimized value R a is about 0.15 µm, with x 2 = 95-110 m/min. and x 3 = 55-58 HRC.
The prediction of the optimized value of surface roughness R a is 0.1070 µm (Figure 7), with x 1 = 0.5 wt %; x 2 = 110 m/min; x 3 = 56.6061 HRC. In Figure 5, it can be clearly seen that the Figure 5. The effects of nano concentration and hardness on surface roughness R a . graph of surface roughness does not show the extreme point with nanoparticle concentration of 0.5 wt %. Therefore, the following investigation is done to find the optimal value.

The Optimized and Validated Experiments
The experiments aim to compare the effects of dry, MQL pure emulsion-based fluid, MQCL pure emulsion-based fluid, and MQCL emulsion-based nanofluid, from which the effectiveness of MQCL emulsion-based nanofluid can be evaluated more accurately in hard machining. Moreover, the experimental study is made to find the optimized concentration of MoS 2 nanoparticles based on the obtained parameters in Figure 7 = 0.130 µm). Interestingly, the value of surface roughness increases from 0.130 µm to 0.202 µm when the nanoparticle concentration rises from 0.5 to 0.8 wt %, which is similar to the study of MoS 2 nanofluid in [52,53]. Studying the microstructure and profile of machined surface in Figures 9-13, the white-layer and burn marks under MQL condition are reduced when compared with dry cutting. The compression phenomena of machined surface under MQL condition is significantly decreased by observing the surface profile. The main reason is that MQL technique with emulsion-based fluid provided the better lubricating condition in the cutting zone. Compared to MQL condition, the surface microstructure under MQCL with pure emulsion-based fluid significantly improves and the surface profile is smoother (Figures 10 and 11). The white-layer and burn marks much reduced due to the better cooling performance. In case of using MQCL with MoS 2 emulsion-based nanofluids, the surface microstructure and profile are similar to those of MQCL with pure fluid, but the value of surface roughness is slightly better (Figures 11-13). MoS 2 nanoparticles are ellipsoidal and they provide the low coefficient of friction up to 0.03-0.05 or even lower caused by the weak binding of sulfur atoms between molecular layers to create "an easy-to-slide plane" [23], by which it proves the better lubricating performance of MoS 2 nanoparticles in MQCL technique.

The Effect of MoS 2 Nanoparticle Concentration on Surface Roughness
The effects of MoS 2 nanoparticle concentration on the value of surface roughness (Figure 8) indicate that the average values of surface roughness are 0.144, 0.130, and 0.202 µm, corresponding to the nano concentration of 0.2, 0.5, and 0.8 wt %. Accordingly, the optimized surface roughness occurs in the concentration range of 0.2-0.8 wt %, so MoS 2 nanoparticle concentration of about 0.5 wt % can be suggested to use in this case.
The microstructure and profiles of machined surface captured by KEYENCE VHX-6000 Digital Microscope are investigated in Figures 9-13 and reveal that MoS 2 nanoparticle concentration has a strong effect. The novel observation is that the so-called "micro bubbles" remain on the machined surface, which increase with the rise of nanoparticle concentration shown in Figures 12 and 13. The morphology of MoS 2 nanoparticles is ellipsoidal and they possess the large surface area; therefore, they remain on the machined surface and form a thin protective film, which amplifies when the nanoparticle concentration increases [53] and contributes to form the tribofilm easily [38]. In addition, the oil mist containing MoS 2 nanoparticles plays an important role in improving the cooling and lubricating characteristics in cutting zone [29,[54][55][56]. However, when the concentration of MoS 2 nanoparticles in emulsion-based fluid rises to 0.8 wt %, it causes a negative effect on surface roughness [53]. Accordingly, the proper concentration in this case is 0.5 wt %, but more investigations need to study and explain this observation.

The Effect of MQCL Technique Using MoS 2 Nanofluid on Cutting Performance
The cutting performance of normal carbide tools is significantly improved in hard milling due to the better cooling and lubricating effects of MQCL technique. In the previous studies, the cutting speed of carbide inserts increased to 110 m/min. during MQL milling of hardened steel (50-52HRC), which is 2 times higher than the recommended cutting speed of the manufacturer [22,57]. In this work, the carbide tools still show the effectiveness during cutting the difficult-to-cut steel with the range of hardness of 56-60 HRC, while remaining the cutting speed of 110 m/min., which is about 2.75 times higher than that of the manufacturer's recommendation. It clearly reveals the superior cooling and lubricating effects on cutting zone by using MQCL method with MoS 2 nanofluid. From that, the manufacturing cost significantly reduces and the cutting applicability of carbide tools enlarges.

Conclusions
ANOVA analysis is applied for the Box-Behnken experimental design to evaluate the effects of variables on the objective functions, from which the directions of further studies can be made. In this study, the influence of MQCL parameters including the nanoparticle concentration, cutting speed, and hardness is investigated in terms of surface roughness. The interaction effects of hardness and nanoparticle concentration reveal the strongest influence on the investigated function.
The MQCL tool named by Frigid-X Sub-Zero Vortex Tool Cooling Mist System based on the principle of Ranque-Hilsch vortex tube in combination with MQL is applied in the hard milling of SKD 11 tool steels (52-60 HRC) using cemented carbide tools. This is the first attempt to investigate the novel MQCL using MoS 2 nanofluid. The surface roughness, surface microstructure, and surface profile of machined surface are better under the MQCL technique using MoS 2 additives in emulsion-based fluid when compared to dry, MQL pure fluid, and MQCL pure fluid. The enhancement of lubricating characteristic is observed due to the presence of MoS 2 nanoparticles, which contribute to form oil mist.
The form of white layers and burn marks are much reduced by using MQCL technique due to the better cooling and lubricating performance, which can overcome the main drawback of MQL method. The application of MQCL technique in hard machining brings out a promising alternative solution for dry and wet, MQL conditions, enlarges the cutting applicability of difficult-to-cut materials, and contributes to prevent climate change.
The normal carbide inserts can be effectively used in hard milling under MQCL technique with MoS 2 nanofluid, which contributes to enlarge the cutting applicability and decrease the manufacturing cost. Moreover, MQCL tool only needs ordinary air for cooling, which will reduce the expense of using other cooling gases.
The MoS 2 nanoparticle concentration in emulsion-based fluid for MQCL technique is investigated and optimized by experiments. The optimized value is 0.5 wt %. When the concentration is larger than 0.5 wt %, the surface roughness becomes worse. The observation will provide the necessary technical guideline on using MoS 2 nanofluids and hybrid nanofluid more efficiently.
In further research, more investigations need to be focused on the effects of nanoparticle concentration on surface quality. The influences of other properties like nanoparticle morphology, nanoparticle size are necessary to be studied. In addition, more focus will be given to investigate the impact of feed rate and the parameters of MQCL using nanofluid.

Conflicts of Interest:
The authors declare no conflict of interest.