Evaluation of MQCL Technique Using MoS2 Nanofluids During Hard Milling Process of SKD 11 Tool Steel

The current study demonstrates the effect of minimum quantity cooling lubrication (MQCL) using MoS2 emulsion-based nanofluid on hard milling of SKD tool steel (52-60 HRC) with coated cemented carbide inserts. The input machining parameters including nanoparticle concentration, cutting speed and hardness on cutting forces are investigated in term of cutting force components by using ANOVA analysis applied for the Box-Behnken experimental design. The results indicate that the hardness and nanoparticle concentration have a strongest influence on cutting forces. The interaction effects of investigated parameters are studied in detail and provide the important direction for using MoS2 nanofluid efficiently with the proper concentration of 1.0-1.1 wt%. Moreover, the cutting performance of carbide tools is significant improved during hard milling process due to the better cooling and lubricating effects of MQCL technique.


Introduction
SKD11 tool steel is among the high-carbon and highchromium alloy steel and has good wear resistance and size ability after heat treatment. Accordingly, it is used for making long-life high-precision cold-work dies, but it is grouped in difficult-to-cut materials [1]. In conventional approach, the finishing process of hardened steels is grinding with flood coolant, but the growing demand for high productivity, machining of complex parts, good surface quality and especially the elimination of cutting fluids due to the concerns with the environmental and health problems needs to find the alternative solution. Recently, hard cutting processes are developed to machine the difficult-to-cut materials and are proven to replace many of traditional finish grinding operations. These processes directly use the geometrically defined cutting edges to cut the steels in hardened state with the hardness of 45-70 HRC [2]. In the earliest type, hard machining is carried out in dry condition, which is also considered the environmental friendly process. There are numerous studies of cutting performance of dry hard machining. C. Y. Wang et al [3] studied the wear and breakage of coated carbide tool during high-speed hard milling processes in dry condition. The results showed the type of dominant wear patterns and modes of breakage. The authors also pointed out the influence of rake angle on the cutting forces and friction coefficient. K. Zhang et al [4] investigated the effect of nano-scale textures on cutting performance of WC/Co-based Ti55Al45N coated tools in dry hard turning. The study aimed to reduce the contact length at tool-chip interface at rake face, from which the friction coefficient, cutting forces, cutting temperature, and tool wear reduced. The similar observation had been made for studying hard turning performance using coated carbide tools [5]. However, the necessity of using high-grade inserts such as coated cemented carbide, ceramics, (P) CBN, PCD tools due to the high hardness materials as well as the enormous amount of generated heat [4][5][6][7][8][9]. Moreover, the use of flood coolant in hard machining faces the difficulty due to the thermal shock, and the high generated temperature from cutting zone causes the thermal distortion of machined parts, handling, and testing process. Hence, minimum quantity lubrication (MQL) technique has been developed to be an alternative solution for flood and dry machining. This technique provides the high lubricating effectiveness, which contributes to the decrease of friction coefficient, cutting forces, cutting temperature and tool wear as well as the improvement of surface quality and tool life [10][11][12][13][14]. The low cooling effect is the main drawback of MQL method, which demands new methods for developing the cooling and lubricating performance of MQL. Recently, MQL technique using nanofluids and minimum quantity cooling lubrication (MQCL) have been studied and gained much attention of researchers around the world. Many studies focus on the use of nano additives, such as Al 2 O 3 , MoS 2 , SiO 2 , TiO 2 , CNT, CuO, hBN and so on, suspended in MQL fluid and prove the promising results because of the enhancement of the tribological property and viscosity of the based fluid. Accordingly, the cutting forces, cutting temperature and friction coefficient are reduced, and the improvement of surface quality and cutting performance are reported [14][15][16][17][18][19][20][21][22][23][24][25][26]. Besides, minimum quantity cooling lubrication (MQCL) is considered to be an attractive solution to improve the cooling effect of MQL method. R. W. Maruda et al. [27][28][29] studied MQCL parameters and chip formation during hard turning process. The results indicated that the surface quality and surface topography improved when compared to dry machining due to the better cooling and lubricating performance. S. Pervaiz et al. [30] investigated the influence of MQCL during turning process of Ti6Al4V alloy, a difficult-to-cut material. The obtained results indicated that cutting forces and tool wear decreased and surface quality improved when compared to dry and flood cutting. The main reason is the better cooling and lubricating effect of MQCL technique. Nevertheless, the cooling effect of MQCL technique is based on the property of the based fluids. There is a little information about the use of real cooling method combined with MQL technique to form MQCL used in hard machining. Hence, the authors are motivated to study the effectiveness of MQCL technique during hard milling process of SKD 11 tool steels (52-60 HRC). Furthermore, the study also investigates the effect of MoS 2 nanoparticles suspended in emulsion-based fluids in MQCL hard milling.

Experimental Set Up
The experimental set up is shown in Figure 1. The experiments were done in Mazak vertical center smart 530C. The APMT 1604 PDTR LT30 PVD submicron carbide inserts of LAMINA Technologies (made in Switzerland) was utilized ( Figure 2). The MQCL system includes Frigid-X Sub-Zero Vortex Tool Cooling Mist System (made by Nex Flow™, compressed air, pressure stabilization device, water-based emulsion 5.0% and MoS 2 nanoparticles. The cutting forces are directly measured by Kistler quartz three-component dynamometer (9257BA) connected to A/D DQA N16210 (made by National instruments, USA), which is linked to the computer having DASYlab 10.0 software [33]. MoS 2 nanoparticles made by Luoyang Tongrun Info Technology Co., Ltd with the size of 30nm (average) were used. (Figure 3). KEYENCE VHX-6000 Digital Microscope is used to study the surface topography ( Figure 4). In this research, the SKD 11 tool steels with hardness of 52÷60 HRC were used. The chemical composition is shown in Table 1. To ensure uniform suspension of MoS 2 nanoparticles in emulsion-based fluids, the prepared nanofluids are kept in Ultrasons-HD ultrasonicator (JP SELECTA in SPAIN) (seen in Figure 5), generating 600W ultrasonic pulses at 40 kHz for 6 hours to ensure the uniform distribution of the nanoparticles.  1. The experimental set up [33].

Experiment Design
Minitab 18.0 software is applied for the Box-Behnken experimental design with three control parameters and their values on three levels are listed in Table 2. Table 3 summarizes the design of experiment with test run order and output in term of cutting forces. 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 0.5 ml/min; 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.

Results and Discussion
The ANOVA analysis is carried out at a confidence level of 95% (i.e. 5% significance level). The regression models of cutting forces F x , F y , F z are given below in Eqs. 1-3.
The Pareto chart of the standardized effects with α = 0.05 for the response parameter R a is shown in Figures 6-8. The hardness has a strongest influence on cutting forces F x , F z (shown in Figures 6-8, but nanoparticle concentration has a strongest effect on cutting force F y in Figure 7. The nanoparticle concentration contributes a significant effect on the cutting forces F y , F z (Figures 7-8). The effects of these input machining parameters are also reflected by the corresponding coefficients in Eqs. 1-3. In contrast, the cutting speed has a very little effect on cutting forces. 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 is not investigated in the model. From the analysis of the effects of three input machining parameters.  The interaction effects of nanoparticle concentration, cutting speed, and hardness on cutting force F x are shown in the surface plots and contour plots (Figures 9-11). The nanoparticle concentration has a strong influence on cutting force component F x . When considering the interaction effects of cutting velocity and hardness, the proper value of concentration is about 1.0 wt%, which leads to the reduction of cutting force F x (Figures 9-10). The main reason is that MoS 2 nanoparticles have the layer structure of a hexagonal crystal system combining Mo and S through a covalent bond, and the bond between them is short, but the spacing between sulfur atoms is large. Accordingly, the bond between two adjacent sulfur atom layers is weak to form "an easy-to slide plane" from weak binding of sulfur atoms between molecular layers by cutting forces, from which the friction coefficient in cutting zone reduces [15,34]. The formation of oil mist is decreased by the reduction of MoS 2 concentration, but the oil mist disappears when increasing the concentration over the proper value [23,31]. The concentration should be used by 0.75 -1.25 wt% when changing cutting speed shown in Figure 9. From Figure 10, the MoS 2 nanoparticle concentration has a little effect with the change of hardness, so it can be used in the range of 0.5-1.5wt%. From Figure 9 and Figure 11, the cutting force F x is largest at the cutting speed of 100 m/min, and it reduces when increasing the cutting speed to 110 m/min and increases with the rise of hardness. Depending on the specific requirements, the input parameters could be selected to fulfill.       The interaction effects of nanoparticle concentration, cutting speed, and hardness on cutting force F y are shown in the surface plots and contour plots (Figures 12-14). The lowest values of F y occur in the concentration range of 1.0-1.25 wt% (Figures 12-13) and the cutting speed of 95-105 m/min (Figures 12-14). The cutting force F y is largest at the hardness of 54-57HRC, which is the distinguishing characteristic of hard machining and is suitable to the previous study [2,33]. The similar observation can be made with the cutting force F z , and the difference is only the level and value (Figures 15-17).

Conclusion
The effects of nanoparticle concentration, cutting speed and hardness on cutting forces are investigated by using ANOVA analysis applied for the Box-Behnken experimental design. The study provides the necessary directions of further studies in MQCL hard milling process. The interaction effects of hardness and nanoparticle concentration show the strongest influence on the investigated function. The influence of each input machining parameter on the cutting force components F x , F y , F z is studied to propose the usage suggestion of MoS 2 nanofluid in MQCL technique.
The cutting performance of normal carbide inserts is significantly improved by using MoS 2 additives in emulsionbased fluid with MQCL technique in terms of cutting forces. The significant improvement of cooling and lubricating is observed and gives out a promising alternative solution for dry, wet, MQL conditions, from which the applicability of machining difficult-to-cut materials is enlarged. In addition, using MQCL technique can overcome the low cooling effect of MQL.
The concentration of MoS 2 nanoparticles is investigated and should be used in the range of 1.0 -1.1 wt%, which provides the very important guide for using MoS 2 nanofluid efficiently with the target of low cutting forces. The cutting forces should be chosen at the high level of 110 m/min to achieve the productivity.
In further research, more investigations need to focus on the influences of other parameters like nanoparticle morphology, nanoparticle size, MQCL parameters in order to bring it to industrial practice.