Temperature Analysis in Cubic Boron Nitrate Cutting Tool during Minimum Quantity Lubrication Turning with a Coconut-Oil-Based Nano-Cutting Fluid Using Computational Fluid Dynamics

Abstract

The minimum quantity lubrication (MQL) approach is used for improving tool life at a low cost, and it is environmentally friendly. When compared to traditional flood cooling technology, the flow rate in MQL is thought to be 10,000 times lower. The workpiece’s surface smoothness is enhanced by continuous chip formation during turning, but because the tool is always in touch with the chip, a crater wear zone is formed on the rake face due to high friction and thermal stress. While adding nanoparticles to MQL enhances cutting performance, a high concentration of these nanoparticles causes burr adhesion and decreased chip evacuation capability due to the agglomeration of nanoparticles, which affects the surface finish of the workpiece. A novel “coconut-oil-based SiC–MWCNT nano-cutting fluid for a CBN insert cutting tool” is proposed in this approach to overcome these issues. Silicon carbide (SiC) and multi-walled carbon nanotubes (MWCNTs) are added to coconut oil with an appropriate volume fraction for better lubrication. The thermal properties of the proposed nano-cutting fluid are compared with those of some existing nano MQL cutting fluids, and it was found that the MQL cutting fluid under consideration exhibits an elevated thermal conductivity and convective heat transfer coefficient that efficiently reduce tool temperature and improve tool life. The comparative study between the Finite Element Simulation using computational fluid dynamics (CFD) predicted variation in tool temperature and the corresponding experimental values revealed a remarkable alignment with a marginal error ranging from 1.27% to 3.44%.

1. Introduction

Mineral oil often serves as the foundation fluid in traditional cutting fluids, and the use of these fluids enhances product quality. To enhance productivity, they reduce the cutting temperature and provide lubrication to the cutting zone, as well as assist in chip removal during machining processes. MQL, also known as near-dry machining, involves a combination of a small quantity of base cutting oil with compressed air to generate aerosols, which are then applied to the cutting area. The interactions between the chip and the tool have a big impact on how quickly and what kind of tools wear out. With higher cutting speeds, it becomes increasingly difficult for the coolant to gain access to the cutting edge due to the cutting motion. Additionally, a “seizure zone” forms at the tool–chip interface because of high compressive stresses, strain hardening, and strong chemical action. Due to their superior lubrication and tribological and thermal transfer capabilities, nano-size solid lubricants are added to cutting fluid, which considerably increases the system’s machining efficiency [1,2].

According to the experimental findings in turning operations, the cutting speed has less of an impact on the chip macromorphology than the feed rate. When the feed rate is increased, the chip length drops, but as the cutting speed is increased, it practically stays the same. The primary drawback of the MQL technique, especially for intense machining operations that generate significant heat in the cutting area, is the inadequate cooling rate associated with MQL. Nanoscience has been used to increase the effectiveness of cooling in metal-cutting processes. Producing a homogenous nanofluid without the base fluid’s particles clumping together is essential when employing MQL. This well-mixed nanofluid is created using either a two-step or a one-step process. The nanoparticles are created utilizing chemical or physical techniques in the two-step method. The chemical procedures include thermal spray, microemulsions, and chemical vapor deposition (CVD). The nanofluid preparation steps may include mixing, mechanical stirring at a desired temperature for a required time, and ultrasonic dispersion [3,4,5].

The lubricating and cooling capabilities of nanofluids depend on three crucial factors: the contact angle, viscosity, and thermal conductivity. Changes in nanoparticle concentration or volume fraction have an impact on the cutting temperature and tool wear, in addition to the surface roughness and specific cutting energy. While using a nano-cutting fluid, the oil-penetrated particles stick to the machined surface and encourage plastic to flow on the chip in reverse. Consequently, MQL technology greatly enhances machinability, and when the cutting fluid nozzle is projected on the tool’s flank [6] surface, a greater amount of cutting fluid can penetrate the tool [7,8].

K. Khanafer et al. [9] looked at how utilizing an MQL nanofluid affected the cutting tool’s average temperature using a finite element analysis and a discrete phase model. The results showed that adding Al₂O₃ nanoparticles to MQL decreased the cutting tool’s average temperature from 443 K (0% volume percent) to 420 K (2% volume fraction). The experimental verification of the numerical findings revealed a relative inaccuracy of 2.7% for nanoparticles with a volume fraction of between 2% and 9.7% in the absence of nanoparticles.

In their study, W. El-Bouri et al. [10] created an ANSYS Fluent-based computational fluid dynamics (CFD) model to simulate the temperature outline and oil droplet behavior in the cutting zone. A numerical frictional model produced the tool temperatures used in the CFD model. However, the findings of the arithmetical model for dry, flood coolant, and MQL were confirmed experimentally using a CNC lathe of Grade 2 (ADI), with relative errors of 1.25%, 1.33%, and 6.56%, respectively.

Vedant Joshi et al. [11] combined Al₂O₃ (alumina), Al (aluminum), and SWCNT (single-walled carbon nanotube) nanoparticles with mineral oil to create a nanocoolant. By altering the nanoparticle volume percentage and coolant flow rate, the heat transfer capabilities of several nanocoolants were examined. The findings showed a sharp decrease in the cutting tool temperature with the increasing dispersed nanoparticle volume percentage and flow rate of the coolant. Additionally, it was shown that the SWCNT nanocoolants outperformed the Al and Al₂O₃ nanocoolants in terms of thermal performance and heat removal rate. However, there were some variations in the results with varying mesh sizes in the simulation.

Khalil Khanafer and Kambiz Vafaito [12] investigated the effect of incorporating Al₂O₃ nanoparticles into the MQL cooling fluid on the temperature dispersion of the cutting tool. The path of the mist in the fluid domain was described using a discrete phase model (DPM). The fluid region’s outside surfaces were given symmetric boundary conditions. The results of this experiment showed that introducing Al₂O₃ nanoparticles caused the cutting tool’s average temperature to drop. Modifying the pressure of the compressed air had a positive effect on the temperature distribution of the cutting tool, resulting in a considerable decrease in the temperature of the cutting tool as the compressed air pressure increased. But this method did not consider the temperature increase caused due to flank wear by continuous chip formation.

Amr Salem et al. [13] used minimal quantity lubrication (MQL)-based nano-cutting fluids with multi-walled carbon nanotubes (MWCNTs) and Al₂O₃ particles to analyze the machining of an Inconel alloy. The outcomes of the performed machining experiments and the various MQL-NCF life cycle phases of two distinct nano-additives were used in two different milling techniques. To find the best cutting conditions for both procedures, modeling and multi-objective optimization were used. Additionally, non-dominated sorting was applied to these improved cutting circumstances in order to determine whether the technique was preferable. However, the effect of material adhesion was not considered for nanoparticle addition in this analysis.

In their study, Ahmed Mohamed et al. [14] used rice bran oil, an environmentally friendly vegetable oil, as a lubrication fluid. Both cutting fluids with and without the addition of ZnO nanoparticles were analyzed to examine the impact of turning factors, such as the cutting speed, feed rate, and depth of cut, on the response variables using a Taguchi L16 orthogonal array. But this method did not consider the effect of the contact angle of the base fluid, which is an important factor to be considered for better heat removal.

Munish Kumar Gupta et al. [15] introduced an integration system called ANFIS that uses neural networks to enhance the fuzzy inference system for MQL applications. They used the Response Surface Methodology (RSM), which is based on the design of experiments, for creating experiments and maximizing the influence of process variables. In order to generate multi-response optimization of the process parameters, the desirability function approach was applied in this study. The limitation of fuzzy systems used in this approach is that their adaptive and self-learning abilities are shallow.

Cagri Vakkas Yildirim et al. [16] analyzed the performance of nano-MQL in Inconel 625 turning by adding hBN nanoparticles to pure MQL. The machining studies were conducted utilizing four distinct cutting conditions: dry cutting (without coolant), pure MQL, and nano-MQL with 0.5 vol% hBN and 1 vol% hBN additive ratios. The study employed nickel-based Inconel 625 for challenging applications. The addition of hBN in the nanolubricant to the MQL system, particularly in the chip removal processes, remained restricted.

Jose Claudio Lopes et al. [17] examined the results of three techniques: the profuse application of a cutting fluid (flood), regular MQL, and MQL combined with a wheel cleaning jet (MQL + WCJ). They examine the utilization of the MQL + nanofluid (MQL + Nano) technology. This can be explained by the ineffective cooling system and limited capacity to remove the chips produced, both of which contribute to increased adhesiveness.

Fatih Gunan et al. [18] determined the ideal rate of Al₂O₃ nanoparticle distribution with regard to machining parameters and exceptional characteristics. The experimental design involved using three different cutting speeds (60, 75, and 90 m/min) and three separate feed rates (0.10, 0.15, and 0.20 mm/rev) to assess the performance of nanofluids. While all cutting circumstances resulted in chipping/fractures, attrition wear, and coating peeling, there was no indication of workpiece material adherence in the 1 vol% and 1.5 vol% Al₂O₃-based nanofluid MQL. But when the concentration ratio was increased to 1.5 vol%, there was a minor drop in tool life.

Overall, in previous studies, [16,17] found relative errors between the analytical and experimental results. In the paper [18], the variation in mesh sizes produced variations in the results of the simulation. In the study [19], the flank damage of the tool due to the continuous chip formation was not considered while designing the tool. In the paper [19], the effect of burr adhesion due to the high nanoparticle concentration was not noticed, and in the study [20], the effect of the contact angle on heat transfer was not considered. In the study [21], the shortcoming of fuzzy systems was found to be that they lack adaptiveness and have limited capacity for self-learning. In the paper [22], specifically, the chip removal procedures in the MQL system were constrained by the addition of hBN to the nanolubricant. In the paper [23], the stickiness was increased due to its inadequate cooling and its restricted ability to remove the chips generated, and in the study [24], a little diminution in tool life was seen when the concentration ratio reached 1.5 vol%.

Hence, to overcome the limitations of the previous methods and to improve heat removal during machining operations using the MQL technique, a proper material selection process for cutting tools and a proper selection process for nano-cutting fluids are necessary.

The protective tool–chip interface film grows faster as nanoparticle concentrations increase, and the film that forms on the workpiece’s machined surface improves surface quality while lowering the cutting temperature and force. If the composite concentration is too high, the nanofluid dispersion worsens, and viscosity causes the intermolecular forces between the components to rise, which raises the contact angle. Nanoparticles of various materials, including alloyed nanoparticles, metallic oxides, fullerene, carbide ceramics, nanodiamonds, nitride ceramics, semiconductors, and metals, have been used. Additionally, compared to other base fumes, CNT–water nanofluid has a higher thermal conductivity [9,10,11]. Since they have the highest thermal conductivity of any nanoparticle now in use (3000 W/m K), CNT [12,13,14,15] carbon nanotubes have been regarded as one of the best nanoparticles. Further attention needs to be given to nano-cutting-fluid-based MQL for efficient heat removal. So, there is a need for a novel strategy in the selection of cutting tools and cutting fluids to increase the heat transmission of a tool with cost-effective and environmentally acceptable nano-cutting fluids.

The major contributions in this paper are given as follows:

• A CBN insert cutting tool, selected for its exceptional heat and corrosion resistance, high hardness, thermal stability, and chemical inertness, is utilized with an obstruction-type chip breaker to eliminate continuous chipping during machining to avoid tool wear and improve the tool life.
• A vegetable-oil-based nano-cutting fluid in MQL is used to improve the thermal conductivity of the nano-cutting fluid.

These contributions mentioned above have been considered for improving the tool life and thermal conductivity of cutting fluids. Section 2 describes the materials and methods.

2. Materials and Methods

A cutting tool with a chip breaker that obstructs the formation of continuous chips was chosen for the turning of an Inconel alloy. The cutting tool is made of cubic boron nitrate (CBN). The choice of pure coconut oil as the base fluid is due to its non-reactivity with the Inconel alloy. The research method utilized in this study is minimum quantity lubrication (MQL) with nanofluids (SiC, MWCNTs, and coconut oil).

Nanoparticles are incorporated into the base fluid in minimum quantity lubrication (MQL) to enhance its heat dissipation capability. The coconut oil is enhanced with the addition of silicon carbide and multi-walled carbon nanotubes as nanoparticle additions. Pure coconut oil is combined with a mixture of silicon carbide and multi-walled carbon nanotubes. The concentration of nanoparticles is adjusted to control the volume portion of the nano-cutting fluid. The composition of the Inconel 718 superalloy is mostly iron, nickel, and chromium. It demonstrates remarkable thermal stability and can maintain its mechanical properties at elevated temperatures, typically exceeding 700 °C. The CBN insert cutting tool was selected for its exceptional heat and corrosion resistance, as well as its high levels of hardness, thermal stability, and chemical inertness. The thermal characteristics of the Inconel alloy 718 are displayed in

Table 1. Thermal properties of Inconel 718 alloy.

Properties	Inconel Alloy 718
Density (kg/m3)	8190
Thermal conductivity (W/mK)	11.4
Specific heat (J/kg·K)	435

.

Table 2. Thermal properties of CBN insert.

Properties	CBN
Density (kg/m3)	4280
Thermal conductivity (W/mK)	44
Specific heat (J/kg·K)	750

illustrates the characteristics of the CBN insert. The CBN tool/insert material is grade CBN-L, or low-content CBN. CBN cutting inserts (TNMG160404) were used in the experimental work. The cutting inserts have single edges, and the tool holder utilized in the experiments is shown in Figure 1.

A ceramic phase is added to the material, which is usually titanium nitride. The thermal properties of SiC, MWCNTs, and coconut oil are given below in

Table 3. Thermal Properties of SiC, MWCNTs, and coconut oil.

Sr. No	Properties	SiC	MWCNTs	Coconut Oil
1	Density (Kg/m3)	3210	1400	920
2	Thermal conductivity (W/mK)	120	3000	0.321
3	Modulus of elasticity (GPa)	410	270	-

. By studying changes in the viscosity and thermal conductivity of the NCF, the temperature distribution on the tool, and the rate of heat removal, it is possible to determine the optimal concentration by varying the volume fraction of these nanoparticles from 0.5% to 5% during the examination. The design and temperature analysis of the CBN cutting tool and the performance of the proposed nano-cutting fluid in improving the tool’s life and heat removal are explained in the next section, Section 4.

The processing conditions of the experiments for the turning of the Inconel 718 workpiece are given as follows: air pressure: 6 bars; fluid flow rate: 60 mL/h; distance from nozzle: 20 mm; nose radius (mm): 0.4; machine: lathe machine; and type of insert: CBN insert (TNMG insert). The machine parameters selected for the experiments are shown in

Table 4. Processing conditions for turning of Inconel 718 alloy.

Sr. No	Variable Parameters	Level of Parameters
		I	II	III
1	Cutting speed (m/min)	50	75	100
2	Depth of cut (mm)	0.14	0.16	0.18
3	Feed rate (mm/rev)	0.04	0.06	0.08

.

The overall process of the proposed nano-MQL method or the methodology adopted for the experiments is elaborated with the help of a flow chart, as given in Figure 2.

3. Coconut-Oil-Based SiC–MWCNT Nano-Cutting Fluid for CBN Insert Cutting Tool

MQL uses a very small quantity of environment-friendly lubricant mixed with compressed air to minimize the huge quantities of water and mineral-oil-based cutting fluids. In order to overcome the limitations of present MQL approaches and to improve the tool life and heat removal capacity, a novel “coconut-oil-based SiC–MWCNT nano-cutting fluid for a CBN insert cutting tool” is proposed. Continuous chip formation in the turning process of an Inconel alloy improves the surface finish, but a crater wear zone is formed on the rake face of the tool due to high thermal stress. So, a cubic boron nitrate (CBN) insert-type cutting tool with an obstruction-type chip breaker was selected for breaking continuous chips formed during the turning of the Inconel alloy. The CBN cutting tool insert remains stable at temperatures up to 1400 °C. The selection of a base fluid is an important factor in the context of minimum quantity lubrication. The small contact angle of a nano-cutting fluid (NCF) results in a large tool surface area and better lubrication. Also, the chemical interaction of the base fluid with the machining material needs to be considered while selecting the proper base fluid for MQL. In this method, pure coconut oil is selected as the base fluid because it does not chemically react with the Inconel alloy. Nanoparticles are added to the base fluid in MQL to improve heat removal capacity, but they affect the viscosity of the base fluid. A high concentration of nanoparticles increases the viscosity and heat conductivity of the nano-cutting fluid (NCF). Moreover, it causes material adhesion (burr) and leads to decreased chip evacuation capability due to the agglomeration of nanoparticles. In this proposed method, silicon carbide and multi-walled carbon nanotubes are selected as nanoparticle additives in the coconut oil.

The chip formation at different cutting speeds for a CBN insert with a nose radius of 0.4 mm is shown in Figure 3. Silicon carbide and multi-walled carbon nanotubes are mixed with pure coconut oil. The volume fraction of the nano-cutting fluid is controlled by varying the concentration of nanoparticles. The thermal conductivity of the nano-cutting fluid depends on the volume fraction of nanoparticles. The proposed NCF effectively removes heat from the tool and workpiece during machining, so the tool lifetime is increased and the surface smoothness of the workpiece is also increased. The obstruction-type chip breaker on the tool is used for continuously breaking the chips without disturbing the surface finish of the workpiece. The layout of the minimum quantity lubrication (MQL) setup, the turning experimental setup with an actual photograph, and a line diagram for the turning of the Inconel 718 alloy with the cutting tool insert are shown in Figure 4. In the next section, Section 3.1, the cutting tool and the work material used in the MQL turning process are discussed.

3.1. CBN Cutting Tool Insert for Inconel Metal Alloy

In the manufacturing field, the long-term use of a cutting tool is the most significant factor in increasing the production of components, so improving the cutting tool’s life is a developing area. The Inconel 718 superalloy has a high concentration of iron, nickel, and chromium. It exhibits exceptional thermal stability and is capable of maintaining mechanical qualities at high temperatures, usually above 700 °C. Due to these characteristics, the alloy can be used for a variety of purposes, including liquid rockets, cryogenic tankages, aircraft parts, and turbine engines. But this Inconel 718 alloy is difficult to cut and has poor machinability. Therefore, a harder cutting tool is needed for machining the Inconel alloy. The surface smoothness of the workpiece is improved by continuous chip production during the turning process, but since the tool is always in touch with the chips, a crater wear zone forms on the rake face due to high thermal stress, which raises the specific cutting energy (SCE). The ratio of cutting power to the material removal rate is known as the specific cutting energy. The heat removal rate is also impacted by this continuous chip formation, as it covers the tool–workpiece interface until it breaks. Yet, forced chip breaking, which occurs frequently, creates vibrations on the workpiece and degrades the surface smoothness. So, the CBN insert cutting tool was chosen due to its great resistance to heat and corrosion, hardness, thermal stability, and chemical inertness. The CBN insert has an incorporated obstruction-type chip breaker that prevents continuous chip generation, preventing the crater wear of the tool and lowering the specific cutting energy. Cubic boron nitrate (CBN) is the toughest material for cutting tools. Because of its high levels of hardness, abrasion resistance, and chemical stability, it is employed as an excellent cutting tool material.

An obstruction-type chip breaker is provided on the top surface of the CBN insert to forcefully stop the continuous formation of chips. A nano-cutting fluid with high thermal conductivity and good lubricative properties needs to be selected to decrease vibrations due to forced chip breaking and improve heat removal from the tool and workpiece. Next, Section 3.2 discusses the selection of a nano-cutting fluid with the required properties.

3.2. Coconut-Oil-Based SiC–MWCNT Nano-Cutting Fluid

The important properties of a coolant required for lubrication are good thermal conductivity, environmental friendliness, and low cost. In MQL, the amount of cutting fluid applied to the cutting zone is significantly reduced as the cutting fluid is sprayed on the workpiece surface with the help of compressed air. The contact angle of coconut oil at room temperature is 9.06°, and it is comparatively lower than that of the other vegetable oils. Also, pure coconut oil is environmentally beneficial; it has the highest thermal conductivity at room temperature; and it does not chemically react with cubic boron nitrate (CBN) inserts or Inconel alloys.

A high-volume fraction of nanoparticles increases the viscosity and heat conductivity of an NCF, but a high-viscosity cutting fluid causes material adhesion (burr) and decreased chip evacuation capability. So, we need to go for nanoparticles with the desired dispersion capability that are eco-friendly and have high thermal conductivity. In this proposed nano-cutting fluid, SiC and MWCNTs are the two nanoparticle combinations that have been chosen because of their highly desirable thermal conductivity and ease of dispersion in the base fluid. The contact angle of coconut oil and the agglomeration phenomenon [25] are shown in Figure 5.

Silicon carbide (SiC) is a material that exhibits great thermal shock resistance and has a high thermal conductivity, minimal thermal expansion, and high hardness. Multi-walled carbon nanotubes (MWCNTs) are a distinct variety of carbon nanotubes characterized by the presence of multiple single-walled carbon nanotubes arranged concentrically. MWCNTs also have good thermal conductivity, lubricative properties, and high strength.

Table 5. Thermal properties of SiC coconut-oil-based SiC–MWCNT nano-cutting fluid.

Sr. No	Properties	2%	3%	4%	5%
1	Density (Kg/m3)	980	1040	1130	1190
2	Thermal conductivity (W/mK)	0.321	0.43	0.49	0.59

represents the properties of the SiC coconut-oil-based SiC–MWCNT nano-cutting fluid. By studying changes in the viscosity and thermal conductivity of the NCF, the temperature distribution on the tool, and the rate of heat removal, it is possible to determine the optimal concentration by varying the volume fraction of these nanoparticles from 0.5% to 5% during the examination. Figure 6a shows the ultrasonic bath for the dispersion of the nanoparticles into the base fluid, and Figure 6b shows the thermal conductivity measurement equipment. The design and temperature analyses of the CBN cutting tool and the performance of the proposed nano-cutting fluid in improving the tool’s life and heat removal are explained in the next section, Section 4.

3.3. Experiment Conditions

The turning tests were carried out on an Inconel 718 alloy rod with a 30 mm diameter and a 70 mm length with a CBN cutting tool insert. During the tests, the turning length was set at 25 mm. All the tests were carried out under the same machine conditions: cutting speed: 100 m/min; depth of cut: 0.18 mm; feed rate: 0.08 mm/rev; air pressure: 6 bars; and flow rate: 60 mL/h. Three times, each test was conducted under the same machine conditions. The Testo 872 infrared camera data acquisition system is shown in Figure 7. The present investigation used a Testo 872 brand infrared thermal camera to measure the machined workpiece’s surface temperature. The temperature measurement range of the infrared camera lies in between −20 °C and +650 °C. The infrared camera was operated at 55 to 60 cm from the tool–work contact zone. A fast image refresh rate of 9 Hz was used for real-time thermal imaging. The thermal camera maintains a high level of emissivity accuracy with the automatic recognition of emissivity and the determination of the reflected temperature (RTC). The accuracy for temperature measurements is within ±2%.

4. Result and Discussion

This section provides the design of the selected CBN cutting tool insert and the process of its implementation, along with results and a performance comparison of the proposed nano-cutting fluid method to ensure that the proposed method performs well.

4.1. Design of Cubic Boron Nitrate (CBN) Cutting Tool

Using Autodesk CFD (version), the CBN tool insert was designed for our requirements, and a simulated image of the tool with a control volume is shown in Figure 8.

Figure 8 represents the CBN insert and the inlet port through which the nano-cutting fluid in MQL is sprayed, and after removing heat from the tool, it leaves through the outlet port provided on the selected path.

4.2. Applying Mesh to the Tool Design

Figure 9 shows a mesh image taken using Autodesk CFD.

Figure 9a represents the tetrahedron element mesh applied to the CBN insert cutting tool. While progressing towards an ideal shape, a design process may necessitate tens to thousands of geometrical shape adjustments. A valid CFD mesh that complies with the revised geometry must exist for each of these modifications. The accuracy of a computational fluid dynamics (CFD) simulation is determined by the grid point collections generated by meshing techniques. Table 1, Table 2 and Table 3 present the mechanical and thermal properties of the insert, the work material, and the lubricant material used during the FEM analysis, respectively. Once the meshing was completed, the boundary criteria listed below were implemented.

The following boundary conditions were used during this analysis:

• The initial temperature of the entire model is maintained at 20 °C.
• The heat transfer coefficient for the outer boundaries of the insert that are exposed to the air is h = 20 W/m².
• Machine conditions: cutting speed: 100 m/min; depth of cut: 0.18 mm; feed rate: 0.08 mm/rev; air pressure: 6 bars; flow rate: 60 mL/h; nose radius: 0.4 mm; and 2% nano-cutting fluid.

4.3. Temperature Variation in CBN Insert

Figure 10 depicts the fluctuation in temperature in the CBN insert tool. The nozzle emits a fine mist of MQL nanofluid that settles on the rake face of the tool, creating a boundary film layer. This layer significantly improves the transmission of heat by convection. As a result, the increased nanoparticle concentration causes a reduction in the heat distribution on the rake face.

4.4. Performance Metrics of the Proposed Nano-MQL Approach

This section provides a comprehensive explanation of the performance of the suggested nano-MQL technique and the resulting results.

Figure 11 represents a comparison of the FES-predicted variation in tool temperature vs. the experimental value of tool temperature that varies with time. It can be seen from Figure 8 that the error between the FES-predicted variation in tool temperature and the experimental value of tool temperature is in the range of 1.27 to 3.44%. Initially, when machining is started, the temperature of the tool is 159 °C and then gradually decreases when the nano MQL comes into play. The tool temperature reaches 155 °C at the time of 3 s, and then it drops significantly. The enhanced thermal conductivity of the proposed nano-MQL removes heat from the tool and improves the tool’s life.

Figure 12 represents the reduction in the tool temperature according to the change in the volume fraction of nanoparticles. The tool temperature is 139 °C at a volume fraction of 2%, and when increasing the volume fraction to 5%, the tool temperature is reduced to 131 °C. With the desired lubrication and thermal conductivity of the nano-cutting fluid, the tool temperature is reduced.

Figure 13 represents the thermal conductivity of the nano-cutting fluid, which varies with temperature. The thermal conductivity of the proposed nano-cutting fluid is 0.22 W/mK at a temperature of 25 °C, and when increasing the temperature to 75 °C, the thermal conductivity increases to 0.3 W/mK. The improved thermal conductivity of the nano-cutting fluid helps in effectively increasing the heat reduction rate, thereby improving the life of the tool.

Figure 14 represents the temperature distribution on the cutting tool from the tool’s tip during the turning process. The tool tip temperature is 144 °C, and when moving away from the tool’s tip, the temperature decreases. There is a huge variation in temperature at a distance of 0.5 mm from the tool’s tip, after which it gradually decreases. The proposed nano-MQL with high thermal conductivity limits the tool temperature and prevents the tool from overheating to improve the tool’s life.

The comparison of the performance of the proposed nano-cutting fluid for the MQL approach with other existing approaches is discussed in the next section, Section 4.5.

4.5. Comparison of Proposed Method with Previous Methods

This section demonstrates the efficacy of the suggested nano-MQL method by comparing it to existing approaches and presenting their results using different metrics.

Figure 15 represents a comparison of the variation in the thermal conductivity of existing nano-MQL fluids [26] with that of the proposed nano-MQL fluid. For Al₂O₃/HOSO-4.0, at a temperature of 75 degrees Celsius, the thermal conductivity is 0.28 kW/mK. The thermal conductivity of Al₂O₃/HOSO-3.0 is found to be 0.25 kW/mK at a temperature of 75 degrees Celsius. From this comparison, it is clear that the proposed nano-cutting fluid has the highest thermal conductivity among other NCFs, and the maximum value is found to be 0.31 kW/mK at a temperature of 75 degrees Celsius.

Figure 16 compares the variation in the maximum tool temperature with different volume fractions of existing nanoparticles and the proposed nanoparticle-based cutting fluid. In this figure, a performance comparison of mineral oil, mineral oil + Al, mineral oil + Al₂O₃, mineral oil + SWCNTs, and the proposed base fluid [18] is given. The maximum temperature of mineral oil + SWCNTs is 907 K at a volume fraction of 2%, and when increasing the volume fraction to 5%, the temperature is reduced to 903 K. Meanwhile, when the volume fraction of the proposed nanoparticle combination is increased from 2% to 5%, the temperature of the cutting tool reduces from 904 to 898 K. This indicates that the proposed nano-cutting fluid greatly reduces the tool temperature when compared to other cutting fluids, and also, for increased volume fractions, the tool temperature is reduced considerably.

Figure 17 depicts the temperature gradient (measured in Kelvin per meter) of the cutting fluid at various volume fractions. The temperature gradient refers to the change in temperature per unit length of a cutting tool. It provides insight into the pace at which temperature changes over a given distance. In this comparison, mineral oil, mineral oil + Al, mineral oil + Al₂O₃, mineral oil + SWCNTs [18], and the suggested nano-cutting fluid are taken. The temperature gradient when the mineral oil + SWCNT combination is used is 2.75 × 106 K/m at a volume fraction of 2%, and when increasing the volume fraction to 5%, the temperature gradient is reduced to 2.74 × 106 K/m. Likewise, when the volume fraction of the proposed nanoparticle combination is increased from 2% to 5%, the temperature gradient reduces from 2.72 × 106 K/m to 2.67 × 106 K/m. The proposed nano-MQL approach has a reduced temperature gradient due to its high thermal conductivity and efficient heat elimination capacity.

Figure 18 shows the variation in the heat transfer coefficient between the nano-cutting fluid and the tool while increasing the volume fraction. Here, the convective heat transfer coefficient for the recommended nano-cutting fluid is compared with that of other existing fluids such as mineral oil, mineral oil + Al, mineral oil + Al₂O₃, and mineral oil + SWCNTs [18]. The convective heat transfer coefficient of mineral oil + SWCNTs is 1.67 × 104 W/m²K at a volume fraction of 2%, and when increasing the volume fraction to 5%, the convective heat transfer coefficient is increased to 1.75 × 104 W/m²K. When the volume fraction of the proposed nanoparticle combination is increased from 2% to 5%, the convective heat transfer coefficient increases from 1.7 × 104 W/m²K to 1.79 × 104 W/m²K. The proposed nano-MQL approach increased the convective heat transfer coefficient due to its enhanced thermal properties and efficient heat removal capacity.

Overall, the proposed method has a good result compared to existing methods, and the friction is reduced because of the better lubricative properties of the nano-cutting fluid. The tool temperature is reduced because of the high thermal conductivity (0.31 kW/mK) and the improved heat removal rate of the proposed NCF. The temperature gradient is found to be 2.67 × 106 K/m when varying the volume fraction to 5%, and it is lesser when using existing nano-cutting fluids. The coconut-oil-based SiC and MWCNT nano-cutting fluid at a 5% volume fraction reduces the tool tip’s temperature to 898 K. As a result, the tool’s life is increased, and better heat removal with the desired lubrication is achieved using the proposed nano-MQL cutting fluid.

5. Conclusions

This paper presents a temperature analysis of a CBN cutting tool during MQL turning with a coconut-oil-based nano-cutting fluid using CFD.

From this study, the following conclusions can be made:

• The comparison between the FES-predicted variation in tool temperature and the corresponding experimental values reveals a remarkable alignment, with a marginal error ranging from 1.27% to 3.44%.
• We found a significant reduction in tool temperature with an increase in the volume fraction of nanoparticles within the cutting fluid.
• The thermal conductivity study of the proposed nano-cutting fluid revealed that an increase in thermal conductivity is a key factor in enhancing the heat dissipation capabilities of the nano-cutting fluid.

Minimum quantity lubrication (MQL) is used to minimize the usage of cutting fluid, and when nanoparticles are added to the base fluid in MQL, it further improves the tool life, machining process, and heat removal capacity. The continuous chip production during the turning of harder materials, such as Inconel 718 alloys, improves the surface finish, but it also leads to crater wear on the rake face of the tool, affecting the tool’s life. A CBN insert cutting tool with an obstruction-type chip breaker is used to avoid continuous chip formation from the workpiece, so the tool’s life is improved. In the proposed nano-cutting fluid, coconut oil is used as a base fluid, and SiC–MWCNT nanoparticles are added to the base fluid with varying volume fractions. Overall, the proposed nano-MQL fluid has a better thermal conductivity of about 0.31 kW/mK and an improved heat removal capacity, with a high convective heat transfer coefficient of 1.79 × 104 W/m²K at a 5% volume fraction, compared to other existing nano-cutting fluids used in various MQL approaches.