**1. Introduction**

The inner circular surface machining quality of the components, such as engine cylinder, shaft sleeve, hydraulic cylinder and connecting rod, is one of the key factors that significantly affects their service performance. The inner circular surface, represented by two types named through-hole and blind-hole, are mainly machined by turning, drilling or boring processes. The internal turning is widely used to machine the inner circular surface. However, chatter vibration may happen due to the large overhang of the toolholder when turning the inner circular surface, which will affect the machining accuracy. Many methods aiming to restrain this vibration have been developed to increase the inner circular surface quality [1,2]. In addition, the high cutting temperature has a grea<sup>t</sup> influence on the machining quality [3]. It is well known that the heat generated in the turning process is mainly transmitted by chips, turning tools and workpieces [4,5]. However, the turning tool operates inside the workpiece when turning the circular surface, and the chips are difficult to evacuate. This causes an extremely high cutting temperature inside the workpiece and accelerates tool wear, and consequently deteriorate the inner circular surface quality. Thus, designing cutting tools, reducing cutting friction and controlling the cutting temperature during the machining process are important when facing the semi-closed heat dissipation space and low thermal conductivity of the workpieces.

Currently, the flood cooling is widely used in the process of cutting temperature reduction. However, this method is known as having low coolant utilization, high pollution levels and processing cost of waste coolant [6,7]. To improve the cooling efficiency and achieve green and environmentally friendly cutting, various cooling methods have been developed and utilized in the cutting process, such as the cryogenic cooling [8], heat pipe cooling [9], closed internal cooling [10], minimum quantity lubrication (MQL) [11], and cryogenic minimum quantity lubrication (CMQL) [12]. Therein, the MQL, or so-called spray cooling can avoid the utilization of complex and expensive equipment of cryogenic

**Citation:** Liu, L.; Shu, S.; Li, H.; Chen, X. Design, Optimization and Cutting Performance Evaluation of an Internal Spray Cooling Turning Tool. *Coatings* **2022**, *12*, 1141. https://doi.org/10.3390/ coatings12081141

Academic Editor: Diego Martinez-Martinez

Received: 14 July 2022 Accepted: 3 August 2022 Published: 8 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

cooling, and improve the insufficient cooling and lubrication capacity. Li et al. [13] carried out the sliding tests between YG8 cemented carbide and austempered ductile iron under dry, cold air and the MQL conditions by using a tribometer. Results show that the tool wear rate was the lowest under the MQL cooling condition. Das et al. [14] evaluated the machining performance of the austenitic stainless steel under the dry cutting, compressed air, flood and MQL conditions. They found that the tool life under the MQL conditions was improved compared to other conditions, and the chip separation speeds were also improved.

The external spray cooling can be applied on conventional tools when turning externally, therein having no specific limitation on the cutting space. However, the internal spray cooling can be more efficient and effective in drilling and internal turning. Zeilmann et al. [15] carried out the drilling experiment and found that compared to the external spray cooling, the internal spray cooling significantly reduced the cutting temperature. Jessy et al. [16] tested the cutting performance of the GERP composite material under external and internal cooling conditions. Results show that the internal cooling method reduces the average temperature by 66% compared to the external cooling method. Li et al. [17] reviewed the drilling machinability of CGI under the dry cutting, compressed air and MQL conditions. They summarized that compared to the dry cutting condition, the tool life could be greatly improved under the compressed air and MQL (5 mL/h) conditions.

A well-designed internal cooling cutting tool can precisely send the coolant into the cutting zone to improve the cooling and lubrication efficiency [18–20]. Obikawa et al. [21] conducted CFD simulations on the internal cooling tools with three types of nozzle structures. They found that the cover-type nozzle provided the best performance. Duchosal et al. [22] conducted the optimization on the milling tools and found that the spray angle had significant influence on the distribution of the oil mist. The spray angle of 75◦ provided the best cooling performance. Zhang et al. [23] compared the milling performance with H13 steel using three types of flow channels in the internal cooling milling tools. Results show that double straight channel was most beneficial to the tool life improvement. Peng et al. [24] designed and optimized an internal cooling external turning tool with microchannel structures. The cutting experiments reveal that compared to the conventional flood cooling and external cooling turning tools, the internal cooling technology led to lower cutting temperature, and improved the surface topography as well.

The aforementioned studies mainly focused on the cooling method design for the external turning tools, drilling tools and milling tools, rather than on the development of the environmentally friendly cooling structure for the internal turning tool. Thus, in this work, an internal spray cooling turning tool structure was proposed based on the requirements on the internal surface turning of the workpiece. The Taguchi orthogonal design method based on CFD simulations were conducted to optimize the cooling structure parameters of internal spray cooling turning tool, and the turning tool was prepared according to the optimization results. A cutting test was carried out on the inner cylinder of the QT500-7 workpiece. The influence of internal spray cooling turning tool parameters on the cutting temperature, workpiece surface roughness and chip morphology were investigated.

#### **2. Cooling Structure Design of the Internal Spray Cooling Turning Tool**

Significantly high cutting temperature is often observed on the internal surface of the hollow cylindrical workpiece during the turning process due to the semi-closed space and poor material thermal conductivity, and thus deteriorates the workpiece surface quality and tool life. Compared to the tools with a single nozzle designed on the front or flank face, the internal cooling tools with two spray cooling nozzles can significantly improve the cooling efficiency and tool wear performance [25].

Figure 1 shows the structure of the designed internal spray cooling turning tool. The S25R-MCLNR12 internal turning tool was selected as the prototype with a toolholder diameter of 25 mm. The green part shown in Figure 1 represents the flow channel of the

coolant flowing through the tool, the main coolant flow channel is designed in the center of the toolholder, and the cooling nozzles are designed on the rake and flank faces of the tool, respectively. The coolant inlet is connected with the external spraying cooling equipment, which designed at the end of the toolholder. During the turning process, the compressed air carrying a certain amount of coolant flows into the tool via the inlet, passes through the internal cooling channel and then sprays out from the nozzles to lubricate the tool-chip and tool-workpiece interfaces. In addition, the dynamic pressure generated by the compressed air are expected to contribute to the chip removal.

**Figure 1.** The structure schematic of internal spray cooling turning tool.

In the turning process with spray cooling, the number of nozzles, diameter of the nozzles, distances between nozzles and cutting zone are the key parameters that significantly affecting the cutting temperature variation [26,27]. In Section 3, a numerical simulation model of the internal turning was established by using the ANSYS Fluent to study the influence of the nozzle diameter, the distance between the nozzle and tool tip on the cutting performance. Then the parameters of the tool cooling structure were optimized by Taguchi method based on the CFD simulations.

#### **3. Structure Optimization of Internal Spray Cooling Turning Tool**

#### *3.1. Establishment of Simulation Model*

The inner surface diameter and length of the workpiece were set as 45 mm and 200 mm, respectively; the outer diameter of the workpiece was set as 70 mm to improve the simulation efficiency. The fluid domain was established inside the tool and workpiece considering the flow channel of the compressed air and coolant droplets. Figure 2 shows the geometrical model of the internal spray cooling turning tool. The small square surface (L1 × L2) on the tool rake face represents the tool-chip interface, which was set as 1.0 mm × 0.5 mm in all simulations. The heat flux was set as 40 W/mm<sup>2</sup> and was applied on this square surface to simulate the cutting heat transfer.

**Figure 2.** The geometric model of numerical simulation for the internal spray cooling turning tool.

Since the volume fraction of the coolant droplets in the fluid domain is much smaller than air, the discrete phase model (DPM) was chosen to simulate the flow behaviour of the coolant droplets. Considering the evaporation and boiling of the droplets after being sprayed into the cutting zone, the species transport model is essential to be activated.

Because of the entrainment occurring during the spraying and the disturbance due to the high-speed fluid impingement on the irregular surfaces of tool and workpiece, the realizable k-ε turbulence model, highlighted by its efficient prediction on the circular jet and plane jet, was applied to simulate the turbulent flow of fluid [28]. The wall-film model was utilized to simulate the liquid droplets colliding with the surfaces of the tools and workpiece, thus forming thin films, splashing, boiling and vaporization.

As a green cooling method, the amount of coolant used in the spray cooling process is usually within 50–500 mL/h [29]. Here, water was used as the coolant in the spray cooling, and the flow rate of coolant was 50 mL/h. The initial temperature of the cutting was 20 ◦C in the simulations. Table 1 lists the input parameters of the thermal-fluid-solid coupling simulations for the internal spray cooling turning tool.

**Table 1.** Simulation parameters.


To improve the simulation efficiency and ensuring the accuracy simultaneously, the grids were meshed densely near the cutting tip and flow channel wall, while were meshed coarsely in other regions. A mesh independence study was carried out based on varying meshed grid sizes of different parts in each design scheme of the Taguchi method. Meanwhile, the CFD simulation was conducted with varying mesh grid sizes. Figure 3 shows that the effect of the grid number on the cutting temperature is insignificant within the range of 2.56–3.2 million. Figure 4 shows the meshed simulation model of the fluid-solid coupling heat transfer during the internal turning with internal spray cooling.

**Figure 3.** Mesh independence calculating result.

**Figure 4.** The mesh of simulation model of fluid-solid coupling heat transfer.

#### *3.2. Optimization of Tool Cooling Structure Parameters*

The Taguchi orthogonal design method based on the CFD simulations was used to optimize the parameters of the tool cooling structure. Figure 5 shows the structure parameters that to be optimized. The lower nozzle-tip distance (LND) was 8.5 mm in this design. The upper nozzle-tool tip distance (UND), the upper nozzle diameter (UD) and the lower nozzle diameter (LD) were the variables that to be optimized. Note that the influence of the distance from the lower nozzle to tool tip on the cutting temperature was not considered due to the space limitation of the toolholder lower end.

Considering the influence of the cooling structure layout on the tool rigidity and machinability, each design parameter had three levels as listed in Table 2. The maximum temperature of the tool-chip interface was taken as the output value of the Taguchi test.


**Table 2.** List of the geometric design parameters and their levels.

#### *3.3. Simulation Results Analysis*

For each design scheme in the orthogonal table L9(34), the maximum temperature of the tool was obtained through the simulations, and the range analysis of the orthogonal test was carried out as shown in Table 3. Therein, the range value of the effects of the upper nozzle diameter, upper nozzle-tool tip distance and lower nozzle diameter on the maximum temperature are 1.61 ◦C, 1.73 ◦C and 5.17 ◦C, respectively, indicating that compared to the upper nozzle diameter and upper nozzle-tool tip distance, the lower nozzle diameter has greater influence on the cooling performance of the internal spray cooling turning tool.


**Table 3.** Simulation results and range analysis of L9(34) orthogonal test.

Figure 6 shows the effect of each parameter on the mean of the maximum temperature. The maximum temperature of the tool-chip contact area increases with the increasing UND (A) from 532.55 ◦C to 534.28 ◦C, because the increasing distance between the upper nozzle and tool tip results in the decrease in the coolant delivery to the tip, and thus increases the coolant evaporation efficiency. Therefore, less air and liquid droplets flows into the cutting area, resulting in less convective heat transfer. The temperature of the tool-chip contact area decreases with the increasing UD (B) from 533.97 ◦C to 532.36 ◦C. Larger upper nozzle diameter leads to larger flow rate of coolant droplets when the spray pressure is constant, thus more droplets flow into the cutting zone and more heat is transferred. With increasing LD (C), the maximum temperature decreases to 530.09 ◦C till 1.5 mm and obtains the best cooling performance. Then it increases to 534.44 ◦C at 2.0 mm. This is mainly because the lower nozzle diameter affects the amount of the droplets ejected from both the lower and upper nozzles.

**Figure 6.** The diagram of main effects of the three variables on maximum temperature. (**a**) Influence of UND to maximum temperature, (**b**) Influence of UD to maximum temperature, (**c**) Influence of LD to maximum temperature.

According to the range analysis results, the optimal combination of the structure parameters is A1, B3, and C2, namely the UND (A) is 18.5 mm, the UD (B) is 3 mm, and the LD (C) is 1.5 mm. The numerical study was conducted for the optimized internal spray cooling turning tool under the same boundary condition. Figure 7a shows the temperature distributions of the tool with the optimal structure parameters combination. The maximum temperature is 528.48 ◦C at the tool tip, which is lower than the minimum temperature listed in Table 1. This demonstrates that the cooling performance of the tool with optimized structure parameters combination was improved. Figure 7b illustrates the fluid pathline of

the internal spray cooling process. The compressed air is sent through the inner channel of the tool and then is sprayed out rapidly from the upper and lower nozzles. The air is sprayed on the tool tip and then diffused rapidly.

**Figure 7.** Temperature contour and fluid pathline of the optimized tool. (**a**) Temperature contour of the optimized tool; (**b**) Fuid pathline of the internal spray cooling process.

#### *3.4. Influence of Inlet Pressure on Cooling Performance*

Inlet pressure is an important parameter that directly affecting the spray cooling performance [22]. The maximum spray pressure provided by the spray cooling equipment was 0.6 MPa, thus the spray pressure was selected as 0.05 MPa, 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa and 0.6 MPa to investigate the influence of the spray pressure on the cutting temperature of the optimized tool.

Figure 8 shows the coolant droplets velocity under different inlet pressures ranging from 0.1 MPa to 0.6 MPa. Overall, with increasing tool inlet pressure from 0.1 MPa to 0.6 MPa, the droplet velocity increases from 367.7 m/s to 861.6 m/s, and the droplet distribution range changes from the local area near the tip to the entire internal surface of the workpiece, which results in more rapid heat transfer.

**Figure 8.** Droplets velocity under different inlet pressures, (**a**) 0.1 MPa, (**b**) 0.2 MPa, (**c**) 0.3 MPa, (**d**) 0.4 MPa, (**e**) 0.5 MPa, (**f**) 0.6 MPa.

Figure 9 shows the influence of tool inlet pressure on the maximum temperature of the tool-chip interface. The maximum temperature first decreases rapidly but then mildly with the increase in spray pressure. This indicates that the spray cooling effectively decreases the cutting temperature even with low inlet pressure such as 0.05 MPa. The high-speed airflow under the spray condition can rapidly facilitate the convective heat transfer, so when the tool inlet pressure is in the low region (<0.1 MPa), the cutting temperature decreasing trend is more significant. Therein, the temperature at 0.05 MPa is approximately 60 ◦C lower than that of the dry condition. When the inlet pressure is larger than 0.3 MPa, the temperature decreasing tends to be insignificant, which is mainly due to the gradual saturation of the convective heat transfer.

**Figure 9.** Maximum temperatures of tool under different inlet pressures.

#### **4. Tool Preparation and Cutting Experiments**
