1. Introduction
With the rapid development of the aerospace industry, many high-temperature alloys with superior performance are gradually becoming necessary materials for processing parts. Nickel-based high-temperature alloys have good structural stability, strong oxidation and corrosion resistance, and low thermal conductivity due to the rich variety of internal elements in their structure. These properties make it adapt high-temperatures, corrosion, oxidation, and other conditions in aerospace operations [
1]. However, nickel-based high-temperature alloys with excellent performance often face high processing temperatures that are not easily diffused, severe tool wear, and high residual stresses in the workpiece during processing, which in turn affect the fatigue life of the parts [
2]. Many studies show that residual tensile stress can lead to the formation of tiny cracks on the surface or inside the parts, causing unpredictable deformation and misalignment, thereby reducing the fatigue limit and fatigue life of the workpiece, decreasing part accuracy, and increasing the crack expansion rate. In contrast, residual compressive stress exerts the opposite effect, in appropriate amounts, it can inhibit crack initiation, enhance fatigue resistance, reduce or prevent surface defects such as slip bands and deformation, and improve surface finish and flatness, leading to an improvement in the dimensional and geometric accuracy of components [
3]. In real production, reducing residual tensile stress helps to decrease cutting heat, allowing mechanical stresses to play a primary role, enhancing the effect of residual compressive stresses, and improving the material’s fatigue limit and machining accuracy. Consequently, studying the variation in residual stress through simulation and experiments is advantageous for enhancing the service life, accuracy, and excellent surface quality of machined parts [
4].
In the study of residual stress, it is found that residual stress is mainly generated by plastic deformation caused by mechanical stress and thermal stress, as well as volume changes caused by phase transformation [
5]. During the cutting process, a large amount of cutting heat is generated, leading to an increase in temperature in the cutting zone. This results in thermal softening expansion of the cutting layer metal and phase transformation of the metallographic structure, leading to the generation of residual stress. Therefore, the variation in residual stress is usually closely related to cutting heat and cutting forces during the cutting process. Researchers at home and abroad have explored the impact of changes in cutting forces and cutting heat on residual stress by applying cutting fluids to alter the cutting environment. In recent years, the concept of green cutting has gradually gained popularity. Green cutting not only achieves lubrication and cooling effects but also reduces harm to the environment and workers. High-pressure cooling, spray cooling, and cold air cooling are among the most common methods. Researchers have found that these cooling methods promote the reduction of residual tensile stress on the workpiece surface.
Among these cooling methods, high-pressure cooling is the most studied. Li et al. [
6] conducted experiments on cutting GH4169 (similar to Inconel 718) with PCBN tools under high-pressure cooling conditions. The study found that the influence of high-pressure cooling fluid on the surface roughness of the machined workpiece was relatively small, which was more helpful in reducing residual surface stress. Another study by Li et al. [
7] indicated that under high-pressure cooling conditions, the maximum residual compressive stress strain depth in the cutting depth direction and the maximum residual tensile and compressive stress on the machined surface are both smaller than those in dry cutting. Oliveira et al. [
8] found in the research on milling Inconel 718 that residual stress is closely related to variables such as tool wear, cutting force, and cutting heat. In the study of turning GH4169 under high-pressure cooling conditions, Wu et al. [
9] utilized Deform-3D to construct a thermal–mechanical coupled finite element model for high-pressure cooling machining of GH4169, analyzing the turning temperature and surface residual stress. The results indicate that with an increase in cooling pressure, the residual tensile stress on the machined surface decreases.
There are some studies on air cooling and MQL (minimal quantity lubrication). Ji et al. [
10] found that a larger maximum residual compressive stress was obtained at a low flow rate, small cutting depth, and appropriate gas–oil mixture ratio during the cutting of AISI 413 under air cooling + MQL conditions. Khaliq et al. [
11] investigated the lubrication and cooling effects of a tungsten carbide end milling cutter for the micro-milling of Ti-6Al-4V under the condition of micro-lubrication at a low temperature. It was found that there was no significant difference in residual stress values between dry cutting and low-temperature micro-lubrication during micro-milling due to the low cutting temperature. Muhammad et al. [
12] utilized dry ice jet cooling during the milling of AISI52100 tool steel and compared the cutting performance with that under micro-lubrication. The results indicated that the residual compressive stress values with dry ice cooling increased by 3% and 8% at speeds of 75 m/min and 300 m/min, respectively, compared to MQL.
There are also some related studies on spray cooling. Yao et al. [
13] found that increasing the air pressure could reduce the diameter of the spray droplets, enhancing their penetration ability and enabling easier reduction in the tool’s temperature. Under the condition of spray cooling, Ramanuj et al. [
14] employed coated carbide cutting inserts to machine AISI D2 steel. It was discovered that the spray cooling process effectively absorbed a substantial amount of cutting heat during the evaporation phase, resulting in a reduction in cutting temperature. Consequently, this led to a decrease in tool wear and the attainment of superior surface quality. Ukamanal et al. [
15] conducted an orthogonal experimental study on the influence of spray cooling process parameters on the machining performance of AISI 316 steel. The study revealed that compared to dry turning, spray cooling resulted in better surface finish and reduced tool wear. Furthermore, the most effective spray pressure was identified.
In the study on the influence of GH4169 material properties, Li et al. [
16] analyzed the effects of cutting temperature and load under the mechanical–thermal coupling on the residual surface stress, the plastic deformation degree, and the high-temperature fatigue performance of GH4169 specimens. Yu et al. [
17] studied the microstructural evolution during high-speed cutting of the high-temperature alloy GH4169 and found that the dynamic recrystallization (DRX) grain size and volume fraction increased with the rise in cutting temperature, which was attributed to the promotion of grain growth by the elevated temperature.
In the research process of residual stress, due to the numerous factors influencing residual stress and the cumbersome measurement process, researchers often employ various methods such as establishing predictive models, finite element simulations, and so on to investigate changes in residual stress.
Zhou et al. [
18] employed a stress relaxation analysis algorithm combined with a finite element model to acquire residual stress distribution data. Additionally, an enhanced BP neural network agent model was utilized for the quick estimation of residual stress. The effectiveness of the hybrid model was ultimately confirmed through cutting experiments conducted on H13 steel. In order to investigate the distribution of surface residual stresses along the feed direction during turning, Weng et al. [
19] employed a three-dimensional numerical model based on the Coupled Eulerian–Lagrangian (CEL) method to accurately predict the evolution of residual stresses during multiple consecutive cuts in turning operations. The effectiveness and accuracy of the proposed model were validated through a strong agreement between simulation results and experimental measurements. Ullah et al. [
20] proposed a numerical and experimental approach to comprehensively predict the residual stresses in milled sections of Ti-6Al-4V alloy. Upon establishing the model, excellent correlation was achieved between simulation and experimental results for given milling conditions. Finally, the impact of the white layer on residual stress distribution was investigated. This analytical method provides a thorough understanding of residual stress distribution within milled components.
Li et al. [
21] utilized multiple regression analysis to develop a residual stress prediction model for rough turning Ti-6Al-4V. This model takes into account not only cutting parameters but also cutting force and cutting temperature. Through model analysis, it is evident that the friction coefficient and tool edge radius influence the thickness of the residual stress layer. While cutting speed has minimal impact on the thickness of the residual stress layer, an increase in cutting speed can induce a shift from residual stress to tensile stress, eventually leading to residual stress approaching zero at a specific depth.
Although residual stresses can be obtained through predictive modeling and experimentation, the process often demands significant computational time and material wastage. Utilizing finite element simulation not only allows for precise analysis of the workpiece material’s plastic deformation but also enables visualization of the material’s evolution during the process, leading to more accurate simulation results. For instance, Alok et al. [
22] used a finite element model to simulate the effect of cutting speed and cutting depth on residual stresses when turning Ti-6Al-4V. The accuracy of the model was verified by comparing it with experimental values. Luo et al. [
23] employed the Third Wave AdvantEdge finite element analysis software to conduct simulation experiments on turning 7075-T651 aluminum alloy. The aim was to investigate the influence of cutting parameters, such as cutting depth and feed rate, on cutting force, residual stress, and cutting temperature. The results obtained from the simulations align well with the experimental values. Min et al. [
24] summarized the methods for calculating residual stress from different contact conditions based on temperature, elastic stress, and plastic stress, and analyzed the advantages and disadvantages of analytical methods and finite element methods.
Based on the research conducted by the aforementioned scholars, it is understood that residual stress is closely associated with various factors including turning parameters, tool specifications, workpiece materials, and cooling conditions. The cooling environment directly influences the thermal effects during the cutting process, consequently mitigating residual tensile stresses, fatigue life, cracks, and precision-related phenomena. However, the influence of spray cooling parameters on residual stress changes in GH4169 has received limited attention from researchers. Hence, this paper employs finite element simulation to investigate the variation patterns of residual stress in GH4169 workpieces under different spray parameters (such as pressure and flow rate). This research aims to provide insights for subsequent studies on workpiece surface integrity.
3. Finite Element Simulation Results Analysis
The residual stress on the workpiece surface after the turning process is the result of four stages: turning, unloading, constraint transformation, and cooling. Simulation investigates the effects of pressure and flow rate of spray cooling using a single-factor experimental design, with the accuracy of this model validated through experimentation. The process of residual stress change in turning machining is shown in
Figure 5. The software simplifies the cutting section of the workpiece into a rectangle, where the horizontal axes represent the position of the tool in the cutting direction and the vertical axes denote the cutting depth. The color scale indicates the magnitude of residual stresses, with positive values representing residual tensile stresses and negative values indicating compressive stresses. On the surface of the machined workpiece, a transition from red-colored residual tensile stresses at the surface gradually decreasing towards the interior of the workpiece and becoming blue-colored residual compressive stresses can be observed, eventually reaching a pale blue color indicating a stress-free state.
3.1. Effect of Spray Cooling Pressure on Residual Stresses
When the cutting speed is 100 m/min, feed rate is 0.2 mm/r, and cutting depth is 0.25 mm, the variation trend of residual stress in the cutting depth direction of the workpiece with depth by two-dimensional simulation is shown in
Figure 6. From the figure, it can be observed that the workpiece surface exhibits residual tensile stress. In dry cutting, the residual tensile stress reaches 1208.5 MPa. During spray cooling conditions, the residual tensile stress values at spray pressures of 0.1 MPa and 0.2 MPa are 1115.51 MPa and 1027.05 MPa, respectively. The minimum residual tensile stress value of 933.93 MPa is achieved at a spray pressure of 0.4 MPa, resulting in a relative reduction of 22.72% compared to dry cutting. The residual tensile stress decreases as the spray pressure increases from 0.1 MPa to 0.4 MPa. Compared to dry cutting, spray cooling offers significant advantages in cooling and protecting the workpiece surface. Initially increasing the spray cooling pressure allows the cutting fluid to enter the tool–chip contact area, forming an oil film on the surface that effectively reduces the friction coefficient between the tool and chip, thereby decreasing the friction and cutting heat. Additionally, increasing the pressure can enhance the convective heat transfer coefficient. Since air is the carrier of the cooling pressure, the cooling efficiency primarily depends on the specific heat of the gas. Coolants with higher specific heat values can absorb more heat from the workpiece and cutting tool. Air has a relatively high specific heat, so increasing pressure can expedite cooling, leading to a reduction in the cutting temperature of the machined surface. These factors contribute to a gradual decrease in residual tensile stress with increasing spray pressure between 0.1 MPa and 0.4 MPa.
From
Figure 6, it is evident that the change in residual compressive stress between pressures of 0.1 MPa and 0.4 MPa shows a decreasing trend as pressure increases. This phenomenon can be elucidated as follows: Firstly, during the turning process, the poor thermal conductivity of GH4169 results in significant heat generation at the tool nose, leading to thermal softening on the workpiece surface and subsequently reducing the cutting force. Secondly, the orientation and angle of the spray jet directed towards the rake face facilitate the fracture of serrated chips during turning, thereby reducing cutting forces and mechanical stresses. Consequently, an increase in pressure results in a decrease in residual compressive stresses. The fraction of the cutting fluid on the chip at various moments during spray cooling is depicted in
Figure 7. The horizontal axis represents the position of the tool in the cutting direction, while the vertical axis corresponds to the depth of cut in the axial direction. When observing changes in cutting depth, it is noted that the thermal effect diminishes under a pressure of 0.4 MPa. The depth at which the transition from residual tensile stress to residual compressive stress occurs is advanced to approximately 20 μm, with the maximum residual compressive stress advancing by 10 μm compared to pressures of 0.2 MPa and 0.1 MPa, reaching around 30 μm.
3.2. Effect of Flow Rate of Spray Cooling on Residual Stresses
The variation in residual stress with the flow rate of spray cooling is depicted in
Figure 8. As the spray cooling flow rate increases within the range of 2 L/h to 3.5 L/h, the residual tensile stress value demonstrates a trend of initially decreasing and then increasing. The maximum residual tensile stress values at spray flow rates of 2 L/h, 3 L/h, and 3.5 L/h are 1023.95 MPa, 928.98 MPa, and 1099.15 MPa, respectively. At a flow rate of 3 L/h, the minimum residual tensile stress is achieved, with a relative reduction of 23.13% compared to dry cutting.
Unlike increasing the air pressure, increasing the flow rate not only enhances the cooling effect but also impacts the tool wear rate. An increase in the flow rate enhances the lubrication effect at the cutting point, consequently reducing the tool wear rate. Simultaneously, it diminishes the heat generated during cutting due to wear, thereby reducing the residual stresses on the workpiece’s cutting surface. Moreover, decreasing tool wear hampers the escalation of cutting forces and enhances the quality of the machined surface, ensuring that residual stresses remain minimal and evenly distributed.
Through simulation, it has been observed that an increase in flow rate leads to a decreasing trend in residual tensile stress within the range of 2 L/h to 3 L/h. As illustrated in
Figure 9, during the cutting process, the second deformation zone generates a region with elevated temperature. The cutting fluid sprayed on top of this region rapidly vaporizes and volatilizes under the high temperature, thereby hindering its lubricating function. By augmenting the spray flow rate, a substantial amount of cutting fluid flows into the tool–workpiece contact area, carrying away the heat generated during cutting. Furthermore, excess cutting fluid infiltrates the tool–chip contact area to fully exploit its lubricating effect. The cutting fluid serves to enhance the frictional conditions, reducing both the coefficient of friction and frictional force. The reduction in frictional force subsequently helps to constrain the cutting heat, thereby influencing the tool wear rate. Additionally, by mitigating the cutting heat, it also impacts the tool wear rate.
Residual stress is the result of mechanical–thermal coupling, and the simulation results from spray pressure and flow rate indicate that spray cooling can enhance cutting conditions, optimizing residual stress. Compared to dry turning, at an optimal spray pressure of 0.4 MPa, the residual tensile stress decreased by 22.72%, and the residual compressive stress increased by 26.54%. When the optimal spray flow rate was 3 L/h, the residual tensile stress decreased by 23.13%, while the residual compressive stress increased by 30.84%.
The spray cooling process involves the mixture of gas and cutting fluid to form a mist for lubrication and cooling purposes. As the proportion of cutting fluid continuously increases and reaches an optimal state with the gas, there is a decreasing trend in residual tensile stress within the range of 2 L/h to 3 L/h. Increasing the flow rate of cutting fluid disrupts the effective lubrication in the cutting zone, leading to a decrease in lubrication effectiveness. Therefore, a significant increase in residual tensile stress is observed when the flow rate increases from 3 L/h to 3.5 L/h. And the overcoming of frictional forces between the tool and chips is one of the sources of cutting forces. Changes in frictional forces result in a decrease in cutting forces, leading to a reduction in residual compressive stresses in the workpiece due to the decrease in mechanical stress and thermal stress. As shown in
Figure 8, the maximum residual compressive stress values are all less than those during dry cutting, and they decrease with an increase in flow rate.
By comparing the residual stress variation curves at different spray flow rates, the maximum residual compressive stress at a cutting fluid flow rate of 3.5 L/h is 267.81 MPa. Compared to 2 L/h and 3 L/h, the position of the maximum residual compressive stress increases, which is located near the workpiece surface at around 50 μm. The main reason for this shift is the decrease in effective lubrication, which leads to an increased influence range of cutting heat, thereby altering the position of the maximum residual compressive stress.
3.3. Analysis of the Effect of Spray Cooling
The spray cooling process involves the mixture of liquid and gas, which is then sprayed onto the surface of the turning process through high-pressure atomization. Spray cooling primarily serves to cool and lubricate, as the small oil droplets sprayed from the nozzle reduce the cutting temperature in the cutting zone, on the tool, and on the workpiece, thereby improving the residual stress effect.
From the Leidenfrost phenomenon [
30], it can be known that during metal cutting, the high temperatures generated in the cutting zone create a high-pressure insulating vapor layer around it. When cutting fluid is applied, a portion of it evaporates and is lost due to the lower boiling point of the cutting fluid. The cutting fluid entering the cutting zone is also impeded in heat transfer by the vapor layer, significantly reducing the effectiveness of the cutting fluid.
Spray cooling, supported by gas pressure, can break through the constraints of the vapor layer. As shown in
Figure 10, with enhanced penetration capability, the vapor layer can be penetrated by the lubricant, thereby increasing the convective heat transfer coefficient and heat transfer on the cutting surface. In addition to improving lubrication, spray cooling also enhances the friction coefficient in the tool–chip contact area. The tool–chip contact area is not completely fitted together but can be seen as a capillary phenomenon with small gaps. By applying pressure, the cutting fluid can infiltrate the tool–chip contact area, providing boundary lubrication. Due to the adsorption properties of the boundary lubrication film, it can adhere to the surface of the tool–chip contact, while the lubricating oil film reduces friction by decreasing the cutting heat. In addition to entering the tool–chip contact area, a portion of the spray will also be sprayed onto the contact area between the flank face and the workpiece to reduce the wear and friction on the flank face. Spray and pressure not only improve lubrication but also promote convective heat transfer, lowering the temperature in the cutting zone.
The generation of residual stresses can be considered as a result of mechanical–thermal coupling, caused by the plastic deformation of mechanical stress and thermal stress. Due to frictional forces and plastic deformation, a significant amount of cutting heat is generated in the secondary deformation zone. The use of spray increases the heat transfer rate and improves lubrication and friction, thereby reducing the cutting heat in the secondary deformation zone of the tool–chip contact. This has a promoting effect on reducing residual tensile stresses and can enhance the fatigue life of the workpiece.