Effects of TiAlN Coating Thickness on Machined Surface Roughness, Surface Residual Stresses, and Fatigue Life in Turning Inconel 718

Abstract

The surface roughness and surface residual stresses are affected by the coating thickness of physical vapor deposition (PVD) TiAlN tools, which could alter the service performance of machined components. However, the relationships among the tool coating thickness, the machined surface roughness, the surface residual stress, and the fatigue life are still not sufficiently illustrated. In this research, PVD TiAlN coatings with several thicknesses of 1.6 μm, 2 μm, 2.5 μm, and 3 μm were deposited on carbide tools. Turning experiments using Inconel 718 were conducted with uncoated and various TiAlN tools under flood cooling conditions. The surface roughness could be improved with the selection of thin PVD TiAlN coating thicknesses of 1.6 μm and 2 μm compared to that of uncoated tools. The tensile residual stresses at the machined surface in the directions of cutting speed and feed rate linearly decreased by 148.67% and exponentially decreased by 92.24% when the TiAlN coating thickness was increased from 0 μm to 3 μm. The values of the low-cycle fatigue life of machined Inconel 718 linearly increased by 15.60% with the increase in TiAlN coating thickness from 0 μm to 3 μm, which was mainly due to the improvement of surface residual stresses. The results could provide guidance for the selection of suitable TiAlN coating thicknesses for wet machining Inconel 718 based on the part service conditions.

1. Introduction

Inconel 718 is widely utilized in many applications, including the aerospace and nuclear industries, due to its superior properties such as its high resistance to corrosion, shock, fatigue, and creep at elevated working temperatures over 650 °C [1,2,3]. However, it is classified as a difficult-to-cut material due to its extreme hardness that leads to high cutting forces and temperature, which are further worsened by its strain-hardening effect [4]. The service performance of a machined component of Inconel 718 is strongly dependent on its surface conditions [5,6]. The surface roughness and surface residual stresses are two important factors of the machined surface quality. The reliability and fatigue life of a component of Inconel 718 are closely related to the machined surface quality [7]. There is an urgent need to find suitable cutting tools to improve the machined surface quality of Inconel 718 components [8].

Many researchers analyzed the effects of tool geomaterial angles, shapes, and tool materials on the machined surface quality of Inconel 718 [9,10,11,12]. PVD TiAlN-coated carbide tools were verified as a good choice for machining Inconel 718 by improving the machined surface quality. Arunachalam et al. [13] conducted a high-speed face milling process of Inconel 718 with a PVD TiAIN-coated tool, ceramic, and cubic boron nitride (CBN) cutting tools. They found that the TiAlN and CBN tools generated compressive surface residual stresses and good surface roughness compared with that generated by ceramic tools. The TiAlN tool cost only 4–5% of the cost of a CBN cutting tool. Devillez et al. [2] illustrated that TiAlN tools exhibited a superior antifriction effect and tool life compared to that of uncoated, TiN, and AlN monolayer-coated tools in dry cutting Inconel 718. The high-temperature oxidation of TiAlN coating generated an Al₂O₃ coating with low thermal conductivity. The generation of the Al₂O₃ coating enhanced the tool–chip contact thermal resistance to protect the tool substrate and reduce tool wear [14,15,16]. Grzesik et al. [17,18] also illustrated the formation of protective Al₂O₃ coatings on a TiAlN-coated surface during the machining process of Inconel 718 alloys. Many researchers have identified the improvement effects of TiAlN coatings on tool wear in turning, milling, and drilling processes of Inconel 718 under dry and various cooling conditions [19,20,21,22,23,24,25,26].

The coating thickness and Al contents are two important parameters for PVD TiAlN coatings [27,28]. The varied coating thickness and Al contents could affect the coating antifriction and thermal barrier effects, thus changing the induced mechanical loads and thermal loads in machining Inconel 718 [29,30]. On the one hand, the thermal loads could induce plastic deformations which are responsible for the generation of tensile residual stresses. Uçak and Çiçek [23] found that the suitable selection of cooling conditions decreased the evaluation of thermal loads. The low cutting temperature could improve the machined hole quality in drilling Inconel 718 with TiAlN tools. On the other hand, the mechanical loads could affect the plastic deformation to induce compressive residual stresses [31,32]. Zhao et al. [33] found that TiAlN coatings with higher thermal conductivity could perform massive heat transfer from the contact interface between a tool flank face and a machined surface into the ambient environment air. The lower tensile residual stress was induced in the machined surface layer due to the decreased thermal loads in the turning process of Inconel 718. The relationships between the thermal conductivity and Al content of TiAlN coatings have been measured in the research of Ding et al. [34].

Arunachalam and Mannan [35] illustrated that the deposited coating thickness could affect the machined surface roughness and surface residual stresses after machining Inconel 718 with PVD-coated tools. Zhao et al. [36] and Abdoos et al. [27] also found that thicker TiAlN coatings induced a higher chip temperature and lower internal temperature within the tool substrate. The surface roughness and surface residual stresses are affected by the coating thickness of PVD TiAlN tools, which could alter the service performance of machined components. However, the relationships among the tool coating thickness, the machined surface roughness, the surface residual stress, and the fatigue life are still not sufficiently illustrated.

However, the effects of coating thickness on the fatigue life of machined parts have not been reported until now. The fatigue life of machined parts is related to the machined surface integrity. Therefore, it was valuable to illustrate the effects of coating thickness on the fatigue life combined with the machined surface integrity. In this research, the PVD TiAlN-coated tools with several thicknesses of 0 μm, 1.6 μm, 2 μm, 2.5 μm, and 3 μm were selected to turn the Inconel 718 under flood cooling conditions. The representative indexes of the machined surface integrity were selected as the machined surface roughness and the residual stresses in the directions of cutting speed and feed rate. The low-cycle fatigue life values and the cross-sectional three-dimensional (3D) topographies of fatigue crack initiation zones have been obtained. The relationships among the tool coating thickness, the machined surface roughness, the surface residual stress, and the fatigue life of machined Inconel 718 were determined. The improvement of surface residual stress was the main conducive factor for the improvement of fatigue performance compared to the limited effects of surface roughness on fatigue performance after machining Inconel 718 with TiAlN tools. The thinner TiAlN tools (1.6 μm) could help to improve the surface roughness of the machined parts of Inconel 718. The thicker TiAlN tools (3 μm) could help to improve the low-cycle fatigue life of the machined parts of Inconel 718. Therefore, the suitable selection of TiAlN coating thickness should depend on the service conditions of the machined parts.

2. Workpiece and Tool Materials

2.1. Workpiece of Inconel 718

The heat treatment procedure of Inconel 718 was illustrated in the research [19]. Figure 1a depicts the microstructure surface topography of polished Inconel 718. Figure 1b,c shows the diffraction peaks of Inconel 718 and the dimensional schematic of the standard specimen, respectively. The main elements and their proportions in Inconel 718 were Ni (53.51 wt.%), Cr (18.05 wt.%), Nb (5.43 wt.%), Mo (2.98 wt.%), Ti (1.02 wt.%), Al (0.50 wt.%), Co (0.31 wt.%), and the balanced Fe. The main mechanical properties and thermophysical properties of Inconel 718 are summarized in

Table 1. Mechanical properties of Inconel 718 (Test condition: GB/T228B20).

Properties	Tensile Strength	Proof Strength, Plastic Extension	Percentage Elongation after Fracture	Percentage Reduction in Area	Hardness
Values	1504 MPa	1247 MPa	19.5%	32%	439 HBW

and

Table 2. Thermophysical properties of Inconel 718.

Properties	20 °C	200 °C	400 °C	600 °C	800 °C	1000 °C
Density (kg/m3)	8240	-	-	-	-	-
Young’s modulus (GPa)	199.9	-	-	-	-	-
Poisson’s ratio	0.3	-	-	-	-	-
Thermal conductivity (W/(m·K))	13.4	15.9	18.3	21.2	23.6	30.4
Specific heat (J/(kg·K))	435	460.4	493.9	539	615.3	707.4

, respectively.

There was only one margin of cutting depth remaining on the specimens before the turning experiments with uncoated and TiAlN tools. Figure 1d depicts the standard specimen, which was finally obtained by removing material with only one margin of cutting depth using new cutting tools as referenced in the research [37,38,39]. The tool wear after the initial turning process was difficult to detect due to the small amount of removal material of the workpiece. Therefore, the research was conducted to analyze the relationships among the tool coating thickness, the machined surface roughness, the surface residual stress, and the fatigue life of machined Inconel 718 during the initial cutting process.

2.2. Cutting Tools with Various Coating Thicknesses

TiAlN coatings were deposited on surfaces of uncoated carbide tools (VNMP160404 K313, Kennametal Inc., Latrobe, PA, USA) using the direct-current-arc method in the Multi-Arc PVD 20 system. Figure 2a,b depict the brief formation mechanism and deposition technology of the TiAlN coating. The deposited TiAlN coated tool and its chemical composition were obtained as shown in Figure 2c,d show the cross-sectional micrography of Ti_0.55Al_0.45N tools with various coating thicknesses.

The selected carbide tool substrate contained 89.50% content of the WC phase and 10.50% content of the Co phase. The five types of cutting tool inserts with various coating thicknesses (0 μm, 1.6 μm, 2 μm, 2.5 μm, and 3 μm) possessed the same geometric angles and dimensions, such as a side rake angle of 11°, back rake angle of 3.2°, lead angle of 40°, tool edge radius of 0.04 mm, etc.

Figure 3a–e shows the surface topographies of uncoated and TiAlN tool rake faces. The surface roughness values of the TiAlN tool rake face vs. coating thicknesses are summarized in Figure 3f. The deposited TiAlN coating improved the surface roughness more than 66.67% compared to that of the uncoated tool rake face. The coating surface roughness was increased by about 20% with the increase in coating thickness from 1.6 μm to 3 μm for TiAlN-coated tools.

3. Design of Experiments

3.1. Turning Tests

The turning experiments with water-soluble cutting fluid LN6889 were conducted in a numerical control lathe, namely CK6560A (Bochi Inc., Chongqing, China) (Figure 4). The utilized cutting cemented carbide inserts with various TiAlN coating thicknesses were fabricated using the cutter arbor MVJNR2020K16 (Kennametal Inc., USA). The rough machining parameters were adopted with a cutting speed of 40 m/min, depth of cut of 0.1 mm, and feed rate of 0.1 mm/r. Each type of wet turning experiment of Inconel 718 with PVD TiAlN tools was conducted more than three times to keep the effectiveness of our results. Some pictures and measured values were utilized to analyze the effects of coating thickness on the machined surface roughness, surface residual stress, and low-cycle fatigue life.

3.2. Detection and Characterization

The time-varied cutting forces were timely captured by a three-component measuring system, namely a 9229A dynamometer (Kistler Inc., Zürich, Switzerland). The cross-sectional topography and chemical element composition of the TiAlN-coated tool were measured using a scanning electron microscope (SEM, JSM-6510, JEOL Ltd. Tokyo, Japan) coupled with energy-dispersive X-ray spectroscopy (EDS). The phase composition and crystal structure of Inconel 718 were characterized using X-ray diffraction (XRD, D8 Discoverer, Bruker Inc., Billerica, MA, USA). The phase characteristics were determined using XRD, using Cu Kα radiation with a scanning increment of 0.02 ° in the 2θ ranges of 30–90°. The data processing and analysis on the XRD diffraction results were conducted by using MDI Jade 6.5 software.

Figure 5a depicts the surface residual stresses in the directions of cutting speed and feed rate on machined standard specimens, which were measured using the cosα method in an X-ray diffraction analyzer, μ-X360n. The light source of the X-ray tube utilized Cr-Kβ radiation. The radiation was diffracted on a crystal plane <311>. The tool rake face roughness and the machined surface roughness were measured using the instrument VK-X250K (Keyence Inc., Osaka, Japan), as shown in Figure 5b. The low-cycle fatigue test for the standard specimen of Inconel 718 was conducted using LFV-250HH (W+B Inc., Zürich, Switzerland) in Figure 5c. The applied mechanical load value σ_max and stress ratio R were set to 1380 MPa and 0.1 with the consideration of the actual working conditions of the components of Inconel 718.

4. Results and Discussion

4.1. Measured Cutting Force

The resultant forces F_∑ in turning Inconel 718 with the TiAlN tool with various coating thicknesses can be determined using Equation (1). The tool–chip friction coefficient μ was calculated with cutting force F_c, radial force F_r, and axial force F_z in Equation (2).

$$F_{\sum }=\sqrt{{F_c}^2+{F_r}^2+{F_z}^2}$$

(1)

$$\mu ={\displaystyle \frac{F_c\tan {\gamma }_0+\sqrt{{F_r}^2+{F_z}^2}}{F_c-\sqrt{{F_r}^2+{F_z}^2}\tan {\gamma }_0}}$$

(2)

where γ₀ is the orthogonal rake angle. In a single point turning tool, the side rake angle and orthogonal rake angle are equal.

Figure 6 depicts the measured forces, calculated resultant forces, and tool–chip friction coefficients in the wet turning Inconel 718 process using TiAlN tools with various coating thicknesses. The resultant forces F_∑ in wet turning Inconel 718 using TiAlN tools with thicker coating thicknesses (2 μm, 2.5 μm, and 3 μm) were increased by 11.39 N (8.37%), 19.11 N (14.04%), and 34.49 N (25.33%) compared to that using the TiAlN tool with a thin coating thickness of 1.6 μm. The tool–chip friction coefficients in wet turning Inconel 718 using TiAlN coatings with various coating thicknesses decreased by 23.74–40.29% compared to that by using uncoated tools. On the one hand, the TiAlN coating decreased the surface roughness of tool rake face compared to that of uncoated tool, which improved the tool–chip contact characteristics in wet turning Inconel 718 using TiAlN tools. On the other hand, the antifriction effect of the TiAlN coating could help to improve the tool wear process and decrease cutting forces. In the research of Devillez et al. [2], they found that TiAlN tools exhibited a superior antifriction effect compared to that of uncoated tools in cutting Inconel 718. The formation of protective Al₂O₃ coatings on the TiAlN coating surface could help to illustrate these antifriction effects, as found in the research of Grzesik et al. [17,18]. Similar TiAlN coatings were deposited on the carbide tools to obtain a similar improvement of antifriction effects. In addition, some researchers have identified the improvement effects of TiAlN coatings on tool wear in turning, milling, and drilling processes of Inconel 718 [19,20,21,22,23,24,25,26]. The foundation of these references also verified our results.

4.2. Machined Surface Roughness

Figure 7a–e depicts the 3D surface topographies of machined Inconel 718 after the wet turning process using TiAlN tools. The surface roughness Sa of machined Inconel 718 after the wet turning process using TiAlN tools with various coating thicknesses are summarized in Figure 7f. The sequence for the surface roughness values of machined Inconel 718 with TiAlN tools with various coating thicknesses was 3 μm > 2.5 μm > 0 μm> 2 μm> 1.6 μm. The TiAlN coating with a thickness of 1.6 μm could decrease 22.64% of the machined surface roughness Sa after wet turning Inconel 718 compared to that of the uncoated tool. The surface roughness Sa value after wet turning Inconel 718 using TiAlN tools was linearly increased by 48.32% when the coating thickness increased from 1.6 μm to 3 μm. It may be related to the idea that the thicker coating could increase the tool edge radius, which deteriorates the tool–workpiece contact interface characteristics to increase the machined surface roughness. As illustrated by Zhao et al. [36], they conducted orthogonal cutting experiments of Inconel 718 using TiAlN coated tools with various coating thicknesses. They found that the coating thickness could affect the cutting temperature and cutting forces, which changed the machined surface roughness. The maximum temperature of the generated chip after machining with thicker TiAlN coatings could be increased compared to that using a thin coating thickness. The low thermal conductivity of TiAlN coatings could prevent more heat dissipation into cutting tools. The massive heat would dissipate into the generated chip and the machined surface to deteriorate the friction status between the tool and the machined surface; thus, the machined surface roughness could be increased with the increase in TiAlN coating thickness.

4.3. Machined Surface Residual Stress

Figure 8a,b depict the measured surface residual stresses in the directions of cutting speed and feed rate after wet turning Inconel 718 using TiAlN tools with various coating thicknesses. The tensile surface residual stresses in the direction of cutting speed were induced after wet turning Inconel 718 using TiAlN tools with coating thicknesses of 0–2 μm. The compressive surface residual stresses in the direction of cutting speed were induced after wet turning Inconel 718 using TiAlN tools with coating thicknesses of 2.5 μm and 3 μm. The machined surface residual stresses in the direction of cutting speed decreased by 148.67% for the machined Inconel 718 with TiAlN tools when the coating thickness was increased from 0 μm to 3 μm. The machined surface residual stresses in the direction of cutting speed for the machined Inconel 718 with TiAlN tools decreased linearly with the increase in coating thickness.

The tensile residual stresses in the direction of feed rate were induced after wet turning Inconel 718 using TiAlN tools with coating thicknesses of 0–3 μm. The machined surface residual stresses in the direction of feed rate for the machined Inconel 718 with TiAlN tools decreased exponentially with the increase in coating thickness. The machined surface residual stress in the direction of feed rate decreased by 92.24% for the machined Inconel 718 with TiAlN tools when the coating thickness was increased from 0 μm to 3 μm.

Compared to the results of reference [35], Arunachalam and Mannan illustrated that the deposited coating thickness could affect the surface residual stresses after machining Inconel 718 with PVD-coated tools. The compressive surface residual stresses could be easily induced due to the applied mechanical loads, but the tensile surface residual stress could be easily induced due to the applied thermal loads [32]. On the one hand, the thermal loads could induce plastic deformations, which are responsible for the generation of tensile residual stresses. The thick TiAlN coating could prevent more heat dissipation into cutting tools; thus, more heat flux would dissipate into the machined surface to increase the thermal loads. However, the cooling medium could decrease the evaluated thermal loads and improve high cutting temperatures in the turning process of Inconel 718 under flood cooling conditions, as shown in the research of Uçak and Çiçek [23]. They found that the suitable selection of cooling conditions decreased the evaluation of thermal loads. The thermal barrier effect of thick TiAlN coatings was not evident compared to the thin coating in the wet turning of Inconel 718. A similar phenomenon was obtained in references [29,30].

On the other hand, the thick coating increased the resultant forces compared to that with a thin coating during the wet turning process of Inconel 718 with TiAlN tools. The mechanical loads could affect the plastic deformation to induce compressive residual stresses [31,32]. The compressive surface residual stresses could be easily induced due to the applied mechanical load. Therefore, the measured tensile surface residual stresses were decreased with the increase in coating thickness in the wet turning process of Inconel 718 with PVD TiAlN tools. Increased compressive residual stresses were detected in wet turning Inconel 718 with thick TiAlN-coated tools.

4.4. Fatigue Life of Machined Inconel 718

Figure 9a–e depicted the cross-sectional 3D topographies of fatigue crack initiation zones of machined Inconel 718 using TiAlN-coated tools with various coating thicknesses of 0 μm, 1.6 μm, 2 μm, 2.5 μm, and 3 μm, respectively. The cracks of all specimens germinated on the machined surface of Inconel 718. The cracks exhibited a ladder shape and multiple sources in parallel. Due to that, the feed tool marks on the machined surface could increase the surface roughness. The increased surface roughness caused the local stress concentration on the surface to promote crack initiation. The enlarged graphs of crack source areas show that the cracks were initiated at the surface grain and grain boundary. Massive intergranular fracture and cleavage step characteristics were exhibited near the crack source region. The mixed brittle fracture forms of intergranular and transgranular occurred at the initial stage of crack initiation and propagation. The propagation stripes showed in the crack source area. It was illustrated that the boundary between the crack source area and the propagation area was not evident. The cracks developed rapidly due to the specimens enduring high mechanical loads. The formed cracks immediately entered crack growth stages.

The low-cycle fatigue life values of machined Inconel 718 using TiAlN-coated tools with various coating thicknesses were obtained as shown in Figure 9d. The fatigue life of machined Inconel 718 was increased linearly with the increase in TiAlN coating thickness. The average fatigue life of Inconel 718 after machining using the TiAlN tool with a coating thickness of 3 μm was increased by 15.6% compared to that of uncoated tools. It was showed that the fatigue life could be improved in wet turning Inconel 718 using TiAlN tools with thick coatings.

Figure 10a–e depict the 3D topographies of fatigue crack propagation zones of machined Inconel 718 using TiAlN-coated tools with various coating thicknesses of 0 μm, 1.6 μm, 2 μm, 2.5 μm, and 3 μm, respectively. The fatigue crack propagation zones were obtained 300 μm away from the crack generation area. The evident propagation stripes could be detected in the crack propagation area, which were induced due to the crack tip propagation. The spacing of the propagation stripes reflected the crack propagation speed.

Figure 10f shows the fatigue fringes per micron and the crack propagation speed of machined Inconel 718 using TiAlN tools with various coating thicknesses. The crack propagation speed was decreased with the increase in TiAlN coating thickness in machining Inconel 718. The coating thickness of turning tools has significant effects on the fatigue performance of machined specimens due to the fact that the coated tools could affect the surface quality of machined specimens. The compressive residual stress and small surface roughness could improve the fatigue life of machined workpieces [40,41]. As referred to in the research of Javadi et al. [42], the axial compressive residual stress had the most significant effects on fatigue life. The tangential compressive residual stress would slightly increase the fatigue life.

In the research of Arunachalam and Mannan [35], they illustrated that the deposited coating thickness could affect the machined surface roughness and surface residual stresses after machining Inconel 718 using PVD-coated tools. In our research, the surface roughness was slightly increased with increasing coating thickness, but the fatigue life of the machined specimen was increased with increasing coating thickness. However, the axial residual tensile stress of 360.67 MPa and tangential residual tensile stress of 539.67 MPa were induced on the machined specimen of Inconel 718 using uncoated tools. With the increase in TiAlN coating thickness, the axial residual tensile stress was reduced gradually to 28 MPa, while the tangential residual stress was decreased to −262.67 MPa. It was shown that the improvement in residual stress was the main conducive factor for the improvement of fatigue performance compared to the limited effects of surface roughness on the fatigue performance after machining Inconel 718 with TiAlN tools. This was the new foundation compared to the former research [35].

5. Conclusions

(1)

The deposited TiAlN coatings with various coating thicknesses improved the tool-chip contact friction characteristics in wet turning Inconel 718 compared to that by uncoated tools. The resultant forces F_∑ in wet turning Inconel 718 by TiAlN tools with thicker coating thicknesses (2 μm, 2.5 μm, 3 μm) were increased 11.39 N (8.37%), 19.11 N (14.04%), 34.49 N (25.33%) compared to that by TiAlN tool with thin coating thickness 1.6 μm.

(2)

The surface roughness Sa value of machined Inconel 718 by TiAlN tools with coating thickness 1.6 μm was decreased about 22.64% compared to that by uncoated tool. The surface roughness Sa value of machined Inconel 718 by TiAlN tool with coating thickness 3 μm was linearly increased 48.32% compared to that with coating thickness 1.6 μm. The thinner TiAlN tools (1.6 μm) could help to improve the surface roughness of machined parts of Inconel 718.

(3)

The machined surface residual stresses in the directions of cutting speed and feed rate were decreased linearly 148.67% and decreased exponentially 92.24% for the machined Inconel 718 with TiAlN tools when the coating thickness was increased from 0 μm to 3 μm, respectively.

(4)

The low cycle fatigue life of machined Inconel 718 with TiAlN tool was increased linearly 15.60% when the coating thickness increased from 0 μm to 3 μm. The improvement of surface residual stress was the main conducive factors for the improvement of fatigue performance compared to the limited effects of surface roughness on fatigue performance after machining Inconel 718 with TiAlN tools. The thicker TiAlN tools (3 μm) improved the low cycle fatigue life of machined parts of Inconel 718 compared to the other tools. The suitable selection of TiAlN coating thickness should depend on the service conditions of machined parts.