**1. Introduction**

Inconel 718 nickel superalloys are well known for their superior mechanical properties, retaining them even at elevated temperatures of up to 650 ◦C [1]. In addition to nickel, these alloys also contain chromium, iron, and traces of niobium, molybdenum, titanium, and aluminum. These elements are dispersed through the nickel γ matrix with a face-centered cubic (FCC) lattice structure [2]. These Inconel alloys exhibit high strength, resistance to thermal creep deformation, and high resistance to corrosion and oxidation phenomena. Because of this, these alloys are applied in a wide variety of industries, such as defense, food processing, automotive, and the aeronautical and aerospace industries [2]. They are heavily employed in the latter industries, particularly to produce aircraft engine components (mainly components that are subject to high service temperatures), making up for about 50% of the total weight of these components in aircraft [3], or even as explosive fasters for space shuttles [4].

**Citation:** Sousa, V.F.C.; Fernandes, F.; Silva, F.J.G.; Costa, R.D.F.S.; Sebbe, N.; Sales-Contini, R.C.M. Wear Behavior Phenomena of TiN/TiAlN HiPIMS PVD-Coated Tools on Milling Inconel 718. *Metals* **2023**, *13*, 684. https:// doi.org/10.3390/met13040684

Academic Editor: Badis Haddag

Received: 27 February 2023 Revised: 14 March 2023 Accepted: 28 March 2023 Published: 30 March 2023

**Copyright:** © 2023 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/).

Indeed, Inconel 718 has excellent mechanical properties at high temperatures when compared with other types of alloys, even other nickel-based alloys [5]. However, these high values at high temperatures, coupled with other properties, such as low thermal conductivity and work hardening, make this alloy incredibly hard to process through machining, which is the primary process used to produce engine parts and aircraft components [6]. Furthermore, the final metallurgical structure of Inconel 718 contains a significant number of hard carbides, mainly TiC and NbC, being responsible for the high abrasive behavior registered during machining, causing severe tool wear [7]. This fact, coupled with the tendency of Inconel 718 to adhere to the surface of cutting tools, only make the machining of this alloy even more challenging. The machining of this alloy generates high cutting forces and temperatures, with estimated mechanical stresses reaching peaks of 450 MPa and temperatures of up to 1100 ◦C, as reported by Agmell et al. [8]. These issues, coupled with high abrasive damage and material adhesion to the tools' surfaces, cause the cutting tools to fail quickly, suffering considerable damage after a reduced machining time or distance.

Machining remains a very relevant process, especially for the production of highprecision and quality parts [9]. Over the years, researchers have encountered processing problems when machining various types of materials, with this process suffering constant evolution regarding the production of new tool technologies that can attenuate these problems faced when machining hard-to-cut materials, such as (but not exclusively) Inconel 718. There have been various studies performed on the influence of machining parameters (for various cutting processes, such as drilling, turning, or milling) [9,10]. These studies have focused on the determination of the optimal parameters to produce various parts, analyzing the produced machined surface's quality, and the tool wear sustained by the employed tools in various processes [11,12]. By analyzing these factors, conclusions can be drawn regarding the best set of parameters to machine certain hard-to-cut materials, offering a basis for future works and investigation into the processing of these materials. Another important aspect when it comes to machining process control is the assessment of the cutting forces developed during this process, as these forces provide valuable information regarding the machining process's performance and stability, offering some insight into tool wear and even the machined surface's quality [13,14]. Although these empirical studies offer this basis, they are quite expensive and time-consuming. However, they offer the opportunity to develop prediction methodologies for these processes. For example, in the following study conducted by Zhao et al. [15], the authors studied the influence of cutting parameters on the milling performance of C45E. They focused on the tools' edge optimization for reducing the developed cutting forces and sustained tool wear during machining. The authors performed a simulation using DEFORM V13.0 software to predict the developed cutting forces and tool wear during the machining of this material. Practical tests were also conducted to validate the obtained results. The authors reported satisfactory results, obtaining a deviation in the measured cutting forces from the simulations performed of about 20%. They also determined the influence of the machining parameters, namely the spindle speed and feed rate, on the developed cutting forces and sustained tool wear. These simulations are quite useful, providing valuable information in a faster and more cost-efficient manner when compared with empirical tests. However, special attention is needed when employing these methods, as the configuration of these simulations is quite difficult and depends on previously acquired knowledge [16].

Regarding the mitigation of problems faced when machining hard-to-cut materials, tool coatings or even novel tool geometries can be employed to reduce these problems [10,17]. Hard coatings offer many advantages in improving the wear resistance of various surfaces, being employed not only on cutting tools, but also on other surfaces, such as injection molds [18,19]. As such, cutting tools are no different, heavily benefiting from these hard coatings. These coatings are usually obtained by CVD (chemical vapor deposition) or PVD (physical vapor deposition), conferring different properties based on the coating structure or even deposition method [17]. A recent deposition method shows some

promise, which is PVD HiPIMS (High-Power Impulse Magnetron Sputtering), in which the produced coating structure is more compact and possesses improved mechanical properties. Moreover, this deposition method confers the tools a degree of compressive residual stress, which has a positive impact on the machining performance of the coated tools [20]. This happens mainly because these stresses confer the tools' edges higher strength, producing an overall better-machined surface quality. However, this makes the tools' edges more prone to wear. These novel coatings might prove useful when machining Inconel 718, as they not only confer the tool increased wear resistance and improved mechanical properties, but can also contribute to better surface roughness production. There are studies that have employed these tool coatings in the machining of Inconel 718, both PVD and CVD, as well as uncoated tools. The use of uncoated tools for machining Inconel alloys (namely Inconel 718) is not advised, especially at higher cutting speeds [21]. The main wear mechanisms that these uncoated (WC-Co) tools present are adhesive and abrasive wear, which are considerably higher when compared with those of coated tools tested at higher feed values [22]. Akhtar et al. [23] evaluated the effect of machining parameters on surface integrity in the high-speed milling of Inconel 718. In this study, the authors tested coated and uncoated tools, more precisely, uncoated SiC whisker-reinforced tools and PVD TiAlN-coated carbide tools. Despite the uncoated tools presenting overall lower wear, it was observed that the produced surface roughness for the coated tools was lower, even suggesting that the roughing operations could be conducted using these uncoated tools. This can be attributed to the fine cutting edges commonly produced by the PVD-coating process, as well as the use of TiAlN coatings, which are usually employed for high-speed machining applications [17].

Despite the use of various tool coatings and tool materials, there are still a lot of challenges related to high wear when machining Inconel 718. De Bartolomeis et al. [24] presented a review article in which there was a collection of information regarding the most desired properties for cutting tools when machining Inconel 718. These tools are preferred to have high hardness under elevated temperatures, high strength and toughness, high thermo-chemical stability, high wear resistance, and high thermal shock resistance. There is high importance regarding the thermal properties of these cutting tools, as this alloy has very low thermal conductivity. This usually results in very high cutting temperatures in small punctual areas, primarily at the tool–chip interface [25]. As such, there are some tool materials that show promising behavior when machining these types of alloys, for example, cubic boron nitrides (CBNs), ceramic materials and coatings, such as TiC, TiAlN, or ceramic tools, such as the ones presented in [23], and carbides. However, there are some disadvantages associated with the use of each of these tool materials, such as abrasion and changes in tool geometry for fine edges, or even diffusion phenomena when machining with carbides [26]. These phenomena can be attenuated by introducing a coating layer, such as AlTiN or TiAlN, to act as a chemical/thermal barrier [27]. Indeed, TiAlN-based coatings have proven to be quite promising when machining Inconel 718, as the use of these coatings under high-speed machining conditions can lead to the formation of protective Al2O3 films between the coating and the Inconel 718, as reported in the study by Grzesik et al. [28]. In short, due to the high mechanical strength and elevated temperatures developed in the machining of INCONEL® alloys, the main wear mechanism reported so far is abrasion. However, when the cutting edges are very sharp, the breakage of these edges commonly occurs, dragging the coatings with it, as the fracture occurs essentially through the substrate. Coatings that lead to the formation of oxides on the surface tend to improve chip flow, thereby reducing wear phenomena. Some studies, as previously mentioned, have shown that TiAlN coatings lead to the formation of an Al2O3 film on the surface, which has been proven to be beneficial for reducing wear phenomena.

Another important aspect to improve the machining process of Inconel 718 and the machining processes is the analysis of tool wear and wear mechanisms. The analysis of the tools' sustained wear after machining can yield relevant information regarding the process's productivity, for instance [29], which provides useful information on machining parameter adjustment and, of course, on the quality/suitability of the employed cutting tool [30,31] and even coatings [10,17]. The analysis of the tools' cutting behavior and wear enable researchers to develop new strategies to machine these difficult-to-cut alloys; for example, in the study by Fox-Rabinovich et al. [32], the authors proposed the use of a novel AlTiN/Cu coating to improve the machinability of Inconel 718 superalloy. As previously mentioned, machining tools used to cut Inconel 718 are subject to severe wear, primarily abrasive wear, resulting in early tool failure. This novel tool coating had good self-lubricating properties and reduced thermal conductivity, which resulted in an increase in tool life. The authors assessed not only the tool life and wear behavior, but also analyzed chip formation in the various studied tools. They compared the chips formed with coated and uncoated tools, verifying that the undersurface of the chips created by the coated tools exhibited a very smooth surface, indicating minimal abrasive wear. This information, obtained from analyzing the machining process using different approaches, can be used to create prediction models about tool wear, cutting forces, and even machined surface quality [33]. The configuration of these models is quite difficult and relies on practical/empirical data, either by validation after implementation or by creating a database containing information previously acquired by performing practical tests. There is a lack of these models for milling operations, particularly when considering the machining of Inconel 718 [24].

The machining of Inconel 718 is quite a relevant and popular topic, with most of the studies being performed on the turning of these alloys. However, there have been few studies regarding the milling of these alloys, particularly for finishing operations [24]. Therefore, in the present manuscript, a comparative study on the production quality of TiN/TiAlN-coated tools using different machining parameters is presented. The influence of these parameters on the sustained tool wear and machined surface quality will be assessed. Furthermore, the developed tool-wear mechanisms will be analyzed for each of the devised test conditions. This coating was chosen due to the TiN sublayer that confers high coating adhesion strength [17], which is commonly associated with improved wear behavior [34]. Furthermore, these sublayers highly contribute to the coating's crack resistance [35] and, thus, to its wear resistance. Introducing layers such as these also contributes to the coating's oxidation resistance [36,37], which is important, given that the machining of Inconel 718 causes high cutting temperatures in small areas, thus contributing to coating and tool degradation [17,38]. Test results for uncoated tools will not be presented in this study, as the use of a coated tool to machine this type of alloy is advised and is well documented in the literature. Inconel 718 tends to adhere to uncoated tools' surfaces, causing severe abrasive and adhesive wear; therefore, the authors have opted to study the mentioned tool coating.

Therefore, in this study, the authors hope that the presented findings will be used as a basis for the optimization of the milling processes of Inconel 718, particularly finishing operations. Furthermore, the findings reported in this study can be used for simulation and predictive model configuration. Although this is considered quite difficult for the milling process, as it is a very dynamic machining process, a considerable lack of studies have been conducted on this theme [24]. Moreover, the milling of Inconel 718 alloy has not been heavily explored in the recent literature, especially on finishing operations and conditions [24]. Therefore, this study aims to help fill this gap in the recent literature, as well as provide insight into the milling (finishing) operations of these alloys. Studies such as these have proven to be quite useful when assessing the machinability of various alloys, helping to adjust machining parameters and develop new strategies or even tools and tool coatings [39,40]. As mentioned, the basis to develop simulations about the milling of Inconel will prove to be quite useful. If they are properly calibrated, the simulations can yield interesting and valuable results regarding the generated cutting forces, temperature, and developed tool wear (even providing useful information regarding tool life). However, to calibrate these simulations, studies such as this one are very important as they serve as an experimental basis for these calibrations.

This paper will be divided into three main sections. The following section will be the materials and methods, where the used materials will be presented, as well as the analyses performed and the equipment used for them. Then, in Section 3, the results of the conducted tests will be presented, showing the machined surface quality results followed by the presentation of the wear sustained by the machining tools. This wear analysis will be quantitative and qualitative, showing wear measurements (regarding flank wear) as well as characterizing the sustained wear mechanisms. In Section 3, the obtained results will be discussed based on the current literature. Finally, in Section 4, the conclusions of this work will be presented.
