**1. Introduction**

Nickel superalloys, such as Inconel 718, are well known for their excellent mechanical properties, even at elevated temperatures [1,2]. This makes these superalloys the choice material for applications that are quite aggressive and require high mechanical strength at higher temperatures. For example, these alloys are employed in the making of turbine components that operate at temperatures above 800 ◦C. In addition to their high mechanical properties, these alloys also exhibit high resistance to corrosion and oxidation phenomena [3], making them suited for a wide variety of industries, such as the defense, food processing, automotive, and aeronautical and aerospace industries, in which more Inconel 718 is used [4,5].

Inconel 718 is also known for being a hard material to process, especially with machining operations, and it is classified as a hard-to-cut metal. This is due to the fact of their high mechanical property values coupled with the fact that this alloy has low thermal conductivity and a tendency to work harden [6]. Furthermore, the metallurgical structure of Inconel 718 has a significant number of hard carbides, namely, TiC and NbC, which causes the material to exhibit highly abrasive behavior while being cut [7]. As such, the main problems encountered when machining Inconel 718 is the short tool life caused by the high abrasive wear, high cutting temperatures [8], and tendency of the metal to adhere to the tools' surface. Moreover, the machining of this material damages the workpiece itself at a microstructural level due to the very high cutting forces generated during the process,

**Citation:** Matos, F.; Silva, T.E.F.; Sousa, V.F.C.; Marques, F.; Figueiredo, D.; Silva, F.J.G.; Jesus, A.M.P.d. On the Influence of Binder Material in PCBN Cutting Tools for Turning Operations of Inconel 718. *Metals* **2023**, *13*, 934. https://doi.org/ 10.3390/met13050934

Academic Editor: George A. Pantazopoulos

Received: 13 March 2023 Revised: 17 April 2023 Accepted: 6 May 2023 Published: 11 May 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/).

as well as the surface tearing and distortion of the final machined components [9]. This is a problem as most aircraft engine components are obtained via machining.

A wide variety of strategies can be employed to mitigate the problems faced when machining hard-to-cut materials, such as the use of tool coatings or novel tool geometries [10]. Alternative machining strategies are also employed to mitigate problems that arise from machining Inconel 718, such as laser-assisted machining aimed at improving surface quality [11] or even different machining strategies using robotics with cutting force control to improve the surface quality of Inconel components, including turbines [12]. Hard coating offers many advantages, especially by improving the wear resistance of cutting tools [13]. However, there are some machining strategies that can be employed in the machining of these hard-to-cut materials, such as high-speed machining (cutting speeds over 120 m/min). This strategy has shown some potential in the machining of hardened steels [14,15], titanium alloys [16,17], and super alloys [18,19]. By employing this machining strategy, higher material removal rates can be achieved (compared to conventional machining). Nonetheless, employing this strategy using carbide tools (uncoated or coated) causes severe tool wear and coating delamination, primarily due to the high abrasive wear caused by the Inconel 718. As such, some alternative tool materials can be employed, such as cBN (cubic boron nitride) or PCBN (polycrystalline cubic boron nitride), which has excellent chemical inertness, high thermal stability, and very high hardness (being second to diamond) [20]. CBN can better maintain its mechanical strength values when subjected to higher temperatures compared to other conventional tool materials; moreover, due to the fact of its high hardness values, it has excellent abrasion resistance [21]. Regarding its resistance to diffusion phenomena, PCBN can resist interactions with the iron present in Inconel 718 up to temperatures of 1300 ◦C, both for low and high contents of CBN [22].

PCBN tools can be divided into three categories: low content CBN (50–65%); high content CBN (80–90%); and binderless sintered CBN (no binder) [23]. Several studies have been conducted to determine the most appropriate cutting tools for turning Ni-based alloys at higher cutting speeds. Criado et al. [21] compared the performance of low-content PCBN tools and carbide tools during machining operations of Inconel 718. In this study, it was found that the PCBN tools could handle higher cutting speeds, however, at lower values of depth of cut compared to the uncoated WC cutting tools. However, the authors concluded that even with this parameter's adjustment, the PCBN tools had an increased value for machined volume compared to the WC cutting tools while retaining a very similar level of tool wear. Regarding the machined surface roughness, the best quality was obtained using the PCBN tools, which outperformed the WC cutting tools by up to five times in this regard. Studies such as these highlight the potential of using these PCBN tools, particularly for finish-turning operations of Inconel 718. Dudzinski et al. [19] stated that although carbide tools are suited for machining Inconel 718 at lower cutting speeds (between 20 and 30 m/min), for enhanced productivity, ceramic tools are a more appropriate choice, even when considering that WC tools are less prone to crater wear compared to ceramic cutting tools.

Costes et al. [24] analyzed the influence of CBN content when finishing Inconel 718. The authors found that percentages of CBN content in the range of 45–60% in the cutting tool and used at cutting speeds between 250 m/min and 300 m/min led to the best behavior in terms of the best machined surface quality and least amount of sustained wear. Bushlya et al. [25] compared the performance of coated and uncoated PCBN (50% CBN content) with whisker-reinforced alumina tools in high-speed turning operations of Inconel 718. It was concluded that the tool life was highly sensitive to the variations in the cutting speed. The whisker-reinforced alumina tool life was lower than that of the coated and uncoated PCBN tools. The authors also studied the developed cutting forces during the machining operations, finding that these force values were higher for the whisker-reinforced alumina tools compared to the PCBN cutting tools. However, the cutting forces were slightly higher for the coated PCBN cutting tools.

Still, regarding studies conducted on the use of PCBN tools, Khan et al. [26] reported that a 0.2 mm/rev feed rate and a cutting speed value of 300 m/min offered a good productivity/material removal rate when turning Inconel 718. However, in TiN-coated PCBN inserts, as the cutting speed value increased from 300 m/min to 350 m/min, no significant improvement in the performance of the tool life was noticed. This is due to the rapid oxidation of the coating layer [26,27]. Bushlya et al. [28] also tested TiN-coated PCBN tools, comparing them with uncoated PCBN tools. Again, the coated tools produced higher cutting forces during the turning than those observed for the uncoated PCBN tools. In addition, the coated PCBN tools did not produce the best results in terms of the machined surface quality. Some experimental tests showed that low-content CBN tools with TiN binder correspond to excellent wear resistance because the ceramic binder phase has better chemical stability with respect to nickel [29,30].

Understanding tool wear and the developed wear mechanisms during machining is crucial, especially when optimizing a certain process. Wear generally occurs over time, and this failure is a gradual, cumulative process that affects the tool life [31]. As such, studies evaluating the wear behavior of cutting tools can provide useful information about the optimization of the cutting process. Understanding and predicting the effect of the selected cutting parameters on the tool life and wear behavior are also very important. Cores et al. [24] reported adhesion and diffusion wear as the predominant wear mechanisms sustained by CBN tools when turning Inconel 718. The authors evaluated speeds of 250 m/min, 350 m/min, and 450 m/min. The workpiece was subjected to high temperatures and stresses, suffering superficial plasticization. As such, the alloy spread over the contact area between the insert and the workpiece. Regarding diffusion wear mechanisms, Bushlya et al. [29] also reported that, under both high pressure and temperature, boron and nitrogen diffuse from the CBN into the Inconel 718 while forming solid solutions. A chemical reaction triggers the formation of Ti and Nb nitrides and Cr, Mo, and Nb borides. The authors also found that the presence of TiC binder in PCBN is an obstacle to further diffusional loss of the CBN phase. Despite the diffusional loss of carbon, TiC remains more stable than CBN and acts as an inert obstacle. These results made it possible to explain the significantly lower wear rate of PCBN tools with low CBN content and ceramic binder compared to the high CBN content grades.

In the present study, instrumented turning experiments were devised to evaluate the machinability of Inconel 718 alloy using PCBN tools. Two PCBN tool types were used that had distinct binder phases: TiN and TiC. The effect of the binder on the generated cutting loads and the sustained tool wear is presented and discussed. Although the use of PCBN tools for machining Inconel alloys has been researched to some extent, the influence of the binder phases could use additional exploration, as this may bring some advantages in the use of these tools, especially when aiming to mitigate common problems associated with the machining of Inconel 718.

## **2. Materials and Methods**

Longitudinal cylindrical turning tests were performed using a Mazak Integrex i-200ST multitasking CNC machine instrumented with a load cell (Kistler 9129A piezoelectric dynamometer) coupled to a multichannel charge amplifier (Kistler 5070A) and data acquisition system (Kistler 5697A). In addition to the acquisition of the cutting forces, tool flank wear measurements and image collection of the rake and flank face were conducted using a DinoLite digital microscope. SEM images were taken, and a spectroscopy analysis was also performed. The SEM/EDS analysis was conducted using a high-resolution (Schottky) environmental scanning electron microscope with X-ray microanalysis and electron backscattered diffraction analysis (FEI Quanta 400 FEG ESEM/EDAX Genesis X4M). To avoid contaminants, all samples were submitted to an ultrasonic cleaning bath prior to the SEM analysis. The samples were immersed in ethanol and subjected to ultrasonic waves for 2 min to remove any debris or unwanted particles that may have accumulated on the surface of the samples. Table 1 displays the mechanical properties at both room temperature

(RT) and 649 ◦C, as specified by the alloy manufacturer and determined through tensile testing according to ASTM E8/E8M-16a and ASTM E21.

**Table 1.** Mechanical properties of the tested IN718 alloy as a function of temperature.


A schematic representation of the described experimental setup is shown in Figure 1. A tool holder (ISO DCLNL 2525M 12) was clamped to the dynamometer, resulting in an insert (ISO CNGA 120408) setting angle of 95◦, a rake angle of −6.5, and a clearance angle of 6.5◦. A sampling rate of 1000 Hz was defined for the three components, Fx, Fy, and Fz, which corresponded to the passive, cutting, and feed forces, respectively. Concerning the lubrication conditions, a soluble oil emulsion was used with a pressure of 3.7 bar pointed towards the cutting edge. The tested levels of each cutting parameter are presented in Table 2. The full combination of the presented parameters was used for the two different cutting inserts (with different binder phases). Experimental tests usually vary the cutting speed (*vc*) from 150 to 400 m/s, so five cutting speed levels within this range were selected. A feed (*f*) of 0.1 and 0.2 mm/rev and a depth of cut (*ap*) of 0.15 mm were selected for the experimental runs. This combination of cutting conditions resulted in a matrix of 10 tests for each tool grade, adding up to 20 different experimental runs.

**Figure 1.** Instrumented setup of the longitudinal, cylindrical turning machining operations: (**a**) tool holder and effective rake, clearance, and cutting-edge angles; (**b**) cutting tool and tool holder relative to the position of the workpiece and load cell mount; (**c**) charge amplifier and data acquisition system.

**Table 2.** Tested levels for each cutting parameter.


Two identical cutting inserts were selected that varied the ceramic binder: (i) titanium carbide (TiC) for the PBY603 grade and (ii) titanium nitride (TiN) for the PBY602 grade. These 50% CBN content grades are suitable for the continuous and lightly interrupted cutting of hardened steel and the finishing of abrasive high-strength cast irons, and they can also be used to machine heat-resistant superalloys. It is worth noting that the inserts were

composed of a PCBN tip brazed on a cemented carbide substrate, as shown in Figure 2. The inserts had an AlTiN/TiSiAlN double-compound coating (1.5 μm thickness) applied using the physical vapor deposition (PVD) process. Both inserts had the same negative 80◦ rhombic-shaped cutting insert and a honed-edge preparation with a 0.020 mm radius, which was measured (for thorough control of the cutting conditions) using the Brucker Alicona apparatus based on 3D optical measurement technology (at Palbit S.A.), as shown in Figure 2.

**Figure 2.** Overall geometry of the cutting insert (brazed tip construction) and edge radius measurement (at <0.05 mm from tool tip) of TiN-based and TiC-based PCBN inserts.

The Inconel 718 samples were equally pre-machined (facing and turning) into cylindrical shapes (50 mm in diameter), and a free length of 55 mm was defined for each longitudinal turning pass, ensuring the repeatability of the process. Prior to the machining operations, hardness tests were conducted (using an Emco M4U-075 hardness testing machine, according to ISO 6508) on the cylindrical rods. The following steps were successively repeated for each condition of the turning experiment: (i) placing a virgin (i.e., new), turning the insert on the tool holder; (ii) starting the data acquisition system without the lathe rotation; (iii) starting the turning test while acquiring data; (iv) stopping the test after a determined number of passes; (v) inspecting the tool and measuring the flank wear with a digital microscope; (vi) collecting the chip; (vii) repeating steps (i) to (iv) until a flank wear (*VB*max) of 0.2 mm is achieved or 5 min of cutting time have been completed.

An example of the as-recorded cutting force data is shown in Figure 3. Distinct stages can be noticed. During the first stage (I), the force noise came from the lathe since the spindle was already rotating, but the cutting tool was motionless, whereas during the second stage (II) the cutting tool started its movement, entering the workpiece and beginning to cut. This second stage was characterized by a sudden increase in the forces, which stabilized less than half a second later into the (III) cutting stage itself. This third stage corresponded to the steady-state cutting conditions that were considered for this analysis. The last stage (IV) was characterized by a peak in the forces, possibly due to the fact of inertial effects and occasional chip tangling and winding around the workpiece.

**Figure 3.** Load evolution of the Inconel 718 turning operation using the TiC binder cutting tool (*vc* = 200 m/min, *f* = 0.10 mm/rev, and *ap* = 0.15 mm) while recording the cutting force signals: continuous cutting of (**a**) multiple (3) longitudinal turning passages; (**b**) distinct stages in each passage.
