*3.3. Tool Wear*

The adopted experimental procedure made possible the tracking of the evolution of the tool wear. It is an inevitable, gradual, and complex phenomenon affecting workpiece surface accuracy and integrity and contributing to the cost of the machining process. It occurs due to the fact of geometrical damage, frictional force, and temperature increases at the tool–workpiece interface region. Studying the influence of the cutting parameters and their effect on tool wear allows to optimize the process, achieving both better results on the final product and on the economical side of the operation.

To establish the tool flank wear progression depending on the machining time, wear data were acquired approximately every 30 to 60 s of the machining time. To do so, the test was stopped and the flank wears (*VBmax* and *VBnotch*) were measured according to ISO 3685. Herein, flank wear refers to *VB* for simplicity. Figure 8 shows the monotonic linear wear increase for the CBN cutting tool according to the cutting time (*tc*). These data were then computed into a graph, where the ratio of *VB* and cutting time represents the tool flank wear rate (TWf) in mm/s, as presented in Figure 8.

As the cutting speed increased, the tool wear rate tended to increase as well, which may be explained by the increase in the cutting temperature at the cutting edge for increasing cutting speeds. Those high-speed (and, thus, high-temperature) conditions may lead to softening of the tool material which, in turn, may result in an accelerated tool wear rate (monotonic trend of the curves shown in Figure 9). Still, there seems to be a tendency towards the stabilization of the wear rates for longer cutting speeds. This might be associated with the obtained cutting loads that were more stable for faster cutting speeds (refer to Figure 5), especially for TiC binders. Thus, it is noticeable that within cutting speeds between 250 m/min and 350 m/min, the wear rate tended to a steadier rate when machining with TiC binder inserts. However, when machining the same alloy with the

CBN tool with TiN binder, this effect was less evident, since the tool flank wear rate kept increasing with an increasing cutting speed, while the cutting loads also showed lower stability (Fp >> Fc, Ff), as illustrated in Figure 5.

**Figure 9.** Influence of the cutting speed (*vc*) on the tool flank wear rate for each distinct tested insert: (**a**) TiC binder; (**b**) TiN binder.

Throughout the tests, all inserts experienced noticeable crater wear on the rake face and flank wear due to the fact of abrasion on the flank face. In most of the experimental runs, an increased cutting speed improved the stability of the cut. In Figure 10, two distinguishable wear patterns that were often found in the current experiment are shown. At lower cutting speeds (i.e., between 150 and 200 m/min), the flank wear was extremely irregular with the length variations of the flank itself in addition to the formation of buildup edge and notching. At higher cutting speeds, despite the rapid wear of the tool, this phenomenon was less discernible. Regarding the influence of the feed on flank wear, the results do not show a noticeable influence for the different levels of tested feeds (0.1 and 0.2 mm/rev), which is supported by Cantero et al. [35], who found similar results when machining 45 HRC Inconel 718.

**Figure 10.** Example of cutting inserts (TiC binder) with exceeded flank wear (*VB* > 0.2 mm) for different operational cutting conditions: (**a**) cutting speed (*vc*) of 200 m/min and feed (*f*) of 0.2 mm; (**b**) cutting speed (*vc*) of 350 m/min and feed (*f*) of 0.2 mm.

Knowing the speed at which each tool flank degrades, the flank wear rate, as shown in Figure 9, enables the calculation of the expected tool life, as presented in Figure 11, using a flank wear of 0.2 mm as the wear criterion. It was chosen to display the average tool life of each tool for both feeds, as no minor differences were found for the flank wear with the two tested feeds (0.1 and 0.2 mm/rev). It is also important to note that the tool life at a cutting speed of 150 m/min was extrapolated and could hardly be achieved due to the chipping and notching phenomena, which is further addressed. Moreover, it is important to remark that even though the cutting speed greatly affects the tool life (which shows considerable differences regarding the CBN binder at low cutting speed), it plateaus at 300 m/min. This could be explained by a change in the phenomena, leading the wear to change from a tribological type (i.e., abrasion and adhesion) to a chemical (i.e., diffusion and oxidation). Within the selected range of the tested parameters, the cutting speeds between 250 and 350 m/min resulted in smaller notching, better cutting stability, and chip control despite a short tool life.

**Figure 11.** Influence of cutting speed on tool life prediction based on a maximum flank wear of 0.2 mm for TiC and TiN based binders.

Productivity should not only be assessed by the geometric parameters of the operation (which allow to calculate the MRR) but also in terms of the tool's life, particularly at an industrial level where tool replacement and set-up times are detrimental. Figure 12 exhibits the removed material volume as a function of the feed and cutting speed for the maximum tool life (*VB* > 0.2 mm), enabling an assessment of the tool's performance.

Figure 12 allows for the observation of two distinct conclusions: (i) the highest machined volume was achieved with a feed of 0.2 mm and (ii) the high material removal rates did not necessarily correspond to a lower machined volume (due to the shorter life), constituting improved productivity scenarios.

Notch wear induced by chipping, contrary to flank wear, decreased with an increase in the cutting speed and became indistinguishable from flank wear at *vc* = 250 m/min, which was also reported by several other authors, for the same tool–workpiece material pairs [35,40]. The notching and built-up edge were particularly intense at lower cutting speeds for both tested feeds with the TiC binder insert, as seen in Figure 13, whereas with the TiN binder inserts under the same cutting conditions, less notching occurred. This excessive notch wear at lower cutting speeds led to an accelerated deterioration of the cutting edge and its ability to maintain strength properties, culminating in either (i) premature failure of the tool or (ii) flank wear homogenization covering the initial notching. The second occurrence is a more convenient scenario, resulting in a more gradual tool wear and, thus, the higher stability of the cutting operation. This is consistent with the fact that the tool flank wear pattern is more homogenous at higher cutting speeds, becoming the failure mode for most tools at speeds above 200 m/min. Unexpected tool failure in addition to

an increasing frequency of tool change may also destroy the previously machined surface, causing an inherent impact on the economical side of the process.

**Figure 12.** Influence of the cutting speed, feed, and PCBN binder on the machined volume.

**Figure 13.** Flank surface of the TiC binder insert exhibiting notching under a cutting speed (*vc*) of 150 m/min and (**a**) feed (*f*) of 0.1 mm after 105 s of cutting; (**b**) feed (*f*) of 0.2 mm after 65 s of cutting time.

Tool failure due to the fact of notching located at the depth of cut (see Figure 14c) at cutting speeds within the range of 150–200 m/min and a reduction of notching as the cutting speed increases has also been reported by various authors [26,41]. This tool nose notching at the depth of cut is located at the intersection between the cutting edge and the machined surface, which is a common failure mode when machining nickel-based alloys. Notch formation results from a combination of aggressive cutting conditions involved during the machining process of Ni-based alloys: high temperature, elevated strength, and strain hardening of the workpiece maintained at high temperature and the abrasive chips [42].

**Figure 14.** Rake face after 1.5 min of machining time: (**a**) insert 603; (**b**) insert 602; (**c**) notching occurrence located at depth of cut dimension.

Comparing the tool wear with the TiN binder insert and the TiC binder insert, during the previous turning experiments, it seemed that the first one appeared to have a more homogenous and predictable flank wear. The flank wear rate (TWf) of insert 602 showed less variation than insert 603 for the two tested feeds, except at a cutting speed of 350 m/min, so a further investigation and comparison of these two inserts was performed.

Table 4 shows the measured flank wear (*VB*) for cutting speeds of 250 m/min and feed of 0.1 mm, confirming that TiN binder inserts outperform TiC binder inserts, achieving less flank wear. The rake face images shown in Figure 14 also support the initial premise that the TiC binder insert has a tendency to notch at the depth-of-cut line. The TiN binder inserts outperformed TiC by almost 30%, a value that is in line with the 20% difference reported in the studies by Bushlya et al. [40] when machining Inconel 718 under flood coolant lubrication cutting conditions at a cutting speed of 300 m/min, feed of 0.1 mm/rev, and depth of cut of 0.3 mm. Curiously, it was reported that the oxidation of TiN and TiC start at 700 ◦C and 800 ◦C, respectively, forming TiO2 [43].

**Table 4.** Flank wear after machining for 1.5 min for *vc* of 250 m/min and *f* of 0.1 mm.


Electron backscatter diffraction (EBSD) imaging was used to further investigate and distinguish compositional changes in the tools tested. Different zones usually have different light intensities that correspond to distinct chemical compositions. EDS analyses and SEM images allowed to identify compositional changes on the tool's surface, such as the build-up edge, and to recognize that, as consequence of the abrasion at the tool–workpiece interface, small portions of the workpiece tended to become attached to the tool while the coating was removed.

Figures 15 and 16 exhibit SEM images of the three TiC and TiN binder inserts used to machine alloy 718. All tested inserts exhibited a curvilinear, elongated branded zone where the chip primarily exited at the rake face, resulting in both a loss of the tool coating due to the tool–chip interaction, diffusion, and adhesion of workpiece elements. Regarding the TiC and TiN inserts, there was a noticeable difference among them, which is visible as prominent abrasion wear roughly in the same place on the three TiC inserts, and this is coincident with the notching present on the flank face, as seen in Figure 17. This excessive notching tendency of the TiC binder insert compared to the TiN binder insert has previously been noticed in tool wear rate analyses and further confirmed with SEM analysis.

**Figure 15.** SEM images of the TiC binder insert rake face after 1.5 min of machining.

In addition, the flank wear on the TiN binder inserts was generally more homogeneous than for the TiC binder inserts mainly due to the absence of notching. Moreover, SEM images made it possible to understand that the cutting edge of the TiC binder inserts underwent more workpiece adhesion than the TiN binder inserts. While the workpiece material adhered to the tool, a new edge was created resulting in the loss of geometric and mechanical characteristics. This newly generated layer made the cutting edge less sharp, thus increasing the forces during the cutting, particularly the passive or radial force.

The TiN binder inserts seemed to have a geometrically consistent wear pattern on the rake face and tool, and the TiC binder insert's crater wear appeared to be darker in the images, indicating the presence of greater erosion, consequently reaching the tool's substrate. Both inserts show groove occurrence roughly at the same location, which seems to be related with the cutting depth; although for the TiC binder inserts, the cutting edge appeared to be more affected by the chipping, for the TiN binder inserts, the cutting edge seemed to better withstand its geometrical characteristics.

**Figure 16.** SEM images of the TiN binder insert rake face after 1.5 min of machining.

**Figure 17.** SEM image of a worn TiC binder insert tool flank.
