3.1.2. Turning

Opposed to milling, the turning process takes place in a lathe for components with a revolution axis, i.e., turbine shafts. The workpiece spins in a lathe while the fixed tools, with or without inserts, remove the surplus material. It is patent in Figure 11 the normal movement of the tool while turning a sort of shaft and some intrinsic characteristics of the inserts used. Some of the main problems in turning INCONEL® 718 and 625 are the specific cutting energy (SCE) and rapid augment of surface hardening upon cutting material. Moreover, since shafts must comply with certain geometric specs for the better functionality of the component, *R*a is a key factor to be studied, varying *v*c, *f* and *a*p.

**Figure 11.** (**a**) Turning example; (**b**). Insert A-type (view of basic side cutting edge angle, rake angle, and secondary angles for chip breakage) [84].

Table 7 presents the latest experimental challenges and developments in the machining of INCONEL® with the turning process.


**Table 7.** Critical challenges and developments in the turning process of INCONEL®.



### 3.1.3. Drilling

Drilling is a cutting process where a drill bit is spun to cut a circular hole in a component. In INCONEL® applications, drilling is important to create micro holes that will permit the cooling of gas turbines, as illustrated by Figure 12 and studied by Venkatesan et al. [90] on the hole quality assessment in INCONEL® 625 alloy parts.

**Figure 12.** Gas turbine blade cooling schematic [91].

The INCONEL® 718 alloy has many challenges in deep-hole-drilling as well since the process is prone to drill jamming due to material expansion inside the holes. Table 8 presents the latest experimental challenges and developments in the machining of INCONEL® alloys with the drilling process.


**Table 8.** Critical challenges and developments in the drilling process of INCONEL®.

#### 3.1.4. Boring

Boring is the manufacturing process in which previously drilled holes are enlarged by a single-point cutting tool. Not much information is available about the boring process on INCONEL® alloys whereby it is only presented in the study carried out by Ratnam et al. [94], whose challenges involved the investigation of the machining parameters' effect on *R*a, TW, *F*<sup>c</sup> on the cutting tool and workpiece vibration during dry boring of INCONEL® 718 with TiCN-Al2O3-TiN coated inserts using response surface methodology (RSM). It was found that the use of accelerometers, radioactive sensors and piezoelectric actuators does not make it possible to measure rotating objects' vibrations. On the other hand, the LDVs are capable to measure rotating objects' vibrations with a simple experimental arrangement. Parameters *s* and *f* were found to have a significant influence on *R*a. The parameter

*a*<sup>p</sup> was found to be significant on *VB*, *F*c, and workpiece vibration amplitude (*VA*). The optimal machining parametric combination was obtained using the desirability function. Cutting condition parameters such as *s* = 360 rpm, *f* = 0.14 mm/rev and *a*<sup>p</sup> = 0.4949 mm was obtained for *VA* = 38.7 μm, minimum *F* = 117.8 N, *VB* = 0.3 mm and minimum *R*<sup>a</sup> = 2.55 μm. The proposed RSM approach was an easy method to obtain maximum information with a smaller number of experiments. and successfully used by different authors in the improvement of process parameters.
