*3.4. Tool Materials*

As previously mentioned, due to poor *k* from INCONEL® alloys, which lead to a substantial increase of *T* in the three heat-zones when machining, the used tools are more prone to premature wear since the heat generated will end up creating BUE, which will rapidly degrade coatings and the tool material itself. The TW mechanisms, which include abrasive wear, adhesive wear, and plastic deformation, are following described. Severe TW is one of the key reasons for machining inefficiency [118].

**Figure 14.** The wear causes, wear mechanisms, wear types, and wear consequences in the cutting of Ni-based superalloys [118].

**Figure 15.** (**a**) Hot hardness characteristic curve of CBN, Ceramic and Carbide tool materials compared with the γ structure of INCONEL® 718, (**b**) Thermal conductivity of tungsten carbide (WC), INCONEL® 718 and different coatings for carbide tools against *T* (adapted from [119]).

Tool materials, depending on their application, may vary as hard metal, high-speed steel (HSS, and its variant HSS-Co), cermets, ceramics (where carbides are inserted), PcBN and many more. Table 11 presents the latest experimental observations and performance of non-coated tools in the machining of INCONEL® alloys.

**Table 11.** Machinability performance during non-coated tools assisted machining of INCONEL®.



**Table 11.** *Cont.*

#### *3.5. Tool Coatings*

Some tool materials have enough hardness to cut through INCONEL®, as it is shown in Figure 15, although others require a coating to protect the core material from abrasion when machining. The binomial substrate/coating is selected, as a function of specific requisites, from each application which often demands from the cutting tool antagonistic characteristics like tenacity and hardness. The usage of coated tools is highly advantageous from a production point of view, not because it is only possible to escalate *v*<sup>c</sup> or *s* values, but also to promote better quality to the fabricated components, and some of them can be multilayer, in which each layer has its unique function. The preferred manufacturing processes to make coated tools are metallurgical powder processes, chemical vapour deposition (CVD) or even physical vapour deposition (PVD).

**Figure 17.** Crack propagation behaviour for each of the common coating structures [124].

**Figure 18.** The hardness of different coating materials with a lower limit and suitable performance range [125].

A schematic of how different coatings can appear in a tool is presented in Figure 16, whereas Figure 17 demonstrates how crack propagation occurs inside the coatings due to TW. Figure 18 shows the lower limits and the hardness performance range of some of the most used tool coatings. Examples of preferred coatings to machine INCONEL® 718 include TiAlN, TiAlCrN, TiCN, TiN/AlTiN, and TiAlCrSiYN/TiAlCrN [126].

Table 12 presents the latest experimental observations and performance of coated tools in the machining of INCONEL®.

**Table 12.** Machinability performance during coated tools assisted machining of INCONEL®.


*Metals* **2023**, *13*, 585


#### **Table 12.** *Cont.*
