**1. Introduction**

With the increasing requirement to achieve the best thermal efficiency in the field of aeronautics [1,2] and energy power plants steam turbines [3], applications in which aluminium and steel would succumb to creep [4] as a result of thermally induced crystal

**Citation:** Pedroso, A.F.V.; Sousa, V.F.C.; Sebbe, N.P.V.; Silva, F.J.G.; Campilho, R.D.S.G.; Sales-Contini, R.C.M.; Jesus, A.M.P. A Comprehensive Review on the Conventional and Non-Conventional Machining and Tool-Wear Mechanisms of INCONEL®. *Metals* **2023**, *13*, 585. https://doi.org/ 10.3390/met13030585

Academic Editor: Yadir Torres Hernández

Received: 14 February 2023 Revised: 2 March 2023 Accepted: 9 March 2023 Published: 13 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/).

vacancies [5], nickel-based (Table 1) alloys became a very attractive solution for hightemperature operation [6–9]. Ni-Cr-Fe superalloys (Figure 1, blue zone), better known as INCONEL® (trademark registered by the International Nickel Company of Delaware and New York [10]), are materials resistant to oxidation, caustic and high-purity water corrosion, and stress-corrosion cracking (SCC) [11], optimal for service in extreme environments subjected to high mechanical loads [12], within numerous applications and characteristics (Table 2).


**Table 1.** Some physical properties of nickel (adapted from [7]).

Caption: *B*max—saturation magnetization; *B*r—residual magnetization; *c*p—specific heat at constant pressure; *E*—Young's Modulus; FCC—face-centred cubic; *G*—shear modulus; *H*C—coercive force; *T*C—Curie temperature; *T*m—melting temperature; *Z*—atomic mass; *α*—thermal expansion coefficient; *ρ*—volumetric mass density; *ρ*R—electrical resistivity, *ν*—Poisson's coefficient.

**Figure 1.** Fe-Ni-Cr ternary phase diagram [13] (Caption: *wt*%—element weight percentage).

A brief insight is provided with the most known alloys, including the INCONEL® 600, a Ni-Cr alloy that offers high levels of resistance to several corrosive elements. In high-temperature situations, INCONEL® 600 will not succumb to Cl-ion SCC or general oxidation, but it can still undergo corrosion by sulphuration deterioration in the hightemperature flue gas. This was a research topic by Wei, et al. [14]. INCONEL® 600 also suffers from severe hydrogen embrittlement at 250 ◦C [15]. Nonetheless, this alloy is recommended for use in furnace components and chemical processing equipment [16]. Moreover, INCONEL® 600 is also effectively used in the food industry and nuclear engineering due to its constant crystalline structure in applications that would cause permanent distortion to other alloys [17].

**Table 2.** Summary of applications and characteristics of some nickel-based superalloys (adapted from [18,19]).


The INCONEL® 601 alloy, similarly to INCONEL® 600, resists various forms of high-temperature corrosion and oxidization [20]. Nevertheless, this Ni-Cr alloy has an addition of aluminium which results in higher mechanical properties, even in extremely hot environments. INCONEL® 601 can prevent the significant strains (*ε*) that would appear under operating loads when exposed to high-temperature environments. The applications go from the use in furnaces to heat-treating equipment like retorts, baskets and gas-turbine components [21] to petrochemical processing equipment. The INCONEL® 625, which will have a special focus during this study, is a rare alloy that gains strength without having to undergo an extensive strengthening heat treatment [22]. It is a Ni-Cr-Mo alloy with an addition of Nb. The Nb reacts with Mo, causing the alloy's matrix to stiffen and increase its strength [23]. Like most INCONEL® alloys, the INCONEL® 625 has high resistance to several corrosive elements [24], withstanding harsh environments that would severely affect the performance of other alloys. This alloy is particularly effective when it comes to stavingoff crevice corrosion and pitting. The INCONEL® 625 is a versatile alloy that is effectively used in the marine engineering, aerospace, chemical, and energy industries, among other applications [25,26]. The INCONEL® 690 alloy, unlike others in the group, is a high Ni and Cr alloy (Cr gives it particularly strong resistance to corrosion [15,27] that occurs in aqueous atmospheres). Along with its ability to resist the corrosion caused by oxidizing acids and salts, INCONEL® 690 can also withstand the sulfidation that takes place at

extremely high temperatures (*T*). One of the most known INCONEL® alloys is the 718 alloy. This alloy, along with the formerly mentioned 625 alloy, will also have a special focus. INCONEL® 718 differs from other INCONEL® variants in structure and response, since it is designed for operation at *T* ≤ 650 ◦C [28]. The 718 alloy is obtained by precipitation hardening [29,30]. It contains substantial levels of Fe, Mo, and Nb, as well as trace amounts of Ti and Al [31]. It has good weldability, which is not matched by most INCONEL® alloys [32], and combines anti-corrosive elements with a high level of strength and flexibility. It is particularly resistant to post-weld cracking, maintaining its structure in both hightemperature and aqueous environments as well, being most widely used in different industries, such as petrochemical, aeronautics, energy, and aerospace [33]. INCONEL® alloys tend to form a thick and stable passivating oxide layer to protect the surface from further attack, retaining strength over a wide *T* range, making INCONEL® an attractive material for high-temperature applications [34]. The strength at high-temperatures of INCONEL® alloys may be developed by solid solution strengthening or precipitation strengthening, depending on the alloy [35]. In those processes, small amounts of niobium combine with nickel to form the intermetallic compound Ni3Nb or γ-prime, which consists of small cubic crystals that inhibit slip and creep effectively at elevated *T* [36].

Concerning the roles of major phases or composition elements that contribute to the INCONEL® alloys, an overview is presented in Table 3 on the effects of Ni-alloying to better comprehend the compositions presented in Table 4.


**Table 3.** Major roles of solutes in different types of INCONEL® alloys [26].


### **Table 3.** *Cont.*

**Table 4.** Chemical composition of relevant INCONEL® alloys.


Caption: Bal.—Balance.

As it was possible to find out, some of the consequences of alloying Ni with certain chemical elements make INCONEL® alloys [46] a difficult-to-machine material [47] (Figure 2) and difficult to metal shape [48], identically to stainless steel [49,50]. As opposed to other alloys, like Al-alloys [36,51], Magnesium (Mg) alloys [51], steel alloys [52] or Tialloys [51], INCONEL® alloys do not benefit from better-established wear mechanisms between the pair tool-workpiece.
