**Figure 2.** Relationship between mechanical properties and machining challenges with INCONEL® (adapted from [19]).

Table 5 presents the most relevant and used models of the better-established wear mechanisms referred to, based on physics and experiments for heat partition coefficient *R*chip, for common materials like aluminium and low carbon mild steels.


**Table 5.** Predictive models based on physics and experiment for heat partition coefficient *R*chip [53].


**Table 5.** *Cont.*

Caption: *A*—area shape factor; *l*c—tool-chip contact length; *q*F—frictional heat flux generated in SDZ; *R*chip—heat partition coefficient into the moving chip from the secondary deformation zone (SDZ); *v*ch—chip moving speed; *α*T—tool thermal diffusivity; *w*—tool-chip contact area; αW—workpiece thermal diffusivity; Δ*θ*p max—maximum tool-chip interface temperature rise due to heat generation in PDZ; *θ*0—environment temperature; *λ*T—tool thermal conductivity; *λ*W—workpiece thermal conductivity.

It is suggested to consult the work of Zhao, et al. [53] to better understand the additional variables described in Table 5.

Figure 3 explains how superficial hardness is affected in INCONEL® alloys when machined after cold work processes, compared to some more stable materials like Cu, Al and mild steel. Machining (or surface) cold working may result from mechanical machining (milling, lathing, grinding) [61] or surface treatment (sandblasting, shot-peening), and may introduce residual tensile or compressive stresses into the surface of materials. Compressive stresses generated by shot-peening processes prevent the occurrence of stress corrosion cracks. In the case of plastic strain, tensile stresses appear instead, and the resulting stress levels may be extremely high [62]. A curious detail patent in Figure 3 is the similarity behaviour between INCONEL® 718 and 625 alloys after the 20% cold reduction.

**Figure 3.** Effect of cold work on hardness for different INCONEL® alloys and comparison with other materials [1].

As a consequence of low *k* [36] of nickel-based alloys, which significantly influences heat distribution during the machining process, the surface integrity is affected when applying traditional cold forming techniques, due to the rapid work hardening (Figure 3) in the chip formation region [63]. This phenomenon leads to plastic deformation of either the INCONEL® workpiece or the tool, on subsequent machining passes [64], eventually resulting in built-up-edge (BUE) formation [31] (Figure 4) and consequentially in premature tool failure [65]. For this reason, age-hardened INCONEL® alloys, such as the 718 alloy, are typically machined using an aggressive but slow cut with a hard tool, minimizing the number of passes required [66].

**Figure 4.** Schematic diagram of BUE formation in micromachining processes [67].

The BUE phenomenon occurs because of an accumulation of hot debris generated by the chip-start cutting process and deposited on the tool surface during machining, leading afterwards to adhesion and abrasion TW. From an experimental point of view, some authors noted that the BUE is significantly affected by the state of stress around the tool cutting edge and happens under extreme contact conditions at the tool–chip interface as high friction, high pressure, and high sliding velocity [68]. INCONEL® alloys are well known to abrade tools and develop BUE [69], especially the 718 alloy. Also during the machining of INCONEL® 625, heat concentration is likely to occur at the cutting edges, resulting in early tool failure and consequent BUE [70].

#### **2. Method of Research**

The research and information compiling method are illustrated in the flowchart of Figure 5, which is simple to visually interpret and track down all the inherent steps in the making of this specific paper. In the flowchart, all the consulted databases and most used keywords are found (in the topic of this document), to find information about conventional and non-conventional machining and tool-wear mechanisms of INCONEL® alloys.

Additionally, will be provided three attachments containing abbreviations, symbols and units used within the article.

**Figure 5.** Research method accomplished to achieve a better redacting result to the review paper.
