**3. Literature Review**

*3.1. Conventional Manufacturing Processes*

The machining process of chip-start cutting is a technological process able to transform a wrought stock into a component, using a cutting tool. The surplus material from the wrought stock, or just stock, is removed in the form of chips; a consequence of the mechanical action of a cutting wedge with higher hardness than the material of the component that is meant to manufacture. In the following literature review, Milling, Turning, Drilling and Boring will be the discussed processes, in which chip-start cutting is a key and common factor to all these traditional processes.

**Figure 6.** Evolution of the sliding velocity along the tool-material interface [71].

Making use of Figure 6, and taking into account that the chip-start cutting process is very much equal to the machining of INCONEL® 718 and 625, Bonnet et al. [72] described the different friction parts on the rake face in the machining of steel. Directly behind the cutting edge, the chip velocity rapidly shrinks to zero. For a certain contact length, the chip material has a sliding velocity of zero, which starts to increase for the rest of the tool–chip interface, before the chip loses contact with the tool [72].

Due to the friction created around the chip creation process, three distinct heat zones are created within the vicinities of the cutting wedge. In Figure 7, the three different thermal affected regions between the tool-workpiece are visible. In Figure 7a there is a thermo-mechanical deformation of the primary shear region (or primary deformation zone, PDZ) where the majority of the energy is converted into heat due to the internal friction of the material to be cut. In Figure 7b there is a tool-material interface region, or SDZ, of the tool rake surface and the chip rear face where heat is generated by the rubbing between the chip and the tool and finally. In Figure 7c the contact between the flank of the tool and the already machined surface takes place, called tertiary deformation zone (TDZ).

**Figure 7.** Regions of heat generation during metal orthogonal machining (adapted from [73]): (Caption: *ϕ*—shear plane angle; *γ*0—Rake angle, *F*f—Feed force, *F*t—tangential force).

A novel approach to improve the efficiency of the traditional chip-start cutting process is laser-assisted machining (LAM), illustrated in Figure 8, which consists of preheating the material to cut and lowering the superficial hardness to facilitate tool cutting. This solution is common to turning, milling and grinding. Kim and Lee [74] also worked on a machining preheat approach for the INCONEL® 718 alloy, which includes a magnetic induction coil instead of a laser.

**Figure 8.** (**a**) Schematic of LAM indicating heat-losses by convection and radiation, (**b**) Schematic of LAM turning, (**c**) LAM milling, and (**d**) LAM grinding (adapted from [75]) (Caption: *V*f—feed velocity).

### 3.1.1. Milling

Milling is the nomenclature given to the machining process that uses rotary cutting tools to remove excess material from the wrought stock. Nowadays, with the use of CNCs, milling can be done at a maximum of six degrees of freedom (DOF).

**Figure 9.** Chip formation showing (**A**) chip formation showing cutter tooth entry angle in downmilling and cutter tooth exit angle in up-milling, (**B**) maximum chip thickness, *h*max, and (**C**) chip length, *L*c [76].

Traditional milling tends to have lower *a*<sup>p</sup> values and higher *a*<sup>e</sup> values compared to more advanced milling techniques. However, this would cause a concentration of all heat generated in a small portion of the cutting edge, which in this case is the tip of the tool. It would require more axial passes too. This problem can be well managed in aluminium and steel alloys, but not with refractory materials like INCONEL® alloys. Many milling approaches can be tackled to enhance INCONEL® machining, such as up and downmilling, studied by Hadi et al. [77] in INCONEL® 718 machining, illustrated by Figure 9. Another interesting and efficient technique [78] that enriches milling INCONEL® 718 and 625 is trochoidal milling, illustrated by Figure 10, which consists of making the centre of the cutting tool walk a "helical horizontal" path. This procedure not only prevents tool

jamming due to workpiece heat-dilation, but it also enables cutting bigger *a*e dimensions, with lower *a*p, improving heat-spread over the entire tool with more radial passes.

**Figure 10.** Example of a geometric model of trochoidal milling [79].

Table 6 presents the latest experimental challenges and developments in the machining of INCONEL® with the milling process.




#### **Table 6.** *Cont.*
