## 3 Machinability of AM Ti6 Al4V Alloy
### Machining Forces Generated  
The cutting forces generated during the machining operation are normal force (\(F_{n}\)), tangential force (\(F_{t}\)), cutting force (\(F_{c}\)), and feed rate force (\(F_{a}\)). The coating layer minimizes friction at the tool-chip interface, reducing the cutting forces and heat produced by friction. Additionally, the coating layer serves as a shield protecting against thermal shocks in milling [51]. Cutting stresses (energies) can be measured using a force dynamometer and a load cell to measure forces, or by monitoring power usage [68].  
The cutting force \((F_{C})\) and tangential force \((F_{t})\) can be determined from Equations (3) and (4), respectively. Frictional \((F)\) and resultant force \((R)\) can be obtained from Equations (5) and (6), respectively. SCE can be determined from either Equation (7) or (8), and shear plane angle \((\phi)\) can be determined from Equation (9), while chip compression ratio \((r)\) can be determined from Equation (10) [53; 69; 70; 71; 72].  
\[F_{C}=R\text{cos}(\lambda-\alpha) \tag{3}\]  
\[F_{t}=R\text{sin}(\lambda-\alpha) \tag{4}\]  
\[F=R\text{sin}\,\lambda \tag{5}\]  
\[R=\frac{F_{S}}{\text{cos}\,\theta} \tag{6}\]  
\[SCE=\frac{P_{cut}}{MRR} \tag{7}\]  
\[SCE=\frac{F_{C}}{w\cdot t_{0}} \tag{8}\]  
\[tan\phi=\frac{rcos\alpha}{1-sin\alpha} \tag{9}\]  
\[r=\frac{t_{1}}{t_{2}} \tag{10}\]  
Figure 7: SEM image of chip formation in turning titanium alloy [3].  
The observed weak cutting forces can be attributed to the low friction resulting from the high rake angle of 20 degrees selected for the machining process [53]. In the machining of titanium alloys, the high heat level at the cutting tip may sometimes approach 1100 \({}^{\circ}\)C. Due to this high temperature, the cutting edge may deteriorate rapidly and become dull due to the elevated temperature close to the cutting zone [51]. It was observed that at any cutting speed selected and if the cutting temperature was less than the beta transit of titanium alloy (880 \({}^{\circ}\)C), no phase transition occurred during orthogonal cutting of titanium alloy [53]. The different types of heat generated because of tool-to-workpiece interaction and the deformation zones have been reported [51].  
It was reported that energy stored during a cutting operation generates more softening effects in titanium alloy [53]. Another apparent reason for the heat accumulation in the titanium workpiece is that the workpiece is diffused with oxygen and nitrogen from the air, which hardens the surface layer when the cutting zone temperature surpasses 600-700 \({}^{\circ}\)C [51]. This report lacks a comparative analysis of different cutting tools and materials specifically in the context of machining titanium alloys. Such an analysis would be instrumental in understanding the advantages and disadvantages of various tools under identical conditions, thereby providing valuable insights for optimizing machining processes. It was reported that short contact time between the tool and workpiece due to high cutting velocities affected heat transfer efficiency; in other words, low cutting speed improves heat transfer effectively [68]. Another study showed that an increase in cutting speed results in lower cutting forces that lead to low SCE due to the thermal softening effects during machining [73].