## 3 Machinability of AM Ti6 Al4V Alloy
### Finite Element Analysis  
The Johnson-Cook (J-C) model is the best model for predicting material behavior that involves dynamic loading with enormous strain hardening, high strain rates, and elevated temperature. The J-C model is one of the constitutive equations employed to simulate the thermo-elastoplastic behavior of the work piece's Ti6 Al4V alloy and the WC-Co tool material [56]. This model has the advantage of predicting material behavior at different strain rates and temperatures but with the disadvantage of requiring extensive experimental data [110]. The J-C model, the most popular model used to predict high strain rate deformations, was employed for the workpiece material. The J-C plasticity model contains analytical strengthening rate and temperature dependence models [68]. Several mathematical models created based on the material's plastic deformation principles can express cutting force, temperatures, stress, or strain [7]. The first part of the J-C model is the strain hardening effect, the second part has the strain rate effect or (viscous effect), and the last is the thermal softening effect. The J-C model material parameters (A, B, C, m, and n) are determined by the split Hopkinson pressure bar (SHPB) mechanical test conducted at strain rates from about 800 to 3300 s\({}^{-1}\) and temperatures from 20 to 1100 \({}^{\circ}\)C [111]. SHPB has some imperfections that might compromise the test result validity, including the swinging effects that flow stress shows, especially at low strain values, because the flow stress always depends heavily on the strain pathway [112]. Due to this limitation, some researchers were able to modify the J-C model equation to predict the flow stress in machining operations accurately. For example, the J-C model equation was modified to consider the strain-softening effect to predict the flow stress [113], and the temperature effect was considered in the modified J-C model. The result was acceptable and validated using the modified J-C models [111]. It was observed that no previous model has been reported for predicting the cutting forces and flow stress during the turning operation for cylindrical bars of Ti6 Al4V alloy used in the AM (DMD) process. Therefore, this reviewed article focuses on this critical gap. Table 8 below summarizes the findings of different models from various themes, focusing on the investigated parameters, material type, and model type. The J-C material model is the most used plastic model due to its ability to incorporate strain hardening, strain rates, and thermal softening effects. It was found that the J-C model shows a poor prediction of cutting forces at higher cutting speeds, up to 2000 m/min in orthogonal cutting of Al 6061-T6 [68]. However, regarding the Ti6 Al4V alloy, reports indicate that the J-C model has several limitations. These include inaccuracies in capturing phase transformations and strain hardening saturation, as well as neglecting the effects of fracture or damage accumulation and microstructural variations within the Ti6 Al4V alloy [116; 117]. Future research directions aimed at addressing these limitations may involve the development of a numerical model that modifies the J-C model to better predict microstructural effects, including grain size, phase changes, and texture formation. Additionally, it would be beneficial to incorporate more comprehensive damage criteria, such as micro-crack nucleation, into the modified J-C model. One report showed that the J-C model was able to predict the milling forces when the preheating temperature exceeded 300 \({}^{\circ}\)C, and the milling forces were found to be reduced significantly in the milling operation of Ti6 Al4V alloy, which was due to the thermal softening effect of the workpiece [31]. The J-C model shows a high plastic strain value on the chip contact side with the tool [115]. The modified Arrhenius model was more accurate in predicting flow stress than the Zerilli-Armstrong and Rusinek-Klepaczko models [107].