# Study on the Machinability of Ni-Based Superalloy by
## 3 Result and Discussion
### Effects of Cooling Mode  
The grains near the machined surface of GH4169 were deformed, as shown in Figure 9. The tool tip has a relative slip with the workpiece surface, and the grain rotates along the grain boundary, and the direction of rotation is consistent with the feeding direction of the tool. As a result, the strain concentration phenomenon of GH4169 is intensified and the surface hardness is increased. Compared with nanofluid MQL milling, the grain slip exists in both dry milling and MQL, and more intense work hardening occurs in dry milling, where grain slip is more obvious. Under dry cutting and MQL conditions, the plastic flow capacity and microdamage of the material surface are more severe than that of nanofluid MQL milling, which also indicates that the reduction in the cutting force in the milling direction is extremely beneficial to restrain the change in the material microstructure. The variation rules and values of the surface roughness of GH4169 under different cooling modes are shown in Figure 10a and Table 5, respectively. Air cooling, MQL, and nanofluid MQL have an improved surface roughness compared to dry milling, and the reduction rate of nanofluid MQL is the most significant. When the nozzle sprays air into the machining area for cooling, the pressure of the air can blow away the chips, reducing the possibility of the chips scratching the workpiece surface. During MQL and nanofluid MQL milling, the cutting oil forms a lubricating film between the workpiece surface and the cutting edge, thus reducing the friction between the contact surfaces. Nanoparticles can significantly improve the surface quality of the workpiece by rolling, protecting, repairing, and polishing. Meanwhile, nanoparticles mixed in lubricating oil play a rolling and protective role in the lubricating film. On the other hand, the nanoparticles precipitate in the ravines of the workpiece surface to fill the ravines [8], as shown in Figure 11. The lubricating film formed by the nanoparticles is removed as the cutting continues, but due to the continuous injection of the MQL system during the machining process, a new lubricating film is reformed and attached to the tool blade and workpiece surface. According to the above analysis, the order of the workpiece surface roughness under different cooling modes is dry cutting > air cooling > MQL > nanofluid MQL. Figure 8: Variations in hardness with different milling parameters: (**a**) cutting speed; (**b**) feed per tooth; (**c**) radial cutting depth. Figure 9: Microstructure of different cooling modes: (**a**) dry milling; (**b**) MQL; and (**c**) nanofluid MQL. The variation rules of the cutting force of GH4169 under different cooling modes are shown in Figure 10b. As the heat transfer performance in the cutting area enhanced under the MQL and nanofluid MQL milling, the friction and adhesion between the tool and GH4169 were reduced, resulting in the phenomenon of decreased cutting force. In addition, the atomized cutting oil inhibited the reaction between the air and the cutting layer, reducing the degree of work hardening. Therefore, the order of the cutting force under different cooling modes is dry cutting > air cooling > MQL > nanofluid MQL. Figure 11: Schematic diagram of milling processing with different cooling methods. Figure 10: Surface roughness and cutting force changes with cutting speed and cooling modes: (**a**) surface roughness; (**b**) cutting force.