# Study on the Machinability of Ni-Based Superalloy by
## 3 Result and Discussion
### Effects of Milling Parameters
As the low feed per tooth and radial cutting depth increase and the cutting speed decreases, the milling forces increase, resulting in easier plastic flow of the material and poor surface quality. Figure 4: Variations in GH4169 surface roughness with feed per tooth: (**a**) \(f\)= 4 \(\upmu\)m; (**b**) \(f\)= 13 \(\upmu\)m; (**c**) changing law. Figure 5: Variations in GH4169 surface roughness with radial cutting depth: (**a**) \(a_{p}\) = 0.03 mm; (**b**) \(a_{p}\) = 0.09 mm; (**c**) changing law. Figure 6: Variations in GH4169 cutting force with different milling parameters: (**a**) cutting speed; (**b**) feed per tooth; (**c**) radial cutting depth. During the milling process of GH4169, a large amount heat was generated in the first deformation zone due to the compression and shear deformation. With the continuous deformation of the material, the fibrous tissue was formed, namely, the second deformation zone. In the third deformation zone, the machined surface produced plastic deformation and material rebound, resulting in the work hardening of the workpiece surface. A uniform work hardening layer can improve the wear resistance and fatigue strength of the workpiece, but work hardening often leads to micro cracks on the workpiece surface due to the nonuniformity, which increases the hardness and decreases the surface toughness, resulting in cutting tumor defects, thus reducing the mechanical properties. The work hardening characteristics of the GH4169 superalloy were evaluated using the workpiece surface hardness, work hardening rate, and hardening layer depth. \[N_{H}=\frac{H-H_{0}}{H_{0}}\times 100\% \tag{1}\]  
where \(N_{H}\) is the work hardening rate, and \(H\) and \(H_{0}\) are the Vickers hardness of the machined surface and the substrate surface, respectively. The hardness variation trends of the machined surface are shown in Figure 8. Compared with the average hardness of the GH4169 Ni-based superalloy of 342 HV\({}_{0.5}\), the maximum hardness of the machined surface with different cutting parameters is less than 400 HV\({}_{0.5}\), the work hardening rate is less than 17%, and the work hardening layer depth is about 200 \(\upmu\)m. The different cutting parameters have a nonsignificant effect on the work hardening layer depth, but the hardness of the machined surface increases compared with that of the GH4169 substrate. When the cutting speed increases from 47.1 m/min to 75.4 m/min, the surface hardness and work hardening rate increase from 380 HV\({}_{0.5}\) to 391 HV\({}_{0.5}\) and 111.1% to 114.3%, respectively. This is because the temperature of the machining area increases with the increasing cutting speed, which leads to the phase transformation and recrystallization of the machined surface. The surface hardness and work hardening rate of the GH4169 Ni-based superalloy increased slightly when the feed rate and radial cutting depth were increased to 13 \(\upmu\)m/z and 0.09 mm, respectively. This is because the workpiece removal rate and cutting force increased in the milling process, and the workpiece surface extrusion and plastic deformation intensified, resulting in a change in the workpiece surface hardening. GH4169 Ni-based superalloy is a face-centered cubic structure material which is prone to plastic deformation due to the increased crystal slip. In the process of grain deformation, the number of dislocations between grains increases, and entanglement occurs between the dislocations [25] which leads to work hardening of the machined surface.