## 5 Conclusions and Outlook  
The optimization and control of machined surface integrity is the key technology to ensure the functional performance and service life of parts. The effects of cutting parameters such as cutting speed, feed speed, cutting depth, and tool wear on the machined surface integrity during milling are emphatically reviewed. At the same time, the research progress of the relationship between the  
Figure 10: Fatigue life vs. tool wear and fatigue fracture pattern [42]. Figure 9: Comparison effects of residual stress on fatigue life between electro-discharge machining and milling [105]. machined surface integrity and the component use performance is also revealed. Researchers have made extensive research on the influence of machined surface integrity on fatigue properties and obtained a series of research results. However, there are still some urgent problems to be solved. 1. The effects of surface integrity on performance were mostly determined by experiments and could not be revealed effectively at the mechanism level. 2. No uniform standard has been formed to assess the effect of surface integrity parameters on fatigue performance, which limits the optimization of process parameters and the improvement of service performance of workpieces. 3. The local performance change, design, and service performance state of the parts could not be organically combined to form an interconnected mode. 4. Combined with the current research status and engineering application requirements, the following aspects can be emphatically studied in the future. 5. To strengthen theoretical research on fatigue performance optimization. The existing optimization models assume that the surface integrity evaluation indices (surface morphology, residual stress, microhardness, and metamorphic layer thickness) have the same effect on fatigue performance, so the weight of the coefficient of the optimization model is consistent. In order to make the optimization result more accurate, the corresponding mathematical analysis method can be considered to distinguish the weight of various indexes. 6. To strengthen the research on modeling technology of machined surface integrity and workpiece performance. The analytic model of residual stress of workpiece surface during milling is still a difficult problem. The reason is that the change of chip thickness on cutting edge is more complicated, which results in the nonlinear gradient distribution of cutting force and cutting temperature in machining process. Only some physical factors (vibration, tool/workpiece deformation, and cutting force) are considered in the analytical model of surface topography, while tool wear and the characteristics of interrupted cutting are not taken into account, so there is still a large error in the simulation results. 7. Development of new milling processes. The reasonable selection of process parameters directly affects the machined surface integrity and controls the workpiece performance. How to develop a new milling process based on different machining systems, processing environment, workpiece materials, and performance needs to be further explored. 8. The relationship between the design-manufacturing-service performance of parts is explored. A complete interconnected mode of design, manufacturing, and service performance evaluation of parts was established. 9. The theoretical analysis and experimental research on the response relationship between high temperature fatigue, vibration fatigue, corrosion fatigue performance, and surface integrity of processing under environmental conditions are further improved. The idea of this project was conceived by [PERSON] [PERSON] and [PERSON] consulted to relevant high-level papers on aspects of changes in surface integrity caused by changes in the milling process, and wrote this project. [PERSON] and [PERSON] reviewed this project and proposed constructive guidance to make the article more complete. This research was funded by the National Natural Science Foundation of China (Grant No.