## 5 Conclusions  
In this study, titanium alloy TC21 is milled using a coated tool (coated with TiAlN, and the substrate material is mainly WC) under three lubrication methods (dry, high-pressure air cooling, and MQL). By analyzing the surface topography, roughness, and microhardness of the milled titanium alloy TC21, the main findings may be as follows:  
1. In terms of microscopic topography, the defects on the milled machined surface mainly include feeding tool marks and surface scratches, banding bulges, scalp spurs, tearing dimples and cracks, and secondary material deposition. Under high-pressure air cooling and dry milling conditions, the feeding tool marks on the machined surface are not obvious due to the thermoplastic deformation. Plastic deformation of the milled surface is the main factor affecting surface roughness. When the machining parameters are too large, a great amount of black scars will be formed on the dry milling surface, which affects the surface machining quality. Under the MQL condition, the feeding tool marks are the most important factor affecting the surface quality of titanium alloy TC21.
2. In terms of surface roughness, the effect of radial cutting depth on surface roughness increases under high-pressure air cooling and dry milling conditions. Under the MQL condition, the lubrication and cooling effect of high-pressure oil mist can reduce the scraping between the tool and workpiece surface and reduce the plastic deformation of the machined surface. Although the feeding tool marks are obvious, the surface roughness is still significantly reduced, and the machined surface quality is improved.
3. The factors affecting the surface microhardness of titanium alloy TC21 come from three aspects: the thermal softening of the machined surface caused by cutting heat, the deformation and fibrillation of the internal grains of the material, and the dense oxide layer on the machined surface. The dry milling surface has the largest microhardness of the surface layer, the widest softening layer of subsurface, and the largest microhardness of the rehardening layer.
4. A surface roughness model considering milling vibration is developed. In the orthogonal experiments, the maximum error between the theoretical values and the experimental values is 18.19%. The minimum value of error is 5.98%. The error values of all experiments are within 20%. Therefore, the model can be considered valid and acceptable. It is worth noting that the theoretical values of surface roughness are all smaller than the experimental values because this model does not consider the effects of surface burrs and tool wear.  
Figure 29: The milled surface of titanium alloy TC21 with optimized parameters.  
**Author Contributions:** Z.W.: Conceptualization, methodology, software, validation, formal analysis, data curation, writing--original draft, and writing--review and editing. H.L.: Resources, supervision, and project administration. T.Y.: Funding acquisition and supervision. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was funded by the Major State Basic Research Development Program of China [No. 2017 YFA0701200], and the Science and Technology Planning Project of Shenyang [No. 18006001].  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The datasets supporting the conclusion of this article are included within the article.  
**Conflicts of Interest:** The authors declare no competing interests.