## 4 Conclusions  
In this study, experiments on the UVR of CFRP/Ti stacks were conducted. The experiments studied the influence of different ultrasonic amplitudes on reaming thrust and the influence of tool speed on thrust, dimensional accuracy, and surface roughness under optimal ultrasonic amplitude:  
1. UVR can reduce the roughness of CFRP/Ti stacked holes, improve diameter and roundness, reduce entrance and exit defects of laminated holes, and improve the quality of holes. Therefore, UVR is an effective finishing method;
2. At the same rotational speed, the higher the ultrasonic output power, the more obvious the reduction in thrust forces. When the amplitude reaches 6 \(\upmu\)m, the average thrust forces in UVR were decreased by over 57% (titanium alloy) and 40% (CFRP), respectively, compared to CR. At different tool rotational speeds, the thrust force at  
Figure 21: Diagram of 150\({}^{\circ}\) fiber cutting angle: (**a**) CR; (**b**) UVR.  
Figure 22: SEM images of Ti hole wall drilled by: (**a**) CR; (**b**) UVR.  
UVR is less than CR when machining CFRP/Ti stacks. When the rotational speed is higher, the thrust force is reduced more dramatically;
3. The surface roughness of holes for CFRP and the titanium alloy fabricated by UVR is less than that by CR. In addition, the difference with CR is more obvious under the condition of lower tool rotational speed. When the rotational speed is 200 rpm, the roughness of CFRP is reduced by 20% with UVR and 28% for the titanium alloy;
4. With the introduction of ultrasound, the diameter difference between CFRP (around 11 \(\upmu\)m (CR) vs. 7 \(\upmu\)m (UVR)) and the titanium alloy (around 17 \(\upmu\)m (CR) vs. 5 \(\upmu\)m (UVR)) holes becomes smaller. CFRP/Ti stacked holes machined by UVR have better roundness than those by CR;
5. With CR, the hole of CFRP has fiber pullout and more fiber protrusion at the entrance/exit, while it has fewer defects with UVR. There are burrs and chippings at the entrance and exit of the titanium alloy with CR, while burrs and chippings can be reduced with UVR. Fiber arrangement of CFRP hole walls is uniform by UVR, whereas fiber breakage occurs by CR;
6. UVR is a effective method for reaming CFRP/Ti stacked holes. Concerning the change of feed speed with material, 1000 rpm is more advantageous to the reduction of thrust force and the improvement of surface roughness. Tool speed selection at 600 rpm is more conducive to the reduction of hole size error. Tool speed is selected at 800 rpm, which is favorable for keeping the roundness of hole diameter.  
Conceptualization, Y.W. and S.L.; methodology, Y.W.; software, Y.Q.; validation, R.K., Z.D. and X.G.; formal analysis, Y.W.; investigation, S.L.; resources, Z.D.; data curation, Y.W.; writing--original draft preparation, S.L.; writing--review and editing, Z.D.; visualization, R.K.; supervision, X.G.; project administration, R.K.; funding acquisition, R.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (Grant No. U20A2091), High Level Talents Innovation Plan of Dalian (Grant No. 2020 RD02).  
Not applicable.  
This work very much for the funding of National Natural Science Foundation of China fund and High Level Talents Innovation Plan of Dalian. All the authors are greatly acknowledged for their financial support in making this research possible.  
The authors declare no conflict of interest.