## 2 Experiments  
The experimental setup diagram of UVR for CFRP/Ti stacks is shown in Figure 1a. The adopted experimental platform is a 5-axis vertical machine center (KMC600 SU, Kede Machine Tool Co., Ltd., Dalian, China), which includes the UVR unit and the thrust forces measurement unit, as shown in Figure 1b. The UVR unit consists of an ultrasonic generator, a UAR tool holder, and a carbide reamer. The ultrasonic generator converts conventional (220 v, 50 Hz AC) power into high-frequency electrical energy, which will supply the piezoelectric transducer. The piezoelectric transducer converts the high-frequency electrical energy into mechanical vibrations. The ultrasonic vibrations from the transducer are amplified and transmitted to the reamer, which causes the reamer to vibrate axially at high frequency. It has been proven that the greater the output power, the greater the amplitude. The amplitude of the ultrasonic vibrations can be adjusted by varying the output power of the power supply. The experimental materials are CFRP (T700-12K/AG80) and titanium alloy (Ti6 AI4V) stacks, where CFRP plate and titanium alloy plate are glued together, as shown in Figure 1c. CFRP is at the top and titanium is at the bottom. This stacking method is a common stacking sequence in industry. When the CFRP is at the bottom and the titanium alloy is at the top, the tool passes through the titanium alloy at a high temperature and cokes the CFRP at the bottom. According to the company's requirements, the top CFRP is being placed and the bottom titanium alloy is being placed. This is a commonly used direction for titanium alloys in industry. The thicknesses of the CFRP and the titanium alloy are 7 mm and 8 mm, respectively. The reason for the different thicknesses is that they are bonded with glue in the middle, close to 1 mm, similar to the CFRP bonding method, which is essential to ensure the same thickness of the two layers. The fiber laying direction of CFRP is 0\({}^{\circ}\) and 90\({}^{\circ}\), and the stacking order is \(0/90/0_{5}/90/0_{25}\). The main component of CFRP is carbon fiber, and its resin matrix is epoxy resin. The CFRP plate is placed on the top of the titanium alloy plate, and their compositions and performance parameters are shown in Tables 1-4. Figure 1: Experimental setup: (**a**) the schematic diagram; (**b**) the overall view of platform; (**c**) the side view of platform. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Item** & **Material** & **Substrate** & **Pavement** & **Fiber** & **Fiber** & **Bundles** \\  & **Grades** & **Material** & & & **Fraction/\%** & **Fiber** \\ \hline Value & T700 & AG80 & 10 layers & 60 & 6 \(\mu\)m,12 K \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Compositions of T700-12K/AG80 CFRP. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Element** & **Al** & **V** & **Fe** & **O** & **C** & **N** & **H** & **Ti** \\ \hline Content/wt\% & 5.5–6.75 & 3.5–4.5 & \textless{}0.25 & \textless{}0.2 & \textless{}0.88 & \textless{}0.05 & \textless{}0.01 & Residuals \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Properties of Ti6 Al4V. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **Value** & \\
**Transverse modulus of elasticity/GPa** & 40 \\ Longitudinal modulus of elasticity/GPa & 230 \\ Main Poisson’s ratio & 0.26 \\ Transverse shear modulus/GPa & 14.3 \\ Longitudinal shear modulus/GPa & 24 \\ Longitudinal tensile strength/MPa & 4900 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Properties of T700-12K/AG80 CFRP. \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Value** \\ \hline Density/(g\textless{}cm^{-3}) & 1.55 \\ Transverse modulus of elasticity/GPa & 40 \\ Longitudinal modulus of elasticity/GPa & 230 \\ Main Poisson’s ratio & 0.26 \\ Transverse shear modulus/GPa & 14.3 \\ Longitudinal shear modulus/GPa & 24 \\ Longitudinal tensile strength/MPa & 4900 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical compositions of Ti6 Al4V alloy. Figure 2: The reamer.