## 1 Introduction  
Carbon fiber-reinforced plastic/titanium alloy (CFRP/Ti) stacks are widely used as joint components with a high load-bearing capacity in the wings and fuselages of aircrafts [1; 2; 3; 4]. CFRP/Ti stacks possess not only the advantages of CFRP [5], such as high specific strength, good vibration damping, and good fatigue resistance, but also the advantages of titanium alloys [6], such as superior strength, low density, excellent hardness, good corrosion resistance, extreme temperature resistance, etc. As a result, with the effects of improving the overall strength and achieving the lightweight demand, CFRP/Ti stacks have gradually replaced traditional alloy materials in the advanced aircraft manufacturing industry. Laminated material components are usually fastened via bolt holes, which determine the assembly strength and become vital to flight safety [7]. Aircraft assembly requires extremely high-quality bolt holes, particularly holes that cannot be damaged. As both CFRP and titanium alloys are typical difficult-to-machine materials, the laminated materials prepared from them will face new challenges when machining. CFRP is highly susceptible to defects such as delamination and tearing with conventional drilling [8]. In contrast, titanium alloys are prone to defects such as chipping at the entrance and burrs at the exit. The hole quality from conventional drilling cannot satisfy aircraft assembly requirements. Therefore, reaming finishing of bolt holes is indispensable [9; 10]. Reaming is the process of removing a very thin layer of material from the hole wall with a reamer to improve dimensional accuracy and surface quality. However, the bolt holes of CFRP/Ti stacks made by the existing reaming process still cannot reach the excellent tolerance requirement [11; 12; 13; 14; 15; 16]. Moreover, other problems also exist. The dimension of the machined diameter and roundness is unstable. The hole entrance of CFRP materials is prone to burrs, while residual damage may appear at the hole exit. This can harm the stack's mechanical integrity with negative effects such as the loosening of bolted connections. Ultrasonic-assisted drilling has been widely focused on by researchers in recent years due to its advantages of low thrust forces and enhanced surface quality. [PERSON] et al. [17] drilled CFRP/Ti stacks with variable feed rate machining and concluded that drilling stacks with variable feed rates (low feedrate should be used for machining of Ti, and machining of CFRP could be conducted using feedrate 10 times higher) at different ultrasonic output powers resulted in lower axial forces than with fixed feed rates. In addition, the cycle time using variable feedrate was shorter than that using fixed federate, which led to lower tool wear. [PERSON] et al. [18] carried out an experimental study of the ultrasonic vibration-assisted helical milling drilling process for laminated composites and found that ultrasonic vibration-assisted helical milling reduces thrust forces, hole exit tears, burrs, and other defects. [PERSON] et al. [19; 20; 21] drilled CFRP with ultrasonic longitudinal and torsional vibration, which reduced the thrust forces and hole exit burrs and reduced the surface roughness of the hole wall due to the torsional vibration of the side edge of the drill in longitudinal, and torsional vibration helps strengthen the effect of resin adhesion. It has been proven that the variable stress state makes the fibers more susceptible to shear fracture by kinematic characteristic analysis. [PERSON] et al. [22] introduced ultrasonic elliptical vibration-assisted reaming into finish drilling CFRP/Ti stacked bolt holes and showed that it can reduce axial forces and torques, where the average axial force can be reduced by 30%, and the geometric accuracy of the hole is better compared to conventional reaming (CR). In dry reaming, it is of great value to select ultrasonic power and cutting parameters quickly for efficient practical machining.