## 3 Results and Discussions
### Hole Quality  
CFRP is a typical anisotropic material, which is different from the titanium alloy and belongs to isotropic material with simple influence law. Different layers of CFRP have different material removal during cutting due to the influence of the fiber cutting angle. Figure 15 is a schematic diagram of material removal corresponding to the angle between 0 and 180\({}^{\circ}\). CFRP laminates are composed of carbon fibers and resin matrix, which determine the main material breakage is fiber/resin debonding and fiber breakage. Carbon fibers are mainly brittle before fracture. Fiber/resin debonding occurs close to the 0/180\({}^{\circ}\) fiber cutting angle area. As shown in Figure 16a,b, carbon fibers are fully exposed to the machined surface during ordinary reaming. Some carbon fibers break along the transverse direction because when the fibers/resins are debonded, they break after bending to a certain extent. It can also be seen that a small portion of the fibers break lengthwise due to the extrusion of the carbon fibers under the action of the cutting edge and the flank face. Figure 16c is a break diagram of CFRP fibers under ultrasonic reaming, from which it can be seen that the exposed fibers show significantly less transverse breakage and no longitudinal breakage. Figure 14: Surface morphology of titanium alloy: (**a**) entrances in CR; (**b**) entrances in UVR; (**c**) exit in CR; (**d**) exit in UVR. When the angle of cut of the fiber angle is \(15^{\circ}\), the surface morphology is 3000 times larger under the scanning electron microscope, as shown in Figure 17. From Figure 17a, it can be seen in the fracture diagram of ordinary reamed fibers that the fiber fracture is parallel to the cutting surface. From 5000 times magnification of local fibers, the roughness of the fracture surface can be observed, and extrusion fracture of the fibers is obvious. In addition, some of the carbon fibers were exposed to the surface, indicating that there was also a fiber/resin debonding fracture. When the cutting edge direction of the reamer is the opposite to that of fiber, the debonding and fracture of fiber/resin occurs under the push of cutting edge, but it is shorter than the cutting angle of \(0/180^{\circ}\). A recess in Figure 17a indicates a very uneven breakage during ordinary reaming. Figure 17b shows the breakage pattern of fibers during ultrasonic reaming. Compared with ordinary reaming, the breakage is uniform, and the cross section is smooth. Figure 16: Diagram of \(0/180^{\circ}\) fiber cutting angle: (**a**) CR; (**b**) CR; (**c**) UVR. The area with the most surface pits occurs near the cutting angle of the fibers at 40\({}^{\circ}\), as shown in Figure 18a. With ordinary reaming, the machined surface is serrated, and the surface quality is poor. The pit is the main cause of the formation of the serrated surface. It can be observed from the fracture surface that three fracture modes exist and alternate, including fiber shear fracture, fiber bending fracture, and fiber/resin debonding. Figure 18b shows the shape of the machined surface during ultrasonic reaming. It can be seen that the fiber/resin debonding phenomenon is significantly reduced, the serrated shape is more uniform, and the pit fluctuation is smaller than that of ordinary reaming. As the surface quality at the cutting angle of this fiber is the worst, it is the main reason for the formation of the hole wall morphology. As shown in Figure 19, the 70\({}^{\circ}\) fiber angle, observed 8000 times larger under scanning electron microscopy, indicates the morphology, which is typical of fiber bending fracture and partial extrusion fracture. It can be seen from the diagram that the form of the fiber/resin debonding fracture is limited, so the depth of defects in this surface layer is very limited, and most of the fiber fracture is caused by bending fracture. Figure 19a is the morphology of ordinary reaming.