## 3 Results and Discussion  
### Effect of FR on the 7075 Aluminum Alloy Blank  
Figure 6 shows the macro-morphology of an aluminum alloy blank deformed by the RF process under varied values of FRs. As shown in Figure 6a, the RF-deformed 7075 aluminum alloy blank had four different diameters, indicating the FRs of 30.56%, 42.91%, 52.54%, and 75%, respectively. The value of the FR increases with the decrease in the diameter, which represents an elevation in the strain energy stored in the RF-deformed blank, when the diameter decreases under the RFPD process [52]. As presented in Figure 6b-e, the RF-deformed blanks with different FRs had smooth surfaces without any defects, such as surface cracks or pits, which indicated that the 7075 aluminum alloy with a diameter of 90 mm used in this work can be successfully deformed by the RFPD process, with an FR up to 75%. Moreover, some characteristic lines (marked with a red A) can be obviously seen in Figure 6b-e, which can be attributed to the deformed process of RFPD in this work. During the RFPD process [53; 54; 55], the rotary 7075 aluminum blank is fed along the axial direction. Meanwhile, a high-frequency reciprocating forging for the 7075 aluminum blank is carried out by four hammer dies distributed along the radial direction of this blank. Therefore, the characteristic lines appeared on the outer surface of the RF-deformed 7075 aluminum alloy blank.  
Figure 6: Effect of FR on the macro-morphology of the aluminum alloy blank.  
Figure 7 shows the effect of FR on the metallographic structure of the aluminum alloy blank. The longitudinal and transverse sections of the metallographic structure for the 7075 aluminum alloy are shown in Figure 7a,b, respectively. The directionality was more obvious in the longitudinal microstructure of the 7075 aluminum alloy bar along the extrusion direction, while the microstructure of the transverse section was more uniform. The above phenomenon was mainly determined by the initial state of the microstructure for the 7075 aluminum alloy blank. The 7075 aluminum alloy blank used in this work was in the extruded state, which was formed by the hot-extrusion process. Therefore, the longitudinal section microstructure of this blank might be distributed along the extrusion direction. It was also shown that, despite the subsequent annealing treatment being performed for this extruded blank, it was not possible to completely eliminate the deformed fibrous microstructures obtained by the extrusion process.  
According to the principle of the RFPD process [54; 56], the main deformation mode in the RFPD process is radial reduction rather than torsion deformation. Therefore, the  
Figure 7: Effect of FR on the microstructure of the aluminum alloy blank.  
longitudinal section microstructure of the RF-deformed blank is more important for us to investigate the effect of FR on the microstructures of the RF-deformed 7075 aluminum alloy blank in this work. The microstructures shown in Figure 7c-f are the experimental results corresponding to the longitudinal section microstructure of the RF-deformed blank with different FRs shown in Figure 6b-e, respectively. Comparing Figure 7a,c, it can be seen that, after applying RFPD with an FR of 30.56% to 7075 aluminum alloy blank, its microstructure changed significantly, where the fibrous microstructure distributed along the RFPD direction. The densification degree of the fibrous microstructure in Figure 7c increased significantly compared to Figure 7a. As shown in Figure 7d, the densification degree of the fibrous microstructure further increased when the FR was 42.91%, but the difference in densification degree for the fibrous microstructure was not obvious as the FR further increased to 52.54% or 75%. It showed that, after the RFPD process of 7075 aluminum alloy blank, the densification degree of its fibrous microstructure gradually increased with increasing the FR, but the deformation degree tended to be stable when its FR exceeded 52.54%.