## 3 Results and Discussion
### Effect of the Isothermal Process on the RF-Deformed Blank  
The longitudinal section microstructures of the RF-deformed blank after SSIHT at 620 \({}^{\circ}\)C for 9 min are shown in Figure 8, in which the FRs of the RF-deformed blank are 30.56%, 42.91%, 52.54%, and 75.00%. Comparing Figures 7 and 8, it reveals that, after the RF-deformed blank with different FRs was treated at the temperature of 620 \({}^{\circ}\)C for 9 min, its microstructure transformed from a fibrous deformation microstructure to a semi-solid microstructure, where a uniform distribution of a large number of solid grains was observed in the liquid phase. This was mainly because a large amount of strain energy was accumulated in the aluminum alloy blank after the RFPD process. However, in the subsequent IHT process, this accumulated strain energy was released, which promoted grain recrystallization and growth. At the same time, the above phenomena also indicated that the RFSIMA process including RFPD, and the followed IHT can be successfully used to produce 7075 aluminum alloy SSB with fine and spherical microstructures. Figure 8: Microstructure of the RF-deformed blank after SSIHT at 620 \({}^{\circ}\)C for 9 min. As shown in Figure 8a, when the FR was 30.56%, more solid grains can be observed, but most of them were bonded together. This was because, at a low FR, relatively low energy was accumulated in the material, which resulted in less liquid phase generated during the IHT process. As the FR increased to 42.91%, the AGS in the microstructure was 42.64 \(\upmu\)m with an ASF of 0.78. Thin liquid films can be seen in the microstructure, as shown in Figure 8b, which effectively separated the solid grains. As presented in Figure 8c,d, the thickness of the liquid film increased, and the distribution of the solid grain was more uniform when the FR increased to 52.54% or 75%. Their AGSs were found to be 41.92 and 42.57 \(\upmu\)m, with ASFs of 0.80 and 0.81, respectively. According to the analysis results, it can be seen that, when the FR increased from 42.91% to 75%, the thickness of the liquid film in the microstructure enlarged, where no significant change in the grain size was observed. However, the spheroidizing feature in the microstructure was improved when the FR increased to 75%. This increase in the thickness of the liquid film and the grain spheroidizing effect would contribute to the flowability improvement of the SSB during the SSFM process. Hence, the FR of 75% was a reasonable process parameter for preparing the 7075 aluminum alloy SSB in this work. The microstructures of the RF-deformed blank with 75% FR after SSIHT at 610 \({}^{\circ}\)C to 630 \({}^{\circ}\)C with different duration times are shown in Figure 9. As shown in Figure 9a, the microstructure after IHT with the ST of 610 \({}^{\circ}\)C and the DT of 3 min changed slightly compared with the microstructure with 75% FR shown in Figure 7f. The directionality in the whole microstructure along the RFPD direction was obvious, which was mainly attributed to the fact that the DT was too short for the generation of blank recrystallization. When the duration time was extended to 6 min, a small number of recrystallized grains appeared, which can be seen in Figure 9b. As presented in Figure 9c, when the DT was further extended to 9 min, a large number of fine solid grains appeared in the microstructure, but these grains were bonded together due to the low liquid fraction caused by the low ST. As depicted in Figure 9d-f, when the ST increased to 620 \({}^{\circ}\)C, a large number of fine recrystallized grains bonded together can be seen when the DT is 3 min. When the DT extended to 6 min at an ST of 620 \({}^{\circ}\)C, more obvious fine solid grains, as well as liquid films, can be seen. When the DT further increased to 9 min, the thickness of the liquid film enlarged, which can effectively separate the solid grains, where fine, uniformly distributed semi-solid spherical grains were obtained.