## 3 Results and Discussion
### Experimental Investigation on the Forming Process of Connecting Rod  
The macro-morphology of the connecting rod produced by SSFF and SFF processes is shown in Figure 10, in which three connecting rods are produced by S \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Forging Ratio**} & **Sustaining** & **Duration Time** & **Figure** & **AGS (\(\upmu\)m)** & **ASF** \\  & **Temperature (\({}^{\circ}\)C)** & **(min)** & & & \\ \hline
42 91\% & 620 & 9 & Figure 8b & 42.46 & 0.78 \\
52.54\% & 620 & 9 & Figure 8c & 41.92 & 0.80 \\
75\% & 620 & 9 & Figure 8d/Figure 9f & 42.57 & 0.81 \\
75\% & 610 & 9 & Figure 9c & 15.26 & 0.62 \\
75\% & 620 & 6 & Figure 9e & 26.56 & 0.67 \\
75\% & 630 & 3 & Figure 9g & 25.54 & 0.72 \\
75\% & 630 & 6 & Figure 9h & 41.48 & 0.8 \\
75\% & 630 & 9 & Figure 9i & 56.12 & 0.76 \\ \hline \end{tabular}
\end{table} 
Table 3: Influence of process parameters on the AGS and ASF of the SSB. Figure 9: Microstructure of the RF-deformed blank with 75% FR after SIIHT: (**a**) 610 \({}^{\circ}\)C, 3 min, (**b**) 610 \({}^{\circ}\)C, 6 min, (**c**) 610 \({}^{\circ}\)C, 9 min, (**d**) 620 \({}^{\circ}\)C, 3 min, (**e**) 620 \({}^{\circ}\)C, 6 min, (**f**) 620 \({}^{\circ}\)C, 9 min, (**g**) 630 \({}^{\circ}\)C, 3 min, (**h**) 630 \({}^{\circ}\)C, 6 min, (**i**) 630 \({}^{\circ}\)C, 9 min. with the reasonable operating parameters, respectively. When the process parameters were set as the FR of 75%, ST of 590 \({}^{\circ}\)C, and DT of 6 min in the SFF process, the rod shaft and the small end of the connecting rod can be fully formed, as shown in Figure 10a,c. However, the big end of the connecting rod was not fully filled, which can be seen in Figure 10b. This was mainly because of the poor fluidity of the solid material during the SFF process resulting from the low sustaining temperature. Moreover, the big end of the connecting rod required a higher fluidity for the forming material in the SSF or SFF processes compared with the rod shaft and the small end of the connecting rod. As shown in Figure 10d-f, the semi-solid connecting rod formed by the SSFF process with the RF of 75%, ST of 630 \({}^{\circ}\)C, and DT of 6 min exhibited a better shape profile compared to that from the SSF process. The smooth surface was obtained for the connecting rod from the SSF process with no obvious crack, shrinkage, or any other defect. This indicated that the forming process for 7075 AACR is reasonable under the above process parameters, which in turn, verifies the feasibility and reliability of the SFF process for AACR in this work. Figure 10: Macro-morphology of the connecting rod. Microstructures at different positions of the connecting rod produced by the SFF and SFF process are shown in Figure 11a,b, showing the longitudinal section microstructure of the sample in the rod shaft and the cross-section microstructure of the sample in the small end of the connecting rod formed by the SFF process, respectively. It was revealed that elongated shapes were observed in the grains in the longitudinal section microstructure at the center position of the connecting-rod shaft. However, uniform and fine grains were found in the cross-section microstructure at the position of the small end. It indicated that the grains of the connecting rod transformed into fibrous microstructures during the forging deformation in the SFF process. The longitudinal section microstructure of the rod shaft for the connecting rod formed by the SFF process is provided in Figure 11c while that of the cross-section microstructure of the sample in the small end is shown in Figure 11d. It can be seen that the solid grain sizes in the longitudinal cross-section microstructure at the center position of the connecting-rod shaft and in the cross-section microstructure at the small end of the connecting rod were relatively uniform and spherical compared to that in Figure 11a,b. Moreover, no thick liquid film was observed between the solid grains.