## 2 Materials and Methods
### Experimental Procedure of the SSFF Process for the Connecting Rod  
The experimental press machine and SSFF die for the connecting rod are shown in Figure 4. Table 2 shows the process parameters used in the test process. First, to obtain the reasonable process parameters for the ideal SSB, as shown in Group I, the RF-deformed blanks with different FRs are treated at 630 \({}^{\circ}\)C with a time of 9 min to analyze the effects of FR on the microstructures. In this group, the 7075 aluminum alloy with a diameter of 90 mm is radially forged to obtain an RF-deformed stepped bar with diameters of 75 mm, 68 mm, 62 mm, and 45 mm for the FR of 30.56%, 42.91%, 52.54%, and 75%, respectively. The qualities of the semi-solid blanks with different FRs are analyzed to identify the reasonable FR. Then the RF-deformed blanks with the reasonable FRs are heated at the ST of 610 \({}^{\circ}\)C, 620 \({}^{\circ}\)C, and 630 \({}^{\circ}\)C. At each ST, the blank is treated for three different DTs of 3 min, 6 min, and 9 min. Therefore, the influence of ST and DT on the microstructures can be obtained through experiments in Groups II, III, and IV. Based on the results from experiments in Groups I, II, III, and IV, the optimal parameters for SSB can be identified. Then, in Group V, the AACR is produced by the forging forming of the ideal SSB. In Group VI, the RF-deformed blank under the reasonable FR is heated with the ST of 590 \({}^{\circ}\)C and DT of 10 min, which provides the material for the AACR production by forging forming. In both Groups V and VI, the die temperature is 250 \({}^{\circ}\)C, while the forging speed is 10 mm/s. Groups V and VI are designed to form the AACR by both SFF and SFF tests for the comparison of their quality, which aims to verify the advancement of the SSFF process for manufacturing the connecting rod.  
The quality of the semi-solid billet and the AACR is presented by the microstructure and tensile property analysis. To investigate the effect of process parameters, such as forging ratio, sustaining temperature, and duration time, on the microstructures of the semi-solid blank, the samples are cut from the RF-deformed blank with different FRs. To study the microstructure and tensile property of the AACR, samples are obtained from the positions as shown in Figure 5 for the microstructure and tensile analysis. The sample dimensions for the strength testing are shown in Figure 5b. A universal material testing machine (Instron-5982) is used for the strength testing, and the tensile rate of this machine is 1.25 mm/min. The microstructure samples are ground, polished, and etched for 90 s in the hydrofluoric acid solution. The optical microscope (NIKON ECLIPSE LV 150N) is adopted in the study to analyze the metallographic structure. The AGD and ASF of the metallographic structure can be calculated by Equations (1) and (2), respectively. The tensile property of the forming connecting rod is obtained by a material testing machine.  
\[\text{AGD}=\frac{\sum_{N=1}^{N}\sqrt{4A/\pi}}{N} \tag{1}\]  
\[\text{ASF}=\frac{\sum_{N=1}^{N}4\pi A/P^{2}}{N} \tag{2}\]  
where \(A\) is the effective area of one solid grain, \(P\) is the perimeter of one solid grain, and \(N\) is the grain number.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Group No.** & **Items** & **Forging Ratio** & **Sustaining Temperature (\({}^{\circ}\)C)** & **Duration Time (min)** \\ \hline \multirow{4}{*}{V} & SFF & Forming material & Die temperature (\({}^{\circ}\)C) & Forging Speed (mm/s) \\  & and SFF & Semi-solid billet prepared by IHT & & \\  & process & process with reasonable process & 250 & 10 \\  & & parameters & & \\ \cline{1-1}  & & Billet prepared by isothermal & & \\ \cline{1-1}  & & treatment with the 590 \({}^{\circ}\)C and 10 min & 250 & 10 \\ \hline \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 5: Microstructure and tensile sample position of the forming connecting rod.