## 4 Results and Discussion
### Performance and Microstructure  
Based on the above study, we obtained the influence law of geometric accuracy of truncated pyramid component under different forming angles. For this purpose, further heat treatment processes were carried out to obtain mechanical properties for components. The truncated pyramids SPIFed at three different forming angles are T8 treated and then tested for their tensile properties (Table 2). At the forming angle of 51\({}^{\circ}\), the tensile strength, yield strength and elongation were 542.1 MPa, 532.5 MPa and 5.4%, respectively. When the forming angle is 54\({}^{\circ}\), the tensile strength increases to 551.3 MPa, the yield strength decreases to 523.5 MPa, and the elongation increases to 7.6%. When increasing the forming angle to 57\({}^{\circ}\), the tensile strength, yield strength and elongation increased to 562.3 MPa, 533.6 MPa and 8.6%, respectively. It is found that the sample SPIFed at a forming angle of 57\({}^{\circ}\) has the best mechanical properties; it is compared to that made at low forming angle. In order to understand the differences in mechanical properties between the above samples, observations and analyses from the microstructural point of view were carried out in this paper. EBSD and TEM observations were conducted on the wall area. As can be seen in Figure 11 below, during SPIF, the grains grows along the forming tool movement direction, and a large number of low angle grain boundaries can be observed inside the grains (grain boundaries of 2-15\({}^{\circ}\) are marked white; grain boundaries greater than 15\({}^{\circ}\) are marked black). When the forming angle is 51\({}^{\circ}\), obvious coarse grains (grain size approximately 180 \(\upmu\)m, as shown by the black dashed line in Figure 9a are found). There were not many low angle grain boundaries observed inside the grains, indicating that dynamic recrystallization occurred during the forming process, with some grains experiencing abnormal growth. The uneven grain size indicates that the deformation is not uniform when the forming angle is 51\({}^{\circ}\). Moreover, it is found that the grains are refined with increasing forming angle. It can be considered that at the forming angle of 51\({}^{\circ}\), it is at the critical degree of deformation, after which the recrystallized grains become finer with increasing deformation. When the forming angle is 54\({}^{\circ}\), obvious coarse fibrous grains can be observed. There are fine recrystallized grains forming a chain like structure at the grain boundaries of fibrous grains (as shown by the black dashed line in Figure 11b). The grain orientation of recrystallized grains is not the same as that of the fibrous grains, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Forming Angle** & **Tensile Strength/MPa** & **Yield Strength/MPa** & **Elongation** \\ \hline
51\({}^{\circ}\) & 542.1 & 532.5 & 5.4 \\ \hline
54\({}^{\circ}\) & 551.3 & 523.5 & 7.6 \\ \hline
57\({}^{\circ}\) & 562.3 & 533.6 & 8.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Tensile properties of SPIF angles of 51\({}^{\circ}\), 54\({}^{\circ}\) and 57\({}^{\circ}\) after T8 aging. Figure 11: EBSD grain distribution at different forming angles: (**a**) 51\({}^{\circ}\) (**b**) 54 and (**c**) 57\({}^{\circ}\). because discontinuous dynamic recrystallization occurs during the forming process [27; 28]. Dislocations tend to accumulate at grain boundaries and rearrange to form small angle grain boundaries. With the increase of strain induced dislocations, small angle grain boundaries transform into large angle grain boundaries, forming recrystallized grains. When the formation angle is increased to 57\({}^{\circ}\), the size of the grain decreases and there are a large number of fine equiaxed recrystallized grains formed, with an average grain size of 8.3 \(\upmu\)m, indicating a good grain refinement effect. Earlier studies [24] have shown that the surface quality worsens under large grain size. which is consistent to our observations.