## 5 Discussion and Application  
By analyzing the microstructure of the joint, it can be found that friction stir processing can overcome the difficulties encountered in the fusion route of surface modification like cracks, porosity, segregation, and grain growth. There are obvious boundaries between the WNZ, TMAZ, HAZ, and the base material (BM), and the boundary between the WNZ and the TMAZ is particularly obvious, as shown in Figure 13. The microstructure of each zone is different. The microstructure of the weld nugget zone is relatively small, and the microstructure of the heat-affected zone is slightly larger than that of the weld nugget zone. The microstructure of the heat-affected zone is affected by the rotation of the FSW tool and the heat dissipation of metal rheology during welding, and the grain size grows and coarsens. Compared with the base material structure, the rolling characteristics of the heat-affected zone structure are reduced, and the coarsened grain size is close to the base material [38; 39]. The frictional heat generated by the rapid rotation of the FSW tool promotes dynamic recrystallization of the weld metal to refine the grain, so that the weld has a finer microstructure than the BM. Therefore, the performance improvement of friction stir welding joints is mainly achieved through the fine grain strengthening (FGS) of the weld microstructure. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Tensile Strength Mpa** & **Percentage Elongation after Fracture \(A\) (\%)** \\ \hline
1 & 372.86 & 6.79 \\
2 & 383.52 & 7.04 \\
3 & 381.59 & 7.25 \\ Average value & 379.32 & 7.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Tensile test result of joint. Figure 12: Tensile fracture locations of joint: (**a**) Tensile specimen front face; (**b**) tensile specimen side face. The mechanical properties of the welded joints with the above process parameters were analyzed. Due to the difference of heat input in the upper, middle, and lower parts of the weld, the microstructure of the weld closer to the upper part was refined significantly, and the hardness of the upper part was significantly higher than that of the lower part. The hardness curve shows that the hardness of the advancing side was slightly higher than that of the retreating side. In this paper, the feasibility of obtaining process parameters by means of simulation was discussed first. Considering the large workload and high testing cost of optimizing process parameters by means of experiments alone, if the simulation and experiments have a high matching degree, the finite element model can be reused in the subsequent optimization of process parameters and applied to the preliminary analysis in other welding fields. The finite element model was updated through experimental data collection to improve the simulation accuracy. Considering the cost and rigor, the welding test was carried out on a flat plate first, and the microstructure and mechanical properties of the flat plate were analyzed after the welding test. It was found that it could reach the space application level. Finally, the process parameters were applied to the curved surface welding of the rocket top cover by the friction stir welding robot developed independently. Most of the existing friction stir welding equipment is based on the transformation of numerical control machine tools, which can only be adapted to the connection of regular welds, have a single function, and lack process flexibility. It is difficult to realize the welding of complex weld workpieces. In view of the large thin-walled curved surfaces that need to be welded in the aviation and aerospace fields, the number of welds is large and presents complex curves in three-dimensional space [40].