## 7 Conclusions  
The purpose of this study was to optimize the production process by predicting the deformation and residual stress when FSW is applied to fillet joints in the production process of electric vehicle battery frames and to improve the quality and mechanical performance of the product. The main findings are as follows:  
1. In the case of butt FSW, the optimum process for each joining method is the rotational speed of the tool at 2000 RPM and the welding speed of 1000 mm/min. In the case of fillet joint FSW, the rotational speed of the tool was 1200 RPM and the welding speed was 80 mm/min. In the case of fillet joint FSW, it was considered that sufficient frictional heat was generated for stirring only when the welding speed was reduced by more than 10 times compared to butt joint FSW;
2. After friction stir welding (FSW) of the full-frame structure, the final deformation was less than \(\pm\)0.2 mm, and the residual stress was about 30 MPa. As for deformation tendency, deformation occurred in the length and width directions, but no significant deformation occurred in the height direction. The errors between the measured and analyzed values for deformation were 0.1 mm or less in all of the directions;
3. In the process of specifying the temperature of the heat source, the method of reducing errors through test correlation with the temperature measurement experiment results was effective;
4. Although this study did not conduct comparative experiments on residual stress, the simulation results were considered to be reliable in terms of deformation tendency and similarity. Strength tests were not performed on the welds, but this will be included in additional studies as durability tests on the structure are planned in the future.  
In this study, it was impossible to apply the T-joint FSW method, which was previously studied to perform FSW on the fillet joint, due to the designed cross-sectional structure. The tool was tilted at an angle of 45 degrees, and then fillet joint FSW was performed on the actual structure using a specially manufactured angle head. This attempt was successful, and the deformation and residual stress could be predicted and verified through finite element method analysis. We hope that this study will help reduce costs and time by predicting deformation and stress in advance through simulations when performing similar processes (such as fillet FSW on aluminum extrusions) and assist other studies related to FSW in the future.  
Figure 23: Stress distribution [unit: MPa].  
**Author Contributions:** Conceptualization, S.K. and H.K.; methodology, S.K. and J.K.; software, C.L. and H.K.; validation, H.K. and Y.J.; formal analysis, S.K. and J.K.; investigation, H.K. and K.L.; resources, S.K. and K.L.; data curation, S.K.; writing--original draft preparation, S.K.; writing--review and editing, S.K. and K.L.; visualization, H.K.; supervision, S.K.; project administration, S.K.; funding acquisition, S.K. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by the Ministry of Trade, Industry & Energy (MOTIE), Korea Institute for Advancement of Technology (KIAT) through the Encouragement Program for The Industries of Economic Cooperation Region (No. P0002251).  
**Conflicts of Interest:** The authors declare no conflict of interest.