**5. Conclusions**

To understand the microstructural evolution and behavior of the mechanical properties during FSW with a consequent PWHT process, FSW was conducted using a beryllium-copper alloy and the resulting joints were subjected to PWHT at 315 ◦C for up to eight hours. The authors determined the microstructural and mechanical behaviors and the relationship between both outcomes. The results from this study are summarized below.


**Author Contributions:** Conceptualization, K.L.; Investigation, Y.L.; Project administration, K.L.; Validation, S.M.; Writing–original draft, Y.L.; Writing–review & editing, K.L.

**Funding:** This work was financially supported by R&D program of the Korea Institute of Industrial Technology (JA180012). The authors would like to express their gratitude to KITECH.

**Acknowledgments:** The FSW tools used in this study were supported by R&D program of Ministry of Trade, Industry and Energy (NK180051). The authors would like to express their gratitude to MOTIE.

**Conflicts of Interest:** The authors declare no conflict of interest.
