**4. Conclusions**

In this paper, material flow behavior at the root of the FSW weld is studied through an established 3D thermal–mechanical coupled FSW model. The results are as follows:


**Author Contributions:** J.L.: Supervision, writing, review and editing. J.W.: Formal analysis, methodology, data curation, writing, and original draft. H.L.: Conceptualization, data curation, writing, and reviewing. L.Y.: Software, validation. J.G.: Investigation, software, writing, and reviewing. H.G.: Investigation, writing, editing, and formal analysis. All authors have read and agreed to the published version of the manuscript.

**Funding:** Support Program of State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology (AWJ-20-Z02), the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University (SKLSP201903), and the Minjiang Scholar Support Program of Fujian, China (2016).

**Acknowledgments:** The authors acknowledge the financial support from the State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology (AWJ-20-Z02), the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University (SKLSP201903), and the Minjiang Scholar Support Program of Fujian, China. The authors are also thankful to Qingzhong Liu, Associate Professor in the Department of Computer Science at Sam Houston State University, USA, for his help with some revision suggestions.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
