**4. Conclusions**

AZ31B Mg alloy joints welded through HP-USW, with and without the use of Cu coating interlayer, and the following main results concluded:


**Author Contributions:** Experiments conceived and designed by A.B. and C.I.; Manuscript wrote by A.B.; Supervision and TEM analysis, C.I.; Consultant and EBSD analysis, S.S.; Consultant and performing coating by vapor deposition, S.T.

**Funding:** This research received no external funding.

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