**5. Conclusions**

Dissimilar joints of pure copper and 6061-T6 alloys prepared by RUSW are investigated and compared to those prepared by the conventional HPUSW method. It is found that the additional resistance heat promotes the increase in interface temperature and reduces the thickness of the brittle IMC layer. The short duration of RUSW prevents the formation of cracks on the copper surface. As a result, the mechanical properties of the joint prepared by hybrid welding are significantly improved. Fracture surface morphology exhibits dimples and some cleavage planes, indicating a ductile–brittle hybrid fracture in RUSW.

**Author Contributions:** H.L. conceived, designed, and performed the experiments, and wrote the manuscript; B.C. contributed to the discussion of the experimental data, interpreted the results and revised the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (Grant No. 51175184).

**Acknowledgments:** The authors would like to acknowledge the kind assistance from Caiyou Zeng from the South China University of Technology.

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