**5. Conclusions**

FWS joints of dissimilar 7A52 and 6061 Al alloys were achieved by post- integral-heat-treatment, the effect of heat treatment on the hardness and the microstructures of the weld were systematically investigated. The results are summarized, as follows.

(1) The T6-treated 7A52 and 6061 alloys were joined by FWS, a weakest region was revealed in WNZ, with the lowest hardness of 62 HV being similar to the solid solution of 6061 alloy.

(2) The solutionized 7A52 and 6061 alloys were joined by FWS, the WNZ can be strengthened by the post-aging, but the hardness of the matrixes cannot be improved by the post-aging.

(3) The weld joints of solutionized 7A52 and 6061 alloys were achieved by post-solid-solution and subsequent artificial aging. The best post-aging is designed as two-stage for the whole weld joint: 120 ◦C for 24 h and then 180 ◦C for 30 min. The hardness of the 7A52 matrix reaches approximately 140 HV, and the hardness of the 6061 matrix is higher than 100 HV. The hardness of WNZ slowly drops from 7A52 side to 6061 side, due to the precipitation of η phase and L phase.

**Author Contributions:** Conceptualization, J.L.; Data curation, Y.J., S.L. and Y.Q.; Formal analysis, J.L.; Funding acquisition, K.W.; Investigation, Y.J., S.L., J.L. and Y.Q.; Methodology, S.L. and J.L.; Supervision, J.L. and K.W.; Writing—original draft, Y.J. and J.L.; Writing—review & editing, J.L. and K.W.

**Funding:** This research was funded by the National Key R&D Program of China, grant number 2017YFA0204403; the National Natural Science Foundation of China, grant number 51301064; the open fund supported by State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body at Hunan University, grant number 31715009; the Fundamental Research Funds for the Central Universities, grant number 30918011341.

**Acknowledgments:** All TEM experiments were performed at the Materials Characterization and Research Center of Nanjing University of Science and Technology.

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