**4. Conclusions**

Ultrasonic spot welding of NiTi SMAs with Cu interlayer was performed varying the welding energy levels. The weld morphology, interface characteristics and failure behavior of the joints were analyzed in detail. The following main conclusions can be drawn:


**Author Contributions:** W.Z., S.A. and Z.Z. conceived, designed the work; W.Z. and S.A. conducted the experiments; W.Z. and Y.H. analyzed the data; W.Z. and S.A. contributed to the writing of the original manuscript; J.P.O. and Z.Z. did the revision; Z.Z. and Z.L. supervised the whole procedure.

**Funding:** This research was funded by National Key R&D Program of China under Grant 2018YFB1107900, the National Natural Science Foundation of China under Grant 51775091 and 51575383; Natural Science Foundation of Tianjin City under Grant 18JCQNJC04100; Science and Technology Project of Sichuan Province under Grant 2018GZ0284; Fundação para a Ciência e a Tecnologia (FCT-MCTES) under Grant PEst-OE/EME/UI0667/2014.

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