**4. Conclusions**

This study provides an e ffective and convenient method to repair brake disc cracks and to extend the service life of brake discs and save resources. Thermal fatigue cracks of ductile iron were repaired discontinuously using biomimetic locking units. The e ffects of size, microstructure, and spacing of the units on thermal fatigue crack repair were studied. The following conclusions were obtained:

(1) The units without defects improved both the thermal fatigue resistance and tensile strength. As the laser energy increased, the depth and microhardness of the units gradually increased, while the grain size gradually decreased. The microhardness and depths of the units reached a maximum at a laser input energy of 165.6+<sup>19</sup> −15 J/mm2. In addition, the sample treated at this laser energy exhibited the highest tensile force of 40.68 kN, which was 37.11% higher than the unrepaired specimen.


**Author Contributions:** Conceptualization, T.Z. and B.Z.; data curation, S.M. and G.C.; funding acquisition, H.Z.; methodology, S.M.; project administration, H.Z.; validation, S.W.; writing—original draft, S.M.; writing—review and editing, S.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by Project 985-High Performance Materials of Jilin University, Project 985-Bionic Engineering Science and Technology Innovation, National Natural Science Foundation of China (U1601203), and double first-class project by Jilin Province and Jilin University (SXGJXX2017-14).

**Acknowledgments:** Acknowledgments are made to the Key Laboratory of Automobile Materials of Jilin University for the use of scanning electron microscopy and X-ray diffraction.

**Conflicts of Interest:** The authors declare no conflicts of interest regarding the publication of this paper. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
