**4. Conclusions**

In this study, it was demonstrated that the activity of CsF-RbF-AlF3 flux was clearly enhanced by adding a trace amount of Ga2O3, promoting the further enlargement of the spreading area of the Zn-2Al filler metal over AA5052 and Q235 steel. The major conclusions were as follows.

The spreading tests showed that the addition of trace amounts of Ga2O3 in CsF-RbF-AlF3 flux promoted the enlargement of the spreading area of Zn-2Al filler metal on both Q235 steel and AA5052 matrix, and the optimal content of Ga2O3 was 0.001–0.003 wt.%.

The interfacial e ffect and induction e ffect of Ga2O3 improved the flux activity by reacting with Al atoms to provide Ga and enriched molten filler metal to decrease its interfacial tension and promote the spreading area increase of molten Zn-2Al filler metal over the matrix. Meanwhile, the phosphorus oxidation and active Al removed iron oxidation and e ffectively increased the spreading area of molten Zn-2Al filler metal over the Q235 steel matrix.

As a result of the identified reaction mechanism, it was found that Ga2O3 partially reacted with ZnO and slowed down the e fficiency loss speed of CsF-RbF-AlF3 flux to improve its flowability.

**Author Contributions:** Conceptualization, S.X.; methodology, J.Z.; software, Z.Y.; validation, Z.Y.; formal analysis, Z.Y.; investigation, Z.Y.; resources, S.X.; data curation, Z.Y.; writing—original draft preparation, Z.Y.; writing—review and editing, Z.Y.; visualization, Z.Y.; supervision, J.Y.; project administration, J.Y.; funding acquisition, S.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the National Natural Science Foundation of China, Grant No. 51375233 and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

**Acknowledgments:** We gratefully acknowledge the support of Jiangsu Higher Education Institutions for providing a scholarship for Zhen Yao. Songbai Xue acknowledges financial support from the National Natural Science Foundation of China, Grant No. 51375233.

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