**E**ff**ect of Various Nanoparticles (GaF3, ZnF2, Zn(BF4)2 and Ga2O3) Additions on the Activity of CsF-RbF-AlF3 Flux and Mechanical Behavior of Al**/**Steel Brazed Joints**

#### **Zhen Yao, Songbai Xue \* and Junxiong Zhang**

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; yaozhen123@nuaa.edu.cn (Z.Y.); zhangjunxiong126@163.com (J.Z.) **\***Correspondence:xuesb@nuaa.edu.cn;Tel.:+86-025-8489-6070

Received: 15 July 2020; Accepted: 5 August 2020; Published: 7 August 2020

**Abstract:** In this study, brazing AA6061 to Q235 steel using flame brazing was performed with 70.9 wt.% CsF-0.5 wt.% RbF-28.6 wt.% AlF3 fluxes doped with GaF3, ZnF2, Zn(BF4)2 and Ga2O3 nanoparticles, matched with a Zn-15Al filler metal, and the spreadability of the filler metal and the mechanical properties of brazed joints were investigated at the same time. The results showed suitable amounts of GaF3, ZnF2, Zn(BF4)2 and Ga2O3 doped into the base flux could strengthen the filler metal in wetting and spreading on the surface of aluminum alloy and steel to different degrees. The suitable ranges of GaF3, ZnF2, Zn(BF4)2 and Ga2O3, respectively, were 0.0075–0.01 wt.%, 0.0075–0.01 wt.%, 0.0075–0.01 wt.% and 0.009–0.01 wt.%, and the maximum spreading area was obtained via doping with GaF3. The shear strength of brazed joints reached the peak at 126 MPa when 0.075 wt.% GaF3 was added. Comparative tests proved that the activity of the CsF-RbF-AlF3 flux doped with GaF3 was the best. The reason was that the CsF-RbF-AlF3-GaF3 flux was competent in removing oxides of the base metal and decreasing the interfacial tension, in virtue of the activity of Ga3+ as well as F<sup>−</sup>.

**Keywords:** GaF3; ZnF2; Zn(BF4)2; Ga2O3; flux; Zn-Al filler metal; spreadability; mechanical properties
