Qiang Ma

Brazing a SiO_2f/SiO₂ composite with metals is often faced with two problems: poor wettability with the brazing alloy and high residual stress in the joint. To overcome these problems, we report a combined method of selective etching and depositing reduced graphene oxide (rGO) on the surface of a SiO_2f/SiO₂ composite (3D-rGO-SiO_2f/SiO₂) to assist brazing with TC4. After the combined treatment, a “3D-rGO” buffer layer formed on the surface layer of the SiO_2f/SiO₂, and the contact angle was reduced from 130° to 38°, which meant the wettability of active brazing alloy on the surface of SiO_2f/SiO₂ was obviously improved. In addition, the “3D-rGO” buffer layer contributed to fully integrating the brazing alloy and SiO_2f/SiO₂; then, the infiltration of the brazing alloy into the surface layer of the SiO_2f/SiO₂ was enhanced and formed the reduced graphene oxide with a pinning structure in the three dimensional (“3D-pinning-rGO”) structure. Moreover, the joining area of the brazing alloy and SiO_2f/SiO₂ was expanded and the mismatch degree between the SiO_2f/SiO₂ and TC4 was reduced, which was achieved by the “3D-pinning-rGO” structure. Furthermore, the concentration of the residual stress in the SiO_2f/SiO₂-TC4 joints transferred from the SiO_2f/SiO₂ to the braided quartz fibers, and the residual stress reduced from 142 MPa to 85 MPa. Furthermore, the 3D-pinning-rGO layer facilitated the transfer of heat between the substrates during the brazing process. Finally, the shear strength of the SiO_2f/SiO₂-TC4 joints increased from 12.5 MPa to 43.7 MPa by the selective etching and depositing rGO method.

Reactive air brazing of porous SiO₂ ceramic (p-SiO₂) was achieved using Ag-CuO filler metal. When brazing p-SiO₂, two main problems existed. Firstly, the wettability of the Ag filler metal on the surface of p-SiO₂ was poor. Secondly, the residual stress caused by the mismatch of the coefficient of thermal expansion was high in the joint. In order to solve these problems, the effects of CuO contents on the p-SiO₂ brazed joint were analyzed. In a wetting experiment, the addition of CuO significantly improved the wettability of the Ag-CuO/p-SiO₂ system. With the content of CuO increasing, the contact angle decreased from 90° to 0°. In addition, when the content of CuO increased to 0.5 mol%, the contact angle decreased from 90° to 52°. Then, during brazing p-SiO₂ with the Ag-xCuO filler metal, the typical interfacial microstructure of the joints brazed at 1000 °C for 30 min was p-SiO₂ ceramic/Ag (s,s) + SiO₂ + CuO/Ag (s,s)/Ag (s,s) + SiO₂ + CuO/p-SiO₂ ceramic. Meanwhile, Ag-CuO infiltrated into the p-SiO₂ ceramic and an infiltration layer formed. The infiltration layer was composed of Ag (s,s) + SiO₂ + CuO and the infiltration layer was conductive to form a good gradient transition of the coefficient of thermal expansion (CTE). Then, the residual stress in the joint was released and the shear strength improved. In addition, with the content of CuO increasing, the depth of the infiltration layer increased. Furthermore, when the content of CuO was 0.5 mol%, the maximum shear strength of the joint was 55 MPa.

A high‐quality joint of the parent metal Al‐25Si‐4Mg‐1Cu alloy (AlSi alloy) was brazed to itself using Zn layer as a filler material. The typical microstructure of AlSi/Zn/AlSi joint was Al‐Zn solid solution, primary Si phase and α‐Al phase. When brazing temperature was increased from 400 to 405°C, Zn began to melt and reacted with Al diffused from base material, then Si phase formed. While the increase in brazing temperature to 415°C caused the gradual buring of Zn from Al‐Zn solid solution, and α‐Al remained. Furthermore, the shear strength of the joint increased because of α‐Al phase and Si phase formation. In addition, with brazing time extending from 8 to 12 min, the amount of Al‐Zn solid solution decreased, while the amount of Si phase and α‐Al phase increased, thus the shear strength of the joint was improved. While the increase in holding time extending to 14 min caused many micro‐voids formed, and the shear strength of the joint was reduced. Finally, the shear strength of the joint brazed at 415°C for 12 min reached 65.6 MPa.

A high-quality joint of pressureless sintered SiC ceramic (PS-SiC) and stainless steel brazed by AgCu/AgCuTi brazing alloy was realized. The effects of brazing temperature and holding time on the interfacial microstructure and mechanical properties of PS-SiC-stainless steel were analyzed. During brazing, Fe and Cr diffused into a brazing seam, and the typical interfacial microstructure of joints brazed at 890 °C for 15 min was stainless steel/Cr₅Si₃ + FeSi₂ + Ag(s,s)/Cr₅Si₃ + FeSi₂ + Ag(s,s) + Cu(s,s) + TiSi₂ + TiC/FeSi₂ + Ag(s,s) + Cu(s,s) + TiSi₂/Ag(s,s) + Cu(s,s) + SiC/PS-SiC ceramic. Cu-Fe system improved the toughness of the joint, and the shear strength of the joint increased to ~67 MPa. With increasing brazing temperature and holding time, the element Fe continued diffusing into the brazing seam, and the uniformity of Cu-Fe system became worse. Then Cu(s,s) infiltrated into the surface layer of PS-SiC ceramic and FeSi₂ brittle phase gathered in the brazing seam. Then, the brittleness of the joint increased and the shear strength of the joint decreased.