Effects of Cu content and Ga addition on suppressing intergranular corrosion of copper substrate in stainless steel/copper vacuum brazed joints

The intergranular corrosion of copper substrates caused by molten Ag-Cu based brazing alloys during the vacuum brazing of copper/stainless steel joints significantly compromises the structural integrity of assemblies in microwave vacuum electronic devices. This study presents a metallurgical approach to suppress intergranular liquid filler corrosion by modifying brazing alloy chemistry. It was found that both increasing the Cu content and adding Ga to Ag-Cu28-Ni0.75 brazing alloys effectively inhibit intergranular corrosion. A higher Cu content promotes the formation of a saturated liquid phase, thereby reducing the thermodynamic driving force for dissolution of solid copper. Meanwhile, the addition of Ga alters the composition and properties of the interfacial copper solid solution layer. Gallium facilitates uniform dissolution of the solid copper substrate rather than localized intergranular attack, thereby enhancing the mutual solubility between the solid and liquid. Based on these mechanisms, new quaternary Ag-Cu-Ni-Ga alloys were designed. The optimized Ag-Cu45-Ni-Ga9 and Ag-Cu50-Ni-Ga10 alloys completely suppressed intergranular corrosion at a brazing temperature of 870 ℃, producing sound joints free of cracks and pores. These newly developed brazing alloys consist of blocky copper solid solution and Ag-rich eutectic structure, with moderate hardness and good processability, without intermetallic compounds or liquid phase separation. The solidus and liquidus temperatures are 732–839 ℃ for Ag-Cu45-Ni-Ga9, and 734–847 ℃ for Ag-Cu50-Ni-Ga10. These results confirm that the intergranular corrosion of copper substrates is fundamentally associated with the grain boundary dissolution of solid copper.