Microstructural evolution and interfacial behavior of joints brazed with Sn-modified low-silver Ag-Cu-Ge medium-temperature filler metals

Abstract

Sn-modified low-silver Ag-Cu-Ge brazing fillers were developed to regulate phase evolution and interfacial microstructure during brazing. A series of Ag_32-xCu₃₈Ge₃₀Sn_x (x = 0, 2, 4, 6 at.%) alloys were designed using CALPHAD calculations and systematically characterized by differential scanning calorimetry, X-ray diffraction, and SEM/EDS. All filler metals exhibited melting temperatures between 564 and 575 °C. With increasing Sn content, the Ag-rich constituent evolved from an FCC Ag-based solid solution toward HCP AgSn phases, while the Cu₃Ge phase changed from fine needle-like and short rod-like morphologies to coarser elongated features. During brazing, pronounced elemental redistribution occurred within the braze seam and at the filler metal/Cu interfaces. Ge became enriched near the interfaces and was incorporated into Cu-Ge-rich reaction products, while Cu₃Ge remained the predominant CuGe phase in the Sn₀ and Sn₂ joints and an additional Cu₅Ge phase was detected in the Sn₄ and Sn₆ joints. The interfacial diffusion-reaction layer increased in thickness from 5.30 to 18.65 μm. These microstructural and interfacial changes were accompanied by a decrease in wetting angle from 26.7° to 5.9° and an increase in average shear strength from 291 to 424 MPa. Moreover, the fracture location shifted from the braze seam in the Sn₀ joint to the Cu substrate in the Sn-containing joints. These results demonstrate that Sn addition governs the phase constitution, elemental redistribution, and interfacial microstructural development of low-silver Ag-Cu-Ge brazing filler metals, thereby strongly influencing their brazing behavior and joint performance.