Effect of Ge-doping on the Melting Property, Phase Structures and Spreadability of CsF-AlF3 Flux

CsF-AlF3 based composite fluxes with different Ge contents which are in the form of GeO2 were developed by the method of solution synthesis, and their melting characteristics, phase structures and microstructures were observed and analyzed subsequently. Meanwhile, 2024 aluminum alloy was brazed successfully at 480℃ under the active effect of CsF-AlF3 flux with 4% (mass fraction, the same below) Ge doped. The results show that the liquidus temperature decreases from 477℃ to around 440℃and has the main phase composition of CsAlF4 and Cs2GeF6 when content of Ge exceeds 4%. And the new Cs2GeF6 changes the morphology of CsF-AlF3 flux to more regular polyhedron structure with fewer flocculent agglomerates. On the other hand, the flux with 4% Ge immensely promotes the spreading of Zn-15Al filler metal on the 2024 aluminum alloy at 480℃ and the shear strength of brazing lapping joints reaches 110MPa,with homogeneous structure of the joints,and with no obvious defects observed.