Development and Characterization of Flux-Free Ag-Cu-P-Sn Brazing Alloys for Cu-STS Joining

The demand for reliable dissimilar metal joining is growing with the miniaturization and high-performance needs of modern industries. Brazing between copper (Cu), with excellent thermal conductivity, and stainless steel (STS), with superior corrosion resistance, is widely used in heat exchangers and piping. However, a stable Cr<sub>2</sub>O<sub>3</sub> oxide layer on STS reduces filler wettability, causing incomplete bonding and defects. To solve this problems, a flux-free Ag-Cu-Sn-P filler metal was developed. Phosphorus (P) was added to remove oxides and promote interfacial reactions, and Sn was used to adjust the melting range and reduce solidification defects. Alloys with different Ag and P contents were fabricated and evaluated for microstructure, wettability, and Cu-STS joint strength. Analysis revealed phases of Ag-Cu, Cu<sub>3</sub>P, and Cu<sub>3</sub>Sn, with P addition increasing the Cu<sub>3</sub>P fraction. Alloys containing 5wt.% P showed superior spreading, and joints produced with these alloys had higher tensile strength than commercial BCuP-5. These results highlight the potential of Ag-Cu-Sn-P alloys as cost-effective, flux-free fillers that improve bonding and reliability in Cu-STS dissimilar joints.