Infrared brazing of Ti50Ni50 shape memory alloy using two Ag–Cu–Ti active braze alloys

Microstructural evolution, shape memory effect and shear strength of infrared brazed Ti50Ni50 shape memory alloy using Cusil-ABA® and Ticusil® active braze alloys are investigated. The Ag–Cu eutectic braze alloy can readily wet Ti50Ni50 substrate by minor titanium additions. The brazed Ti50Ni50/Cusil-ABA®/Ti50Ni50 joint is mainly comprised of Cu-rich, Ag-rich and CuNiTi phases. On the other hand, the brazed Ti50Ni50/Ticusil®/Ti50Ni50 joint consists of Ag-rich, Cu-rich and TiCu2 phases. Because the chemical composition of Ticusil braze alloy is located inside the huge miscibility gap, the molten braze tends to be separated into two liquids during brazing. One is rich in Ag, and the other is rich in both Cu and Ti. The Ag-rich liquid does not react with Ti50Ni50 substrate. In contrast, the copper content is depleted from the matrix of brazed joint due to the formation of interfacial TiCu2 phase. The TiCu2 phase is less detrimental to the shape memory effect than CuNiTi phase during the shape recovery bending test. Shear strength of brazed joints exceeds 200 MPa for both braze alloys if the brazing time exceeds 180 s. However, thick interfacial CuNiTi and TiCu2 layers can deteriorate the shear strength.