Infrared brazing of Ti50Ni50 shape memory alloy using pure Cu and Ti–15Cu–15Ni foils

Infrared brazing of Ti50Ni50 using two brazing filler metals was investigated in the study. Three phases, including Cu-rich, CuNiTi (Δ) and Ti(Ni,Cu), were observed in the Ti50Ni50/Cu/Ti50Ni50 joint after brazing at 1150 °C. The Cu-rich phase was rapidly consumed in the first 10 s of brazing, and the eutectic mixture of CuNiTi and Ti(Ni,Cu) phases were subsequently observed in the joint. Samples brazed for longer time resulted in less CuNiTi and more Ti(Ni,Cu) phases in the joint. The existence of CuNiTi phase deteriorated the shape memory effect of the joint, but Ti(Ni,Cu) could still preserve shape memory behavior even alloyed with a large number of Cu. Therefore, higher shape recovery ratio was observed for specimens brazed for a longer time period. Extensive presence of Ti2(Ni,Cu) phase was observed in Ti50Ni50/Ticuni®/Ti50Ni50 joint upon brazing the specimens up to 1150 °C. The bending test could not be performed due to the inherent brittleness of Ti2(Ni,Cu) matrix. Moreover, the stable Ti2(Ni,Cu) phase was difficult to be removed completely by increasing either brazing time and/or temperature.