Brazing of titanium-based alloys with amorphous 25wt.%Ti-25wt.%Zr-50wt.%Cu filler metal
1994
Abstract TiPd (ASTM grade 7) and Ti-6Al-4V alloys (where the composition is in approximate weight per cent) were brazed in a vacuum furnace, each to itself, by using a new rapidly solidified amorphous 25Ti-25Zr-50Cu brazing foil. The joint tensile strength, fatigue resistance and microstructure were determined, the latter by X-ray diffraction analysis, scanning electron microscopy-energy-dispersive spectroscopy and scanning microscopy. The joint tensile strength is close to that of each base metal. The fatigue properties of TiPd (grade 7) joints do not differ from those of this base metal. The microstructure and mechanical properties of the brazed joints depend on the brazing cycle conditions: a fine lamellar eutectic joint microstructure consisting of α-Ti and γ-[Ti(Zr)] 2 Cu (tetragonal MoSi 2 -type) phase is observed after brazing of Ti-6Al-4V alloy at 900 °C for 10 min, followed by fast cooling. This brazing operation results is high strength joints. Brazing at temperatures higher than 900 °C and/or with a relatively low cooling rate results in a coarse dendritic microstructure consisting of γ-[Ti(Zr)] 2 Cu and hexagonal λ Laves Cu 2 TiZr phases. Fine precipitations of γ-tetragonal phase in the α-Ti matrix were also observed in the transition area between the base metal and joint in this case. Joints with such microstructures are brittle and have a low strength. It is shown that fast cooling suppresses formation of the λ Laves brittle phase, thus resulting in high mechanical properties of the brazed joint.
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