BRAZING OF TITANIUM-BASED ALLOYS WITH AMORPHOUS 25WT-PERCENT-TI-25WT-PERCENT-ZR-50WT-PERCENT-CU FILLER METAL

Ti-Pd (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 Ti-Pd (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 alpha-Ti and gamma-[Ti(Zr)]Cu-2 (tetragonal MoSi2-type) phase is observed after brazing of Ti-6Al-4V alloy at 900 degrees C for 10 min, followed by fast cooling. This brazing operation results is high strength joints. Brazing at temperatures higher than 900 degrees C and/or with a relatively low cooling rate results in a coarse dendritic microstructure consisting of gamma-[Ti(Zr)]Cu-2 and hexagonal lambda Laves Cu2TiZr phases. Fine precipitations of gamma-tetragonal phase in the alpha-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 lambda Laves brittle phase, thus resulting in high mechanical properties of the brazed joint.