Repair brazing and protection of titanium turbine components using polymer bonded tapes

摘要

Superior strength-to-weight ratio, excellent corrosion resistance and mechanical strength of titanium alloys make them ideal for various aircraft as well as industrial applications. At high mechanical load however, as it appears by droplet impact erosion on turbine blades, it is desirable to have technologies available for protection and repair of such parts. In analogy to the repair brazing of nickel or cobalt based superalloys commonly used for repairing turbines, diffusion brazing of titanium was investigated. Titanium based brazing filler metals were chosen in this case in combination with pure titanium as well as titanium alloys of different particle size and shape to produce polymer bonded repair tapes for titanium diffusion brazing. Filler metal as well as additive powders were blended together and mixed with small amount of organic binding agent to form repair tapes of different thickness. The chosen filler metal has a narrow melting temperature range (825-835℃) in order to braze below β-transus temperature and to avoid α-β transformation. The porosity of the coatings has been minimized by varying the brazing parameters as well as the morphology of the additive powders and by varying filler metal - to additive metal proportion. The results were evaluated by microscopic examination and image analysis. Moreover, considering excellent wettability of graphite by titanium, fine graphite powders as well as fibers were used in addition to form hard titanium carbide with the titanium of the matrix and filler metal in situ. The resulting TiC phase is significantly hard with improved wear resistant compared to titanium alloys. The reaction layers of titanium carbide were examined by studying microstructure of the brazed coating and TiC formation in the matrix was optimized in accordance to brazing parameters.