HEAT-RESISTANT BRAZING FILLER METALS FOR JOINING TITANIUM ALUMINIDE AND TITANIUM ALLOYS

Titanium alloys and titanium aluminides are being considered as key materials for the manufacture of lightweight compressors and turbines in modern aircraft and rocket engines. The joining technology that provides reliable work above 930 degrees F (500 degrees C) is needed to fabricate functional turbine components. Only then, the full high-temperature strength capabilities of TiAl alloys and titanium matrix composites can be exploited. Two filler metals were tested for brazing heat-resistant joints at 2156-2190 degrees F (1180-1200 degrees C): TiBraze (R) 1200 (Ni-27Ti-10Al) and TiBraze (R) 375 (Ti-37.5Zr-15Cu-10Ni) in the form of transfer tape. Microstructure and shear strength of the brazed joints were studied at 1000, 1200, and 1470 degrees F (540-800 degrees C). Brazing at the temperature substantially above the liquidus of the filler metals resulted in a noticeable gain of both the high temperature and room temperature strengths. Joints of hot-pressed titanium aluminide brazed by Ti-37.5Zr-15Cu-10Ni filler metal exhibited shear strength at 1470 degrees F (800 degrees C) as high as 41.6-47.3 ksi (287-326 MPa) and 63-68 ksi (435-469 MPa) at 1200 degrees F (650 degrees C). Some other methods for improving hot strength of titanium-based alloys are also discussed, particularly: (a) application of composite filler metals and (b) pre-coating of surfaces to be brazed to prevent early oxidation and promote wetting.