Joining of Powder Metallurgy Beta Gamma Titanium Aluminide Through Diffusion Brazing

This thesis presents an experimental study of diffusion brazing powder metallurgy beta gamma titanium aluminide, along with a methodology utilizing pentenary equivalent beta forming effectiveness for the design of post bond heat treatments. Beta gamma titanium aluminides are attractive high temperature materials with low density and high specific strength. Adequate joining and repair methods, and the resultant high temperature creep performance, are fundamental requirements for the adoption of beta gamma titanium aluminides in the aerospace, automotive, and land based gas turbine industries. The developed beta forming effectiveness methodology was utilized to target composition and post bond heat treatment parameters to achieve a fully lamellar microstructure. Experimental results indicate that diffusion brazing beta gamma alloy below the eutectoid temperature of the substrate results in an α2 reaction layer at the solid/liquid interface that retards diffusion of the melting point depressant. When the brazing temperature is raised above the eutectoid temperature, the kinetics are enhanced due to the formation of a disordered α phase at the solid/liquid interface resulting in complete solidification at low holding times. A step cooled post bond solutionizing heat treatment produces a fine near fully lamellar microstructure within the bond zone. Aging of the solutionized microstructure manifests interfacial β0,sec precipitates within the bond zone and parent material lamellar colonies. Intercolony β0 phase regions experience the precipitation of two previously unreported phases which do not significantly alter the microhardness across the bond zone. Creep tests conducted on the parent material reveal improved primary creep resistance and reduced rupture time with aging. Creep tests conducted on the post bond heat treated specimens reveal that high stress levels result in premature rupture due to bond-line porosity acting as sites for void coalescence. Decomposition of the intercolony β0 phase occurred during creep due to the altered composition and hence ω solvus temperature within the bond zone. With proper management of bond-line porosity, diffusion brazing meets or exceeds the performance of the parent material in the primary creep regime at low stress levels.