Resistance Diffusion Bonding of a Titanium Aluminide

Abstract

Diffusion bonds and fusion welds have been produced between sheets of Ti-6wt%AI-4wt%V and Ti-14.8wt%Al-21.3wt%Nb titanium aluminide using the capacitor-discharge resistance spot welding process. Diffusion bonds in Ti-6wt%Al-4wt%V were characterized by beta grain growth across the bond line and the presence of occasional interface defects. Despite fracture through the bond region, optimum tensile-shear strengths for the diffusion bonds were comparable to those of fusion spot welds which failed by “nugget pullout”. High quality, defect-free diffusion bonds were also produced in a Ti-14.8wt%Al-21.3wt%Nb alpha-two titanium aluminide. The bond region was characterized by the growth of equiaxed beta grains across the bond line and an absence of interface defects. Fusion welds in the titanium aluminide exhibited a coarse, columnar beta grain structure. High cooling rates experienced in both the diffusion bonds and fusion welds in the titanium aluminide promoted the retention of beta phase down to room temperature throughout a large proportion of the bond and weld zones. Tensile-shear fracture of the titanium-aluminide welds initiated near the weld outer periphery and propagated principally through the unaffected base metal. The average fracture strength of the diffusion bonds produced at high axial force was markedly superior to that of bonds produced at low axial force and fusion welds. This difference was attributed to a more severe notch geometry at the outer periphery of the latter weld types.