A study of weld fusion zone phenomena in cast gamma titanium aluminides

Welding studies performed to date on gamma titanium aluminide Ti-48Al-2Nb-2Cr (at.%) have shown weldability to be defined by susceptibility to solid-state cracking and mechanical properties of the weld. The purpose of the present study was to systematically investigate the influence of Al content and the weld cooling rate on weld cracking susceptibility, solidification behavior, solid-state phase transformations, mechanical behavior of gamma alloys.; Cast alloys (HIP'ed and heat-treated) with compositions ranging from 45.5 to 47.9% Al were laser beam welded and their weldability was evaluated. Beam power, traversing rate and preheat temperature were systematically varied to generate a series of welds exhibiting a wide range of cooling rates (100-10,000{dollar}spcirc{dollar}C/s). A liquid Sn quenching method was employed to study solidification characteristics of the fusion zone. Postweld heat treatment (PWHT) was applied to investigate the response of the fusion zone to heat treatment.; LB welds in the gamma alloys were susceptible to both solidification and solid-state cracking. As the Al content increased and weld cooling rate decreased, solidification cracking susceptibility increased while solid-state cracking susceptibility decreased.; Weld fusion zones exhibited cored dendrites and an interdendritic gamma segregate. A transition in the solidification mode from primary {dollar}beta{dollar} to a duplex mode ({dollar}beta{dollar} + {dollar}alpha{dollar}) with an increase in Al content and solidification rate was observed. The fusion zone microstructure changed from mixed microstructures, which were comprised of an incipient lamellar structure, and metastable alpha-two and gamma phase, to a fully-lamellar structure with a decrease in cooling rate, with their relative proportions dependent on the Al content. Grain morphology and phase distribution in the PWHT'ed fusion zones were significantly influenced by the solidification structure and Al content.; SEM fractographic analysis indicated that the fracture behavior depends strongly on microstructure. The fractographic analysis revealed that fracture facet size increased with a decrease in heat input and that PWHT increased crack path tortuosity.; In summary, the results of this comprehensive, systematic study showed the Al content and the weld cooling rate to strongly influence both the weld cracking susceptibility and the fusion zone microstructure.