Multi-scale analyses of phase transformation mechanisms and hardness in linear friction welded Ti17( + )/Ti17() dissimilar titanium alloy joint

The Ti17(alpha + beta)-Ti17(beta) dual alloy-dual property blisk produced using Linear Friction Welding (LFW) is considered as high-performance component in advanced aeroengine. However, up to now, microstructure evolution and relationship between microstructure and micro mechanical properties of LFWed Ti17(alpha + beta)/Ti17(beta) dissimilar joint have not been thoroughly revealed. In this work, complex analyses of the phase transformation mechanisms of the joint are conducted, and phase transformations in individual zones are correlated to their microhardness and nanohardness. Results reveal that a dissolution occurs under high temperatures encountered during LFW, which reduces microhardness of the joint to that of Ti17(alpha + beta)/Ti17(beta). In ThermoMechanically Affected Zone of Ti17(alpha + beta) (TMAZ-(alpha + beta)) side joint, a large number of nanocrystalline alpha phases form with different orientations. This microstructure strengthens significantly by fine grains which balances partial softening effect of alpha dissolution, and increases nanohardness of alpha phase and microhardness of TMAZ-(alpha + beta). Superlattice metastable b phase precipitates from metastable beta in Weld Zone (WZ) during quick cooling following welding, because of short-range diffusion migration of solute atoms, especially b stabilizing elements Mo and Cr. The precipitation of the superlattice metastable b phase results in precipitation strengthening, which in turn increases nanohardness of metastable beta and microhardness in WZ. (C) 2023 Production and hosting by Elsevier Ltd. on behalf of Chinese Society of Aeronautics and Astronautics. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).