Comprehensive investigation on linear friction welding a dissimilar material joint between Ti17(α+β) and Ti17(β): Microstructure evolution, failure mechanisms, with simultaneous optimization of tensile and fatigue properties

In order to advance the use of Ti17(α+β)-Ti17(β) dissimilar material blisks produced with linear friction welding (LFW) for advanced aeroengines, detailed studies of failure mechanisms and performance enhancements of such joints were conducted. The changes in microstructure including phase transformations and dynamic recrystallization were investigated. Also, the tensile failure mechanism of the joint was investigated by in-situ tensile test using scanning electron microscopy, while the high-cycle fatigue failure mechanism was studied using quasi-in-situ electron back-scattering diffraction observations combined with transmission electron microscopy characterization. A post-weld annealing heat treatment (PWAHT) was applied for optimizing tensile and fatigue strength. Results shows that α dissolution and β continuous dynamic recrystallization (CDRX) are controlling microstructure changes during welding. Tensile loading failure is related to local dislocation slip concentration on the Ti17(β) side of the thermo-mechanically affected zone with limited α strengthening phase and low degree β CDRX; while fracture under high-cycle fatigue loading is related to equiaxed α/ metastable β boundary microcracks on the Ti17(α+β) side of the heat-affected zone, which was produced by unequal deformation between the two phases. Following PWAHT for 630 °C at 3 hours, re-precipitation of secondary α occurred, which increased dislocation movement resistance in the weak areas, producing tensile and high-cycle fatigue strengths of 1196.4 MPa and 550.2 MPa, respectively, higher than those of BM-(β). In addition, an adequate plasticity of the joint with elongation of 8.7 % (82.1 % of Ti17(β)) was reached.