| Titanium alloys have been used widely as structural materials in the aerospace industry because of their advantages of low density, high fatigue strength and good weldability. Welding was the most common method of joining the structural material, and the fatigue property was important one of the criterias to assessing the welded joint (WJ). Electronic beam welding is a non-equilibrium processing with the rapid heating and solidification, and so the investigation of the effect of the welded joint shape, the feature and variation of the microstructure on the fatigue property has important significance and reference value to optimize the welding technical parameter, evaluate and improve the fatigue property of the welded joint.The microstucture feature was studied, and the microstucture heterogeneity characterized by a new method of the microstructure gradient in the electron beam welded (EBW) joints in thick TC4 and TA15 alloy plates. The effects of the microstructure gradient on the fatigue, especially the fatigue crack growth rate (FCGR), were investigated in the different zones and sections. The microstructure morphologies of the fatigue crack tip in the different zones in TC4 welded joint were obtained by the transmission electron microscope. The texture, the elastic module and the relation between them were analyzed in the WJs in two titanium alloys. The grain orientations and their effect on the fatigue crack growth path in the heat-affected zone (HAZ) and the fuse zone (FZ) in TC4 welded joint were first examined by the electron backscattered diffraction (EBSD) technology. The primary conclusions were as follows:The results of Energy Dispersive Spectrometer (EDS) line scan analysis indicated that the composition distribution changes were very small near the fuse line in TC4 and TA15 alloys, the loss of alloy elements were little. The main microstructure were the a' phase in the FZ in two alloys, only little (3 phase could be observed by TEM in the FZ in TC4 alloy. The calculations by the microstructure gradient model showed that in the funnel-shape, nail-shape, bell-shape and wedge-shape welded joint, the microhardness gradient in the bell-shape weld seam was smaller than the others. The microhardness gradient in the HAZ was obviously higher than that in BM and the FZ. At the different thickness, the microhardness gradient at 5/6 thickness was that at 3/6 and 1/6 thickness. The longitudinal variation gradient of the columnar grains increased from the root to the surface of the weld seam. The value of the longitudinal variation gradient in the bell-shape weld seam was the minimum among the four weld seams; it suggested the minimum microstructure heterogeneity was in the bell-shape weld seam.The bell-shape weld seam with the minimum microstructure gradient had the maximum average fatigue life; conversely, the wedge-shape weld seam had the minimum average fatigue life. The fatigue crack usually originated in the HAZ or near the fuse line with the high microstructure gradient. The FCGR increased with the microstructure gradient. Comparing with the FZ, the HAZ with the larger microstructure gradient had the higher FCGR. The space of the fatigue striation obtained from the SEM fractographs was narrower in the FZ than that in the HAZ. Due to the higher microstructure gradient in the FZ near the root of the weld seam than the top, the FCGR at 5/6 thickness was faster than that at 1/6 thickness. The microstructure gradients in the HAZ at 1/6,3/6 and 5/6 thickness were slight difference, and the FCGRs among them were not observed the important distinction in this experiment.The dislocations existed in the coarse equiaxed primary a grain on the tip of the fatigue crack in the TC4 BM. The piling-up and row of dislocations easily arise from theα/βandα/αinterfaces where the fatigue crack tended to occur. The dislocation concentrated on the martensite interfaces on the tip of the fatigue crack in the FZ, and formed the high-density dislocation net. The microplastic deformation occurred on the tip of the crack and near the crack tip, and it formed the large deformation zone. The fine matensite laths obstructed the motion and emitting, consequently, it brought the stress concentration to induce the shear fracture of the laths. The mismatch of the mechanical property and the deformation between the equiaxed primary a grain and the matensite laths led to the crack initiation within the phase boundary in the HAZ. The microplastic deformation mostly occurred in the equiaxed primary a grain, and the microstucture heterogeneity resulted in the deformation selectivity in some extent.The stronger{0001}<10T0> and{0001}<2110> basal textures formed in the BM in TC4 and TA15, respectively. The weak{1123}<1122> texture was in the FZ in TC4, and the weak{0111}<2110> and{0112}<1212> texture in the FZ in TA15. The test results of the elastic module (E) of the HCP phase by nanoindentation suggested that the values of the average E were maximal in the BM in two titanium alloys due to the basal textures. Under the constant fatigue load parallel to the rolling direction, the changes of E in the welded joint increased the mismatch of the plastic deformation, and decreased the fatigue property.The higher prismatic Schmid factor indicated that the{1010}<1210> prismatic slip with the lower critical resolved shear stress (CRSS) was more easily activated in the crack path in the HAZ. The Schmid factor of the{0001}<1120> basal slip was close to that of prismatic slip, however, the former was difficult to activate because of its higher CRSS. The crack path in the HAZ had few diversions in the coarse grains with the similar orientation. In the FZ, the a'martensite laths with the large misorientation interweaved and formed the high-angle boundaries. The fatigue crack predominantly propagated in transgranular manner, and the path was zigzag. Most grains were oriented for basal slip and had the higher basal Schmid factor. However, the higher CRSS of basal slip raised the crack propagation driving force. Furthermore, the complex competition between basal slip and prismatic slip systems would promoted the activation of multiple slip. So the crack growth consumed a lot of energy, and the FCGR in the FZ could be decreased. |
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