Microstructure, mechanical property and deformation behavior of electron beam weldment of a high-temperature titanium alloy Ti-60 were studied systematically in this thesis by using the techniques of optical microscopy, scanning electron microscopy and transmission electron microscopy and the theories of the welding metallurgy of titanium alloy, fracture mechanics, fatigue and creep, which aimed to supply a beneficial support for reliability design and practical application of electron beam welding (EBW) weldment of Ti-60 alloy.
Metallographic examination of as-welded Ti-60 electron beam welds showed that there exist three zones in the EBW weldment, the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM). These heat-affected zone (HAZ) and fusion zone (FZ) microstructures reflected the effects of rapid heating and cooling weld thermal cycles during electron beam welding, which consists of acicular martensitic α′ laths with different crystallographic orientations due to the diffusionless transformation. Further studies showed that the rare earth phase particles were about 100nm~200nm in dimension and precipitated in the grain boundary by preference. With the increasing content of Nd in Ti-60 alloy, the rare earth phase particles at the edge of the FZ began to precipitate linearly in the columnar grain interior and by the grain boundary, but dispersedly in the center of the FZ. It has also been observed that post-weld heat treatment (PWHT) would have little effect on the precipitation and distribution of the rare earth phase particles in the FZ.
Room temperature tensile testing and analysis indicated that Ti-60 EBW weldment would exhibit the same tensile strength as the BM but inferior ductility than the BM, which may be ascribed to the superior non-deformation ability of the FZ and HAZ than the BM during tensile testing. It has also been found that PWHT would have little effect on the tensile strength of EBW weldment. While PWHT at high temperature (Duplex-PWHT1) could eliminate the martensitic structures and improve the ductility of the FZ, limited improvements were observed on the ductility of EBW weldment. Additionally, PWHT above β transformation temperature (Duplex-PWHT2) would deteriorate the ductility of Ti-60 EBW weldment because of the increasing tendency of intergranular fracture. The results of room temperature impact toughness of the welds indicated that impact toughness of the electron beam welds could be improved after Duplex-PWHT1, which is accompanied by the decomposition of the martensite structure in the FZ.
The failure location of the 600oC tensile specimens of EBW weldment was found to be in the BM region with transgranular fracture, and the weldments all exhibited comparable tensile strength but inferior ductility in relation to base metal. It had also been observed that deformation mechanism of creep rupture was found to be related to the level of creep stress applied, the fracture location shifted from the FZ to BM when the applied stress reached 550MPa, at or above which the deformation mechanism changed from dislocation climb to dislocation glide. Fatigue cracks were found to initiate at the surface or sub-surface of the weldment at the BM, TEM observation found that the deformation in the BM is mainly caused by dislocation gliding through α laths, while the deformation in the FZ is caused by dislocation climb and limited dislocation glide within the laths.
The creep rupture property of the weldment was poor when the temperature of PWHT was between 700℃~900℃, PWHT at 980℃~1050℃ could remarkably increase the creep rupture life of weldment, which was ascribed to the decomposition of martensite structure to α phase and β phase. It had been noted that the creep mechanism of the FZ was mainly the diffusion controlled dislocation climb, and the creep of lamellar-BM was controlled by ‘solute drag’ and dislocation climb related dislocation movements. In addition to the creep mechanism of diffusion controlled dislocation movement, the interface sliding creep mechanism had been found to play an important role during creep of the FZ.
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