| Because of its excellent specific tensile, fatigue strengths and corrosion resistance, Titanium alloy have been mainly used for aircraft structural and engine parts, materials for a petrochemical plants and surgical implants. Since excellent fatigue properties are required from investment titanium alloy to be used for airplane parts, fatigue properties should be taken into critical consideration. Particularly, the fatigue behavior of airplane parts containing welded regions should be closely examined and evaluated before the parts are actually applied, because there were unhomogeneities of microstructure and mechanical property in welded joints. Therefore, it is essentially required to investigate fatigue properties of the welded region which differ in microstructures.Addition of hydrogen has a considerable effect on the mechanical properties of titanium alloys because that titanium alloys have a high affinity for hydrogen, hydrogen can be detrimental to the mechanical properties of titanium alloys, especially for the titanium alloy with welded joints. Much work has been done concerning the effects of hydrogen as an alloying element and its corresponding mechanism. In present study, the fatigue growth behavior and micro-morphologies at crack tips of the welded joints in different region and the effects of small amounts of hydrogen on the fatigue properties of electron beam welded TC4 and TA15 alloys were investigated, which is expected to contribute to fundamental understanding of the fatigue mechanism and the role of hydrogen.The important outcomes are as follows:①The results of fatigue crack growth revealed that: the fatigue crack growth rate of 5/6 from the welded joint surface was highest, followed by the 1/6 from the surface, the lowest rate was 3/6 from the surface which was the middle of the welded joint. The experiment of fatigue growth in different region of welded joints was based on the fatigue crack growth of micro-hardness, the results also revealed that the gradients of micro-hardness have some relationship with the fatigue crack growth: the bigger of the gradient, the faster of the fatigue growth rate. The base metal whose microstructure was exquilax plasticαphase had best crack growth resistance; the orientation of martensite packets in fusion zone differ with each other, which makes the crack growth tortuously. The heat affected zone whose hardness gradient was the highest of the three groups showed the lowest resistance to cyclic crack growth. Theαphase and martensite lash in heat affact zone deformation dismatched during the fatigue cycle, which induced the resistance to the crack growth decreased.②The research on the micro-morphologies at the crack tips of TC4 welded joints revealed that: experiencing from fatigue crack growth, the dislocations which emitted fromα/βinterfaces reached so densely that they inclined to become a source of the fatigue cracks, from which these dislocations were diverged radioactively under the cyclic stress. Dislocations accumulated badly among the small fatigue phases which lay between two martensite lathes, indicating that these small fatigue phases tended to become the source of fatigue crack. What is more, the more narrow the width of martensite lathes, the easier dislocations accumulated and became the fatigue source. The plasticαphase and martensite lash mixed structure in heat affected zone deformation dismatched which decreased the resistance of fatigue crack growth. By comparing the TEM morphologies of different regions, the corresponding evidence that welded joints were more liable to become the source fatigue cracks than the base metal was found.③The influences of hydrogen on fatigue behaviors of electron beam welded TC4 experiment revealed that: no hydride formed when less than 0.120 wt% hydrogen charged in TC4, and the fatigue lives drop evidently with median hydrogen content increase. The compatible deformation ability of the phase boundaries was decreased because of theβphase became coarser after hydrogen charged, and the defects were accumulated at the phase boundaries, then became a source of second crack. Hydrogen atoms accumulated at the phase boundaries because of the cyclic stress, which accelerate the crack growth. With the increase of the hydrogen content, the micro-hardness of TC4 became higher, while the toughness and plasticity was drop badly. The microstructure analysis revealed that: dislocations accumulated in secondaryα/βphase region, which could be suspected of developing a source of crack. Hydrogen atoms soften the PSB at crack tips, which resulted fatigue threshold of the crack growth was decreased, while the crack growth rate was increased.④The influences of hydrogen on fatigue behaviors of electron beam welded TA15 experiment revealed that: no hydride formed when less than 0.105wt. % hydrogen was charged in TA15 welded joints. Fatigue results revealed that: with hydrogen content increasing, the fatigue life in the welded joints was drop remarkably, because the existed hydrogen reduced the toughness remarkably of the TA15 alloy in the welded joint, and increased the fatigue crack growth rates. The substrates presented better fatigue cracking resistance than the welded joint. The morphology analysis of the fatigue fracture demonstrated that, the hydrogen accumulated along the boundaries, and accelerated crack propagation along the martensite packets in the welded joints, resulting of"colony structure"on the fracture surface, the martensite packets performed as a unit, and hydrogen had some effects on the martensite packet during the crack propagation.⑤In the research on micro-morphologies of the fatigue crack tip in TA15, the△K-da/dN curve of TA15 alloy was determined, the curve equation through regression fitting was: da/dN=6.93×10-10(△K)2.61. Through calculation it was found that the size of the plastic zone of the crack tip is 166.17μm. According to findings, the conclusion was finally reached thatαphases of similarly aligned accelerate the crack propagation. Furthermore, the small-crack stage and the micro-morphologies plastic zone of the crack tip was studied by transmission electron microscopy (TEM), indicating that the plastic zone of the crack tip have profound effects on the crack propagation behavior. While the crack propagate through the sharp - blunt - resharp process of the crack tip, the motion and accumulation of dislocation as well as the formation of the slip band were displayed. The slip bands are deflected at the phase or grain boundaries, and the orientation of theαphases of similarly aligned weakes the hindering effects of the phase boundaries. |
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