| Due to the processing characteristics of the higher utilization ratio of the materia,near-net-shape and more freedom in design,the additive manufacturing technology is particularly recommended for aerospace components manufacturing.As a mainstream method of the additive manufacturing,laser selective melting(SLM)technology is the most widely selected in titanium production.Ti6Al4 V,an α+β double-phase alloy,is widely used due to the outstanding comprehensive properties,excellent biocompatibility and high damage-tolerance.Many researches have shown that the tensile properties of SLMed-Ti6Al4 V are comparable with the forged-Ti6Al4 V.However,the fatigue performance shows the significant decline which is attributed to the existence of defects.Therefore,the research on defect tolerance is the indispensable premise and foundation for the safety evaluation of SLMed-Ti6Al4 V components.Based on the acceptable manufacturing process of SLMed-Ti6Al4 V alloy,this paper has conducted a series of research in terms of microstructure,defect distribution,fatigue strength prediction and defect tolerance evaluation by comprehensively utilizing the experimental methods of the microstructure characterization,mechanical testing,Shanghai synchrotron radiation X-ray micro computed tomography(SR-μCT)and laboratory X-ray computed tomography(XCT).The microstructure investigation results confirm that the fine needle-like α’ martensite phase is considered to be the essential reason for the high strength and low plasticity in tense property of SLMed-Ti6Al4 V alloy by means of metallographic microscope and electron backscatter diffraction(EBSD)equipment.The other mechanical properties are also revealed including the hardness test,fatigue test and SR-μCT in-situ fatigue test.The test results show that the size,location and shape of the defects dominate the fatigue properties of the SLMed-Ti6Al4 V.Based on the results of defects reconstruction from SR-μCT and XCT,the extreme value statistical of defect size is performed,a reliable prediction result of the fatigue strength is obtained by using Murakimi model.Utilizing the basic concept of Kitagawa-Takahashi diagram,the fracture tolerance mechanics method is used to evaluate the internal defect tolerance of SLMed-Ti6Al4 V and determine the maximum critical defect size range.To describe the evolution law of the local stress around critical defect with different positions,a finite element simulation analysis is performed.Meanwhile,four different methods of defect classification have been proposed to predict the fatigue crack initiation position of the real defect-containing sample by simulation analysis.The research results provide an important analysis method for evaluating the internal defect tolerance and assessing the effect of defects on fatigue performance and service-life,which also provide a preliminary exploration for applying the lightweight additive manufacturing components into high-speed trains. |
