Evaluation Of Anisotropic Fatigue Performance Of Additative Manufactured Aluminium Alloy Based On 3D X-ray Computed Tomography Of Defects

Metal additive manufacturing(AM),as an advanced high-performance component processing technology,has significant technical advantages such as high flexibility,low energy consumption,strong adaptability to complex geometries fabricating,and has become one of the leading technologies to enhance the core competitiveness of highspeed rail.Aluminum alloy is the appropriate material for lightweight structure design of high-speed train with the advantages of low density,good plasticity,high strength,good corrosion resistance,etc.However,the defects generated during the manufacturing process such as lack of fusion,porosity,cracks,etc.have a great influence on its mechanical properties and fatigue behavior,which is one of the important reasons affecting its development and application.In order to understand the impact of defects on the fatigue behaviour which has long restricted the development and application of AM aluminium alloy,the microstructure,defect characteristics,tensile and fatigue performance of selective Laser Melted(SLM)AlSi10 Mg alloy in X direction(vertical to build direction)and Z direction(parallel to build direction)were studied by various experimental methods such as X-ray computed tomography(X-CT),scanning electron microscope(SEM),high cycle fatigue(HCF)and fatigue crack growth(FCG)testing.A fatigue strength evaluation method is proposed depend on the tensile properties and defect characteristics.Firstly,the improved Kitagawa-Takahashi(K-T)diagram is established based on the fatigue limit of defect-free materials determined by the ultimate tensile strength and the FCG rate determined by the i LAPS model.Then,the statistical assessment of defects is obtained by X-CT and extreme value statistics(EVS).Finally,the assessment method is verified by the FCG and HCF testing data.The results show that the SLM AlSi10 Mg alloy has typical anisotropic microstructures.The static strength of samples in different orientations is equal,but the plasticity is significantly different.The elongation in X direction is about twice as large as that in Z direction.In terms of fatigue properties,X-direction specimens have higher fatigue strength than Z-direction specimens,but with greater dispersion.The significant anisotropy of the size and shape of the defects is the main reason for the anisotropy of fatigue performance.It is found that the predicted fatigue life is in better agreement with the experimental results.The fatigue performance evaluation method proposed in this thesis based on the three-dimensional imaging of defects and the tensile properties is reasonable and feasible.It is a conservative and economical fatigue performance evaluation method,and it can provide an effective way for process optimization and engineering application of metal AM technology.