选区激光熔化是一种常用的增材制造工艺，具有无模具、可定制等优势，拥有良好的应用前景。但其打印过程中产生的缺陷会对其疲劳性能产生不良影响，尤其是对其超高周疲劳（即疲劳周次超过10⁷的疲劳破坏）性能影响巨大。因此对选区激光熔化合金材料超高周疲劳性能的研究有很重要的科学意义和实用价值。

本文研究了打印取向对选区激光熔化合金Ti-6Al-4V高周及超高周疲劳性能的影响。疲劳试样制备取向与加载方向的角度分别为0°，45°，90°。疲劳试验所使用的是超声疲劳试验机（加载频率为20 kHz±500 Hz）,应力比设置为-1（即正负应力幅值相等）。试验结果表明，三种不同打印取向的疲劳性能从高到低依次为0°，45°和90°。无论是高周还是超高周疲劳阶段，疲劳裂纹均萌生于内部缺陷。缺陷可以根据几何形状分为规则缺陷和不规则缺陷，90°样品具有最多的不规则缺陷。在缺陷外呈现“粗糙区 (rough area)”和 “鱼眼区 (fish eye)”形貌，粗糙区和鱼眼区被称为钛合金的超高周疲劳特征区。缺陷的应力强度因子幅值随着疲劳周次的增加而降低。

本文使用了基于正态分布的概率统计模型对S-N数据进行统计分析。该模型认为在同一载荷下，疲劳周次服从正态分布，而诱导裂纹萌生的缺陷服从极值分布。通过概率统计模型可以预测疲劳周次在10⁹时的超高周疲劳强度，超高周疲劳强度的平均值从高到低依次为217 MPa (0°)，201 MPa (45°)和155 MPa (90°)。

超高周疲劳裂纹萌生特征区消耗了百分之九十以上的疲劳寿命。因此，本文研究了特征区的尺寸特征和微结构演化。粗糙区的尺寸随着载荷的增大而逐渐减小。粗糙区的等效应力强度因子的范围在3-7 。使用聚焦离子束技术提取了裂纹源区附近的纵剖面，并使用透射电子显微镜和扫描电子显微镜对样品进行了观察。即使在高周疲劳区域，选区激光熔化Ti-6Al-4V的特征区表面也出现了尺度在100 nm左右的纳米晶且纳米晶不连续。特征区的微结构在循环载荷的作用下从原始片层组织转变为较小晶粒最终形成纳米晶。这一结果证实了“大数往复挤压”机制主导了选区激光熔化Ti-6Al-4V疲劳裂纹的萌生和早期扩展以及超高周疲劳特征区的形成。即循环载荷下，由于裂纹面千万次的挤压而导致晶粒细化。

本文利用有限元方法对缺陷附近的应力集中和裂纹扩展过程中裂纹面的接触应力进行了计算。随着缺陷与加载方向的变化，缺陷处的应力集中分布也发生变化。90°样品有着较高的集中应力和最大的应力集中范围。裂纹扩展过程中，由于裂纹闭合效应的存在，接触应力最大值出现在裂纹萌生处和裂纹尖端。此外，随着裂纹长度增加，裂纹尖端残余塑性变形增大，裂纹尖端的接触应力也随之增大。

本文的研究不仅为增材制造的广泛应用提供了数据支持，同时也扩宽了超高周疲劳的研究范围。

Selective laser melting (SLM) is a widely used additive manufacturing process, which has good application prospects with advantages of no molds and customizable. However, the defect produced during the printing process will have an adverse effect on its fatigue performance, especially on the very-high-cycle fatigue (VHCF) performance. VHCF refers to fatigue with more than 10⁷ cycles to failure. With the development of industry, VHCF has received more and more attention. Therefore, the study on the fatigue performance of SLMed alloy has very important scientific significance and is useful.

The effect of building orientation on the VHCF response of Ti-6Al-4V specimens produced through SLM process with three different building orientations (0°, 45° and 90°) has been experimentally assessed in this dissertation. An ultrasonic fatigue testing machine (20 kHz ± 500 Hz) is used in the fatigue test, and the stress ratio is set as -1 (the stress amplitude of positive and negative are equal). The result of fatigue tests shows that the fatigue performance decreases with building orientations from 0° to 90°. The fatigue crack origin has been found to be always an internal defect both at high-cycle fatigue (HCF) and VHCF regimes independent of building orientations. Defects can be divided into regular defects and irregular defects according to the geometric shape. The 90° sample has the most irregular defects. There are rough area (RA) and fish eye (FiE) outside defect and RA and FiE are called the characteristic region of crack initiation for VHCF. The stress intensity factor range of defects decreases with the number of cycles to failure.

This dissertation uses a statistical model based on the normal distribution to study the VHCF strength. The model assumes that under the same loading, the fatigue cycles obey the normal distribution, while the defects that induce crack initiation obey the extreme value distribution. By considering the VHCF strength at 10⁹ cycles, the median value decreases from 217 MPa (0°) to 201 MPa (45°) and finally to 155 MPa (90°).

The characteristic region of crack initiation for VHCF consumes more than 90% of the fatigue life. This dissertation studies the size and microstructure evolution of the characteristic regions. The size of the RA gradually decreases as the load increases. The equivalent stress intensity factor of the RA is in the range of 3-7. Focused ion beam (FIB) technology was used to extract the longitudinal section near the crack initiation area, and the transmission electron microscope (TEM) and scanning electron microscope (SEM) are used to observe the FIB sample. Even in the HCF regime, discontinuous nanocrystalline with the size of about 100 nm appeared on the surface of RA of SLMed Ti-6Al-4V. In addition, microstructures in the crack initiation region evolved from original lamellar microstructures to small grains and finally to nanoscale grains under cyclic loadings. This validates the numerous cyclic pressing (NCP) model for interpretation of crack initiation and early growth mechanisms in the SLMed Ti-6Al-4V.

The finite element method is used to study the stress concentration near the defect and the contact stress between crack surfaces during the crack propagation process. As the angle of the defect and loading direction change, the stress concentration distribution at the defect also changes. The 90° sample has a higher stress concentration factor and the largest range of stress concentration. During the crack propagation process, due to the existence of the crack closure, the maximum contact stress appears both at the crack initiation region and the crack tip. In addition, as the crack length increases, both the residual plastic deformation and the contact stress at the crack tip increase.

The research in this dissertation not only provides data support for the wide application of additive manufacturing, but also expand the research scope of VHCF.