增材制造（AM）是一种先进的制造技术，通过计算机辅助设计（CAD）添加离散材料层来生产实体零件。与传统工艺相比，AM 最大的优点是可以直接从原料中制备零件，这大大降低了生产的复杂度。大量文献表明，AM 合金的静态力学性能较铸造有很大提升。目前，AM 产品最主要的力学问题就是其超高周疲劳（VHCF）与断裂性能较差，而实际生产中因疲劳、断裂破坏占机械零件失效的 70% 以上，甚至更高。所以，充分了解 AM 合金的工艺参数、力学性能、超高周疲劳及断裂特性对预测其使用至关重要。
铝硅（AlSi10Mg）合金，具有密度小、比强度高、导电导热性好等一系列优良的力学性能，因此在航空航天、生物医疗和模具制造等领域广泛应用。然而，现有的制造工艺普遍存在加工效率低，原料成本高等问题，这极大地限制了铸造铝硅合金的进一步发展。在这种情况下，AMed AlSi10Mg 合金的超高周疲劳与断裂特性研究对 AM 工艺的推广具有重要意义。
在实验方面，本文以激光选区熔化（SLM）AlSi10Mg 合金为研究对象，探究了铺层厚度（20μm，50μm）对超高周疲劳性能及裂纹萌生机理的影响，发现：50μm 铺层厚度制备试样的疲劳性能要优于 20μm，并对层厚的影响展开讨论。而且，随着平均应力的增加，疲劳性能降低，该结论对于两种层厚的试样均适用。另外，在光镜（OM）下观察到 AMed AlSi10Mg 独特的微观组织，其晶粒细小且合金元素分布均匀，共晶 Si 呈网状分布在 Al 基体上，呈现“熔池”特征。接着，结合扫描电镜（SEM）观察大量断口后，发现：20μm 铺层厚度制备的试样含有大量缺陷，未熔化颗粒是其裂纹萌生的主要形式，而50μm 层厚试样的 VHCF 更偏向亚表面起源。最后，从疲劳裂纹萌生的角度归纳了五种裂纹起源方式。
本文基于 AMed AlSi10Mg 的 S-N 数据和裂纹源缺陷尺寸，从统计学的角度对 P-S-N 曲线进行了研究。依次计算了不同应力比下的等效应力幅值，建立了SLMed AlSi10Mg 的失效概率统计模型，发现：如果铺层厚度从 50μm 减小到20μm，则 VHCF 强度中值降低了 32%。另外，统计结果表明，当置信水平大于95%时，铺层厚度对 VHCF 的影响显著，主要体现在：与 50μm 铺层厚度制备试样相比，20μm 对应（N=10⁹）的 VHCF 强度较低。
在模拟方面，本文基于加速算法的基本思想，通过收敛性测试和算法加速，找到了满足收敛条件的最大增量步，并对算例进行了验证。另外，讨论了内聚力模型的收敛性问题，给出了几种常见增加收敛性的方法。这些算例补充了AMed AlSi10Mg 内聚力模型的相关参数，为计算 AM 合金超高周疲劳的损伤演化提供了可能。
本文结合 SLMed AlSi10Mg 的实验结果，归纳了单调拉伸及循环载荷作用下的参数选取方法，得到了考虑循环损伤内聚力模型的全部参数。通过编写内聚力UMAT 子程序，建立了 CT 试样在静态拉伸及超高周次循环载荷作用下的裂纹扩展模型。最后，根据断裂力学原理，拟合了 SLMed AlSi10Mg 裂纹稳态扩展阶段的材料参数。
本研究不仅为增材制造铝合金的工程应用提供了有效的疲劳性能数据。同时，对于探索增材制造铝合金的裂纹萌生及扩展机制奠定了理论基础。

Additive manufacturing (AM) is an advanced manufacturing technology used to produce solid parts by adding discrete layers of materials with computer-aided design (CAD). Compared with traditional manufacturing process, the biggest advantage of AM is that it can prepare parts directly from raw materials, which greatly reduces the complexity of production. A large number of literatures show the static mechanical properties of AM alloy are better than those of casting. At present, the main mechanical 
problem of AM products is its poor very-high-cycle-fatigue (VHCF) and fracture performance. In actual production, fatigue and fracture damage account for more than 70% or even higher of mechanical parts failure. Therefore, it is necessary to fully understand the process parameters, mechanical properties, very-high-cycle-fatigue fatigue and fracture properties of AM alloy.
Aluminum silicon (AlSi10Mg) alloy has a series of excellent mechanical properties, such as low density, high specific strength, good conductivity and thermal conductivity. Therefore, it is widely used in aerospace, biomedicine, mold manufacturing and other fields. However, the existing manufacturing processes generally have problems of low processing efficiency and high raw material cost, which greatly limits the further development of cast Al-Si alloy. In this case, the research on very-high-cycle-fatigue and fracture characteristics of AMed AlSi10Mg alloy is of great significance to popularize AM process.
In aspect of experiment, this paper takes selective laser melting (SLM) AlSi10Mg alloy as the research object, and explores the effect of different layer thickness (20μm,50μm) on VHCF property and crack initiation characteristics. It is found that the fatigue performance of specimens printing with layer thickness of 50μm is better than 20μm, and the influence of layer thickness is discussed. Moreover, with the increase of average stress, the fatigue performance decreases. This conclusion is also applicable to the samples with two different layer thicknesses. In addition, the unique microstructure of AMed AlSi10Mg was observed under optical microscope (OM). Its grains were fine and the alloy elements were evenly distributed. Eutectic Si was distributed on the Al-matrix in a network, showing the characteristics of "melt pool". Then, after observing a large number of fracture surfaces with scanning electron microscope (SEM),it isfound that the sample printing with layer thickness of 20μm contains a large number of defects, and unmelted particles are the main form of crack initiation, while the VHCForigin of the sample printing with layer thickness of 50μm is more inclined to sub-surface. Finally, five crack initiation modes are summarized from the perspective of fatigue crack initiation.
In this paper, the P-S-N curve is studied from S-N date and the size of crack source according to the angle of statistics. The equivalent stress amplitude under different stress ratios is calculated in turn, and the failure probability statistical model of SLMed AlSi10Mg is established. It is found that if the layer thickness reduced from 50μm to 20μm. The median VHCF strength is reduced by 32%. In addition, the statistical results show that when the confidence level is larger than 95%, the layer thickness significantly affects the VHCF response, which is mainly reflected in: compared with the sample printing with layer thickness of 50μm, the VHCF (N=10⁹) strength of 20μm is lower.
In aspect of simulation, based on the basic idea of acceleration algorithm, through convergence test and algorithm acceleration, the maximum incremental step is found which satisfies the convergence condition, and the numerical example is verified. In addition, the convergence of cohesive zone model is discussed, and several common methods to increase the convergence are given. These examples supplement the relevant parameters of AMed AlSi10Mg cohesive zone model, which makes it possible to calculate the damage evolution of very-high-cycle-fatigue of AM alloy.
Combined with the experimental results of SLMed AlSi10Mg, this paper 
summarizes the parameter selection methods under monotonic tension and cyclic load, and obtains all the parameters of the cohesive model considering cyclic damage. By compiling the cohesive UMAT subroutine, the crack propagation model of CT specimen under static tension and very-high-cycle-fatigue load is established. Finally, according to the principle of fracture mechanics, the fitting parameters of crack steady-state propagation stage for SLMed AlSi10Mg are fitted.
This study not only provides effective fatigue performance data for engineering application of AMed aluminum alloy. At the same time, it lays a theoretical foundation for exploring the crack initiation and propagation mechanism of AMed aluminum alloy.
Key Words: Additive manufacturing, AlSi10Mg, Very-high-cycle-fatigue, Cohesive zone model, Crack propagation