Influence of residual stresses on the crack propagation behavior in microstructurally graded samples manufactured by laser based additive manufacturing

在采用SLM（选择性激光熔化）的增材制造中，根据EN ISO ASTM 52900，也称为激光基粉末床金属熔化（PBF-LB/M），通过适当选择工艺参数，可以在组件制造过程中对结构和微观结构进行分级。可以根据组件应用的具体要求调整这种渐变，这样就可以制造具有局部适应微结构梯度的复杂组件几何形状。局部纹理可以用来使局部构件区域的方向相关强度和刚度适应当前的加载情况。在前期工作中已经表明，微观组织的级配也会对疲劳行为产生实质性影响。特别是局部各向异性对循环裂纹扩展行为的影响。然而，到目前为止，过程诱发的残余应力分布对局部裂纹扩展行为的作用，特别是对裂纹与界面之间相互作用的作用仍然完全不清楚。合作开展的该项目的目的是评估PBF-LB/M组件的结构完整性，通过适应的工艺设计具体实现不同的等级。这种评估是通过应用断裂力学概念，考虑到局部微观结构和残余应力来实现的。单相奥氏体钢316L最初被用作系统研究的模型材料。在项目的进一步过程中，将过渡到ni基合金IN718，因此也考虑了多相特性。考虑壁厚为2 mm和4 mm的矩形型材作为模型构件。利用PBF-LB/M方法制备的剖面的侧面，从现有的局部结构、晶体织构以及侧向和纵深方向的残余应力梯度等方面对其进行了详细的表征。在此系统的基本表征之后，考虑到构件的构建方向，从侧面提取用于裂纹扩展分析的样品（CT样品）。基于循环应力强度控制裂纹扩展试验，分析了这些CT试样的裂纹扩展行为，确定了中断试验中裂纹扩展不同阶段的局部残余应力分布的发展情况。在此基础上，在评价裂纹扩展基本特征的过程中，导出了基于局部结构、残余应力状态和微观组织（包括晶粒取向和晶体织构）来描述特定梯度材料损伤演化的模型。

In additive manufacturing employing SLM (Selective Laser Melting), also termed laser based-powder bed fusion of metals (PBF-LB/M) according to EN ISO ASTM 52900, the structure and microstructure can be graded during component manufacturing through the appropriate choice of process parameters. It is possible to adapt this gradation specifically to the requirements that arise from the component application, so that complex component geometries with locally adapted micro-structure gradients can be manufactured. Local texturing can be used to specifically adapt the direc-tion-dependent strength and stiffness of local component areas to the prevailing loading situation. In preliminary work, it has already been shown that the fatigue behavior can also be substantially influenced by gradation of the microstructure. In particular, it has been shown that the cyclic crack propagation behavior can be influenced by local anisotropy. However, the role of the process-induced residual stress distributions on the local crack propagation behavior and, in particular, on the interaction between the crack and the interface remains absolutely unclear so far.The aim of the project, carried out in cooperation, is to assess the structural integrity of PBF-LB/M components, for which different gradations are specifically implemented through an adapted pro-cess design. This assessment is realized through the application of fracture mechanics concepts, tak-ing into account the local microstructure and the residual stresses. The single-phase austenitic steel 316L is initially used as a model material for the systematic investigations. In the further course of the project, the transfer to the Ni-base alloy IN718 takes place, so that a multiphase character is also considered. Rectangular profiles with wall thicknesses of 2 and 4 mm are considered as model com-ponents. The side surfaces of the profiles produced by means of PBF-LB/M are characterized in detail with respect to the existing local structure, crystallographic texture and residual stress gradi-ents in lateral and in depth directions. Following this systematic basic characterization, samples for crack growth analysis (CT samples) are taken from the side surfaces, taking into account the built direction of the components. The crack propagation behavior is analyzed on these CT samples based on cyclic stress-intensity-controlled crack propagation tests, whereby the development of the local residual stress distributions is determined for different stages of the crack propagation in interrupted test runs. On this basis, in the course of evaluating the elementary characteristics of crack growth, a model is derived that allows for describing the damage evolution of the specifically graded materi-als based on the local structure, residual stress state and microstructure (including grain orientations and crystallographic texture).