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

在采用选择性激光熔化的添加剂制造中，根据EN ISO ASTM 52900，也称为基于激光的粉末床金属熔化(PBF-LB/M)，通过选择适当的工艺参数，可以在元件制造过程中对组织和微观组织进行分级。可以特别地使这种渐变适应于部件应用的要求，从而可以制造具有局部适应的微结构梯度的复杂部件几何形状。局部纹理可以用来特别地使局部构件区域的依赖于方向的强度和刚度适应主要的载荷情况。在初步的工作中，已经表明疲劳行为也可以很大程度上受到组织级配的影响。特别是，研究表明，裂纹的循环扩展行为会受到局部各向异性的影响。然而，工艺引起的残余应力分布对局部裂纹扩展行为的影响，特别是对裂纹与界面之间的相互作用的影响，到目前为止仍然是完全不清楚的。该项目的目的是评估PBF-LB/M组件的结构完整性，其中不同的梯度通过适当的工艺设计进行具体实施。这种评估是通过应用断裂力学概念，考虑局部组织和残余应力来实现的。首次采用单相奥氏体钢316L作为模型材料进行了系统的研究。在项目的进一步过程中，发生了向镍基合金IN718的转变，因此也考虑了多相特征。模型构件为壁厚为2 mm和4 mm的矩形型材。用PBF-L B/M法制备的型材侧面详细地表征了存在的局部结构、晶体织构和横向和深度方向的残余应力梯度。在这一系统的基本特征之后，考虑到部件的建造方向，从侧面采集用于裂纹扩展分析的样品(CT样品)。基于循环应力强度控制的裂纹扩展试验，分析了这些CT试件的裂纹扩展行为，从而确定了在间断试运行中裂纹扩展的不同阶段的局部残余应力分布的发展。在此基础上，在评价裂纹扩展基本特征的过程中，推导出了一个基于局部组织、残余应力状态和微观组织(包括晶向和晶体织构)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).