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

• Applicants: Dr.-Ing. Jens Gibmeier; Professor Dr.-Ing. Thomas Niendorf
• Subject Area: Materials in Sintering Processes and Generative Manufacturing Processes ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 505457585

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).

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Gunter Kullmer