Microstructure- and defect-controlled damage tolerance evaluation of lattice structures at room temperature and 650 °C based on the E-PBF processed Ni-based alloy Inconel 718

• Applicants: Professor Dr.-Ing. Thomas Niendorf; Professor Dr.-Ing. Frank Walther
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Applied Mechanics, Statics and Dynamics
• Term: from 2017 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 379213719

The main objective of the project is the establishment of a profound understanding of the structural behavior as well as the microstructural and mechanical properties of lattice structures based on the E-PBF processed Inconel 718 (IN718) alloy. In particular, it will be investigated how far material properties and surface roughness can systematically be adjusted by process parameters and subsequent post-processing (surface and heat treatment). Regarding the surface finish, alternative approaches are focused, since conventional grinding and polishing are only partially suitable for additively manufactured lattice structures. Besides wet chemical treatments such as chemical etching or electrochemical polishing (ECP), mechanical processes such as vibratory finishing and ball-polishing are considered. It has to be ensured that uniform material removal takes place in the near-surface and in the center of the specimen. In order to evaluate the different approaches, resulting surface roughness will be measured using confocal microscope and computer tomography (µ-CT). In addition to 2D/3D pore analysis, supplementary mircostructural investigations will be carried out by means of scanning- (SEM) and transmission electron microscopy (TEM) as well as hardness measurements in the near-surface and the center area of the specimens in order to quantify the influence of subsequent heat treatments (solution annealing + quenching, HIP + solution annealing + quenching). The different material conditions will be investigated by means of quasi-static and cyclic tests up to 107 cycles, using application-oriented measurement techniques such as a direct/alternating current potential drop (DC-/ACPD) system, infrared camera and Optimizer4D sensor. In addition to the tests at room temperature (RT), the relevant high-temperature range of 650 °C will further be investigated in order to determine the high temperature properties of IN718 lattice structures. However, especially the adaption of the different measurement techniques is challenging. For a profound understanding of the material, additional damage evolution tests will be carried out to enable in-depth conclusions on the damage accumulation and propagation, respectively, in additively manufactured lattice structures. In summary, microstructural, computed tomographic as well as mechanical investigations will be correlated with the results of fatigue testing. Based on these correlations process-geometry-microstructure-property relationships will be qualitatively and quantitatively established.

• DFG Programme: Research Grants