PSI - Issue 52

Ivo Šulák et al. / Procedia Structural Integrity 52 (2024) 143–153 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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waste, and become an important consideration for Industry 4.0. The advantages of the LPBF process in high-tech industries include component complexity with new features and designs that are impossible to produce using traditional manufacturing tools. However, the LPBF process is also inextricably connected with printing defects like internal cracks, gas porosity, un-melted particles, etc., that can affect the mechanical properties (Chlupová et al., 2023; Du Plessis et al., 2020; Hosseini and Popovich, 2019; Liu et al., 2022; Šulák et al., 2023) . NiCu-based alloys are due to their chemical composition predestined for application, where high resistance to aggressive environments is required (Jahns et al., 2021). One of the representatives of NiCu-based alloys is Alloy 400, which belongs to the Monel family with relatively low cost and high corrosion resistance. Its typical deployment is in gas and liquid pipelines in power facilities, impellers in the marine sector, pump shafts in offshore platforms, heat exchangers for combustion engines, and the chemical conversion of CO 2 and hydrogen. In these applications, components are exposed to vibrations, considerable thermal gradients and centrifugal forces, which result in different degradation mechanisms like high-temperature fatigue and creep (Devendranath Ramkumar et al., 2012). Since this alloy is ductile and easy to machine, the production process was mainly carried out in conventional ways. Only recently, there has been a demand for complex geometries that cannot be produced except by additive technologies. This brings not only new opportunities in component and microstructural design but also challenges in terms of mechanical and chemical properties (Chlupová et al., 2023; Jahns et al., 2021) . This work is a comparative study of the microstructure and high-temperature fatigue and creep properties of conventionally produced NiCu-based Alloy 400 and its LPBF counterpart. S-N curves, creep lifetime curves, creep deformation curves, and creep activation energies for both material variants were obtained in the temperature range of 400- 880 °C . The effect of grain size, texture, fracture mode and defects on fatigue and creep properties is presented and discussed in terms of the respective material manufacturing process. Nomenclature ̇ creep rate (-) σ applied creep stress (MPa) n creep stress exponent (-) A creep calibration constant (-) EBSD electron backscatter diffraction LPBF laser powder bed fusion N f number of cycles to fracture (-) Q activation energy (kJ.mol -1 ) R gas constant (J.K − 1 ⋅ mol -1 ) SEM scanning electron microscope T temperature (K) TEM transmission electron microscope

2. Experimental 2.1. Material

The material under investigation was Alloy 400 manufactured by the LPBF technique. The LPBF blocks for creep (one piece) and fatigue (two pieces) experiments with dimensions shown in Fig. 1 were fabrica ted at Osnabrück University of Applied Sciences using an EOS M290 machine. The machine operates a green laser system with a wavelength of 532 mm (85 W were applied) and enables a pre- heating of the build plate (80 °C was used in this study). All blocks were manufactured under argon atmosphere and by applicati on of a layer height of 20 µm. As scanning speed, 1050 mm/s was chosen, and the hatch distance amounted to 50 µm. No heat treatment was applied. The density of the LPBF Alloy 400 was evaluated from metallographic sections to 99.5 % (Chlupová et al., 2023) . To compare LPBF material properties, the creep and fatigue specimens were manufactured also from conventionally produced (hot extruded) material supplied by KME Germany GmbH. In this study, hot extruded material is nominated as “bulk”.