On the Tensile Properties of Inconel 718 Fabricated by EBM for As-Built and Heat-Treated Components
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Additive manufacturing (AM) builds solid, often geometrically complex objects, which cannot be produced by means of traditional manufacturing methods like machining, casting, or forming, from a series of layers having a thickness in the order of only several micrometers. AM systems help to achieve production of functional components, require no tooling, and produce minimal waste.[1] Inconel 718 is a precipitation-hardened nickel-based alloy used in gas turbine engines, nuclear reactors, and spacecraft structural components.[2] This alloy is widely used due to its high strength at elevated temperatures (up to about 750 C), high corrosion and oxidation resistance, and good creep and fatigue behaviors.[3]
LAITH ABDULLAH AL-JUBOORI is with the Mechanical Engineering Division, Higher Colleges of Technology, Fujairah, UAE. Contact e-mail: [email protected] THOMAS NIENDORF and FLORIAN BRENNE are with the Institut fu¨r Werkstofftechnik, Universita¨t Kassel, Metallische Werkstoffe, Mo¨nchebergstraße 3, 34125 Kassel, Germany. Manuscript submitted June 13, 2018. METALLURGICAL AND MATERIALS TRANSACTIONS B
Although quite a number of metallic alloy systems are currently in focus for additive manufacturing in the literature (e.g., Ti-6Al-4V, TiAl, stainless steel, Inconel 625/718, and Al-Si-10Mg), the majority of publications is on the mechanical properties of Ti-6Al-4V.[4] In the literature, several authors have reported on the microstructure and mechanical properties of Inconel 718 processed by EBM. In Reference 5, the influences of the processing parameters, i.e., beam power, deflection speed, and beam focus, on density and grain structure were investigated. The results showed that fully dense parts can be obtained over a large range of processing parameters. The microstructure consisted of columnar-shaped grains with the long axis parallel to the building direction (BD). This was more pronounced in case of a defocused beam, which was attributed to a shallower melt pool and eventually a thermal gradient being oriented more strongly towards BD. The development of a coarse columnar-like grain structure having a strong h001i texture with respect to BD was confirmed by Strondl et al.[6] In addition, these authors found that a h001i texture parallel to the scanning vectors can be obtained when these are not rotated in subsequent layers. Transmission electron microscopy (TEM) analysis of the as-built material conducted in Reference 6 proved formation of y¢ and y¢¢ precipitates already during the build process indicating that an in situ heat treatment is effective during EBM. The investigations on the mechanical performance of Inconel 718 after EBM and a standard solution annealing and aging treatment in Reference 7 showed that both the ultimate and yield strength are in the range of values obtained by the conventionally processed counterpart. However, elongation at fracture was found to be fairly low, especially when tested parallel to BD, which was related to the presence of process-induced agglomeration of pores. The effect of e
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