Aerospace Technic and Technology

Results of low cycle fatigue (LCF) testing, investigation of microstructure, and fractures of specimens, which was, obtain by selective laser melting process (SLM) of the powders from Inconel 718 alloy have been carried out in the present article. Chemical composition of the considered specimens, microstructure before and after testing, results at room and elevated temperatures, which were built in XY direction (horizontal), were carrying-out. The specimens in the as-build state were exposed by hot isostatic pressing (HIP) with subsequent inherent Inconel 718 heat treatment. It was established that in specimens in as-built state observe accurately expressed zones of layer-by-layer smelting with 100 μm height. After HIP with subsequent heat treatment strengthening of the considered alloy provides by intermetallic γ'' – Ni3Nb phase, γ'-phase and carbides, also was identified lamellar δ-phase in the microstructure. LCF-testing was conducted in a so-called «soft» cycle of loading with a predetermined interval of the strain for providing of fatigue life in 10³, 5х10³, 10⁴ cycles. The obtained data was built logarithmic curve in coordinates «Strain σ – Number of cycles N», that allows, with sufficient reliability, determine a rational strain level at LCF-testing for providing a predetermined number of cycles. The following ratios of σ – N were established (Rσ=0, υ=1 Hz): for providing N=10³, σ≤1020 МPа, N=5х10³, σ=931…961 МPа, N= 10⁴, σ≤941 МPа. Results of the fractures investigation after LCF-testing at 20°С and 550°С shows that during the cyclic elastoplastic deformation at room temperature multicentricity of crack initiations zones observes. Mainly fixed a viscous pattern of fracture with a fatigue grooves presence, which size increases in the process of crack advance to the zone of fracture area. At elevated temperature in the zones of fatigue crack initiation and spreading quasi-brittle character of destruction observes, and turns in viscous in the zone of fracture area. Authors should be pointed out that the application of additive technologies in the manufacture of aerospace parts requires extensive R&D works, and testing efforts to confirm the repeatability of alloy characteristics. 

superalloy; low cycle fatigue; additive technologies; selective laser melting; hot isostatic pressing; heat-treatment

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