On the Dwell-Fatigue Crack Propagation Behavior of a High-Strength Ni-Base Superalloy Manufactured by Selective Laser Me
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DUCTION
SELECTIVE laser melting (SLM) is one of the most widely used additive manufacturing (AM) techniques for metallic materials. The extremely rapid and localized solidification during the SLM process results in quite different as-built microstructures compared with conventional cast or hot-worked ones. For critical Ni-base superalloy engine components, SLM shows overwhelming advantages over other non-AM processes for geometric complexities.[1] The attempts to manufacture Ni-base superalloy (IN718,[2–4] IN738LC,[5,6] Hastelloy X,[7–9] CM247LC[10–12] and IN625[13–15] components) via SLM have attracted much attention in the AM field in the past years. Most of these studies focused on microstructural studies, process parameter optimizations and monotonic mechanical properties (tensile test and hardness). Though promising, the reliability of these SLM Ni-base superalloys under dynamic and complicated service conditions still needs to be demonstrated. IN718 is a c00 strengthened Ni-base superalloy, and it is widely used for turbine disk materials. Turbine disk materials are usually subjected to dwell-fatigue loading DUNYONG DENG, RU LIN PENG, and JOHAN MOVERARE are with the Division of Engineering Materials, Department of Management and Engineering, Linko¨ping University, 58183 Linko¨ping, Sweden. Contact e-mail: [email protected] ROBERT ERIKSSON is with the Division of Solid Mechanics, Department of Management and Engineering, Linko¨ping University, 58183 Linko¨ping, Sweden. Manuscript submitted July 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
during real engine operation, i.e., there is a dwell period at peak loading, in addition to the cyclic ramping up and down. By prolonging the dwell period and/or increasing the test temperature, the fatigue crack propagation in conventional IN718 has been largely accelerated and becomes time-dependently intergranular[16–25] compared with the pure-cyclic fatigue condition. Two possible theories regarding environmentally assisted grain boundary attack, namely dynamic embrittlement (DE)[26] and stress-assisted grain boundary oxidation (SAGBO),[27] are suggested to explain this dwell effect, but there is still debate about which of these two theories is the actual one. On the other hand, it is also possible that creep happens during the dwell-fatigue test, depending on the relative resistance of creep and environmentally assisted grain boundary degradation for the specific microstructure under the test condition. IN718 has been intensively studied in the AM field because of its excellent weldability. The typical SLM microstructure is very different from that of the hot-worked counterpart; please see the detailed microstructural study of SLM IN718 reported in our previous work.[3] This motivated the authors to study the dwell-fatigue cracking behaviors in the present study and demonstrate the reliability of SLM components in service conditions, which is important but has rarely been reported so far. Specific focuses will be on (1) the damage mechanism, (2) effects of heat trea
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