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Fatigue crack initiation and propagation in Alloy 718 with a bimodal grain morphology built via electron beam-powder bed fusion
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. Multi-Scale Additive Manufacturing Lab (MSAM), The University of Waterloo, Ontario (CAN). (PTW)ORCID iD: 0000-0002-7663-9631
Technology Access Centre for Aerospace & Manufacturing, Red River College, Manitoba (CAN).
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. Multi-Scale Additive Manufacturing Lab (MSAM), The University of Waterloo, Ontario (CAN). (PTW)ORCID iD: 0000-0001-6610-1486
Division of Engineering Materials, Linköping University, Linköping (SWE).
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2021 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 827, article id 142051Article in journal (Refereed) Published
Abstract [en]

A unique melting strategy was implemented in electron beam-powder bed fusion (EB-PBF) of Alloy 718, resulting in the formation of a bimodal grain morphology consisting of fine equiaxed and columnar grains. The microstructure was preserved following various thermal post-treatments. The post-treated specimens were exposed to low cycle fatigue (LCF), and fatigue crack growth (FCG) tests in ambient air at 600 °C under pure and dwell-time (120 s) fatigue cycles. Clustered inclusions spanned a region of 100-600 µm in length acted as the crack initiation site, reducing the specimens' total fatigue life. When compared to pure fatigue cycles, dwell-time fatigue cycles reduced LCF life by approximately 35%, regardless of the thermal post-treatments. Due to a high fraction of grain boundary area in the as-built EB-PBF specimens, oxygen diffusion across the grain boundaries was enhanced. The intergranular fracture mode was favored in the plastic zone ahead of the crack tip, leading to rapid crack growth. No unbroken ligaments behind the crack front were found by high-resolution X-ray computed tomography, which was consistent with a large crack opening displacement linked to severe deformation around the crack tip. 

Place, publisher, year, edition, pages
Elsevier Ltd , 2021. Vol. 827, article id 142051
Keywords [en]
3D printers; Additives; Computerized tomography; Crack tips; Electron beams; Fatigue of materials; Grain boundaries; Morphology; Superalloys; Textures, Alloy 718; Bimodal grains; Dwell time; Electron-beam; Fatigue crack initiation; Fatigue cycles; Grain morphologies; Grain-boundaries; Powder bed; Thermal post-treatments, Fatigue crack propagation
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-17513DOI: 10.1016/j.msea.2021.142051ISI: 000713681600001Scopus ID: 2-s2.0-85115023272OAI: oai:DiVA.org:hv-17513DiVA, id: diva2:1599190
Note

Financial supports of the Knowledge Foundation for the SupREme project (2018-0203) and Åforsk for the SCC-SUMAN project (18-296) are highly acknowledged. The authors would like to express their gratitude to Mr. Johannes Gårdstam from Quintus Technologies AB for performing the thermal post-treatment on the specimens. Mr. Mats Högström and Mr. Håkan Bäckström of University West deserve special recognition for their significant contributions to the room temperature LCF experiments. Chalmers Materials Analysis Laboratory (CMAL) at the Chalmers University of Technology is appreciated for the assistance with the EBSD/SEM analysis. Dr. Nitesh Raj Jaladurgam of Chalmers University of Technology and Mr. Vui Mai Nguyen of Red River College are greatly acknowledged for the analysis of the EBSD and X-ray CT results, respectively. The authors would like to thank Dr. Niklas Israelsson (Arcam-EBM), and Dr. James Shipley (Quintus Technologies AB), and Mr. Ron Morford (MetCut) for their significant contribution to the specimen manufacturing and post-processing.

Available from: 2021-09-30 Created: 2021-09-30 Last updated: 2022-03-31Bibliographically approved

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Sadeghi, EsmaeilKarimi Neghlani, PariaHansson, Thomas

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