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Fatigue Properties of Additively Manufactured Alloy 718
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (PTW)ORCID iD: 0000-0002-8664-4573
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Additive Manufacturing (AM), commonly known as 3D Printing, is a disruptive modern manufacturing process, in which parts are manufactured in a layer-wise fashion. Among the metal AM processes, Powder Bed Fusion (PBF) technology has opened up a design space that was not formerly accessible with conventional manufacturing processes. It is, now, possible to manufacture complex geometries, such as topology-optimized structures, lattice structures and intricate internal channels, with relative ease. PBF is comprised of Electron Beam Melting (EBM) and Selective Laser Melting (SLM) processes.

Though AM processes offer several advantages, the suitability of these processes to replace conventional manufacturing processes must be studied in detail; for instance, the capability to produce components of consistent quality. Therefore, understanding the relationship between the AM process together with the post treatment used and the resulting microstructure and its influence on the mechanical properties is crucial, to enable manufacturing of high-performance components. In this regard, for AM built Alloy 718, only a limited amount of work has been performed compared to conventional processes such as casting and forging. The aim of this work, therefore, is to understand how the fatigue properties of EBM and SLM built Alloy 718, subjected to different thermal post-treatments, is affected by the microstructure. In addition, the effect of as-built surface roughness is also studied.

Defects can have a detrimental effect on fatigue life. Numerous factors such as the defect type, size, shape, location, distribution and nature determine the effect of defects on properties. Hot Isostatic Pressing (HIP) improves fatigue life as it leads to closure of most defects. Presence of oxides in the defects, however, hinders complete closure by HIP. Machining the as-built surface improves fatiguelife; however, for EBM manufactured material, the extent of improvement is dependent on the amount of material removed. The as-built surface roughness, which has numerous crack initiation sites, leads to lower scatter in fatigue life. In both SLM and EBM manufactured material, fatigue crack propagation is transgranular. Crack propagation is affected by grain size and texture of the material.

Place, publisher, year, edition, pages
Trollhättan: University West , 2018. , p. 49
Series
Licentiate Thesis: University West ; 24
Keywords [en]
Fatigue; Additive Manufacturing; Powder Bed Fusion; Superalloy; Microstructure; Surface roughness
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
URN: urn:nbn:se:hv:diva-13186ISBN: 978-91-88847-13-3 (print)ISBN: 978-91-88847-12-6 (electronic)OAI: oai:DiVA.org:hv-13186DiVA, id: diva2:1268435
Presentation
2018-12-05, University West, Trollhättan, 10:00 (English)
Supervisors
Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2018-12-05Bibliographically approved
List of papers
1. Influence of defects and as-built surface roughness on fatigue properties of additively manufactured Alloy 718
Open this publication in new window or tab >>Influence of defects and as-built surface roughness on fatigue properties of additively manufactured Alloy 718
2018 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 735, p. 463-474Article in journal (Refereed) Published
Abstract [en]

Electron beam melting (EBM) and Selective Laser Melting (SLM) are powder bed based additive manufacturing (AM) processes. These, relatively new, processes offer advantages such as near net shaping, manufacturing complex geometries with a design space that was previously not accessible with conventional manufacturing processes, part consolidation to reduce number of assemblies, shorter time to market etc. The aerospace and gas turbine industries have shown interest in the EBM and the SLM processes to enable topology-optimized designs, parts with lattice structures and part consolidation. However, to realize such advantages, factors affecting the mechanical properties must be well understood – especially the fatigue properties. In the context of fatigue performance, apart from the effect of different phases in the material, the effect of defects in terms of both the amount and distribution and the effect of “rough” as-built surface must be studied in detail. Fatigue properties of Alloy 718, a Ni-Fe based superalloy widely used in the aerospace engines is investigated in this study. Four point bending fatigue tests have been performed at 20 Hz in room temperature at different stress ranges to compare the performance of the EBM and the SLM material to the wrought material. The experiment aims to assess the differences in fatigue properties between the two powder bed AM processes as well as assess the effect of two post-treatment methods namely – machining and hot isostatic pressing (HIP). Fractography and metallography have been performed to explain the observed properties. Both HIPing and machining improve the fatigue performance; however, a large scatter is observed for machined specimens. Fatigue properties of SLM material approach that of wrought material while in EBM material defects severely affect the fatigue life. © 2018 Elsevier B.V.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Additive manufacturing, Alloy 718, Fatigue, Surface roughness, Hot isostatic pressing, 3D printers, Bending tests, Binary alloys, Fatigue testing, Fighter aircraft, Fracture mechanics, Gas turbines, Hot isostatic pressing, Iron alloys, Mechanical properties, Melting, Nickel alloys, Sintering, Surface roughness, Alloy 718, Conventional manufacturing, Fatigue performance, Gas turbine industry, Lattice structures, Manufacturing complex, Post-treatment method, Selective laser melting, Fatigue of materials
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-12956 (URN)10.1016/j.msea.2018.08.072 (DOI)000447117300055 ()2-s2.0-85052655828 (Scopus ID)
Note

Available online 23 August 2018

Available from: 2018-09-27 Created: 2018-09-27 Last updated: 2018-12-05Bibliographically approved

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