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On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering. 202100-3096.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing (AM), also known as 3D printing, offers great design flexibility for manufacturing components with complex geometries, and has attracted significant interest in the aero and energy industries in the past decades. Among the commercial AM processes, selective laser melting (SLM) and electron beam melting (EBM) are the two most widely used ones for metallic materials. Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties, weldability and low cost. Due to its excellent weldability, IN718 has been intensively applied in the AM filed, to gain more understanding of the AM processes and fully realize AM’s potentials.

The study objects in the present thesis include both EBM and SLM IN718. The solidification conditions in EBM and SLM are very different and are different to that of conventional cast, leading to unique microstructures mechanical properties. Therefore, this thesis aims to gain better understanding of the microstructures and anisotropic mechanical behaviours of both EBM and SLM IN718, by detailed characterizations and by comparisons with the forged counterpart.

The as-built microstructure of EBM IN718 is spatially dependent: the periphery (contour) region has a mixture of equiaxed and columnar grains, while the bulk (hatch) region has columnar grains elongated along the building direction; the last solidified region close to the top sample surface shows segregation and Laves phases, otherwise the rest of the whole sample is well homogenized. Differently, the as-built microstructure of SLM IN718 is spatially homogeneous: the grains is rather equiaxed and with subgrain cell structures. These microstructures also respond differently to the standard heat treatment routines for the conventional counterparts.

Anisotropic mechanical properties are evident in the room temperature tensile tests and high temperature dwell-fatigue tests. The anisotropic tensile properties of EBM IN718 at room temperature are more likely due to the directional alignment of porosities along the building direction rather than the strong crysiii tallographic texture of 100 _ building direction. While for SLM IN718, the anisotropy is more likely attributed to the different extents of ‘work-hardening’ or dislocations accumulated between the horizontally and vertically built specimens. The anisotropy mechanisms in dwell-fatigue crack propagations at 550 C for EBM and SLM IN718 are identical: higher effective stress intensity factor when intergranular cracking path is perpendicular to the loading direction, but lower effective stress intensity factor when intergranular cracking path is parallel to or slightly deviated from the loading direction.

The 2160s dwell-fatigue cracking behaviours at 550 C are of significant interest for AM IN718, of which test condition is similar to that of real service for IN718 disk in turbine engine. Generally, after conventional or short-term heat treatments, EBM IN718 shows better dwell-fatigue cracking resistance than SLM IN718. The damage mechanism is different for EBM and SLM IN718: the intergranular cracking in EBM IN718 is due to environmentally assisted grain boundary attack, while creep damage is active for SLM IN718. The considerably ‘deformed’ microstructure, specifically the subgrain cell structures in SLM IN718 resulted from the manufacturing process, is believed to activate creep damage even at a low temperature of 550 C. And for SLM IN718, heat treatment routine must be carefully established to alter the ‘deformed’ microstructure for better time dependent cracking resistance at elevated temperature.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. , p. 52
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2019
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-161706DOI: 10.3384/diss.diva-161706ISBN: 9789179299910 (print)OAI: oai:DiVA.org:liu-161706DiVA, id: diva2:1368420
Public defence
2019-12-06, ACSA, Hus A, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2019-11-08 Created: 2019-11-07 Last updated: 2019-11-08Bibliographically approved
List of papers
1. Microstructure and Anisotropic Mechanical Properties of EBM Manufactued Inconel 718 and Effects of Post Heat Treatment
Open this publication in new window or tab >>Microstructure and Anisotropic Mechanical Properties of EBM Manufactued Inconel 718 and Effects of Post Heat Treatment
2017 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 693, p. 151-163Article in journal (Refereed) Published
Abstract [en]

Materials manufactured with electron beam melting (EBM) have different microstructures and properties to those manufactured using conventional manufacturing methods. A detailed study of the microstructures and mechanical properties of Inconel 718 manufactured with EBM was performed in both as-manufactured and heat-treated conditions. Different scanning strategies resulted in different microstructures: contour scanning led to heterogeneous grain morphologies and weak texture, while hatch scanning resulted in predominantly columnar grains and strong 〈001〉 building direction texture. Precipitates in the as-manufactured condition included γ′, γ″, δ  , TiN and NbC, among which considerable amounts of γ″ yielded relatively high hardness and strength. Strong texture, directionally aligned pores and columnar grains can lead to anisotropic mechanical properties when loaded in different directions. Heat treatments increased the strength and led to different δ precipitation behaviours depending on the solution temperatures, but did not remove the anisotropy. Ductility seemed to be not significantly affected by heat treatment, but instead by the NbC and defects inherited from manufacturing. The study thereby might provide the potential processing windows to tailor the microstructure and mechanical properties of EBM IN718.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Electron beam melting; Nickel based superalloy; Microstructure; Anisotropy; Mechanical properties; Heat treatments
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-137289 (URN)10.1016/j.msea.2017.03.085 (DOI)000401384400018 ()2-s2.0-85016252903 (Scopus ID)
Note

Funding agencies: Sandvik Machining Solutions AB in Sandviken, Sweden; Faculty Grant SFO-MAT-LiU at Linkoping University [2009-00971]; Chinese Scholarship Council; Agora Materiae

Available from: 2017-05-10 Created: 2017-05-10 Last updated: 2019-11-07Bibliographically approved
2. On the formation of microstructural gradients in a nickel-base superalloy during electron beam melting
Open this publication in new window or tab >>On the formation of microstructural gradients in a nickel-base superalloy during electron beam melting
2018 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 160, p. 251-261Article in journal (Refereed) Published
Abstract [en]

Electron beam melting (EBM) is one of the most widely used additive manufacturing (AM) methods for metallic components and has demonstrated great potential to fabricate high-end components in the aerospace and energy industries. The thermal condition within a melt pool and the complicated thermal cycles during the EBM process are of interest but not yet well-understood, and will significantly affect the microstructural homogeneity of as-manufactured nickel-base superalloy components. To establish the thermal profile evolution during electron beam melting of nickel-base superalloys, Inconel 718 (IN718) is manufactured and characterized in the as-manufactured condition, on account of its representative segregation and precipitation behaviours. The microstructure gradient within a build, specifically the Laves phase volume fraction evolution, is rationalized with the solidification condition and the following in-situ annealing. Precipitations of carbide/nitride/carbonitride, delta and gamma/gamma are also discussed. Hardness is measured and correlated to the Laves phase volume fraction evolution and the precipitation of gamma/gamma . The results of this study will (i) shed light on microstructure evolution during the EBM process with regard to thermal history; and (ii) deepen the current understandings of solidification metallurgy for additive manufacturing of Ni-base superalloys. (C) 2018 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2018
Keywords
Electron beam melting; Inconel 718; Laves; Solidification; In-situ annealing; Thermal history
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-153669 (URN)10.1016/j.matdes.2018.09.006 (DOI)000453008100023 ()
Note

Funding Agencies|Sandvik Machining Solutions AB in Sandviken, Sweden; Linkoping University [SFO-MAT-LiU#2009-00971]; Chinese Scholarship Council; Agora Materiae; Swedish Governmental Agency for Innovation Systems (Vinnova) [2016-02675]

Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2019-11-07
3. Microstructure and mechanical properties of Inconel 718 produced by selective laser melting: Sample orientation dependence and effects of post heat treatments
Open this publication in new window or tab >>Microstructure and mechanical properties of Inconel 718 produced by selective laser melting: Sample orientation dependence and effects of post heat treatments
2018 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 713, p. 294-306Article in journal (Refereed) Published
Abstract [en]

Inconel 718 produced by selective laser melting (SLM) has been characterized with focus on the microstructure, the dependence of sample orientation on the mechanical properties and the effects of post heat treatments. The as-manufactured IN718 has a very fine cellular-dendritic structure with fine Laves phases precipitating in the interdendritic region, and electron backscatter diffraction (EBSD) analysis shows that both the vertically and horizontally built samples have relatively weak texture. The vertically built samples show lower tensile strength but higher ductility than the horizontally built samples, and the mechanism is shown to be partly due to the crystallographic feature but more importantly due to the different amount of residual stress and dislocations accumulated in these two kinds of samples. Applying heat treatments can significantly increase the strength while decrease the ductility correspondingly, and difference in yield strength between the vertically and horizontally built samples decreases with increasing the heat treatment temperatures, mainly due to the removal of residual stress and dislocations.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2018
Keywords
Selective laser melting; Nickel based superalloy; Microstructure; Mechanical properties; Crystallographic orientation; Residual stress
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-145799 (URN)10.1016/j.msea.2017.12.043 (DOI)000425557900036 ()
Note

Funding Agencies|Siemens AG in Berlin, Germany [IN718]; Linkoping University [2009-00971]; Chinese Scholarship Council; Agora Materiae

Available from: 2018-03-22 Created: 2018-03-22 Last updated: 2019-11-07
4. On the dwell-fatigue crack propagation behavior of a high strength superalloy manufactured by electron beam melting
Open this publication in new window or tab >>On the dwell-fatigue crack propagation behavior of a high strength superalloy manufactured by electron beam melting
2019 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 760, p. 448-457Article in journal (Refereed) Published
Abstract [en]

To demonstrate the reliability of additively manufactured superalloys for critical turbine engine components, dynamic tests simulating in-service condition are required. The present study aims to study the dwell-fatigue crack propagation behaviors of IN718 manufactured via electron beam melting (EBM). The textured and columnar-grained microstructure of EBM IN718 shows anisotropic dwell-fatigue cracking resistance when loading axis is aligned parallel and perpendicular to the columnar grains. High and low angle grain boundaries interact differently with the dwell-fatigue cracking path. The effect of different heat treatments on the cracking behavior is also discussed. The dwell-fatigue crack propagation rate of EBM IN718 is compared with forged IN718 under both dwell-fatigue test condition and pure fatigue test condition. The superiority of dwell-fatigue cracking resistance of EBM IN718 to forged IN718 is shown and discussed.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
IN718; Electron beam melting (EBM); Dwell; Fatigue; Crack propagation
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-159148 (URN)10.1016/j.msea.2019.06.013 (DOI)000474501200044 ()
Note

Funding Agencies|Sandvik Machining Solutions AB in Sandviken, Sweden; Chinese Scholarship Council; Swedish Governmental Agency for Innovation Systems (Vinnova) [2016-05175]; Faculty grant SFO-MAT-LiU at Linkoping University [2009-00971]; Agora Materiae

Available from: 2019-07-30 Created: 2019-07-30 Last updated: 2019-11-07

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