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Deng, Dunyong
Publications (5 of 5) Show all publications
Deng, D. (2019). On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718. (Doctoral dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718
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:nbn:se:liu:diva-161706 (URN)10.3384/diss.diva-161706 (DOI)9789179299910 (ISBN)
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
Xu, J., Gruber, H., Deng, D., Peng, R. L. & Moverare, J. (2019). Short-term Creep Behavior of an Additive Manufactured Non-weldable Nickel-base Superalloy Evaluated by Slow Strain Rate Testing. Acta Materialia, 179, 142-157
Open this publication in new window or tab >>Short-term Creep Behavior of an Additive Manufactured Non-weldable Nickel-base Superalloy Evaluated by Slow Strain Rate Testing
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2019 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 179, p. 142-157Article in journal (Refereed) Published
Abstract [en]

Additive manufacturing (AM) of high γ′ strengthened Nickel-base superalloys, such as IN738LC, is of high interest for applications in hot section components for gas turbines. The creep property acts as the critical indicator of component performance under load at elevated temperature. However, it has been widely suggested that the suitable service condition of AM processed IN738LC is not yet fully clear. In order to evaluate the short-term creep behavior, slow strain rate tensile (SSRT) tests were performed. IN738LC bars were built by laser powder-bed-fusion (L-PBF) and then subjected to hot isostatic pressing (HIP) followed by the standard two-step heat treatment. The samples were subjected to SSRT testing at 850 °C under strain rates of 1 × 10−5/s, 1 × 10−6/s, and 1 × 10−7/s. In this research, the underlying creep deformation mechanism of AM processed IN738LC is investigated using the serial sectioning technique, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). On the creep mechanism of AM polycrystalline IN738LC, grain boundary sliding is predominant. However, due to the interlock feature of grain boundaries in AM processed IN738LC, the grain structure retains its integrity after deformation. The dislocation motion acts as the major accommodation process of grain boundary sliding. Dislocations bypass the γ′ precipitates by Orowan looping and wavy slip. The rearrangement of screw dislocations is responsible for the formation of subgrains within the grain interior. This research elucidates the short-creep behavior of AM processed IN738LC. It also shed new light on the creep deformation mechanism of additive manufactured γ′ strengthened polycrystalline Nickel-base superalloys.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Nickel-base superalloy, laser processing, creep, grain boundary sliding, dislocations
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-161742 (URN)10.1016/j.actamat.2019.08.034 (DOI)000488417400013 ()2-s2.0-85071224254 (Scopus ID)
Note

Funding Agencies|Swedish Governmental Agency for Innovation Systems, (Vinnova)Vinnova [2016-05175, 2018-00804]; Linkoping University [2009-00971]

Available from: 2019-11-08 Created: 2019-11-08 Last updated: 2019-12-13Bibliographically approved
Deng, D. (2018). Additively Manufactured Inconel 718: Microstructures and Mechanical Properties. (Licentiate dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Additively Manufactured Inconel 718: Microstructures and Mechanical Properties
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing (AM), also known as 3D printing, has gained significant interest in aerospace, energy, automotive and medical industries due to its capabilities of manufacturing components that are either prohibitively costly or impossible to manufacture by conventional processes. Among the various additive manufacturing processes for metallic components, electron beam melting (EBM) and selective laser melting (SLM) are two of the most widely used powder bed based processes, and have shown great potential for manufacturing high-end critical components, such as turbine blades and customized medical implants. The futures of the EBM and SLM are doubtlessly promising, but to fully realize their potentials there are still many challenges to overcome.

Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties and low cost. Though IN718 is being mostly used as a turbine disk material now, the initial introduction of IN718 was to overcome the poor weldability of superalloys in 1960s, since sluggish precipitation of strengthening phases λ’/λ’’ enables good resistance to strain-age cracking during welding or post weld heat treatment. Given the similarity between AM and welding processes, IN718 has been widely applied to the metallic AM field to facilitate the understandings of process-microstructure-property relationships.

The work presented in this licentiate thesis aims to better understand microstructures and mechanical properties EBM and SLM IN718, which have not been systematically investigated. Microstructures of EBM and SLM IN718 have been characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and correlated with the process conditions. Monotonic mechanical properties (e.g., Vickers microhardness and tensile properties) have also been measured and rationalized with regards to the microstructure evolutions before and after heat treatments.

For EBM IN718, the results show the microstructure is not homogeneous but dependant on the location in the components, and the anisotropic mechanical properties are probably attributed to alignment of porosities rather than texture. Post heat treatment can slightly increase the mechanical strength compared to the as-manufactured condition but does not alter the anisotropy. SLM IN718 shows significantly different microstructure and mechanical properties to EBM IN718. The as-manufactured SLM IN718 has very fine dendritic microstructure and Laves phases in the interdendrites, and is “work-hardened” by the residual strains and dislocations present in the material. Mechanical properties are different between horizontally and vertically built samples, and heat treatment can minimize this difference. Results from this licentiate thesis provide the basis for the further research on the cyclic mechanical properties of EBM and SLM IN718, which would be the focus of following phase of the Ph.D. research.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. p. 69
Series
Linköping Studies in Science and Technology. Licentiate Thesis, ISSN 0280-7971 ; 1798
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-144491 (URN)10.3384/lic.diva-144491 (DOI)9789176853832 (ISBN)
Supervisors
Note

Information about opponent and seminar are missing.

Available from: 2018-01-24 Created: 2018-01-24 Last updated: 2019-10-12Bibliographically approved
Deng, D., Moverare, J., Peng, R. L. & Söderberg, H. (2017). Microstructural Heterogeneity Along the Building Direction of Inconel 718 Produced by Electron Beam Melting (EBM). In: : . Paper presented at EUROMAT17, Thessaloniki 17-21 September 2017,Greece (pp. 1-1).
Open this publication in new window or tab >>Microstructural Heterogeneity Along the Building Direction of Inconel 718 Produced by Electron Beam Melting (EBM)
2017 (English)Conference paper, Oral presentation with published abstract (Refereed)
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-143975 (URN)
Conference
EUROMAT17, Thessaloniki 17-21 September 2017,Greece
Available from: 2018-01-01 Created: 2018-01-01 Last updated: 2018-01-16Bibliographically approved
Deng, D., Moverare, J., Peng, R. L. & Söderberg, H. (2017). Microstructure and Anisotropic Mechanical Properties of EBM Manufactued Inconel 718 and Effects of Post Heat Treatment. Materials Science & Engineering: A, 693, 151-163
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
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