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Thin-wall Effects and Anisotropic Deformation Mechanisms of an Additively Manufactured Ni-based Superalloy
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
ANSTO Australian Nuclear Science and Technology Organization, Australia.
Bundesanstalt für Materialforschung and -prüfung (BAM), Berlin, Germany.
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2020 (English)In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 36, article id 101672Article in journal (Refereed) Published
Abstract [en]

Laser powder bed fusion (LPBF) of Ni-based superalloys shows great potential for high temperature applications, for example, as a burner repair application for gas turbines where the thin-walled structure is important. It motivates this work to investigate the evolution of microstructure and the anisotropic mechanical behavior when plate-like specimens are built with a thickness from 4 mm down to 1 mm. By performing texture analysis using neutron diffraction, a clear transition in fiber texture from <011> to <001> is indicated when the specimen becomes thinner. The residual stress shows no thickness dependence, and at the subsurface the residual stress reaches the same level as the yield strength. Due to the rough as-built surface, a roughness compensation method for mechanical properties of thin-walled structures is outlined and demonstrated. Tensile tests from room temperature up to 700 °C have been carried out. Anisotropic mechanical behavior is found at all temperatures, which is strongly related to the anisotropic texture evolution. Stronger texture evolution and grain rotations are discovered when the tensile loading is applied along the building direction. The mechanical behavior has been compared to a wrought material, where the high dislocation density and the subgrain structure of the LPBF material result in a higher yield strength. Combining the statistical texture analysis by neutron diffraction with mechanical testing, EBSD grain orientation mapping and the investigation of dislocation structures using transmission electron microscopy, this work illustrates the significance of texture for the thin-wall effect and anisotropic mechanical behavior of LPBF materials.

Place, publisher, year, edition, pages
Elsevier, 2020. Vol. 36, article id 101672
Keywords [en]
Hastelloy X; Hot tensile test; Crystallographic texture, roughness; Residual stress; Dislocation density
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-171983DOI: 10.1016/j.addma.2020.101672ISI: 000600807800193Scopus ID: 2-s2.0-85095614801OAI: oai:DiVA.org:liu-171983DiVA, id: diva2:1510523
Note

Funding agencies: Swedish Governmental Agency for Innovation Systems (Vinnova)Vinnova [2016-05175]; Center for Additive Manufacturing-metal (CAM2); AFM at Linkoping University; faculty grant SFO-MATLiU [2009-00971]

Available from: 2020-12-16 Created: 2020-12-16 Last updated: 2023-09-08Bibliographically approved
In thesis
1. Anisotropic Mechanical Behaviours and Thin-wall Effects of Additively Manufactured Austenitic Alloys
Open this publication in new window or tab >>Anisotropic Mechanical Behaviours and Thin-wall Effects of Additively Manufactured Austenitic Alloys
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing (AM), also known as 3D printing, is a general concept of building a three-dimensional object layer-by-layer. AM breaks through the manufacturing limitations in conventionally subtractive manufacturing, leading to a great design freedom of components with complex geometries. The potential of integrating AM into existing manufacturing process with additional functionality raises interest in various fields, such as aerospace, automotive and medical applications. To ensure robust AM applications, this PhD project has carried out investigations on the mechanical behaviours of AM components with respect to the characteristic microstructure and the geometrical effects. The investigated materials include Hastelloy X (HX, a solid-solution strengthened Ni-based superalloy) and stainless steel 316L (SS 316L, a solid-solution strengthened austenitic stainless steel) manufactured by laser powder bed fusion (LPBF). The high temperature tensile behaviours, short-term creep resistance and low cycle fatigue performance have been examined. The aim of this thesis is to conduct a fundamental studies that can be applied to different material grades with single phase face-centred cubic (FCC) crystallographic structure. 

LPBF HX shows a great potential for the burner tip repair application in gas turbines. Due to the complex geometry of the burner and the requirement of high temperature mechanical performance, the tensile properties have been systematically examined. Multiple testing variables have been applied, including the specimen geometry, the elevated temperature, the strain rate and the loading direction (LD). Combined with the prior and post microstructural analysis, the deformation and fracture mechanisms have been investigated. For the thin-walled specimens, a clear texture transition is found when it comes to the thinner specimen, and it leads to the lower yield strength as a result. In addition, as the high surface roughness of the LPBF as-built specimen can cause inaccuracy of the yield strength determination due to the overestimated loading cross section, especially for the thin-walled specimen, a calibration method based on the crystallographic texture results has been proposed. Meanwhile, anisotropic tensile behaviours are observed at all the testing conditions due to the elongated grain structure and the characteristic texture along the building direction (BD). At elevated temperatures, the grain boundary embrittlement takes place at 700 °C that leads to the ductility loss in the horizontal loading (LD ⊥ BD). Slow strain rate tensile testing (SSRT) has been performed to probe the short-term creep resistance at 700 °C, since it is a useful tool to address the strain rate dependent in elastic strain accumulation. Surprisingly, the ductility of the vertical loading (LD // BD) remains at a high level not only at 700 °C but also at SSRT condition, and the high ductility results from the evident texture evolution and crystallographic orientation dependent deformation twinning. The good ductility of the vertical loading indicates a better creep performance compared to the horizontal loading. In-situ and ex-situ neutron diffraction measurements upon loading have also been applied for a full-length investigation on the anisotropic tensile behaviours. 

Thin-wall effects on strain-controlled low cycle fatigue (LCF) behaviours of LPBF SS 316L have been investigated by using the tubular fatigue specimens with different wall thicknesses. The comparison between the machined and as-built surface conditions have been drawn. The fully reversed LCF tests were successfully performed without the buckling problem in thin-walled structures owing to the tubular geometry. The surface roughness and the distinct microstructure at the surface region lead to the inferior fatigue strain-life, especially with the low applied strain range. The combined effects have been quantified by estimating the fatigue notch factor, Kf . The LCF tests have also been performed on the regular cylindrical specimens and compared to the wrought SS 316L. A comparable fatigue strain-life is found between the LPBF and the wrought SS 316L. Yet, the secondary hardening caused by strain-induced martensitic phase transformation is only observed in the wrought SS 316L, while continuous cyclic softening is shown in the LPBF SS 316L. In addition, as high level of residual stress (RS) is commonly found in the as-built specimen, the effect of stress relief heat treatment (600 °C /4 hours) on the LCF behaviours has been examined. A great reduction of RS is found after the heat treatment, and higher responding stresses are shown in the stress relieved specimen, which indicates a better fatigue stress-life. 

In summary, the deformation and fracture mechanisms of LPBF HX and SS 316L under different loading conditions have been systematically investigated. Via increasing deeper knowledge of LPBF material behaviours, the LPBF applications can be expanded to a greater extent. 

Abstract [sv]

Additiv tillverkning (AM), även kallat 3D-printning, är ett allmänt koncept där man bygger ett tredimensionellt objekt lager för lager. AM bryter igenom tillverkningsbegränsningar i konventionell subtraktiv tillverkning, vilket leder till stor designfrihet för komponenter med komplex geometri. Potentialen med att integrera AM i befintliga tillverkningsprocesser med ytterligare funktionalitet väcker intresse inom flera olika områden, såsom flyg-, fordons- och medicinska tillämpningar. För att säkerställa robusta AM-tillämpningar har det i detta doktorandprojekt genomförts undersökningar av de mekaniska egenskaperna hos AM-komponenter med avseende på den karakteristiska mikrostrukturen och de geometriska effekterna som uppstår vid tillverkningen. De undersökta materialen inkluderar Hastelloy X (HX, en lösningshärdad Ni-baserad superlegering) och rostfritt stål 316L (SS 316L, ett lös-ningshärdat austenitiskt rostfritt stål) tillverkat med laser-pulverbäddsomsmältning (LPBF). Dragprovning vid förhöjd temperatur, korttidskrypmotstånd och lågcykelutmattning har studerats. Syftet med denna avhandling är att genomföra grundläggande studier som kan tillämpas på olika materialtyper med en yt-centrerad kubisk (FCC) kristallstruktur.

LPBF av HX visar en stor potential för reparation av brännarmunstycken i gasturbiner. På grund av brännarmunstyckenas komplexa geometri och kravet på mekanisk prestanda vid hög temperatur har dragegenskaperna systematiskt undersökts. Flera testvariabler har undersökts, inklusive provets geometri, den förhöjda temperaturen, töjningshastigheten och belastningsriktningen (LD). I kom-bination med mikrostrukturanalys före och efter testerna har deformations- och brottmekanismerna undersökts. Med avseende på väggtjocklek finner man en tydlig förändring i kristallografisk textur när man går mot tunnare provstavar vilket leder till lägre sträckgräns. Eftersom den höga yt-ojämnheten hos LPBF-material kan orsaka en felaktig bestämning av sträckgränsen, särskilt för tunnväggiga strukturer, har en kalibreringsmetod baserad på texturresultaten föreslagits. Dock så ob-serveras anisotropa dragprovsegenskaper vid alla testförhållanden på grund av den utsträckta kornstrukturen och den karakteristiska kristallografiska texturen längs byggriktningen (BD). Vid förhöjda temperaturer sker en korngränsförsprödning vid 700 °C vilket leder till en duktilitetsförlust i den horisontell belastningsriktning (LD ⊥ BD). Slow strain rate tensile testing (SSRT) har utförts för att undersöka det korttidskrypmotståndet vid 700 °C eftersom det är ett användbart verktyg för att hantera den töjningshastighetsberoende oelastiska töjningsackumuleringen vid förhöjda temperaturer. Överraskande nog förblir duktiliteten hög för den vertikala belastningen (LD //BD), inte bara vid dragprov vid 700 °C utan även vid den långsammare SSRT-provningen. Den höga duktiliteten är ett resultat av den utpräglade texturutvecklingen och en kristallografiskt orienteringsberoende tvilling-bildning. Den höga duktiliteten för den vertikala belastningsriktningen indikerar ett bättre krypmotstånd jämfört med den horisontella belastningsriktningen. In-situ och ex-situ neutrondiffraktionsmätning vid belastning har också använts för en mer djupgående undersökning av de anisotropa egenskaperna.

Inverkan av väggtjocklek vid töjningskontrollerad lågcykelutmattning (LCF) har undersökts för LPBF SS 316L genom att använda rörformiga utmattningsprovstavar med olika väggtjocklekar samt genom en jämförelse mellan bearbetade och icke-bearbetade ytförhållanden. De i drag/tryck-last symmetriska LCF-testerna kunde genomföras framgångsrikt utan bucklingsproblem tack vare den rörformiga geometrin. Ytojämnheten och den distinkta mikrostrukturen i de ytnära områdena ledde till sämre utmattningslivslängd, speciellt vid låga applicerade töjningsomfång, och de kombinerade effekterna har kvantifierats genom att uppskatta anvisningsfaktorn, Kf . LCF-tester har även utförts på konventionella cylindriska provstavar samt så har en jämförelse gjorts med konventionellt (smidd) SS 316L. En jämförbar livslängd i termer av töjningskontrollerad utmattning finns mellan LPBF och kon-ventionell SS 316L. Dock observerades ett sekundärt hårdnande för konventionell SS 316L som orsakas av töjningsinducerad martensitisk fastransformation, medan LPBF SS 316L uppvisade ett kontinuerligt cyklisk mjuknande. Dessutom, efter-som en hög nivå av restspänningar (RS) vanligtvis förekommer i LPBF-material, har effekten av avspänningsglödgning (600 °C /4timmar) på LCF-beteendet undersökts. En stor minskning av RS noterades efter värmebehandlingen och de avspänningsglödgade proverna hade en högre spänningsrespons, vilket indikerar bättre utmattningsegenskaper i termer av spänningsomfång mot livslängd.

Sammanfattningsvis har deformations- och brottmekanismerna för LPBF HX och SS 316L under olika belastningsförhållanden systematiskt undersökts. En djupare kunskap om materialbeteenden hos LPBF-legeringar kan leda till att tillämpningsområdet för LPBF-tekniken breddas.  

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2022. p. 80
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2227
Keywords
Laser powder bed fusion, Slow strain rate tensile testing, Short-term creep, Low cycle fatigue, Neutrons, Anisotropy, Crystallographic texture, Microstruc-ture, Dislocation structure, Lasersmältning i pulverbädd, Provning med långsam töjningshastighet, korttidskryp, Lågcykelutmattning, Neutroner, Anisotropi, Kristallografisk textur, Mikrostruktur, Dislokationsstruktur
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-184803 (URN)10.3384/9789179293154 (DOI)9789179293147 (ISBN)9789179293154 (ISBN)
Public defence
2022-06-10, ACAS, A-building, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Note

Funding agencies: This research is financially supported by the Swedish Governmental Agency for Innovation Systems (Vinnova grant 2016-05175) and the Center for Additive Manufacturing-metal (CAM2). The support also comes from AFM at Linköping University and the faculty grant SFO-MATLiU2009-00971.

Available from: 2022-05-06 Created: 2022-05-06 Last updated: 2022-05-09Bibliographically approved

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Yu, Cheng-HanPeng, Ru LinCalmunger, MattiasMoverare, Johan

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