Development of Oxide Dispersion Strengthened Titanium Aluminides for Additive Manufacturing

216 Pages·2017·10.64 MB·English

ETH Library
Development of Oxide Dispersion
Strengthened Titanium Aluminides
for Additive Manufacturing
Doctoral Thesis
Author(s):
Kenel, Christoph
Publication date:
2016
Permanent link:
https://doi.org/10.3929/ethz-a-010860058
Rights / license:
In Copyright - Non-Commercial Use Permitted
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DISS. ETH NO. 23785
Development of Oxide Dispersion Strengthened Titanium
Aluminides for Additive Manufacturing
A thesis submitted to attain the degree of
DOCTOR OF SCIENCES of ETH ZURICH
(Dr. sc. ETH Zurich)
presented by
CHRISTOPH KENEL
MSc ETH
born on 06.08.1987
citizen of Arth SZ
accepted on the recommendation of
Prof. Dr. Konrad Wegener, examiner
Dr. Christian Leinenbach, co-examiner
Prof. Dr. Gordon Tatlock, co-examiner
2016

Acknowledgement
This thesis was written during my time at the Swiss Federal Laboratories
for Materials Science and Technology (Empa) in the group Alloy Design for
AdvancedProcessingTechnologies(ADAPT)fromFebruary2013untilAugust
2016 in collaboration with the Institute of Machine Tools and Manufacturing
(IWF) of ETH Zurich. The work was conducted in the frame of the multi-
national EU FP7 research project Oxide Dispersion Strengthened Materials
for the Additive Manufacture of High Temperature Components in Power
Generation (OXIGEN).
IwouldliketothankmysupervisorProf. Dr. KonradWegener(IWF-ETH)
for giving me the opportunity to do my PhD on this topic. His inputs and, in
a positive sense, critical questions beyond the material science part of this
study are highly appreciated.
I am very grateful to my supervisor at Empa, Dr. Christian Leinenbach,
headoftheADAPTgroup,forgivingmehistrustandthefreedomtopursue
myresearchinatrulyconstructiveandopen-mindedenvironment. Heprovided
me with the necessary infrastructure, his guidance and scientific input to
successfully complete this thesis.
Thisthesiswouldnothavebeenpossibleinthisformwithoutcontributions
of many people, working at Empa and elsewhere. They provided materi-
als, technical support, measurements, access to equipment, scientific input,
pleasant and fruitful discussions and advice throughout my time at Empa.
Special thanks to Toni Ivas, Adrian Lis, Nico Weyrich, Ariyan Arabi-Hashemi,
Xiaoshuang Li, Georgia Dasargyri, Wookjin Lee, Hansruedi Elsener, Michael
Koster, Sara Romer, Martin Sauder, Sascha Populoh, Remo Widmer, Erwin
Pieper,andBarbaraZeps,aswellasthepastandpresentmembersoftheLabs
"Joining Technologies and Corrosion" and "Advanced Materials Processing".
I also thank Thomas Bauer and Adriaan Spierings (Inspire AG, CH), Gordon
i
Tatlock,KeithArnoldandKarlDawson(UniversityofLiverpool,UK),Alberto
Colella (Matres, Italy) and Carl Hauser and Josh Barras (TWI, UK) from the
OXIGEN consortium for their support and discussions throughout our project.
Finally, I express my gratitude to Daniel Grolimund, Vallerie Ann Samson,
Julie Fife, Steven van Petegem and Helena van Swygenhoven from PSI for
their support with the in situ diffraction experiments.
I would like to thank my family for their support allowing me to pursue my
path in science. A special thank you to my partner Johanna Nyffeler for her
advice and patience throughout these stressful years.
TheresearchleadingtotheseresultshasreceivedfundingfromtheEuropean
UnionSeventhFrameworkProgram[FP7/2007-2013]undergrantagreement
no. 310279 (OXIGEN).
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Contents
1 Introduction 1
2 State of the art 5
2.1 Constitution of titanium aluminides . . . . . . . . . . . . . 5
2.2 Microstructure formation and related properties . . . . . . . 8
2.3 State-of-the-art higher-order titanium aluminide alloys . . . 17
2.4 Powder-based processing of titanium aluminides . . . . . . . 19
2.5 Reinforced titanium aluminides . . . . . . . . . . . . . . . . 31
2.6 Aim of the thesis . . . . . . . . . . . . . . . . . . . . . . . 34
3 Experimental and analytical methods 37
3.1 Materials and consolidation methods . . . . . . . . . . . . . 37
3.2 Materials characterization. . . . . . . . . . . . . . . . . . . 42
3.3 Materials testing . . . . . . . . . . . . . . . . . . . . . . . 44
3.4 Computational thermodynamics . . . . . . . . . . . . . . . 49
4 Microstructure formation of Ti-Al alloys at high cooling rates 51
4.1 Experimental setup and concept . . . . . . . . . . . . . . . 51
4.2 Computational framework. . . . . . . . . . . . . . . . . . . 61
4.3 Influence of cooling rate and composition in binary Ti-Al . . 66
4.4 Influence of the ternary alloying elements Nb and Mo . . . . 81
4.5 Alloy design of titanium aluminides for additive manufacturing 95
4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5 Microstructure and oxide particle stability in an ODS γ-TiAl alloy 101
5.1 Alloy design and selection . . . . . . . . . . . . . . . . . . . 101
5.2 Processing results and discussion . . . . . . . . . . . . . . . 107
5.3 Summary and conclusions . . . . . . . . . . . . . . . . . . . 124
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Contents
6 Performance of ODS titanium aluminides 125
6.1 High temperature degradation behavior in air . . . . . . . . 125
6.2 Mechanical properties at low and high temperature . . . . . 140
6.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 147
7 Selective laser melting of a β-solidifying ODS γ-TiAl alloy 149
7.1 Experimental details . . . . . . . . . . . . . . . . . . . . . . 149
7.2 SLM processing results . . . . . . . . . . . . . . . . . . . . 153
7.3 Summary and conclusion . . . . . . . . . . . . . . . . . . . 169
8 Summary and Outlook 171
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Abstract
Titanium aluminides gained a lot of interest over the last decades due to a
potential weight reduction and increased performance of high temperature
components. Further increasing their performance at high temperatures is a
crucialfactortocompetewiththeheavierNi-basesuperalloys. Processingthis
class of structural intermetallics using additive manufacturing is also of great
interest to produce novel parts with unprecedented complexity. In this work
an oxide dispersion strengthened (ODS) titanium aluminide alloy is developed
takingintoaccountthenon-equilibriumconditionsduringbeam-basedadditive
manufacturing. In order to understand the influence of cooling rate and
composition on microstructure formation, rapid solidification experiments on
binary Ti-Al and ternary Ti-Al-Nb and Ti-Al-Mo alloys are performed using a
novelexperimentalliquiddropletquenchingtechnique. Incombinationwithin
situ X-ray diffraction experiments at a synchrotron light source, the observed
behavior of Ti-Al-(Nb, Mo) is mapped and compared to thermodynamic
simulations based on the CALPHAD approach taking into account the non-
equilibrium nature of the process. Based on this fundamental study, a matrix
alloy is selected for production of ODS titanium aluminide by mechanical
alloying. Consolidation tests using spark plasma sintering, selective laser
meltinganddirectmetaldepositionshowtheinfluenceoftheprocessingroute
on the formed microstructure and the resulting dispersoid size. Apart from
the effects of alloy composition and ODS particle addition on microstructure
andprocessingalsotheinfluenceonmechanicalperformanceanddegradation
behaviorisstudied. Comparedtothedispersoid-freealloy, theODSvariantis
superiorintermsofyieldstressandultimatetensilestrengthat293to1073K
as well as more oxidation resistant. Finally it is demonstrated that complex
lattice structures can be produced from the ODS alloy by selective laser
melting. Additionally, the beneficial effect of adapted re-scanning strategies
on the occurrence of cracking is discussed.
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Zusammenfassung
Seit rund drei Jahrzehnten sind Titanaluminide sind im Fokus der Forschung,
da ihr Einsatz Gewichtseinsparungen und bessere Eigenschaften in Hochtem-
peraturkomponenten verspricht. Eine kontinuierliche Verbesserung dieser
Eigenschaften ist ein entscheidender Punkt um eine Konkurrenz zu den deut-
lich dichteren Ni-Basis Superlegierungen zu schaffen. Die additive Fertigung
dieserKlassevonintermetallischenStrukturwerkstoffenistzudemseitkurzem
von grossem Interesse um Komponenten mit bisher unerreichter Komplexität
zuproduzieren. ImRahmendieserArbeitwurdeeineoxid-dispersionsverstärkte
(ODS) TiAl Legierung entwickelt, wobei die Nichtgleichgewichtsbedingun-
gen während der strahlbasierten additiven Fertigung miteinbezogen wurden.
Eine neuartige Methode, basierend auf abgeschreckten Metalltropfen, erlaubt
den Einfluss der Kühlrate als auch der Legierungszusammensetzung auf die
ausgebildete Mikrostruktur in Ti-Al, Ti-Al-Nb und Ti-Al-Mo zu untersuchen.
In Kombination mit in situ Röntgenbeugungs-Experimenten an einer Syn-
chrotronquelle wurde das Verhalten der Ti-Al-(Nb,Mo) Systeme bestimmt
undunterBerücksichtigungderNichtgleichgewichtsbedingungenderProzesse
mit thermodynamischen Berechnungen verglichen. Basierend auf dieser Un-
tersuchung wurde eine Zusammensetzung für die Produktion einer ODS
Legierung durch mechanisches Legieren ausgewählt. Konsolidierungstests
mittels Spark Plasma Sintern, Selektivem Laserschweissen und Laserauftrags-
schweissen zeigen den Einfluss des Prozesses auf die Mikrostrukturbildung
und die resultierende Partikelgrösse. Zusätzlich wurde auch der Einfluss der
Oxidpartikel auf die mechanische Festigkeit und Oxidationsbeständigkeit bes-
timmt. Die verstärkte Variante zeigt eine höhere Dehn- und Streck-Grenze
sowie Oxidationsbeständigkeit verglichen mit der unverstärkten Variante. Am
Ende der Arbeit wird die Herstellung von komplexen Gitterstrukturen mittels
Selektiven Laserschweissen demonstriert. Der positive Einfluss von diversen
Wiederaufschmelz-Strategien auf die Rissbildung wird ebenfalls diskutiert.
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