Ph D Thesis
Production Technology 2020 No. 42
Towards understanding the
fatigue behaviour of Alloy 718
manufactured by Powder Bed
Fusion processes
Tryck: Stema Specialtryck AB, October 2020 Trycksak 3041 0234 SVANENMÄRKET Trycksak 3041 0234 SVANENMÄRKET
PhD Thesis
Production Technology 2020 No. 42
Towards understanding the
fatigue behaviour of Alloy 718
manufactured by Powder Bed
Fusion processes
University West SE-46186 Trollhättan Sweden
+46 520-22 30 00 www.hv.se
© Arun Ramanathan Balachandramurthi 2020 ISBN (Printed)
978-91-88847-79-9
ISBN (Electronic)
978-91-88847-78-2
Trollhättan, Sweden, 2020
iii
University West SE-46186 Trollhättan Sweden
+46 520-22 30 00 www.hv.se
© Arun Ramanathan Balachandramurthi 2020 ISBN (Printed)
978-91-88847-79-9
ISBN (Electronic)
978-91-88847-78-2
Trollhättan, Sweden, 2020
iii
v
“If I have seen further it is by standing on the shoulders of giants.” – Sir Isaac Newton
“I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth
v
“If I have seen further it is by standing on the shoulders of giants.” – Sir Isaac Newton
“I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth
vii
Acknowledgements
At the time of writing this thesis, four years have passed since I embarked on this journey as a PhD student. During these years, many things have happened — both professionally and personally — all of which have contributed in some way to the result, this book. It has been an exciting and thoroughly rewarding journey, albeit requiring a few course corrections along the way. Despite build failures, ruined tests, and other technical glitches that led to frustration, I never — for one moment — doubted whether this was my path to go.
During the work that has led to this thesis, many people have been involved — both directly and indirectly — for whom I would like to express my sincere grat-itude. First and foremost, I would like to thank my supervisors, Per Nylén, Robert Pederson and Johan Moverare. Thank you for all your guidance and support so far!
You trusted me, provided me with the freedom I had hoped for, and offered an excellent environment for me to grow as a professional. Per, you supervised me only during the first year, but that set the tone for the years that followed. Robert, thanks for letting me pursue almost all the ideas and design those expensive test matrices. Johan, thanks for sharing your expertise in the fields of fatigue and sup-eralloys with me. You amaze me every time I visit your lab – only you can keep so many fatigue testing machines busy at the same time! I would also like to thank
Thomas Hansson for the guidance in setting up and performing fatigue testing at
PTC. Thomas, you gave me my first opportunity to work in the field of fatigue, as a Master thesis student in 2014, and I have been hooked on to it ever since! I would like to extend my gratitude to all the members of the AM group. It has been a pleasure working with you. I really enjoyed all the discussions during our meetings. Sincere thanks to Shrikant Joshi for reviewing the thesis. I also would
like to thank Jonas Olsson, Mats Högström, Kenneth Andersson, Björn Särnerblom and Håkan Backström, who have helped to coordinate various project tasks and to work
in the lab. Thanks for sharing your valuable experience and — most of all — for pushing my boundaries with tricky questions and comments. I would like to express my gratitude to fellow PhD students and collaborators for all the meaningful brainstorming sessions throughout these years. You guys have been a cool gang to hang out with! Chamara Kumara and Fabian Hanning, thanks for being
my go-to superalloy metallurgy people. Sincere thanks to Ana Bonilla for
proofreading the thesis. To the ever-helpful Eva Bränneby and Victoria Sjöstedt –
thanks for all the administrative help; without your support, things might not have been as easy! A collective acknowledgement goes to all the colleagues at PTC for the positive working atmosphere; I have enjoyed my time as a PhD student. It
vii
Acknowledgements
At the time of writing this thesis, four years have passed since I embarked on this journey as a PhD student. During these years, many things have happened — both professionally and personally — all of which have contributed in some way to the result, this book. It has been an exciting and thoroughly rewarding journey, albeit requiring a few course corrections along the way. Despite build failures, ruined tests, and other technical glitches that led to frustration, I never — for one moment — doubted whether this was my path to go.
During the work that has led to this thesis, many people have been involved — both directly and indirectly — for whom I would like to express my sincere grat-itude. First and foremost, I would like to thank my supervisors, Per Nylén, Robert Pederson and Johan Moverare. Thank you for all your guidance and support so far!
You trusted me, provided me with the freedom I had hoped for, and offered an excellent environment for me to grow as a professional. Per, you supervised me only during the first year, but that set the tone for the years that followed. Robert, thanks for letting me pursue almost all the ideas and design those expensive test matrices. Johan, thanks for sharing your expertise in the fields of fatigue and sup-eralloys with me. You amaze me every time I visit your lab – only you can keep so many fatigue testing machines busy at the same time! I would also like to thank
Thomas Hansson for the guidance in setting up and performing fatigue testing at
PTC. Thomas, you gave me my first opportunity to work in the field of fatigue, as a Master thesis student in 2014, and I have been hooked on to it ever since! I would like to extend my gratitude to all the members of the AM group. It has been a pleasure working with you. I really enjoyed all the discussions during our meetings. Sincere thanks to Shrikant Joshi for reviewing the thesis. I also would
like to thank Jonas Olsson, Mats Högström, Kenneth Andersson, Björn Särnerblom and Håkan Backström, who have helped to coordinate various project tasks and to work
in the lab. Thanks for sharing your valuable experience and — most of all — for pushing my boundaries with tricky questions and comments. I would like to express my gratitude to fellow PhD students and collaborators for all the meaningful brainstorming sessions throughout these years. You guys have been a cool gang to hang out with! Chamara Kumara and Fabian Hanning, thanks for being
my go-to superalloy metallurgy people. Sincere thanks to Ana Bonilla for
proofreading the thesis. To the ever-helpful Eva Bränneby and Victoria Sjöstedt –
thanks for all the administrative help; without your support, things might not have been as easy! A collective acknowledgement goes to all the colleagues at PTC for the positive working atmosphere; I have enjoyed my time as a PhD student. It
viii
has been a real pleasure to supervise thesis and internship students. Thanks for having
worked with me and good luck with your careers!
Financial support from KK Foundation, The Swedish Agency for Economic and Regional Growth, Region Västra Götaland and European Regional Development Fund for carrying
out this research work are greatly acknowledged. The industrial collaboration with
Arcam EBM, Element, GKN Aerospace Engine Systems, Sandvik Machining Solutions, Siemens Industrial Turbomachinery and Quintus Technologies made this project
fascinat-ing. I sincerely thank the representatives from the industrial collaborators for the material support, technical services, and inspiring discussions throughout. I would also like to thank Peter Emvin and Joakim Skoog at GKN Aerospace Engine Systems for having supported my application to the doctoral studies. I express my
sincere thanks to Andreas Ottosson for recommending me to apply and work at
PTC. You were right – I had an enriching experience here! I also thank my man-agers and colleagues at RnT, Materials Laboratory and AMC for the advice and
sup-port that I have received during these years.
To all my friends — here in Sweden, back in India and spread across the rest of the
world — thank you for all the love, care, and advice all along. You made all the good times better and the tough times bearable. Without you, my time in Sweden would not have been the same!
I express my deepest gratitude to my parents, Nirmala and Balachandramurthi. It is
your love, care, constant encouragement, and endless sacrifices that have made all the academic accomplishments possible! To my aunt Sasikala, thanks for your
love and encouragement throughout these years. Finally, my dear wife, Priya – I
don’t have enough words to thank you for your unconditional love, support and patience. Thanks for continually reminding me about what really matters in life and showing me that there is a beautiful world outside, waiting to be explored. Our journey through life has just begun, and I look forward to spending the future with you. அன்ேப, என்�ம் ஒன்றாய் நாம்!
Arun Ramanathan Balachandramurthi December 2020
ix
Populärvetenskaplig Sammanfattning
Titel: Utmattningsegenskaper hos Alloy 718 framställd med pulverbädd additiv
tillverkning
Nyckelord: Utmattning; Additiv tillverkning; Pulverbädd; Superlegering;
Mikrostruktur; Ytfinhet
ISBN:
978-91-88847-79-9
(Tryckt)978-91-88847-78-2
(Elektronisk)Additiv tillverkning (eng. Additive Manufacturing, AM) involverar olika metoder där komponenter tillverkas lager för lager. Bland de befintliga AM-processerna för metall har metoden med pulverbäddsteknik, i vilket en energistråle smälter metallpulver lager-på-lager, möjliggjort tillverkning av geometrier som inte tidigare varit möjliga att tillverka med konventionella metoder. De två huvudpulverbäddsteknikerna är Electron Beam Powder Bed Fusion (EB-PBF), i vilket en elektronstråle smälter pulvret, och Laser Beam Powder Bed Fusion (LB-PBF), i vilket en laserstråle smälter pulvret. Pulverbäddtekniken erbjuder flera fördelar; emellertid måste dessa processers lämplighet undersökas i detalj innan de kan ersätta konventionella processer. För att kunna tillverka högpresterande komponenter med denna teknik är det därför viktigt att förstå pulverbäddsprocessen – efterbehandling – mikrostruktur – egenskaper, samt sambanden mellan dessa. Detta arbete är avgränsat till att undersöka och förstå sambandet mellan utmattningsegenskaperna och mikrostrukturen hos legeringen Alloy 718 tillverkad med pulverbäddstekniken. Dessutom har även inverkan på egenskaperna av den grova byggytan hos pulverbäddtillverkat material undersökts.
Den uppkomna <100> texturen i det byggda materialet leder till en anisotropisk elasticitetsmodul, som i sin tur resulterar i ett anisotropiskt utmattningsbeteende vid töjningsstyrda amplituder. Oxidinneslutningar och defekter så som icke smälta områden (eng. lack of fusion, LoF) samt porositet ifrån krympning, har en dramatiskt försämrande inverkan på utmattningslivslängden. Het isostatisk pressning (HIP) leder till att de flesta defekter sluts samman med förbättrad utmattningshållfasthet som resultat. Genom att ta bort den grova ytan hos byggt material förbättras utmattningslivslängden. Den grova ytan har många sprickinitieringspunkter och leder till en mindre spridning i data för utmattningslivslängden. Sprickpropageringen i pulverbäddstillverkad Alloy 718 påverkas av kornstorlek och textur.
viii
has been a real pleasure to supervise thesis and internship students. Thanks for having
worked with me and good luck with your careers!
Financial support from KK Foundation, The Swedish Agency for Economic and Regional Growth, Region Västra Götaland and European Regional Development Fund for carrying
out this research work are greatly acknowledged. The industrial collaboration with
Arcam EBM, Element, GKN Aerospace Engine Systems, Sandvik Machining Solutions, Siemens Industrial Turbomachinery and Quintus Technologies made this project
fascinat-ing. I sincerely thank the representatives from the industrial collaborators for the material support, technical services, and inspiring discussions throughout. I would also like to thank Peter Emvin and Joakim Skoog at GKN Aerospace Engine Systems for having supported my application to the doctoral studies. I express my
sincere thanks to Andreas Ottosson for recommending me to apply and work at
PTC. You were right – I had an enriching experience here! I also thank my man-agers and colleagues at RnT, Materials Laboratory and AMC for the advice and
sup-port that I have received during these years.
To all my friends — here in Sweden, back in India and spread across the rest of the
world — thank you for all the love, care, and advice all along. You made all the good times better and the tough times bearable. Without you, my time in Sweden would not have been the same!
I express my deepest gratitude to my parents, Nirmala and Balachandramurthi. It is
your love, care, constant encouragement, and endless sacrifices that have made all the academic accomplishments possible! To my aunt Sasikala, thanks for your
love and encouragement throughout these years. Finally, my dear wife, Priya – I
don’t have enough words to thank you for your unconditional love, support and patience. Thanks for continually reminding me about what really matters in life and showing me that there is a beautiful world outside, waiting to be explored. Our journey through life has just begun, and I look forward to spending the future with you. அன்ேப, என்�ம் ஒன்றாய் நாம்!
Arun Ramanathan Balachandramurthi December 2020
viii
has been a real pleasure to supervise thesis and internship students. Thanks for having
worked with me and good luck with your careers!
Financial support from KK Foundation, The Swedish Agency for Economic and Regional Growth, Region Västra Götaland and European Regional Development Fund for carrying
out this research work are greatly acknowledged. The industrial collaboration with
Arcam EBM, Element, GKN Aerospace Engine Systems, Sandvik Machining Solutions, Siemens Industrial Turbomachinery and Quintus Technologies made this project
fascinat-ing. I sincerely thank the representatives from the industrial collaborators for the material support, technical services, and inspiring discussions throughout. I would also like to thank Peter Emvin and Joakim Skoog at GKN Aerospace Engine Systems for having supported my application to the doctoral studies. I express my
sincere thanks to Andreas Ottosson for recommending me to apply and work at
PTC. You were right – I had an enriching experience here! I also thank my man-agers and colleagues at RnT, Materials Laboratory and AMC for the advice and
sup-port that I have received during these years.
To all my friends — here in Sweden, back in India and spread across the rest of the
world — thank you for all the love, care, and advice all along. You made all the good times better and the tough times bearable. Without you, my time in Sweden would not have been the same!
I express my deepest gratitude to my parents, Nirmala and Balachandramurthi. It is
your love, care, constant encouragement, and endless sacrifices that have made all the academic accomplishments possible! To my aunt Sasikala, thanks for your
love and encouragement throughout these years. Finally, my dear wife, Priya – I
don’t have enough words to thank you for your unconditional love, support and patience. Thanks for continually reminding me about what really matters in life and showing me that there is a beautiful world outside, waiting to be explored. Our journey through life has just begun, and I look forward to spending the future with you. அன்ேப, என்�ம் ஒன்றாய் நாம்!
Arun Ramanathan Balachandramurthi December 2020
ix
Populärvetenskaplig Sammanfattning
Titel: Utmattningsegenskaper hos Alloy 718 framställd med pulverbädd additiv
tillverkning
Nyckelord: Utmattning; Additiv tillverkning; Pulverbädd; Superlegering;
Mikrostruktur; Ytfinhet
ISBN:
978-91-88847-79-9
(Tryckt)978-91-88847-78-2
(Elektronisk)Additiv tillverkning (eng. Additive Manufacturing, AM) involverar olika metoder där komponenter tillverkas lager för lager. Bland de befintliga AM-processerna för metall har metoden med pulverbäddsteknik, i vilket en energistråle smälter metallpulver lager-på-lager, möjliggjort tillverkning av geometrier som inte tidigare varit möjliga att tillverka med konventionella metoder. De två huvudpulverbäddsteknikerna är Electron Beam Powder Bed Fusion (EB-PBF), i vilket en elektronstråle smälter pulvret, och Laser Beam Powder Bed Fusion (LB-PBF), i vilket en laserstråle smälter pulvret. Pulverbäddtekniken erbjuder flera fördelar; emellertid måste dessa processers lämplighet undersökas i detalj innan de kan ersätta konventionella processer. För att kunna tillverka högpresterande komponenter med denna teknik är det därför viktigt att förstå pulverbäddsprocessen – efterbehandling – mikrostruktur – egenskaper, samt sambanden mellan dessa. Detta arbete är avgränsat till att undersöka och förstå sambandet mellan utmattningsegenskaperna och mikrostrukturen hos legeringen Alloy 718 tillverkad med pulverbäddstekniken. Dessutom har även inverkan på egenskaperna av den grova byggytan hos pulverbäddtillverkat material undersökts.
Den uppkomna <100> texturen i det byggda materialet leder till en anisotropisk elasticitetsmodul, som i sin tur resulterar i ett anisotropiskt utmattningsbeteende vid töjningsstyrda amplituder. Oxidinneslutningar och defekter så som icke smälta områden (eng. lack of fusion, LoF) samt porositet ifrån krympning, har en dramatiskt försämrande inverkan på utmattningslivslängden. Het isostatisk pressning (HIP) leder till att de flesta defekter sluts samman med förbättrad utmattningshållfasthet som resultat. Genom att ta bort den grova ytan hos byggt material förbättras utmattningslivslängden. Den grova ytan har många sprickinitieringspunkter och leder till en mindre spridning i data för utmattningslivslängden. Sprickpropageringen i pulverbäddstillverkad Alloy 718 påverkas av kornstorlek och textur.
viii
has been a real pleasure to supervise thesis and internship students. Thanks for having
worked with me and good luck with your careers!
Financial support from KK Foundation, The Swedish Agency for Economic and Regional Growth, Region Västra Götaland and European Regional Development Fund for carrying
out this research work are greatly acknowledged. The industrial collaboration with
Arcam EBM, Element, GKN Aerospace Engine Systems, Sandvik Machining Solutions, Siemens Industrial Turbomachinery and Quintus Technologies made this project
fascinat-ing. I sincerely thank the representatives from the industrial collaborators for the material support, technical services, and inspiring discussions throughout. I would also like to thank Peter Emvin and Joakim Skoog at GKN Aerospace Engine Systems for having supported my application to the doctoral studies. I express my
sincere thanks to Andreas Ottosson for recommending me to apply and work at
PTC. You were right – I had an enriching experience here! I also thank my man-agers and colleagues at RnT, Materials Laboratory and AMC for the advice and
sup-port that I have received during these years.
To all my friends — here in Sweden, back in India and spread across the rest of the
world — thank you for all the love, care, and advice all along. You made all the good times better and the tough times bearable. Without you, my time in Sweden would not have been the same!
I express my deepest gratitude to my parents, Nirmala and Balachandramurthi. It is
your love, care, constant encouragement, and endless sacrifices that have made all the academic accomplishments possible! To my aunt Sasikala, thanks for your
love and encouragement throughout these years. Finally, my dear wife, Priya – I
don’t have enough words to thank you for your unconditional love, support and patience. Thanks for continually reminding me about what really matters in life and showing me that there is a beautiful world outside, waiting to be explored. Our journey through life has just begun, and I look forward to spending the future with you. அன்ேப, என்�ம் ஒன்றாய் நாம்!
Arun Ramanathan Balachandramurthi December 2020
xi
Abstract
Title: Towards understanding the fatigue behaviour of Alloy 718 manufactured by Powder Bed Fusion processes
Keywords: Fatigue; Additive Manufacturing; Powder Bed Fusion; Superalloy; Microstructure; Surface Roughness
ISBN:
978-91-88847-79-9
(Printed)978-91-88847-78-2
(Electronic)Additive Manufacturing (AM) is a disruptive modern manufacturing process in which parts are manufactured in a layer-wise fashion. Among the metal AM pro-cesses, Powder Bed Fusion (PBF) technology — comprised of Electron Beam Powder Bed Fusion (EB-PBF) and Laser Beam Powder Bed Fusion (LB-PBF) — has opened up a design space that was formerly unavailable with conventional manufacturing processes. PBF processes offer several advantages; however, the suitability of these processes to replace the conventional processes must be inves-tigated in detail. Therefore, understanding the AM process – post-processing – microstructure – property relationships is crucial for the manufacturing of high-performance components. In this regard, only limited work has been done to-wards understanding the fatigue behaviour of PBF Alloy 718. The aim of this work, therefore, is to understand how the fatigue behaviour of PBF Alloy 718 is affected by its microstructure. Besides, the influence of the rough as-built surface is also investigated.
In general, the <100> fibre texture along the build direction that resulted from PBF processing of Alloy 718 led to anisotropy in Young’s modulus. Conse-quently, the fatigue performance under controlled amplitudes of strain was aniso-tropic such that the low-modulus direction had longer fatigue life and vice versa. This texture-induced elasticity-dependent anisotropic strain-life behaviour could be normalized by the pseudo-elastic stress vs fatigue life approach.
Inclusions and defects had a detrimental effect on fatigue performance. Numer-ous factors, such as their geometry, volume fraction, and distribution, determined the effect on fatigue behaviour. Hot Isostatic Pressing (HIP) eliminated most de-fects and led to an improvement in fatigue performance. However, HIP did not alter the inclusions, which acted as crack initiation sites and reduced fatigue life. The rough as-built surface, which had numerous notch-like crack initiation sites,
deteriorated fatigue performance; however, it lowered the scatter in fatigue life. Machining off the as-built surface improved fatigue life but increased the scatter.
xi
Abstract
Title: Towards understanding the fatigue behaviour of Alloy 718 manufactured by Powder Bed Fusion processes
Keywords: Fatigue; Additive Manufacturing; Powder Bed Fusion; Superalloy; Microstructure; Surface Roughness
ISBN:
978-91-88847-79-9
(Printed)978-91-88847-78-2
(Electronic)Additive Manufacturing (AM) is a disruptive modern manufacturing process in which parts are manufactured in a layer-wise fashion. Among the metal AM pro-cesses, Powder Bed Fusion (PBF) technology — comprised of Electron Beam Powder Bed Fusion (EB-PBF) and Laser Beam Powder Bed Fusion (LB-PBF) — has opened up a design space that was formerly unavailable with conventional manufacturing processes. PBF processes offer several advantages; however, the suitability of these processes to replace the conventional processes must be inves-tigated in detail. Therefore, understanding the AM process – post-processing – microstructure – property relationships is crucial for the manufacturing of high-performance components. In this regard, only limited work has been done to-wards understanding the fatigue behaviour of PBF Alloy 718. The aim of this work, therefore, is to understand how the fatigue behaviour of PBF Alloy 718 is affected by its microstructure. Besides, the influence of the rough as-built surface is also investigated.
In general, the <100> fibre texture along the build direction that resulted from PBF processing of Alloy 718 led to anisotropy in Young’s modulus. Conse-quently, the fatigue performance under controlled amplitudes of strain was aniso-tropic such that the low-modulus direction had longer fatigue life and vice versa. This texture-induced elasticity-dependent anisotropic strain-life behaviour could be normalized by the pseudo-elastic stress vs fatigue life approach.
Inclusions and defects had a detrimental effect on fatigue performance. Numer-ous factors, such as their geometry, volume fraction, and distribution, determined the effect on fatigue behaviour. Hot Isostatic Pressing (HIP) eliminated most de-fects and led to an improvement in fatigue performance. However, HIP did not alter the inclusions, which acted as crack initiation sites and reduced fatigue life. The rough as-built surface, which had numerous notch-like crack initiation sites,
deteriorated fatigue performance; however, it lowered the scatter in fatigue life. Machining off the as-built surface improved fatigue life but increased the scatter.
xiii
Table of Contents
Acknowledgements ... vii
Populärvetenskaplig Sammanfattning ... ix
Abstract ... xi
Table of Contents ... xiii
Acronyms ... xv
List of publications ... xvii
Part I Background & Theory ... 1
1 Introduction ... 3 Background ... 3 Research gap ... 5 Scope of research ... 7 Research Questions ... 7 Research Approach ... 8 Delimitations ... 8 Thesis Outline ... 8 2 Alloy 718... 11 Nickel-based superalloys ... 11 Microstructure of Alloy 718 ... 11 2.2.1 γ matrix ... 12 2.2.2 γ" and γ’ phases ... 13 2.2.3 δ phase ... 13 2.2.4 Laves phase ... 13 2.2.5 NbC ... 14
Heat treatments for Alloy 718 ... 16
3 Additive Manufacturing of Alloy 718 ... 17
Additive Manufacturing ... 17
3.1.1 AM process steps ... 17
3.1.2 Benefits of AM ... 18
3.1.3 Limitations of AM ... 18
Powder bed fusion processes ... 19
EB-PBF of Alloy 718 ... 22
LB-PBF of Alloy 718... 23
Thermal post-treatment of PBF Alloy 718 ... 24
Surface roughness ... 26
4 Fatigue ... 29
Introduction to fatigue ... 29
xiii
Table of Contents
Acknowledgements ... vii
Populärvetenskaplig Sammanfattning ... ix
Abstract ... xi
Table of Contents ... xiii
Acronyms ... xv
List of publications ... xvii
Part I Background & Theory ... 1
1 Introduction ... 3 Background ... 3 Research gap ... 5 Scope of research ... 7 Research Questions ... 7 Research Approach ... 8 Delimitations ... 8 Thesis Outline ... 8 2 Alloy 718... 11 Nickel-based superalloys ... 11 Microstructure of Alloy 718 ... 11 2.2.1 γ matrix ... 12 2.2.2 γ" and γ’ phases ... 13 2.2.3 δ phase ... 13 2.2.4 Laves phase ... 13 2.2.5 NbC ... 14
Heat treatments for Alloy 718 ... 16
3 Additive Manufacturing of Alloy 718 ... 17
Additive Manufacturing ... 17
3.1.1 AM process steps ... 17
3.1.2 Benefits of AM ... 18
3.1.3 Limitations of AM ... 18
Powder bed fusion processes ... 19
EB-PBF of Alloy 718 ... 22
LB-PBF of Alloy 718... 23
Thermal post-treatment of PBF Alloy 718 ... 24
Surface roughness ... 26
4 Fatigue ... 29
Introduction to fatigue ... 29
xiv
4.1.2 Strain–life approach ... 31
4.1.3 Factors affecting fatigue performance ... 34
Fatigue behaviour of Alloy 718 ... 34
4.2.1 Effect of γ grain morphology... 34
4.2.2 Effect of strengthening precipitates ... 36
4.2.3 Effect of δ phase ... 36
4.2.4 Effect of carbides and nitrides ... 37
4.2.5 Effect of surface finishing ... 37
Fatigue behaviour of PBF metals ... 38
Fatigue behaviour of PBF Alloy 718 ... 40
5 Materials and Methods ... 43
Specimen manufacturing ... 43 5.1.1 EB-PBF process ... 44 5.1.2 LB-PBF process ... 44 Thermal post-treatment ... 44 Fatigue testing ... 46 Characterization methods ... 46
5.4.1 Surface roughness measurement ... 46
5.4.2 Metallography ... 47 5.4.3 Fractography ... 48 6 Summary of Papers ... 49 Paper A ... 49 Paper B ... 50 Paper C ... 52 Paper D ... 54 Paper E ... 54 Paper F ... 55 7 Discussion ... 57
8 Conclusions and Outlook ... 65
Conclusions ... 65
Outlook ... 66
Bibliography ... 69
Part II Appended papers ... 79
xv
Acronyms
2D Two dimensional 3D Three dimensional AB As-built AM Additive ManufacturingAMS Aerospace Material Specification
ASTM American Society for Testing and Materials
ATP Advanced Turboprop
BCT Body-Centred Tetragon
BJ Binder Jetting
CAD Computer-Aided Design
CSS Cyclic Stress-Strain
DA Direct Aged
DED Directed Enery Deposition
EB-PBF Electron Beam Powder Bed Fusion
EBSD Electron Backscattering Diffraction
EDM Electrical Discharge Machining
EDS Energy Dispersive X-ray Spectroscopy
FCC Face Centered Cubic
FCGR Fatigue Crack Growth Rate
FEG Field Emission Gun
GA Gas Atomised
GE General Electric
xiv
4.1.2 Strain–life approach ... 31
4.1.3 Factors affecting fatigue performance ... 34
Fatigue behaviour of Alloy 718 ... 34
4.2.1 Effect of γ grain morphology... 34
4.2.2 Effect of strengthening precipitates ... 36
4.2.3 Effect of δ phase ... 36
4.2.4 Effect of carbides and nitrides ... 37
4.2.5 Effect of surface finishing ... 37
Fatigue behaviour of PBF metals ... 38
Fatigue behaviour of PBF Alloy 718 ... 40
5 Materials and Methods ... 43
Specimen manufacturing ... 43 5.1.1 EB-PBF process ... 44 5.1.2 LB-PBF process ... 44 Thermal post-treatment ... 44 Fatigue testing ... 46 Characterization methods ... 46
5.4.1 Surface roughness measurement ... 46
5.4.2 Metallography ... 47 5.4.3 Fractography ... 48 6 Summary of Papers ... 49 Paper A ... 49 Paper B ... 50 Paper C ... 52 Paper D ... 54 Paper E ... 54 Paper F ... 55 7 Discussion ... 57
8 Conclusions and Outlook ... 65
Conclusions ... 65
Outlook ... 66
Bibliography ... 69
Part II Appended papers ... 79
xv
Acronyms
2D Two dimensional 3D Three dimensional AB As-built AM Additive ManufacturingAMS Aerospace Material Specification
ASTM American Society for Testing and Materials
ATP Advanced Turboprop
BCT Body-Centred Tetragon
BJ Binder Jetting
CAD Computer-Aided Design
CSS Cyclic Stress-Strain
DA Direct Aged
DED Directed Enery Deposition
EB-PBF Electron Beam Powder Bed Fusion
EBSD Electron Backscattering Diffraction
EDM Electrical Discharge Machining
EDS Energy Dispersive X-ray Spectroscopy
FCC Face Centered Cubic
FCGR Fatigue Crack Growth Rate
FEG Field Emission Gun
GA Gas Atomised
GE General Electric
xvi
HCP Hexagonal Close Packed
HIP Hot Isostatic Pressing
HSA HIP + Solution treated + Aged
LB-PBF Laser Beam Powder Bed Fusion
LCF Low Cycle Fatigue
LEAP Leading Edge Aviation Propulsion
LoF Lack of fusion
LOM Light Optical Microscope
NDE Non Destructinve Evaluation
PA Powder Atomised
PBF Powder Bed Fusion
ppm parts per million SA Solutioning and Ageing
SEM Scanning Electron Microscope
SGT Siemens Gas Turbine
SLam Sheet Lamination
SR Stress Relieving
STA Solution Treatment and Ageing
SWT Smith Watson Topper
TCP Topologically Close Packed
TEM Transmission Electron Microscope
URQ Uniform Rapid Quenching
XCT X-ray Computed Tomography
xvii
List of publications
This thesis is based on the following appended publications:
Paper A. Arun Ramanathan Balachandramurthi, Johan Moverare, Thomas
Hansson, Robert Pederson, “Anisotropic fatigue properties of Alloy 718 man-ufactured by Electron Beam Powder Bed Fusion”, International Journal of Fatigue, vol. 141, e. 105898, 2020.
Author’s contribution: Principal author and idea originator. Planned the experi-ments and performed all the testing and characterization. Analyzed the data and compiled the results.
Paper B. Arun Ramanathan Balachandramurthi, Nitesh Raj Jaladurgam,
Chamara Kumara, Johan Moverare, Thomas Hansson, Johannes Gård-stam, Robert Pederson, “On the microstructure of Laser Powder Bed Fusion Alloy 718 and its influence on the low cycle fatigue behaviour”, under review in
Materials.
Author’s contribution: Principal author and idea originator. Planned the
experi-ments and performed all the testing and characterization. Analyzed the data and compiled the results.
Paper C. Arun Ramanathan Balachandramurthi, Johan Moverare, Nikhil Dixit,
Robert Pederson, “Influence of defects and as-built surface roughness on fatigue properties of additively manufactured Alloy 718”, Material Science and En-gineering A, vol. 735, pp. 463-474, 2018.
Author’s contribution: Principal author and idea originator. Planned the experi-ments and material characterization methods. Performed major parts of fractog-raphy and characterization of defects. Analyzed the data and compiled the results.
Paper D. Arun Ramanathan Balachandramurthi, Johan Moverare, Nikhil Dixit,
Dunyong Deng, Robert Pederson, “Microstructural influence on fatigue crack propagation during high cycle fatigue testing of additively manufactured Al-loy 718”, Materials Characterization, vol. 149, pp. 82-94, 2019.
Author’s contribution: Principal author and idea originator. Planned the experi-ments and material characterization methods. Performed major parts of fractog-raphy and material characterization. Analyzed the data and compiled the results.
Paper E. Arun Ramanathan Balachandramurthi, Johan Moverare, Satyapal
xvi
HCP Hexagonal Close Packed
HIP Hot Isostatic Pressing
HSA HIP + Solution treated + Aged
LB-PBF Laser Beam Powder Bed Fusion
LCF Low Cycle Fatigue
LEAP Leading Edge Aviation Propulsion
LoF Lack of fusion
LOM Light Optical Microscope
NDE Non Destructinve Evaluation
PA Powder Atomised
PBF Powder Bed Fusion
ppm parts per million SA Solutioning and Ageing
SEM Scanning Electron Microscope
SGT Siemens Gas Turbine
SLam Sheet Lamination
SR Stress Relieving
STA Solution Treatment and Ageing
SWT Smith Watson Topper
TCP Topologically Close Packed
TEM Transmission Electron Microscope
URQ Uniform Rapid Quenching
XCT X-ray Computed Tomography
xvii
List of publications
This thesis is based on the following appended publications:
Paper A. Arun Ramanathan Balachandramurthi, Johan Moverare, Thomas
Hansson, Robert Pederson, “Anisotropic fatigue properties of Alloy 718 man-ufactured by Electron Beam Powder Bed Fusion”, International Journal of Fatigue, vol. 141, e. 105898, 2020.
Author’s contribution: Principal author and idea originator. Planned the experi-ments and performed all the testing and characterization. Analyzed the data and compiled the results.
Paper B. Arun Ramanathan Balachandramurthi, Nitesh Raj Jaladurgam,
Chamara Kumara, Johan Moverare, Thomas Hansson, Johannes Gård-stam, Robert Pederson, “On the microstructure of Laser Powder Bed Fusion Alloy 718 and its influence on the low cycle fatigue behaviour”, under review in
Materials.
Author’s contribution: Principal author and idea originator. Planned the
experi-ments and performed all the testing and characterization. Analyzed the data and compiled the results.
Paper C. Arun Ramanathan Balachandramurthi, Johan Moverare, Nikhil Dixit,
Robert Pederson, “Influence of defects and as-built surface roughness on fatigue properties of additively manufactured Alloy 718”, Material Science and En-gineering A, vol. 735, pp. 463-474, 2018.
Author’s contribution: Principal author and idea originator. Planned the experi-ments and material characterization methods. Performed major parts of fractog-raphy and characterization of defects. Analyzed the data and compiled the results.
Paper D. Arun Ramanathan Balachandramurthi, Johan Moverare, Nikhil Dixit,
Dunyong Deng, Robert Pederson, “Microstructural influence on fatigue crack propagation during high cycle fatigue testing of additively manufactured Al-loy 718”, Materials Characterization, vol. 149, pp. 82-94, 2019.
Author’s contribution: Principal author and idea originator. Planned the experi-ments and material characterization methods. Performed major parts of fractog-raphy and material characterization. Analyzed the data and compiled the results.
Paper E. Arun Ramanathan Balachandramurthi, Johan Moverare, Satyapal
xviii
Beam Melting: Effect of Post-Treatment on the Microstructure and Mechanical Properties”, Materials, vol. 12(1), e. 68, 2019.
Author’s contribution: Principal author and idea originator. Planned the
experi-ments and material characterization methods. Performed major parts of fractog-raphy and metallogfractog-raphy. Analyzed the data and compiled the results.
Paper F. Arun Ramanathan Balachandramurthi, Jonas Olsson, Joakim Ålgårdh,
Anders Snis, Johan Moverare, Robert Pederson, “Microstructure tailoring in Electron Beam Powder Bed Fusion additive manufacturing and its potential consequences”, Results in Materials, vol. 1, e. 100017, 2019.
Author’s contribution: Principal author and idea originator. Planned the
experi-ments and material characterization methods. Performed major parts of metal-lography and texture analysis. Analyzed all the data and compiled the results. Other publications that are not appended to the thesis:
1. Chamara Kumara, Arun Ramanathan Balachandramurthi, Sneha Goel, Fabian Hanning, Johan Moverare, Per Nylén, “Toward a better understanding of phase transformations in additive manufacturing of Alloy 718”, Materialia,
vol. 13, e. 100862, 2020.
1
Part I
Background &
Theory
xviii
Beam Melting: Effect of Post-Treatment on the Microstructure and Mechanical Properties”, Materials, vol. 12(1), e. 68, 2019.
Author’s contribution: Principal author and idea originator. Planned the
experi-ments and material characterization methods. Performed major parts of fractog-raphy and metallogfractog-raphy. Analyzed the data and compiled the results.
Paper F. Arun Ramanathan Balachandramurthi, Jonas Olsson, Joakim Ålgårdh,
Anders Snis, Johan Moverare, Robert Pederson, “Microstructure tailoring in Electron Beam Powder Bed Fusion additive manufacturing and its potential consequences”, Results in Materials, vol. 1, e. 100017, 2019.
Author’s contribution: Principal author and idea originator. Planned the
experi-ments and material characterization methods. Performed major parts of metal-lography and texture analysis. Analyzed all the data and compiled the results. Other publications that are not appended to the thesis:
1. Chamara Kumara, Arun Ramanathan Balachandramurthi, Sneha Goel, Fabian Hanning, Johan Moverare, Per Nylén, “Toward a better understanding of phase transformations in additive manufacturing of Alloy 718”, Materialia,
vol. 13, e. 100862, 2020.
1
Part I
Background &
Theory
3
1 Introduction
Firstly, this chapter provides the necessary background information to situate the research topic of this thesis. It prepares the readers to appreciate the relevance of the chosen research problems to the manufacturing industry. Secondly, it sum-marizes the identified research gap and the scope of this thesis, along with the limitations. Furthermore, it presents the research questions addressed within this thesis as well as the adopted research approach. Finally, it presents an outline of the thesis.
Background
Additive Manufacturing (AM) has disrupted the manufacturing industry in such a way that it is considered to be the core of the fourth industrial revolution, Indus-try 4.0 [1]. The AM indusIndus-try has grown by $10.8 billion in the past decade and is
expected to reach $47.7 billion by 2025 [1]. As the name suggests, AM is a man-ufacturing method that is additive in nature, whereby components are built
pro-gressively by adding layers of material in a step-by-step fashion. In AM, a Computer-Aided Design (CAD) file corresponding to the part to be manufac-tured, is first digitally sliced and then each slice is printed one upon the other using
an AM machine. This method is different from conventional subtractive
manufac-turing in which components are manufactured by progressive material removal. Powder Bed Fusion (PBF) processes for metals, a sub-class of AM technology, have brought about a variety of changes in the manufacturing industry, e.g. new design possibilities and relatively easier manufacturing of parts with difficult-to-process materials. In metal PBF, parts are built in a layer-upon-layer fashion — usually with pre-alloyed metal powder as a precursor — by selectively melting each layer of powder according to the required geometry, while ensuring fusion to the previously built layers. Depending on the heat source for melting, the pro-cess is termed as Laser beam Powder Bed Fusion (LB-PBF) that utilizes a laser beam, or Electron beam Powder Bed Fusion (EB-PBF) that utilizes an electron beam [2].
AM processes are capable of manufacturing complex geometries in near-net-shape. PBF processes have opened-up an entirely new design space, which previ-ously has been unavailable with conventional manufacturing processes. Commer-cial examples that have capitalized on this design space, in the aerospace and gas turbine industries, include fuel nozzles manufactured for CFM LEAP engines,
3
1 Introduction
Firstly, this chapter provides the necessary background information to situate the research topic of this thesis. It prepares the readers to appreciate the relevance of the chosen research problems to the manufacturing industry. Secondly, it sum-marizes the identified research gap and the scope of this thesis, along with the limitations. Furthermore, it presents the research questions addressed within this thesis as well as the adopted research approach. Finally, it presents an outline of the thesis.
Background
Additive Manufacturing (AM) has disrupted the manufacturing industry in such a way that it is considered to be the core of the fourth industrial revolution, Indus-try 4.0 [1]. The AM indusIndus-try has grown by $10.8 billion in the past decade and is
expected to reach $47.7 billion by 2025 [1]. As the name suggests, AM is a man-ufacturing method that is additive in nature, whereby components are built
pro-gressively by adding layers of material in a step-by-step fashion. In AM, a Computer-Aided Design (CAD) file corresponding to the part to be manufac-tured, is first digitally sliced and then each slice is printed one upon the other using
an AM machine. This method is different from conventional subtractive
manufac-turing in which components are manufactured by progressive material removal. Powder Bed Fusion (PBF) processes for metals, a sub-class of AM technology, have brought about a variety of changes in the manufacturing industry, e.g. new design possibilities and relatively easier manufacturing of parts with difficult-to-process materials. In metal PBF, parts are built in a layer-upon-layer fashion — usually with pre-alloyed metal powder as a precursor — by selectively melting each layer of powder according to the required geometry, while ensuring fusion to the previously built layers. Depending on the heat source for melting, the pro-cess is termed as Laser beam Powder Bed Fusion (LB-PBF) that utilizes a laser beam, or Electron beam Powder Bed Fusion (EB-PBF) that utilizes an electron beam [2].
AM processes are capable of manufacturing complex geometries in near-net-shape. PBF processes have opened-up an entirely new design space, which previ-ously has been unavailable with conventional manufacturing processes. Commer-cial examples that have capitalized on this design space, in the aerospace and gas turbine industries, include fuel nozzles manufactured for CFM LEAP engines,
4
burners for SGT-800 turbine, parts for Advanced Turboprop (ATP) engine, parts for CT7-2E1 helicopter engine, and injection head of Ariane rocket engine to name a few [3–6]. The newly redesigned fuel nozzle for the LEAP engine by GE, while sporting an intricate system of cooling channels, is a single part in contrast to the older design that was fabricated by welding and brazing together 18 sepa-rate parts [3]. The new burners for the Siemens SGT-800 are also made as a single part instead of a 13-part assembly. It is redesigned, with internal fuel and air pipes, to reduce the risk of damages. Besides, the burner front has an integrated lattice structure that can only be produced by PBF processes [4]. Both these parts are manufactured by the LB-PBF process and are 25 % lighter than the older designs. Similar — even more striking — examples of parts-integration and other AM en-abled advantages, are found in the ATP engine that has 12 AM parts replacing 855 conventionally manufactured parts [5], in the CT7-2E1 engine that has 16 AM parts replacing more than 900 traditional parts [5] and the all-in-one injection head of the Ariane 6 propulsion system that replaces 248 conventional parts [6]. Such an extent of parts-integration not only makes these products lighter but also cost-efficient and, most importantly, dramatically affects the supply chain. Also, the use of AM in some of these cases has expedited product design and develop-ment. At GE Aviation, EB-PBF has become the first choice process to manufac-ture Low-Pressure Turbine (LPT) blades in titanium aluminide (TiAl), which is a relatively difficult-to-process material [7]. TiAl blades are used in the LPT of GEnx [8] and GE9X [9] engines and contribute to a 10 % reduction in fuel con-sumption [10]. Numerous other products that exploit the AM enabled advantages exist beyond the aerospace and gas turbine applications such as in medicine, au-tomotive, nuclear, tooling, etc.
From an industrial standpoint, near-net-shaping, parts integration, and flexibility in design translate to lowering the manufacturing costs. So, appropriate use of metal AM processes, using different materials, has been explored by a variety of industries. At the current level of technical maturity, PBF processes suffer from considerably lower productivity compared to conventional processes. Therefore, these processes are better suited for industries with low-volume and high-value production environments. Though PBF processes offer numerous design and functional 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 and reliability must be thoroughly in-vestigated. Therefore, understanding the PBF process – post-treatment – micro-structure – mechanical properties relationship is crucial to enable the manufacturing of high-performance components.
INTRODUCTION
5
Research gap
Metal AM has gained significant traction in the academic research community concomitantly with its industrial adoption. The number of publications per year in metal AM has increased dramatically in the last decade, as shown in Figure 1.
Alloy 718, being the workhorse superalloy, has been the most investigated super-alloy using the different metal AM processes.
Figure 1. Publication trend in metal AM (Source: Scopus).
The microstructure of Alloy 718 manufactured by PBF processes is typically co-lumnar, with <100> texture, along the build direction [11,12]. Such a textured microstructure gives rise to anisotropy in mechanical properties, such as Young’s modulus, yield strength and ductility [13,14]. It is, however, possible to manufac-ture Alloy 718 parts with equiaxed microstrucmanufac-ture by carefully tuning the PBF process parameters to control the melting strategies [15,16]. The mechanical properties associated with such an equiaxed microstructure are different from that of the columnar microstructure [14,16]. Therefore, adopting different melting strategies could enable site-specific microstructure control within a part, which remains to be investigated in detail to adopt such design strategies for manufac-turing of components.
Different melting strategies, however, give rise to different defect distribution pat-terns within a manufactured part [17]. Hot Isostatic Pressing (HIP), which is rou-tinely employed as a post-process step to eliminate porosity and other process-induced defects in cast and powder metallurgy superalloy parts [18], is a crucial post-treatment step for PBF parts to eliminate porosity and lack of fusion defects.
INTRODUCTION
5
Research gap
Metal AM has gained significant traction in the academic research community concomitantly with its industrial adoption. The number of publications per year in metal AM has increased dramatically in the last decade, as shown in Figure 1.
Alloy 718, being the workhorse superalloy, has been the most investigated super-alloy using the different metal AM processes.
Figure 1. Publication trend in metal AM (Source: Scopus).
The microstructure of Alloy 718 manufactured by PBF processes is typically co-lumnar, with <100> texture, along the build direction [11,12]. Such a textured microstructure gives rise to anisotropy in mechanical properties, such as Young’s modulus, yield strength and ductility [13,14]. It is, however, possible to manufac-ture Alloy 718 parts with equiaxed microstrucmanufac-ture by carefully tuning the PBF process parameters to control the melting strategies [15,16]. The mechanical properties associated with such an equiaxed microstructure are different from that of the columnar microstructure [14,16]. Therefore, adopting different melting strategies could enable site-specific microstructure control within a part, which remains to be investigated in detail to adopt such design strategies for manufac-turing of components.
Different melting strategies, however, give rise to different defect distribution pat-terns within a manufactured part [17]. Hot Isostatic Pressing (HIP), which is rou-tinely employed as a post-process step to eliminate porosity and other process-induced defects in cast and powder metallurgy superalloy parts [18], is a crucial post-treatment step for PBF parts to eliminate porosity and lack of fusion defects.
4
burners for SGT-800 turbine, parts for Advanced Turboprop (ATP) engine, parts for CT7-2E1 helicopter engine, and injection head of Ariane rocket engine to name a few [3–6]. The newly redesigned fuel nozzle for the LEAP engine by GE, while sporting an intricate system of cooling channels, is a single part in contrast to the older design that was fabricated by welding and brazing together 18 sepa-rate parts [3]. The new burners for the Siemens SGT-800 are also made as a single part instead of a 13-part assembly. It is redesigned, with internal fuel and air pipes, to reduce the risk of damages. Besides, the burner front has an integrated lattice structure that can only be produced by PBF processes [4]. Both these parts are manufactured by the LB-PBF process and are 25 % lighter than the older designs. Similar — even more striking — examples of parts-integration and other AM en-abled advantages, are found in the ATP engine that has 12 AM parts replacing 855 conventionally manufactured parts [5], in the CT7-2E1 engine that has 16 AM parts replacing more than 900 traditional parts [5] and the all-in-one injection head of the Ariane 6 propulsion system that replaces 248 conventional parts [6]. Such an extent of parts-integration not only makes these products lighter but also cost-efficient and, most importantly, dramatically affects the supply chain. Also, the use of AM in some of these cases has expedited product design and develop-ment. At GE Aviation, EB-PBF has become the first choice process to manufac-ture Low-Pressure Turbine (LPT) blades in titanium aluminide (TiAl), which is a relatively difficult-to-process material [7]. TiAl blades are used in the LPT of GEnx [8] and GE9X [9] engines and contribute to a 10 % reduction in fuel con-sumption [10]. Numerous other products that exploit the AM enabled advantages exist beyond the aerospace and gas turbine applications such as in medicine, au-tomotive, nuclear, tooling, etc.
From an industrial standpoint, near-net-shaping, parts integration, and flexibility in design translate to lowering the manufacturing costs. So, appropriate use of metal AM processes, using different materials, has been explored by a variety of industries. At the current level of technical maturity, PBF processes suffer from considerably lower productivity compared to conventional processes. Therefore, these processes are better suited for industries with low-volume and high-value production environments. Though PBF processes offer numerous design and functional 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 and reliability must be thoroughly in-vestigated. Therefore, understanding the PBF process – post-treatment – micro-structure – mechanical properties relationship is crucial to enable the manufacturing of high-performance components.
INTRODUCTION
5
Research gap
Metal AM has gained significant traction in the academic research community concomitantly with its industrial adoption. The number of publications per year in metal AM has increased dramatically in the last decade, as shown in Figure 1.
Alloy 718, being the workhorse superalloy, has been the most investigated super-alloy using the different metal AM processes.
Figure 1. Publication trend in metal AM (Source: Scopus).
The microstructure of Alloy 718 manufactured by PBF processes is typically co-lumnar, with <100> texture, along the build direction [11,12]. Such a textured microstructure gives rise to anisotropy in mechanical properties, such as Young’s modulus, yield strength and ductility [13,14]. It is, however, possible to manufac-ture Alloy 718 parts with equiaxed microstrucmanufac-ture by carefully tuning the PBF process parameters to control the melting strategies [15,16]. The mechanical properties associated with such an equiaxed microstructure are different from that of the columnar microstructure [14,16]. Therefore, adopting different melting strategies could enable site-specific microstructure control within a part, which remains to be investigated in detail to adopt such design strategies for manufac-turing of components.
Different melting strategies, however, give rise to different defect distribution pat-terns within a manufactured part [17]. Hot Isostatic Pressing (HIP), which is rou-tinely employed as a post-process step to eliminate porosity and other process-induced defects in cast and powder metallurgy superalloy parts [18], is a crucial post-treatment step for PBF parts to eliminate porosity and lack of fusion defects.
INTRODUCTION
5
Research gap
Metal AM has gained significant traction in the academic research community concomitantly with its industrial adoption. The number of publications per year in metal AM has increased dramatically in the last decade, as shown in Figure 1.
Alloy 718, being the workhorse superalloy, has been the most investigated super-alloy using the different metal AM processes.
Figure 1. Publication trend in metal AM (Source: Scopus).
The microstructure of Alloy 718 manufactured by PBF processes is typically co-lumnar, with <100> texture, along the build direction [11,12]. Such a textured microstructure gives rise to anisotropy in mechanical properties, such as Young’s modulus, yield strength and ductility [13,14]. It is, however, possible to manufac-ture Alloy 718 parts with equiaxed microstrucmanufac-ture by carefully tuning the PBF process parameters to control the melting strategies [15,16]. The mechanical properties associated with such an equiaxed microstructure are different from that of the columnar microstructure [14,16]. Therefore, adopting different melting strategies could enable site-specific microstructure control within a part, which remains to be investigated in detail to adopt such design strategies for manufac-turing of components.
Different melting strategies, however, give rise to different defect distribution pat-terns within a manufactured part [17]. Hot Isostatic Pressing (HIP), which is rou-tinely employed as a post-process step to eliminate porosity and other process-induced defects in cast and powder metallurgy superalloy parts [18], is a crucial post-treatment step for PBF parts to eliminate porosity and lack of fusion defects.
6
An appropriate choice of pressure and temperature helps eliminate most de-fects [19]; however, re-growth of porosity could occur during subsequent heat treatments or service conditions [20–23]. Furthermore, it is essential to consider the simultaneous effect of HIP parameters on the different phase constituents. For instance, it has been shown that an apt temperature during HIP could recrys-tallize the as-built microstructure of LB-PBF Alloy 718. The recrystallization leads to an equiaxed microstructure that has a lower texture intensity and isotropic me-chanical properties, irrespective of the different as-built texture intensities at-tained with different melting strategies [24].
The thermal signatures of PBF processes are different from that of conventional processes; hence, some microstructural features are different from those observed in castings and forgings. Therefore, for standalone PBF parts, alternative thermal post-treatment routines would have to be designed to ensure that the desired mi-crostructure, with appropriate mechanical properties, is achieved. Such newer post-treatments could be achieved by tuning the process variables of HIP, Solu-tion treatment and Ageing (SA) [25]. However, if PBF parts are to be used in tandem with conventionally manufactured parts as welded assemblies, conven-tional thermal post-treatment protocols would have to be used. Therefore, under-standing the effect of thermal post-treatments on microstructures, and hence on mechanical properties, is essential.
Figure 2. Publication trend in fatigue of metal AM (Source: Scopus). The work for this PhD thesis began in September 2016.
INTRODUCTION
7
With numerous PBF process parameters and melting strategies that can be uti-lized, along with several process variables that could be altered in the subsequent post-processing steps such as HIP, SA, surface treatments etc., extensive research must be carried out to achieve the same extent of understanding and knowledge on the process–microstructure–property relationships as for cast or forged loy 718. For context, the number of research publications in fatigue of AM Al-loy 718 compared to that of fatigue of AlAl-loy 718 is shown in Figure 2. The
knowledge gap in understanding the fatigue behaviour between conventionally manufactured Alloy 718 and AM Alloy 718 is significant. Besides, it is vital to determine the knockdown on fatigue properties due to the characteristic rough-ness of the as-built surface of PBF parts and to explore ways of reducing this knockdown to improve the overall performance further.
Scope of research
Alloy 718 is a superalloy that is widely used within the aerospace and gas turbine industries, usually, in the cast or wrought form. Apart from the static properties, the dynamic (fatigue) and time-dependent (creep) behaviours are crucial in such applications. Understanding how the fatigue behaviour of PBF Alloy 718 is af-fected by the microstructure — amount and distribution of different phase con-stituents and defects, texture — resulting from PBF processes and subsequent thermal post-treatments is the focus of this research. Furthermore, the effect of the as-built surface topography on the fatigue properties is investigated.
Research Questions
The following research questions are formulated to address the identified research gaps and form the basis of the work presented in this thesis.
RQ1. How does the microstructure of PBF Alloy 718, in as-built and/or
post-treated condition, affect its fatigue behaviour? What is the influence of
i. the different phase constituents? ii. the crystallographic texture? iii. the different defects?
RQ2. How does the as-built surface roughness of PBF Alloy 718 affect its
fatigue behaviour?
6
An appropriate choice of pressure and temperature helps eliminate most de-fects [19]; however, re-growth of porosity could occur during subsequent heat treatments or service conditions [20–23]. Furthermore, it is essential to consider the simultaneous effect of HIP parameters on the different phase constituents. For instance, it has been shown that an apt temperature during HIP could recrys-tallize the as-built microstructure of LB-PBF Alloy 718. The recrystallization leads to an equiaxed microstructure that has a lower texture intensity and isotropic me-chanical properties, irrespective of the different as-built texture intensities at-tained with different melting strategies [24].
The thermal signatures of PBF processes are different from that of conventional processes; hence, some microstructural features are different from those observed in castings and forgings. Therefore, for standalone PBF parts, alternative thermal post-treatment routines would have to be designed to ensure that the desired mi-crostructure, with appropriate mechanical properties, is achieved. Such newer post-treatments could be achieved by tuning the process variables of HIP, Solu-tion treatment and Ageing (SA) [25]. However, if PBF parts are to be used in tandem with conventionally manufactured parts as welded assemblies, conven-tional thermal post-treatment protocols would have to be used. Therefore, under-standing the effect of thermal post-treatments on microstructures, and hence on mechanical properties, is essential.
Figure 2. Publication trend in fatigue of metal AM (Source: Scopus). The work for this PhD thesis began in September 2016.
6
An appropriate choice of pressure and temperature helps eliminate most de-fects [19]; however, re-growth of porosity could occur during subsequent heat treatments or service conditions [20–23]. Furthermore, it is essential to consider the simultaneous effect of HIP parameters on the different phase constituents. For instance, it has been shown that an apt temperature during HIP could recrys-tallize the as-built microstructure of LB-PBF Alloy 718. The recrystallization leads to an equiaxed microstructure that has a lower texture intensity and isotropic me-chanical properties, irrespective of the different as-built texture intensities at-tained with different melting strategies [24].
The thermal signatures of PBF processes are different from that of conventional processes; hence, some microstructural features are different from those observed in castings and forgings. Therefore, for standalone PBF parts, alternative thermal post-treatment routines would have to be designed to ensure that the desired mi-crostructure, with appropriate mechanical properties, is achieved. Such newer post-treatments could be achieved by tuning the process variables of HIP, Solu-tion treatment and Ageing (SA) [25]. However, if PBF parts are to be used in tandem with conventionally manufactured parts as welded assemblies, conven-tional thermal post-treatment protocols would have to be used. Therefore, under-standing the effect of thermal post-treatments on microstructures, and hence on mechanical properties, is essential.
Figure 2. Publication trend in fatigue of metal AM (Source: Scopus). The work for this PhD thesis began in September 2016.
INTRODUCTION
7
With numerous PBF process parameters and melting strategies that can be uti-lized, along with several process variables that could be altered in the subsequent post-processing steps such as HIP, SA, surface treatments etc., extensive research must be carried out to achieve the same extent of understanding and knowledge on the process–microstructure–property relationships as for cast or forged loy 718. For context, the number of research publications in fatigue of AM Al-loy 718 compared to that of fatigue of AlAl-loy 718 is shown in Figure 2. The
knowledge gap in understanding the fatigue behaviour between conventionally manufactured Alloy 718 and AM Alloy 718 is significant. Besides, it is vital to determine the knockdown on fatigue properties due to the characteristic rough-ness of the as-built surface of PBF parts and to explore ways of reducing this knockdown to improve the overall performance further.
Scope of research
Alloy 718 is a superalloy that is widely used within the aerospace and gas turbine industries, usually, in the cast or wrought form. Apart from the static properties, the dynamic (fatigue) and time-dependent (creep) behaviours are crucial in such applications. Understanding how the fatigue behaviour of PBF Alloy 718 is af-fected by the microstructure — amount and distribution of different phase con-stituents and defects, texture — resulting from PBF processes and subsequent thermal post-treatments is the focus of this research. Furthermore, the effect of the as-built surface topography on the fatigue properties is investigated.
Research Questions
The following research questions are formulated to address the identified research gaps and form the basis of the work presented in this thesis.
RQ1. How does the microstructure of PBF Alloy 718, in as-built and/or
post-treated condition, affect its fatigue behaviour? What is the influence of
i. the different phase constituents? ii. the crystallographic texture? iii. the different defects?
RQ2. How does the as-built surface roughness of PBF Alloy 718 affect its
fatigue behaviour?
6
An appropriate choice of pressure and temperature helps eliminate most de-fects [19]; however, re-growth of porosity could occur during subsequent heat treatments or service conditions [20–23]. Furthermore, it is essential to consider the simultaneous effect of HIP parameters on the different phase constituents. For instance, it has been shown that an apt temperature during HIP could recrys-tallize the as-built microstructure of LB-PBF Alloy 718. The recrystallization leads to an equiaxed microstructure that has a lower texture intensity and isotropic me-chanical properties, irrespective of the different as-built texture intensities at-tained with different melting strategies [24].
The thermal signatures of PBF processes are different from that of conventional processes; hence, some microstructural features are different from those observed in castings and forgings. Therefore, for standalone PBF parts, alternative thermal post-treatment routines would have to be designed to ensure that the desired mi-crostructure, with appropriate mechanical properties, is achieved. Such newer post-treatments could be achieved by tuning the process variables of HIP, Solu-tion treatment and Ageing (SA) [25]. However, if PBF parts are to be used in tandem with conventionally manufactured parts as welded assemblies, conven-tional thermal post-treatment protocols would have to be used. Therefore, under-standing the effect of thermal post-treatments on microstructures, and hence on mechanical properties, is essential.
Figure 2. Publication trend in fatigue of metal AM (Source: Scopus). The work for this PhD thesis began in September 2016.
