the characterisation and modelling of porosity formation in electron beam welded titanium alloys

201 Pages·2012·13.58 MB·English

THE CHARACTERISATION AND MODELLING
OF POROSITY FORMATION IN ELECTRON
BEAM WELDED TITANIUM ALLOYS
By
JIANGLIN HUANG
A dissertation submitted to
The University of Birmingham
for the degree of
DOCTOR OF PHILOSOPHY
School Metallurgy and Materials
The University of Birmingham
September 2011
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Abstract
This thesis is concerned with the porosity formation mechanism during electron
beam welding of titanium-based alloys. During the welding of titanium alloys for
structural applications, porosity is occasionally found in the solidified welds. Hence
thekeyfactorsresponsible forporosityformation needtobe identified, andguidance
to minimise porosity occurrence needs to be provided.
The aim of this project is twofold. First, porosity formed in electron beam welded
titanium samples is characterised to rationalise the porosity formation mechanism.
Second, models based on sound physical principles are built to aid understanding
of porosity formation, and to provide predictive capability. Chapter 1 contains the
introductionandbackgroundoftheproject. Aliteraturereviewisreportedinchapter
2 which covers the metallurgy of titanium and its alloys, the electron beam welding
process, and porosity formation in titanium welds.
Inchapter3,electronbeamweldsofcommerciallypuretitanium(CP-Ti),Ti-6Al-4V,
Ti-6246, IMI 834 are characterised to rationalise the porosity formation mechanism
by using metallographic sectioning, high resolution X-ray tomography, residual gas
analysis, scanning electron microscopy (SEM) and energy and wavelength dispersive
spectroscopy (EDS/WDS) analysis. The results confirm porosity formed in electron
beamweldedtitanium-basedalloysisassociatedwithgasdynamics; hydrogenisvery
likely to be responsible for porosity formation.
In chapter 4, numerical models are developed to improve the understanding of the
electron beam welding process, including heat cycling, weld pool and keyhole for-
mation, which are prerequisites for further investigation of the physical phenomena
occurring, such as hydrogen behaviour and bubble formation and entrapment.
In chapter 5, based on the numerical models for electron beam welding process, a
coupled thermodynamic/kinetic model is proposed to study the hydrogen migration
behaviour. The modelling results confirm hydrogen migrates from the cold region
towards the hot region and thus causes hydrogen accumulation inside the weld pool.
This model enables the prediction of hydrogen content inside the weld pool. The
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comparison between the predicted hydrogen distribution and previous experimental
data is shown to be reasonable.
In chapter 6, a hydrogen driven bubble growth model is proposed to study the
hydrogen effect on porosity formation, in which bubble is assumed to be initiated
due to the effect of asperities at the joint surfaces. This model is used to estimate
the hydrogen effect on stationary bubble growth in the melt, and thus to make
predictions of the hydrogen concentration barrier needed for pore formation. The
effects of surface tension of liquid metal and the radius of pre-existing micro-bubble
size on the barrier are also investigated.
In chapter 7, to study the effect of hydrogen on porosity formation and to confirm
whether hydrogen is the root cause for porosity formation, Ti-6Al-4V samples were
electrochemically charged to achieve different hydrogen levels before welding. The
results confirm that bubbles are nucleated at the melting front during the welding
process. With optimised electron beam parameters and perfect joint alignment,
porosity can be suppressed even at a very high hydrogen levels; on the other hand,
porosityisexacerbatedwhenasmallbeamoffset(BOF)isemployed. Thisisbecause
any BOF alters the size of the liquid zone at the melting front, where joint edges
are melted. Thus the thickness of the liquid film at the melting front is crucial for
bubble nucleation and their survival in the weld pool. It would appear that the
nucleation rate in the liquid zone at the melting front determines the likelihood of
porosity occurrence. This suggests that BOF is likely to be one factor influencing
porosity formation in these circumstances.
Finally, in chapter 8, conclusions are drawn and suggestions for further work made.
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To my family
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Acknowledgements
I am grateful to the Engineering and Physical Sciences Research Council (EPSRC)
of the United Kingdom and to Rolls-Royce plc for sponsorship of this work, via a
Dorothy Hodgkin Postgraduate Award (DHPA).
I would like to thank my supervisor Professor R. C. Reed, for all the help and direc-
tion he gave me throughout the course of this work. I benefitted a great deal from
his insight and encouragement during the discussions about this work. His enthusi-
asm and hard-working really impressed me, which will be an inspiration throughout
my future life. I also want to thank my co-supervisor Dr Martin Strangwood; I am
really impressed by his profound professional knowledge and scientific rigour and en-
thusiasm. Without his guidance and great patience, this work could not have been
accomplished.
A special thank you must go to Dr Jean-Christophe Gebelin and Dr Nils Warnken,
members of Partnership for Research in the Simulation and Manufacturing and Ma-
terials Group (PRISM2), for the considerable help and guidance I received from
them.
IwouldliketothankallmembersofPRISM2group, mycolleaguesandfriendsinthe
department, whomakemylifemoreenjoyableduringthiswork. Theassistancefrom
the workshop, the technicians and staff of Metallurgy and Materials is appreciated.
Support from Dr Steve Beech and Alistair Smith at Rolls-Royce plc are appreciated
for sharing their knowledge and experience of electron beam welding. Special thanks
toAlistairSmithforhishelponalltheelectronbeamweldingworkperformedduring
thisstudy. IalsothankProfessorIanSinclairandDrMarkMavrogordatoforprovid-
ing X-ray Computed Tomography (CT) support at the University of Southampton.
I would like to dedicate this work to Yangzi Hu, withour her, it would have been
impossible for me to make my decision to study abroad. I greatly appreciate the
encouragement from her, which has changed my life forever.
At last and not least, I thank my family for their continuous love and support.
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Preface
This dissertation is submitted for the degree of the Doctor of Philosophy at the Uni-
versity of Birmingham. It describes research carried out in School of Metallurgy and
Material Science between October 2007 and September 2011, under the supervision
ofProf. R.C.ReedandDr. M.Strangwood. Exceptwhereappropriatelyreferenced,
this work is original and has not been submitted for any other degree, diploma and
other qualification. It does not exceed 50,000 words in length.
Parts of this dissertation have been published or submitted for publication in:
• JianglinHuang,A.Smith,N.Warnken,J-C,Gebelin,M.StrangwoodandR.C.
Reed. On The Mechanism of Porosity formation During Welding of Titanium
Alloys. Acta Materialia (Accepted)
• Jianglin Huang, N. Warnken, J-C, Geblin, M. Strangwood and R. C. Reed.
Hydrogen Transport and Rationalisation of Porosity Formation during Welding
ofTitaniumAlloys. Metallurgy and Materials Transaction A.(Accepted)
• Jianglin Huang, N. Warnken, J-C, Geblin, M. Strangwood and R. C. Reed. A
Coupled Thermodynamic/Kinetic Model for Hydrogen Transport during Elec-
tronBeamWeldingofaTitaniumAlloy. Material Science and Technology.
(Accepted)
• JianglinHuang,N.Warnken,J-CGeblin,M.StrangwoodandR.C.Reed(2010).
Modeling of Hydrogen Effect on Porosity Formation in Electron Beam Welded
Titanium-based Alloys. paper presented at 4th International Conference
on Thermal Process Modelling and Computer Simulation, Shanghai,
China, 1-3 June, 2010. paper No. G01
• Jianglin Huang, A.Smith, N. Warnken, J-C, Geblin, M. Strangwood and R. C.
Reed. On Porosity Formation in Electron Beam Welding of Titanium Alloys.
The 12th World Conference on Titanium, Beijing, China, 19-24 June,
2011.
• Jianglin Huang, J-C, Gebelin, N. Warnken, M. Strangwood, R. C. Reed. The-
oretical and Experimental Investigation of Hydrogen Effect on Porosity Forma-
tion during Electron Beam Welding of Titanium Alloys. The 9th Interna-
tional Conference on Trends in Welding Research, June 4-8, 2012,
Chicago, Illinois USA. (Submitted)
Jianglin Huang
September 2011
Contents
List of Figures xi
List of Tables xvii
1 Introduction 1
1.1 Research Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Titanium Alloys in The Jet Engine . . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 Compressors in The Jet Engine . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.3 Compressor Discs Assembly Using Electron Beam Welding . . . . . . . . 5
1.1.4 The Problem of Porosity Formation in Electron Beam Welds . . . . . . . 6
1.2 Aims and Scope of This Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3 Thesis layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Literature Review 11
2.1 Titanium and Its Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1 Metallurgy and Classification of Titanium Alloys . . . . . . . . . . . . . . 11
2.1.2 Processing and Microstructures . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.2.1 Fully Lamellar Structure . . . . . . . . . . . . . . . . . . . . . . 14
2.1.2.2 Bimodal Microstructures . . . . . . . . . . . . . . . . . . . . . . 16
2.1.2.3 Fully Equiaxed Structure . . . . . . . . . . . . . . . . . . . . . . 17
2.1.2.4 Alpha Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.1.2.5 Martensite Formation . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.3 Ti-6Al-4V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.4 Ti-6246 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.1.5 IMI 834 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2 Electron Beam Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.1 Principles of Electron Beam Welding Process . . . . . . . . . . . . . . . . 23
2.2.2 Deep Penetration Welding Effect . . . . . . . . . . . . . . . . . . . . . . . 25
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CONTENTS
2.2.3 Influence of Welding Parameters . . . . . . . . . . . . . . . . . . . . . . . 27
2.2.4 Modelling of Electron Beam Welding Process . . . . . . . . . . . . . . . . 29
2.3 General Reasons for Porosity Formation in Welds . . . . . . . . . . . . . . . . . . 35
2.4 Porosity Formation in Electron Beam Welded Titanium . . . . . . . . . . . . . . 36
2.5 Hydrogen in Titanium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3 Characterisation of Electron Beam Welds of Titanium Alloys and Porosity
Formation 45
3.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.2 Experiment Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.2.1 EB welds for Porosity Characterisation . . . . . . . . . . . . . . . . . . . 47
3.2.2 X-Ray Detection of Porosity. . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.2.3 Metallographic Investigation . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.2.4 Energy and Wavelength Dispersive Spectroscopy Analysis (EDS/WDS) . 50
3.2.5 Residual Gas Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.3.1 Characteristics of Titanium EB Welds . . . . . . . . . . . . . . . . . . . . 52
3.3.2 Pore Morphology and Distribution . . . . . . . . . . . . . . . . . . . . . . 68
3.3.3 Chemical Composition Analysis around Porosity Edges . . . . . . . . . . 73
3.3.4 Gas Composition inside Porosity . . . . . . . . . . . . . . . . . . . . . . . 74
3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.4.1 Hydrogen Effect on Porosity Formation . . . . . . . . . . . . . . . . . . . 75
3.4.2 Bubble Initiation and Growth . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.4.3 Fusion Zone Shapes and Bubble Escape . . . . . . . . . . . . . . . . . . . 77
3.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4 Numerical Process Models for Electron Beam Welding 79
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.2 Modelling the Heat Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.2.1 Three-Dimensional Conical (TDC) Heat Source Model . . . . . . . . . . . 81
4.2.2 Modified Three Dimensional Conical (MTDC) Heat Source Model . . . . 83
4.2.3 Determination Parameters in Heat Source Model . . . . . . . . . . . . . . 84
4.3 Keyhole Profile Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.3.1 Assumptions for Keyhole Modelling . . . . . . . . . . . . . . . . . . . . . 85
4.3.2 Electron Beam Focus Properties . . . . . . . . . . . . . . . . . . . . . . . 86
4.3.3 Energy Balance at Keyhole Wall . . . . . . . . . . . . . . . . . . . . . . . 87
viii