Modelling of the effects of entrainment defects on mechanical properties in Al-Si-Mg alloy castings

354 Pages·2014·8.01 MB·English

by  Yang Yue

MODELLING OF THE EFFECTS OF
ENTRAINMENT DEFECTS ON
MECHANICAL PROPERTIES IN
AL-SI-MG ALLOY CASTINGS
By
YANG YUE
A dissertation submitted to
University of Birmingham
for the degree of
DOCTOR OF PHILOSOPHY
School of Metallurgy and Materials
University of Birmingham
June 2014
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Abstract
Liquid aluminium alloy is highly reactive with the surrounding atmosphere
and therefore, surface films, predominantly surface oxide films, easily form
on the free surface of the melt. Previous researches have highlighted that
surface turbulence in liquid aluminium during the mould-filling process
could result in the fold-in of the surface oxide films into the bulk liquid,
and this would consequently generate entrainment defects, such as double
oxide films and entrapped bubbles in the solidified casting. The formation
mechanisms of these defects and their detrimental e↵ects on both mechani-
cal properties and reproducibility of properties of casting have been studied
over the past two decades. However, the behaviour of entrainment defects
in the liquid metal and their evolution during the casting process are still
unclear, and the distribution of these defects in casting remains di�cult to
predict. An algorithm developed by Reilly, named the oxide film entrain-
ment model (OFEM), could be used to predict the formation and distribu-
tion of entrainment defects in castings. The model was integrated into the
computational fluid dynamics (CFD) software FLOW-3D, and applied face
normals of free surface and their interaction to capture the entrainment of
surface films.
In this research, validation and development of the OFEM were carried
out. Preliminary simulations were run to investigate the entrainment crite-
riadefinedintheOFEMandtounderstandthecapabilityofthemodel. The
initialvalidationofOFEMusedpreviousreportedexperimentaldataonthe
fatigue and tensile properties of castings, but did not succeed owing to the
quality of the data. Then modelling of three common entrainment mech-
anisms in fluid flow, namely plunging jet, return wave and rising jet, were
conducted, and the predicted defect quantities in the samples were com-
pared with the bending strengths of the castings from the Al-7Si-0.4Mg
alloy. Directly observation of transient flows in moulds by real-time X-
ray radiography showed good correlation between real filling scenario and
simulation results. However, the attempt to establish a quantitative rela-
tionship between the predicted quantities of the entrainment defects and
the mechanical properties of the real castings, were adversely a↵ected by
shrinkage porosity found in the castings. A further validation eliminated
the influence of shrinkage porosity on the casting properties. The tensile
strength of the cast test bars from this validation was compared with ei-
ther the number of defects, or the defect concentration within the bars
i
obtained from the simulation. A general relationship between the mechan-
ical strength of the cast test bars and the quantity of estimated defects was
apparent.
The validation test for OFEM algorithm has been conducted, but strong
correlation between the modelling prediction and the experimental results
have not been achieved yet. A series of modelling work undertaken in this
research highlighted the potential of the method as an indicator of the en-
trainment severity in di↵erent mould designs. Comparison of the Weibull
moduli obtained from the modelling and experimental results also showed
that the model had the potential to predicted the reliability of castings.
The e↵ects of di↵erent modelling conditions on the modelling results were
discussed, and some useful courses were suggested, such as proper settings
of the particle properties should be used, to increase the accuracy of future
simulations.
This research also assessed the behaviour of entrainment defects in the
liquid state and during solidification. The samples that contained entrain-
ment defects were scanned using ultra-fast synchrotron X-ray radiography.
The defects were directly viewed at both room temperature before melting
and in a fully liquid state. The reconstructed images showed three di↵erent
pore morphologies, namely entrained pores, tangled double oxide films and
closed cracks. The morphology evolution of the entrainment defects were
studied, which showed that the oxidation of the internal atmosphere was
the main driving force of the pore shape changes, but has less e↵ect on the
morphology change of tangled double oxide films. The reconstructed three-
dimensionalimagesgavedirectevidenceofthemechanismofmorphological
change of defect and the interaction between entrainment defects and mi-
croporosity.
ii
To my family
iii
Acknowledgements
I would like to express my sincere gratitude to both my supervisors, Prof.
Nick Green, for his kind guidance and encouragement in the first two year
of my research, and Dr. Bill Gri�ths, for his constructive criticisms and
continuous support throughout the course of this work. Although they
only shared a fraction of their wisdom, I benefited a great deal from their
insight and knowledge. Without their help and great patience, this work
could not have been accomplished.
I thank the financial support from the School of Metallurgy and Materials
at University of Birmingham for my program, without which I would never
start my PhD here.
I wish to acknowledge Dr. Carl Reilly, who initiated the modelling method
used in this study and introduced it to me, and Dr. Jean-Christophe Gebe-
lin, whoo↵eredtechnicalassistanceforthemodellingwork; thetechnicians,
Adrian Caden and Peter Craemer in the Foundry, and David Price in the
Mechanical Testing Lab, who provided a wealth of experience to facili-
tate the experimental work contained within this thesis. Their assistance
is critical to this work. I thank Julie Fife in Swiss Light Source (SLS)
at Paul Scherrer Institut (PSI) in Switzerland for her help on conducting
synchrotron X-ray experiments.
I am also grateful to my colleagues in School of Metallurgy and Materials,
especially, Dr. Hang Wang and Dr. Jianglin Huang, for their continuously
support both in academics and after-hour life, and friends at University
of Birmingham, Mofei Guo, Xi Liu, Guangxiong Wang, Yang Yang and
others, their “distractions” made my PhD life in Birmingham varied and
colourful.
The last mention must go to my family: my father YUE Zhihong and my
mother ZHANG Huiping. Their unconditional support and love in this as
with every endeavour are what made my success possible. Now and always,
they are everything to me.
iv
Preface
This dissertation is submitted for the degree of the Doctor of Philosophy at
the University of Birmingham. It describes research carried out in School
of Metallurgy and Material Science between October 2009 and September
2013, under the supervision of Dr. W.D. Gri�ths and Prof. N.R. Green.
Except where appropriately referenced, 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:
Y. Yue, W.D. Gri�ths and N.R. Green, ”Modelling of the e↵ects of
•
entrainment defects on mechanical properties in Al-Si-Mg alloy”, in
Materials Science Forum, Vol.765, pp.225-229, 2013.
Y. Yue, W.D. Gri�ths, J.L. Fife, and N.R. Green, ”In-Situ Char-
•
acterization of Entrainment Defects in Liquid Al-Si-Mg Alloy,” in 1
International Conference on 3D Materials Science, 2012, pp.131–136.
Y. Yue and N.R. Green, ”Modelling of Di↵erent Entrainment Mech-
•
anisms and Their Influences on the Mechanical Reliability of Al-Si
Castings,” in IOP Conference Series: Materials Science and Engi-
neering, 2012, vol. 33, paper No.012072
Yang Yue
September 2013
Contents
List of Figures x
List of Tables xxiii
1 Introduction 1
1.1 Objectives of This Work . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Thesis Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Literature Review 5
2.1 Entrainment Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Formation of Entrainment Defects . . . . . . . . . . . . . . . . . 8
2.1.2 Evolution of Entrainment Defects . . . . . . . . . . . . . . . . . 11
2.1.2.1 Furling and Unfurling . . . . . . . . . . . . . . . . . . 11
2.1.2.2 Relationship between Entrainment Defects and Porosity 13
2.1.3 E↵ects on the Mechanical Properties of Castings . . . . . . . . . 17
2.1.4 Healing of Entrainment Defects . . . . . . . . . . . . . . . . . . 20
2.2 Computational Modelling of Entrainment Defects . . . . . . . . . . . . 21
2.2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.2 Integrated Modelling of Entrainment Defects . . . . . . . . . . . 25
2.2.2.1 Cumulative Entrained Free Surface Area Method . . . 25
2.2.2.2 Cumulative Surface Scalar Method . . . . . . . . . . . 26
2.2.2.3 Air Entrainment Model . . . . . . . . . . . . . . . . . 28
2.2.2.4 Dimensionless Number Criterion . . . . . . . . . . . . 29
2.2.2.5 Bubble Entrainment Model . . . . . . . . . . . . . . . 34
2.2.3 Modelling of Discrete Defects . . . . . . . . . . . . . . . . . . . 36
2.2.3.1 Modelling of Reoxidation Inclusion in Steel . . . . . . 37
2.2.3.2 Modelling of the Folding Mechanisms . . . . . . . . . . 39
2.2.3.3 Modelling of Oxide Entrainment . . . . . . . . . . . . 41
2.2.4 Modelling of Oxide Film Deformation . . . . . . . . . . . . . . . 45
2.2.5 Validation of Modelling . . . . . . . . . . . . . . . . . . . . . . . 47
2.3 X-Ray Tomography of Materials and Defects . . . . . . . . . . . . . . . 48
2.3.1 Introduction of X-ray Tomography . . . . . . . . . . . . . . . . 49
2.3.1.1 Principles of X-ray Tomography . . . . . . . . . . . . . 49
2.3.1.2 Tomography Mode and Radiation Source . . . . . . . . 53
2.3.1.3 Artefacts in X-ray Tomography . . . . . . . . . . . . . 55
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CONTENTS
2.3.2 The Application of X-ray Tomography . . . . . . . . . . . . . . 56
2.3.2.1 3D Visualisation and Image Analysis . . . . . . . . . . 57
2.3.2.2 Quantitative Characterisation . . . . . . . . . . . . . . 57
2.3.2.3 In Situ Experiments . . . . . . . . . . . . . . . . . . . 58
2.3.2.4 Simulation of Tomography Data. . . . . . . . . . . . . 59
2.3.2.5 X-ray Tomographical Study of Entrainment Defects . . 59
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3 Computational Modelling 62
3.1 Hardware and Software . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.2 Implementation of the OFEM in FLOW-3D . . . . . . . . . . . . . . . 64
3.3 The OFEM Algorithm Investigation . . . . . . . . . . . . . . . . . . . . 65
3.3.1 Preliminary Modelling of Di↵erent Entrainment Events . . . . . 65
3.3.2 Modelling with Gravity along the Di↵erent Axis . . . . . . . . . 69
3.3.3 Colliding Fronts and Shear Flows . . . . . . . . . . . . . . . . . 71
3.3.4 Mesh Sensitivity of OFEM Algorithm . . . . . . . . . . . . . . . 71
3.4 Validation of OFEM by Previous Experiments . . . . . . . . . . . . . . 72
3.4.1 Fatigue Life Validation Model . . . . . . . . . . . . . . . . . . . 72
3.4.1.1 Experimental Procedure and Fatigue Test Results . . . 72
3.4.1.2 Set-up for the Validation Model . . . . . . . . . . . . . 77
3.4.2 Froude Number Validation Model . . . . . . . . . . . . . . . . . 78
3.4.2.1 Set-up for Validation Model . . . . . . . . . . . . . . . 80
3.5 The Three Entrainment Mechanisms . . . . . . . . . . . . . . . . . . . 83
3.5.1 Mould Geometries and Mesh Setup . . . . . . . . . . . . . . . . 83
3.5.2 General Model Setting . . . . . . . . . . . . . . . . . . . . . . . 85
3.5.3 Comparative Model Setting . . . . . . . . . . . . . . . . . . . . 86
3.6 X-ray Tomography Heating Profile . . . . . . . . . . . . . . . . . . . . 88
3.7 The Tensile Bar Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.7.1 Mould Geometries and Mesh Setup . . . . . . . . . . . . . . . . 92
3.7.2 General Model Setting . . . . . . . . . . . . . . . . . . . . . . . 94
4 Experimental Methods 96
4.1 Casting and Filling Observation . . . . . . . . . . . . . . . . . . . . . . 96
4.1.1 Pattern and Mould-making . . . . . . . . . . . . . . . . . . . . 96
4.1.2 Casting Alloy Preparation . . . . . . . . . . . . . . . . . . . . . 96
4.1.3 Heat Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.1.4 Real Time X-ray Radiographic Observation . . . . . . . . . . . 97
4.2 Three Entrainment Mechanisms Experiments. . . . . . . . . . . . . . . 98
4.2.1 Casting Sample Preparation . . . . . . . . . . . . . . . . . . . . 98
4.2.2 Bend Testing and Defects Characterisation . . . . . . . . . . . . 99
4.3 In situ Characterisation of Entrainment Defects . . . . . . . . . . . . . 102
4.3.1 Mould Geometries . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.3.2 Casting Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 103
4.3.3 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . 103
4.3.4 Synchrotron X-Ray Radiography . . . . . . . . . . . . . . . . . 105
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CONTENTS
4.3.5 Image Reconstruction and Analysis . . . . . . . . . . . . . . . . 106
4.4 Tensile Bar Mould Experiments . . . . . . . . . . . . . . . . . . . . . . 107
4.4.1 Casting Procedure and Sample Preparation . . . . . . . . . . . 107
4.4.2 Tensile Testing and Defect Characterisation . . . . . . . . . . . 109
5 Results 110
5.1 OFEM Algorithm Investigation . . . . . . . . . . . . . . . . . . . . . . 110
5.1.1 Preliminary Modelling of Di↵erent Entrainment Events . . . . . 110
5.1.2 The Rule of Gravity in Placing Particles . . . . . . . . . . . . . 112
5.1.3 Modelling of Colliding Fronts and Shear Flows . . . . . . . . . . 113
5.1.4 Mesh Sensitivity of the Algorithm . . . . . . . . . . . . . . . . . 117
5.2 Modelling vs. Previous Experiments . . . . . . . . . . . . . . . . . . . . 126
5.2.1 Fatigue Life Validation Model . . . . . . . . . . . . . . . . . . . 126
5.2.1.1 Modelled Flow Structure . . . . . . . . . . . . . . . . . 126
5.2.1.2 Comparison between Particle Count and Fatigue Life
of Castings . . . . . . . . . . . . . . . . . . . . . . . . 130
5.2.1.3 ComparisonbetweenFatigueLifeVariationofTestBars
and Predicted Particle Distribution . . . . . . . . . . . 134
5.2.2 Froude Number Validation Model . . . . . . . . . . . . . . . . . 137
5.2.2.1 Modelled Flow Structure . . . . . . . . . . . . . . . . . 137
5.2.2.2 Comparison between Particle Count in the Model and
UTS of the Test Bar . . . . . . . . . . . . . . . . . . . 142
5.3 Modelling of the Three Entrainment Mechanisms . . . . . . . . . . . . 148
5.3.1 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . 149
5.3.1.1 Filling Structures in the Three Moulds . . . . . . . . . 149
5.3.1.2 Bend Test Properties and Weibull Analysis . . . . . . 149
5.3.1.3 Defect Characterisation . . . . . . . . . . . . . . . . . 156
5.3.2 Computational Modelling Results . . . . . . . . . . . . . . . . . 156
5.3.2.1 Modelled Filling Structures in Three Moulds . . . . . . 156
5.3.2.2 Weibull Analysis of the Predicted Defects Population . 167
5.3.2.3 Model Predicted Defects Distribution . . . . . . . . . . 170
5.3.2.4 Validation with Di↵erent Particle Properties . . . . . . 174
5.3.2.5 Validation with Old Oxide Films . . . . . . . . . . . . 178
5.3.2.6 Final Validation of Three Entrainment Mechanisms . . 181
5.4 Synchrotron X-ray Characterisation of Entrainment Defects . . . . . . 183
5.4.1 Entrainment Defect Classification . . . . . . . . . . . . . . . . . 183
5.4.2 Evolution of Entrainment Defects . . . . . . . . . . . . . . . . . 186
5.4.2.1 Modelling of the Heating Profile. . . . . . . . . . . . . 186
5.4.2.2 Morphological Evolution of Entrainment Defects . . . 194
5.4.3 BehaviourofEntrainmentDefectsintheLiquidStateandduring
Solidification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
5.5 Tensile Test Bar Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
5.5.1 Modelled Flow Structure . . . . . . . . . . . . . . . . . . . . . . 199
5.5.2 Defect Characterisation . . . . . . . . . . . . . . . . . . . . . . 202
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