Optimisation of the mechanical properties in an investment cast aluminium alloy

178 Pages·2017·9.67 MB·English

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Optimisation of the
mechanical properties in an
investment cast aluminium
alloy
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Additional Information:
• A Doctoral Thesis. Submitted in partial fulfilment of the requirements
for the award of Doctor of Philosophy of Loughborough University.
Metadata Record: https://dspace.lboro.ac.uk/2134/26147
Publisher: (cid:13)c Levy Siaminwe
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Please cite the published version.

OPTIMISATION OF THE MECHANICAL PROPERTIES
IN AN INVESTMENT CAST ALUMINIUM ALLOY
by
Levy Siaminwe BEng, MSc.
A Doctoral Thesis
submitted in partial fulfilment of the requirements
for the award of
Doctor of Philosophy
. of Loughborough University
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SYNOPSIS
The thesis reports an experimental investigation concerned with the optimisation of
mechanical properties in investment cast LM25 aluminium alloy. The effects of melt
hydrogen content, filter usage, grain refinement, eutectic silicon modification, heat
treatment, pouring and shell preheat temperatures on the structure and tensile properties of
LM25 investment cast tensile test specimens were studied. Four series of experiments were
conducted to assess the effects of the parameters studied on the structure and tensile
properties.
The first series established the effect of melt hydrogen content, pouring temperature and
shell preheat temperature on the casting porosity, pore morphology and tensile properties.
The second series investigated the effect of using a ceramic foam filter on the tensile
properties. Pouring temperature and shell preheat temperature were variable parameters in
this part of the investigation. The objective of the third series of experiments was to
investigate the effect of grain refinement and eutectic silicon modification on the structure
and tensile properties. The interaction of these melt treatments with shell preheat
temperature and filtration was also studied. In the final series of experiments the effect of
heat treatment cycles on the samples produced in the third series of experiments was
evaluated in terms of structure and tensile properties.
The principal findings of the research were that: shell preheat temperature and hydrogen
content are the most important process variables detennining the total porosity content;
shell temperature affects the structure and, hence, the tensile properties; grain refinement is
enhanced as the titanium content increases to about 0.28% but the tensile properties remain
unaffected; a modified eutectic Si structure is achieved with 0.01 -0.02% Sr with optimum
Sr addition, based on tensile properties, being 0.01%; and, as would be expected, heat
treatment improves the tensile properties of investment cast LM25. On the basis of the
inter-relationships between process variables, structural changes and tensile properties
observed, an optimum processing route was proposed. Grain refined and modified
specimens produced with low hydrogen content and ambient shell temperature had
optimum tensile properties when fully heat treated to produce eutectic Si transformation.
ACKNOWLEDGEMENTS
The author wishes to express his gratitude to the following:
Dr AJ. Clegg, Research Supervisor, for his invaluable guidance, support, encouragement,
constructive suggestions and making my life both in and outside the laboratory an
enjoyable and interesting experience.
Mr R.I. Temple, Mr J. Jones, and Mr J.T.W. Smith for their engineering skills, technical
support and assistance during experimentation, and for being such a good people to work
with and for making my project work manageable and fun.
Mr R. Price and Mr D. Hardwick, technical staff, for their part in the manufacture of the
die and ingot moulds.
Dr C.S. Lim, Dr S.H. Kim, Mr M. Hasbullah and Mr M.S. Yong, former and present office
and laboratory mates, for their encouragements, discussions and sharing their experiences
as researchers.
The author's parents Mr and Mrs Siaminwe for their support and for sending me to school,
my wife Petronella and daughter Esnati Siaminwe, family, and many friends for their
constant support and encouragements that made this an unforgettable experience. May God
bless them all.
Finally, the author gratefully acknowledges the financial support of the Association for
Commonwealth Universities and the University of Zambia for enabling him to take up this
opportunity.
iii
CONTENTS
Page
Synopsis (i)
Certificate of Originality (ii)
Acknowledgements (iii)
Contents Page (iv)
Chapter 1 Introduction 1
1.1 Introduction 1
1.2 Advantages of the Investment Casting Process 2
1.3 Investment Casting of Aluminium Alloys 3
1.4 Cast Metal Quality 5
1.5 Improvement of Mechanical Properties 7
1.5.1 Pre-Casting Process 8
1.5.2 Casting Process 11
1.5.3 Post-Casting Process 12
1.6 The Need for Research 13
1.7 Summary of Research Objectives 15
Chapter 2 The Investment Casting Process 17
2.1 Introduction 17
2.2 How the Process Works 17
2.2.1 Pattern Production 18
2.2.2 Pattern Assembly 20
2.2.3 Investment and Stuccoing 20
2.2.4 Dewaxing 21
2.2.5 Firing and Casting 22
2.2.6 Knockout and Finishing 22
2.3 Process Capabilities 23
2.3.1 Material Selection 23
2.3.2 Casting Size and Weight Limits 23
2.3.3 Section Thickness 24
2.3.4 Surface Finish 24
2.3.5 Dimensional Accuracy 24
iv
2.4 Summary 25
Chapter 3 Aluminium Alloys 27
3.1 Introduction 27
3.2 Improvement of Mechanical Properties in Al-Si Alloys 28
3.2.1 Achievement of Soundness in Aluminium Alloy Castings 29
3.2.1.1 Control of Shrinkage Porosity 29
3.2.1.2 Control of Gas Porosity 31
3.2.1.3 Control of Inclusions 36
3.2.2 Microstructure Control in AI-Si Alloy Castings 40
3.2.2.1 Melt Condition 41
3.2.2.2 Cooling Rate 43
3.2.2.3 Grain Refinement 44
3.2.2.4 Modification of Eutectic Silicon 47
3.2.3 Melt Treatment and Porosity 50
3.3 Heat Treatment 52
3.3.1 Solution Heat Treatment 53
3.3.2 Quenching 55
3.3.3 Age (Precipitation) Hardening 56
3.4 Summary 58
Chapter 4 Experimental Design and Procedures 60
4.1 Introduction 60
4.2 Investment Cast Test Bars 60
4.2.1 Wax Patterns and Assembly 60
4.2.2 Ceramic Shell Moulds Manufacture 61
4.3 Experimental Material 64
4.4 Experimental Procedures 64
4.4.1 Series I: Melt Hydrogen Content 65
4.4.2 Series II: Filtration 66
4.4.3 Series ill: Grain Refinement and Modification 67
4.4.4 Series IV: Heat Treatment 68
4.5 Mechanical Properties Testing 70
4.5.1 Tensile Testing 71
v
4.5.2 Metallography 71
ChapterS Results and Observations 72
5.1 Introduction 72
5.2 Series I: Melt Hydrogen Content 72
5.2.1 Porosity Formation 72
5.2.1.1 Effect of Hydrogen 72
5.2.1.2 Effect of Pouring and Shell Preheat Temperatures 72
5.2.2 Pore Morphology and Size 73
5.2.3 Microstructure 74
5.2.4 Tensile Properties 74
5.2.4.1 Effect of Porosity 74
5.2.4.2 Effect of Pouring and Shell Preheat Temperatures 74
5.3 Series II: Filtration 92
5.3.1 Microstructure 92
5.3.2 Tensile Properties 92
5.3.2.1 Effect of Filtering 92
5.3.2.2 Effect of Pouring and Shell Preheat Temperatures 94
5.4 Series ill: Grain Refinement and Modification 101
5.4.1 Grain Refinement 101
5.4.1.1 Macrostructure 101
5.4.1.2 Tensile Properties 105
5.4.2 Eutectic Silicon Modification 107
5.4.2.1 Microstructure 107
5.4.2.2 Tensile Properties 108
5.4.3 Combined Grain Refinement and Modification 115
5.4.3.1 Structure 115
5.4.3.2 Porosity 115
5.4.3.3 Tensile Properties 116
5.5 Series IV: Heat Treatment 118
5.5.1 Microstructure 118
5.5.2 Tensile Properties 119
vi
Chapter 6 Discussion 123
6.1 Introduction 123
6.2 Casting Integrity 123
6.2.1 Effect of Hydrogen Content 123
6.2.2 Effect of Pouring and Shell Preheat Temperatures 125
6.2.3 Effect of Grain Refinement and Modification 127
6.2.4 Pore Morphology and Size 128
6.3 Casting Structure 130
6.3.1 Effect of Pouring and Shell Preheat Temperatures 130
6.3.2 Effect of Grain Refinement 131
6.3.3 Effect of Eutectic Silicon Modification 132
6.3.4 Effect of Heat Treatment 132
6.4 Casting Tensile Properties 133
6.4.1 Effect of Porosity 133
6.4.2 Effect of Pouring and Shell Preheat Temperatures 134
6.4.3 Effect of Ceramic Foam Filters 135
6.4.4 Effect of Grain Refinement 135
6.4.5 Effect of Eutectic Silicon Modification 136
6.4.6 Effect of Heat Treatment 136
6.5 Summary 138
Chapter 7 Conclusions 140
7.1 Conclusions 140
7.1.1 Casting Integrity 140
7.1.2 Casting Structure 140
7.1.3 Tensile Properties 142
ChapterS Further Work 144
8.1 Suggestions for Further Work 144
References 146
Appendices 156
1 Heat Treatment of Aluminium Alloys 156
vii