Development of Hypereutectic Al-Si based P/M Alloys

265 Pages·2012·17.89 MB·English

by  Shei Sia Su

Development of Hypereutectic Al-Si based
P/M Alloys

By

Shei Sia Su

A thesis submitted to the University of Birmingham
for the degree of

Doctor of Philosophy

School of Metallurgy & Materials
College of Engineering and Physical Sciences
The University of Birmingham
Birmingham B15 2TT
England
April 2012
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Synopsis

The compaction, sintering and heat treatment processing conditions for the
commercial powder, Ecka Alumix 231 with the nominal composition of Al-15Si-
2.5Cu-0.5Mg(wt%) have been optimized in this study. It has been found that
densification of Ecka Alumix 231 depended largely on the sintering atmosphere
and the amount of liquid phase presented. Sintering in nitrogen atmosphere was
found to be beneficial due to the formation of AIN which later induced pore filling
effect. Presence of hydrogen in the sintering atmosphere, however, was not
desirable. Decomposition of MgH at higher temperature was suggested to
2
increase the pore pressure and caused detrimental effect on compact
densification.

The effects of alloy additions (i.e. Cu and/or Ni) on sintering and heat treatment
response were also investigated in this study. Addition of copper was found to
enhance the sintering response by a significant increase in the sintered density,
reduced the peak ageing temperatures and time. Nickel addition, however, was
detrimental to both sintering and heat treatment response since it reduced the
amount of copper content dissolved in the α-aluminum due to the formation of
Al (Ni,Cu) phase. The precipitation sequence for Al-Si-Cu-Mg-(Ni) was suggested
3 2
to be: Supersaturated solid solution (SSS)→GP zones → θ” → θ’ → θ.
Preface

The work described in this thesis was carried out by the author in the School of
Metallurgy and Materials, The University of Birmingham from September 2008 to
February 2012, under the supervision of Dr. I T H Chang

To the best of my knowledge, the material presented in this thesis is original,
except otherwise noted within the text. None of this research has been submitted
for any degree at this or any other university.

Part of the present work was presented in international conference and published
as follows:
[1] Su.S.S. and I.T.H. Chang, “ Effect of Processing Conditions on
Hypereutectic Aluminum-Silicon P/M Alloys”, World PM2010 Proceedings,
Vol. 4, pp 49-57 (2010)
[2] Su.S.S. and I.T.H. Chang, “ Development of Hypereutectic Al-Si based
alloys”, World PM2010 Proceedings, Vol.4, pp 58-62 (2010)
Acknowledgements

This dissertation would not have been completed without the support of many
wonderful individuals. In particular, I would like to acknowledge the enthusiasm
and dedication of my supervisor, Dr I T H Chang. I am deeply grateful for his
valuable guidance, support and constant encouragement throughout my PhD. His
patience, wisdom and excellent supervision have made this PhD a very rewarding
journey.

My sincere gratitude also goes to Dr Winson Kuo from Oxford Instruments who
has always offered insightful suggestions and gave significant contribution to boost
my knowledge in electron microscopy and enhance my analyzing skills. I would
also like to thank Mr. Hang Wang in offering lots of valuable inputs on
thermodynamics and is always willing to lend an attentive ear and a critical eye.

I would also like to express my sincere gratitude to many other friends and
colleagues who have contributed in countless ways to my dissertation. My sincere
thanks go to Dr Martin Strangwood for offering training in optical microscope and
Thermocalc software, Dr Yulung Chiu and Dr Rengen Ding for providing valuable
advices on TEM sample preparations, Dr Ming Chu for offering training in TEMs,
Mr. Paul Stanley for offering training in SEMs/EDS, Mr. David Price for assistance
in tensile test, Mr. Frank Biddelstone for assistance in DSC, Mr. Jeff Sutton for
assistance in XRD and to Mr. Jas Singh for assistance in the grinding and
polishing. I am also grateful to all technical staff of Met & Mat; particularly Tony
Burbery, Geoff Dolman, Mick Cunningham, Avril Rogers, Paul Osborne and Anne
Cabezas, for their support and help.

Special thanks also go to my group mates: Nick Harrison, Andy Poole, Andy Ward
and for providing a pleasant working environment, to Yina Guo, Jiayun Jiang,
Chao Yang and all my bible study members for their friendship and unfaltering
encouragement over the years of my academic pursuit. I would like to take this
opportunity to also thank Overseas Research Scholarship Fund and Professor
Paul Bowen, School of Metallurgy and Materials for providing financial support.

Last but not least, I would like to express my gratitude to all of my family. Their
support, patience and encouragement have significantly contributed to the
completion of my PhD.
Content
Chapter 1 Introduction ......................................................................................... 1
Chapter 2 Literature Review
2.1 Introduction ....................................................................................................... 4
2.2 Powder Production Methods ............................................................................. 7
2.2.1 Gas Atomization .................................................................................. 7
2.2.2 Water Atomization ............................................................................... 9
2.2.3 Centrifugal Atomization ..................................................................... 11
2.2.4 Mechanical Milling ............................................................................. 12
2.2.5 Electrolysis ........................................................................................ 13
2.2.6 Thermal decomposition ..................................................................... 14
2.3 Compaction ..................................................................................................... 15
2.3.1 Uniaxial Compaction .......................................................................... 17
2.4 Sintering .......................................................................................................... 20
2.4.1 Driving Force for Sintering ................................................................. 21
2.4.2 Wetting Characteristic ....................................................................... 25
2.4.3 Solid State Sintering .......................................................................... 28
2.4.4 Liquid Phase Sintering ....................................................................... 30
2.4.4.1 Particles Rearrangement ................................................................ 34
2.4.4.2 Solution-Reprecipitation ................................................................. 35
2.4.4.3 Pores Filling .................................................................................... 38
2.4.4.4 Particle Coarsening ........................................................................ 40
2.4.5 Transient Liquid Phase Sintering ....................................................... 42
2.4.6 Supersolidus liquid phase sintering ................................................... 44
2.5 Effects of Sintering Variables .......................................................................... 46
2.5.1 Particle Size ....................................................................................... 46
2.5.2 Particle Shape ................................................................................... 48
2.5.3 Sintering Atmosphere ........................................................................ 50
2.5.4 Sintering Temperature ....................................................................... 54
2.5.5 Sintering Time ................................................................................... 55
2.6 Aluminum Alloys Powder Metallurgy ............................................................... 56
2.6.1 Al-Si P/M alloys ................................................................................. 59
2.6.2 Effect of Alloy Additions ..................................................................... 61
2.6.3 Heat Treatment of Aluminum Alloys .................................................. 63
2.6.4 Major Precipitation Hardening Alloy Systems .................................... 70
2.6.4.1 Al-Cu System ................................................................................. 73
2.6.4.2 Al-Cu-Mg System .......................................................................... 74
2.6.4.2 Al-Mg-Si System ............................................................................ 75
2.6.4.3 Al-Cu-Mg-Si System ...................................................................... 76
2.6.4.4 Al-Zn-Mg-Cu System ..................................................................... 76
2.7 References ...................................................................................................... 78
Chapter 3 Experimental Methods
3.1 Introduction ..................................................................................................... 85
3.2 Starting Materials ............................................................................................ 86
3.3 Material Processing ........................................................................................ 87
3.3.1 Powder Blending ............................................................................... 87
3.3.2 Compaction ....................................................................................... 88
3.3.2.1 Thermal Analysis Specimen ........................................................... 88
3.3.2.2 Tensile Specimens ......................................................................... 89
3.3.3 Sintering ............................................................................................ 89
3.3.4 Heat Treatment .................................................................................. 91
3.4 Characaterization Techniques ........................................................................ 92
3.4.1 Apparent Density and Powder Flow Rate Measurement ................... 92
3.4.2 Density Measurement ........................................................................ 92
3.4.3 Differential Scanning Calorimetry (DSC) ........................................... 94
3.4.4 Dilatometry Test ................................................................................ 95
3.4.5 Optical and Scanning Electron Microscopy (SEM) ............................ 95
3.4.6 Transmission Electron Microscopy (TEM) ......................................... 96
3.4.7 X-ray Diffraction (XRD) ...................................................................... 97
3.5 Mechanical Testing ......................................................................................... 98
3.5.1 Vickers Hardenss .............................................................................. 98
3.5.2 Tensile Test ....................................................................................... 99
3.6 Thermodynamic Calculations .......................................................................... 99

Chapter 4 Optimizing the Processing Parameters for Ecka Alumix 231
4.1 Introduction ................................................................................................... 100
4.2 Characterization of Ecka Alumix 231 ............................................................ 101
4.3 Optimization of Compaction Pressure .......................................................... 105
4.4 Optimization of Sintering Temperature ......................................................... 107
4.5 Influence of Processing Conditions on Sintered Density .............................. 109
4.5.1 Effects of Sintering Temperatures on Sintered Density ................... 109
4.5.2 Effects of Sintering Atmospheres on Sintered Density .................... 112
4.5.3 Effects of Sintering Times on Sintered Density ............................... 115
4.6 Thermal Expansion Profile ............................................................................ 117
4.7 Influence of Processing Condition on Microstructures .................................. 121
4.7.1 Microstructural Development of Ecka Alumix 231 at Different
Sintering Temperatures ................................................................... 121
4.7.2 Microstructural Change at Different Sintering Atmospheres ........... 129
4.7.3 Microstructural Change at Different Sintering Times ....................... 136
4.8 Influence of Processing Conditions on Mechanical Properties ..................... 138
4.8.1 Effects of Sintering Temperatures on Mechanical Properties ......... 138
4.8.2 Effects of Sintering Atmospheres on Mechanical Properties ........... 140
4.9 X-ray diffraction Analysis .............................................................................. 141
4.10 Thermo-Calc Calculations ............................................................................ 143
4.11 Discussions .................................................................................................. 146
4.11.1 Compaction behavior of Ecka Alumix 231 ....................................... 146
4.11.2 The role of sintering atmospheres on Ecka Alumix 231 .................. 149
4.10.3 Influence of the Sintering Temperatures on Ecka Alumix 231 ......... 157
4.10.4 Influence of Sintering Time on Ecka Alumix 231 ............................. 163
4.10.5 Phase transformation during sintering of Ecka Alumix 231 ............. 165
4.12 References ................................................................................................... 168
Chapter 5 Alloy Additions and Heat Treatment of Hypereutectic Al-Si based P/M
Alloys
5.1 Introduction ................................................................................................... 171
5.2 Effect of Compaction Pressure on Green Density ........................................ 174
5.3 Determination of Suitable Sintering Temperature ......................................... 177
5.4 Effects of Alloying Additions on Sintered Density ......................................... 180
5.5 Determination of Optimal Heat Treatment Parameters ................................. 183
5.5.1 Determination of Solution Treatment Temperatures ....................... 183
5.5.2 Effects of Solution Heat Treatment Temperatures on Ageing
Response ........................................................................................ 186
5.5.3 Effects of Solution Heat-Treatment Times on Ageing Response .... 191
5.5.4 Age Hardening Characteristic for Al-Si based Alloys ...................... 193
5.6 Precipitation Behaviors for Al-Si based Alloys .............................................. 199
5.7 Microstructural Development ........................................................................ 202
5.7.1 Microstructural Characterization of the Sintered Compacts ............ 202
5.7.2 Microstructural Characterization of the Sintered and Solution
heat- treated Compacts ................................................................... 209
5.7.3 Microstructural Characterization of the Sintered and
Age-hardened Compacts ................................................................. 217
5.8 Thermo-Calc Calculations ............................................................................. 229
5.9 Discussion ..................................................................................................... 232
5.9.1 Influence of Alloy Additions on Compressibility ............................... 232
5.9.2 Influence of Alloy Additions on Sinterability ..................................... 235
5.9.3 Influence of Solution Heat Treatment for Different Al-Si based Alloys ..
......................................................................................................... 240
5.9.4 Artificial Ageing Response for Different Al-Si based Alloys ............. 245
5.10 References ................................................................................................... 248
Chapter 6 Conclusions and Future Work
6.1 Conclusion .................................................................................................... 251
6.2 Future Work .................................................................................................. 254
6.3 References .................................................................................................... 256
Chapter 1
Introduction

With the growing demand to lower carbon emission, the developments of
lightweight materials have continued to gain much attention in both automotive and
aerospace industries. Wrought and ingot metallurgy metal forming techniques have
been used extensively in the manufacturing industries to produce the metallic
components. Despite its flexibility in engineering designs, the limited range of alloy
chemistry that can be employed and inadequate dimensional tolerances posses a
major threat to the industries due to its high secondary machining costs.

Power metallurgy (P/M) approach has received considerable attention as an
alternative to wrought and ingot technologies. P/M technology allows fabrication of
near net shape products at high production volume and low manufacturing costs.
Apart from cost-effective processes, the process variables in P/M can be adjusted
to produce parts which are analogous to conventional cast and wrought materials.
Other advantages of aluminum P/M are listed below:
l Cost effective manufacturing for complex or unique shapes application
l Minimal waste during machining
l Energy savings since it requires lower sintering temperature and use of
cheaper sintering gas, i.e. nitrogen gas
l Good workability
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