Solidification Behaviour of Al-Sn-Cu Immiscible Alloys and Al-Si Cast Alloys Processed under ...

181 Pages·2010·24.63 MB·English

Solidification Behaviour of Al-Sn-Cu
Immiscible Alloys and Al-Si Cast Alloys
Processed under Intensive Shearing

By
Hiren R. Kotadia

BCAST (Brunel Centre for Advanced Solidification Technology),
Brunel University, Uxbridge, UB8 3PH, UK

A thesis submitted for the degree of
Doctor of Philosophy
at the Brunel University
January 2010
This thesis is dedicated to
my grandmother and parents
for their love, endless support and encouragement.

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"Once you start working on something, don't be afraid of failure
and don't abandon it. People who work sincerely are the happiest."
-
Chanakya quotes (Indian politician, strategist and writer, 350 BC-275)

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Abstract
Alloy castings are usually solidified with a coarse columnar grain structure under normal
casting conditions unless the mode of the solidification is carefully controlled. It is
desirable for the grain structure to be fine and equiaxed to improve their mechanical
performance as finished castings. It is possible to develop a fine and equiaxed grain
structure either by increasing the number of nucleation sites or by grain multiplication.
Immiscible alloys with a microstructure in which a soft phase is dispersed
homogeneously in a hard matrix have significant potential applications in advanced

bearing systems, especially for the automotive industry.
Despite considerable efforts made worldwide, including extensive space experiments, no
casting techniques so far can produce the desired immiscible microstructure of alloys.
Experimental results on Al-Sn-Cu immiscible alloys have confirmed that intensive
shearing using melt conditioning by an advanced shearing technology (MCAST) unit, is
an effective way to achieve a fine and uniform dispersion of the soft phase without
macro-demixing, and that such a dispersed microstructure can be further refined in alloys
with precipitation of the primary Al phase prior to the demixing reaction. In addition, it
was found that melt shearing at 200 rpm for 60 s will be adequate to produce a fine and
uniform dispersion of the Sn phase, and that a higher shearing speed and prolonged
shearing time can only achieve further minor refinement.
A study of Al-Si hypoeutectic and hypereutectic alloys presents the effects of the
processing temperature and intensive shearing on the microstructural and mechanical
properties which have been investigated systematically. Attempts have been made to
explain the solidification mechanism with intensive melt shearing. The sheared melt was
cast into tensile test samples by high pressure die caster (HPDC) to examine the
microstructures and mechanical properties. The experimental results reveal that
significant grain refinement and uniformity of grains was achieved by the intensive
shearing and also a considerable increase in mechanical properties with pouring
temperature by changing intermetallic particles morphology, the position of defect band
and reduced microscopic defects.
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Preface

This thesis is a description of work that I performed in the Brunel Centre for Advanced
Solidification Technology, Brunel University, West London from 13th May 2006 to 30th
April 2009. To the best of my knowledge, this work is original, except where suitable
references are made to previous work. Neither this, nor any substantially similar
dissertation has been submitted to any other institution. It is the result of my own work
and includes a thermodynamic calculation that is the outcome of collaboration. The
content of the dissertation does not exceed 60,000 words.

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Related Publications

International Journal Papers
• Kotadia HR, Patel JB, Fan Z, Doernberg E, Schmid-Fetzer R.: “Processing of Al-
45Sn-10Cu Based Immiscible Alloy by a Rheomixing Process”, Solid State
Pheno., 2008, 141-143: 529.
• Kotadia HR, Doernberg E, Patel JB, Fan Z, Schmid-Fetzer R.: “Solidification of
Al-Sn-Cu based immiscible alloys under intense shearing”, Metall. Mater. Trans.
A, 2009, 40 (9): 2202-2211.
• Kotadia HR, Hari Babu, Fan Z.: “Microstructural refinement of Al–10.2%Si
alloy by intensive shearing”, Materials Letters, 2010, 64 (6), 671-673.
• Kotadia HR, Hari Babu N, Zhang H, Fan Z.: “Microstructure and mechanical
properties of Al-9.3%Si alloys under intensive shearing”, to be submitted Metall.
Mater. Trans. A.
• Kotadia HR, Hari Babu N, Zhang H, Fan Z.: “Solidification Behaviour of Al-Si
High Pressure Die Castings” to be submitted Journal of Materials Processing
Technology.
Conference Papers
• Kotadia HR, Patel JB, Fan Z, Doernberg E, Schmid-Fetzer R.: “Solidification and
processing of aluminium based immiscible alloys”, Aluminium Alloys:
Fabrication, Characterization and Applications II 2009, San Francisco, CA,
2009, W. Yin, S. K. Das, and Z. Long, ed., TMS, Warrendale, PA, 2009, pp. 81-
86.
• Kotadia HR, Patel JB, Fan Z.: “Processing of Al-Sn-Cu ternary alloys by the
rheomixing process”, research conference, 2008, Brunel University.
Poster
• Kotadia HR, Patel JB, Fan Z.: “Al-Sn-Cu based immiscible alloys process by
rheomixing”, Brunel University, West London, 2007 (Best poster prize award)

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Acknowledgements

I am grateful to the EPSRC, UK and DTI for funding this project. I would like to express
my sincere gratitude to my supervisor, Prof. Z. Fan, for his guidance and encouragement.
I am grateful to Dr. N. Hari Babu for his encouragements and useful discussion
throughout my PhD programme and also I would like to thank my collaborators Miss. E.
Doernberg and Prof. R. Schmid-Fetzer for important pioneering thermodynamic
calculations and useful discussion.
I have received technical help/suggestions from many other people from BCAST during
the course of this work. I want to specially mention Dr. J. Patel, Dr. A. Das, Dr. M. Xia,
Dr. H. Zhang, Dr. S. Arumuganathar, Dr. S. Kumar, Mr. S. Cook, and other BCAST
members for their technical support and scientifically helpful and discussions. I have
received much help from fellow PhD students Mr. S. Tzamtzis and Dr. H. Li.
My family have sacrificed the most during my studies but they have always been with me
and never lost faith in me. The support I have got from my parents cannot be expressed in
words. My parents, Manjulaben and Ramnikbhai always wanted me to become
successful in all my endeavours and it is their blessings that took me this far. I am also
grateful to my brothers Yagnesh and Samir for their support. The help from my friends
Alpesh and Manoj and their family have been outstanding and unforgettable during my
life in London.
I have obtained much encouragement and support from my friends here at Brunel
University. I am indebted to Miss. Htaik Htar and many others for their friendship and
support. I have always enjoyed the company of my friends. Many thanks to my flat
mates, especially for the kitchen discussions.

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Table of Contents

Abstract ............................................................................................................................................. iii
Preface ............................................................................................................................................... iv
Related Publications ..................................................................................................................... v
Acknowledgments ........................................................................................................................ vi
Table of Contents ...........................................................................................................vii
Nomenclature ................................................................................................................... xi
Chapter 1 Introduction ................................................................................................ 1
1.1 General Introduction ....................................................................................................... 1
1.2 Rheomixing of Al-Sn-Cu immiscible alloys .................................................................... 2
1.3 Intensive shearing of Al-Si cast alloys ............................................................................. 3
1.4 Layout of thesis ............................................................................................................... 4
Chapter 2 Literature Review ...................................................................................... 6
2.1 Immiscible alloys ............................................................................................................ 6
2.1.1 Introduction .............................................................................................................. 6
2.1.2 Solidification characteristics of immiscible alloys ..................................................... 7
2.1.3 Liquid-liquid phase separation of immiscible alloys .................................................. 9
2.1.3.1 Marangoni migration ........................................................................................ 10
2.1.3.2 Collision and coagulation ................................................................................. 11
2.1.3.3 Droplet sedimentation ...................................................................................... 13
2.1.4 Possible techniques to produce a well-dispersed microstructure .............................. 15
2.1.4.1 Rapid solidification/ cooling techniques ........................................................... 16
2.1.4.2 Stir casting ....................................................................................................... 17
2.1.4.3 Controlled casting ............................................................................................ 17
2.1.4.4 Drop tube technique ......................................................................................... 19
2.1.4.5 Rheomixing ..................................................................................................... 19
2.1.4.5.1 Droplet breakdown ........................................................................................ 20
2.1.5 Application of immiscible alloys ............................................................................ 21
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2.2 Al-Si Alloy ................................................................................................................... 22
2.2.1 Introduction ............................................................................................................ 22
2.2.2 Effect of alloying elements on binary Al-Si alloys .................................................. 23
2.2.3 Microstructural control ........................................................................................... 26
2.2.4 High pressure die-casting process ........................................................................... 28
2.3 Summary ...................................................................................................................... 29
Chapter 3 Experimental Techniques and Procedures ........................................... 31
3.1 Introduction .................................................................................................................. 31
3.2 Materials preparation .................................................................................................... 31
3.3 Melt conditioning by an advanced shear technology (MCAST) unit .............................. 33
3.4 Casting processes .......................................................................................................... 36
3.4.1 Copper mould ......................................................................................................... 36
3.4.2 High pressure die casting (HPDC) .......................................................................... 37
3.4.3 Test procedure -1 (TP-1) mould .............................................................................. 39
3.4.4 Shrinkage mould (Tatur test) .................................................................................. 40
3.5 The ‘melt conditioning high pressure die-casting’ (MC-HPDC) process ........................ 42
3.6 Characterization methods .............................................................................................. 43
3.6.1 Chemical composition analysis ............................................................................... 43
3.6.2 Sample preparation for metallographic studies ........................................................ 43
3.6.3 Optical microscopy (OM) ....................................................................................... 44
3.6.4 Scanning electron microscopy (SEM) ..................................................................... 44
3.6.5 Differential scanning calorimetry (DSC) ................................................................. 45
3.6.6 Electro-etching ....................................................................................................... 46
3.7 Quantitative metallography ........................................................................................... 46
3.8 Mechanical property tests and method ........................................................................... 48
3.8.1 Hardness tests ......................................................................................................... 48
3.8.2 Tensile tests ............................................................................................................ 49
3.8.3 Heat treatment ........................................................................................................ 49
Chapter 4 Rheomixing of Al-Sn-Cu immiscible alloys: Results ..................... 50
4.1 Introduction .................................................................................................................. 50
4.2. Thermodynamics of the Al-Sn-Cu system .................................................................... 51
4.2.1 Phase equilibrium in the Al-Sn-Cu alloys .............................................................. 51
4.2.2 Semi-solid processing temperature and temperature sensitivity of solid
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fraction .................................................................................................................. 52
4.2.3 Solidification range ............................................................................................... 52
4.2.4 Alloy selection using computational thermodynamics ............................................ 54
4.3 Conformation of the phase transformations with DSC ................................................... 56
4.4 Microstructural evolution .............................................................................................. 58
4.4.1 Phase identification ................................................................................................. 58
4.4.2 Microstructural evolution in gravity casting ............................................................ 60
4.4.3 Microstructure of HPDC and MC-HPDC samples ................................................... 64
4.4.3.1 Effect of shearing time ..................................................................................... 65
4.4.3.2 Effect of shearing speed ................................................................................... 65
4.4.4 Effect of cooling rate on the Sn particle size ............................................................ 68
4.5 Effect of Sn concentration on size of particles ............................................................... 72
4.6 Mechanical properties ................................................................................................... 74
4.7 Summary ...................................................................................................................... 74
Chapter 5 Intensive shearing of Al-Si cast alloys: Results ............................... 77
5.1 Introduction .................................................................................................................. 77
5.2 Microstructural evolution of Al-Si alloys ....................................................................... 77
5.2.1 Solidification microstructure of the conventional HPDC samples .......................... 77
5.2.2 Solidification microstructure of the MC-HPDC samples ........................................ 80
5.3 Hypoeutectic Al-Si alloys ............................................................................................. 82
5.3.1 Fraction of ESCs ..................................................................................................... 82
5.3.2 Defect band ............................................................................................................ 83
5.3.3 Size, shape and distribution of intermetallic particles .............................................. 92
5.3.4. Porosity ................................................................................................................. 95
5.3.4.1 Shrinkage porosity measurements .................................................................... 97
5.3.5 Grain size ............................................................................................................. 102
5.3.6 The mechanical properties of HPDC and MC-HPDC tensile samples ................... 104
5.4 Hypereutectic Al-Si alloy ............................................................................................ 108
5.4.1 Shape, size and distribution of the Si phase ........................................................... 108
5.4.2 Mechanical properties of HPDC and MC-HPDC samples ..................................... 113
5.5 Summary .................................................................................................................... 114
Chapter 6 Al-Sn-Cu and Al-Si alloys: Discussion ............................................ 116
6.1 Introduction ................................................................................................................ 116
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