Corrosion Behaviour on Powder Metallurgy Aluminium Matrix Composite Reinforced
with Alumina Saffil™ Short Fibres
By
Normah binti Cheman 0630410099
A thesis submitted
In fulfilment of the requirements for the degree of Master of Science (Materials Engineering)
School of Materials Engineering UNIVERSITY MALAYSIA PERLIS
2009
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ACKNOWLEDGEMENT
I am grateful to Allah S.W.T. With the name of Allah s.w.t Praise to Allah as our God and our Prophet, Nabi Muhammad s.a.w.
First of all, I would like to express my sincere gratitude to my supervisor, Dr. Mohd Nazree Derman and co-supervisor, Mrs. Nur Maizatul Shima Adzli, for their patience, guidance and constant encouragement.
I wish to thank the academic staffs and technicians at School of Materials Engineering;
Prof. Dr. Shamsul Baharin Jamaludin, Mr. Azmi, Mr. Che Idrus, Mr. Ahmad Hadzrul Iqwan, Mr. Ku Hasrin, Mr. Mohd Nasir, Mr Norzaidi for their kind help. Thanks also to my colleagues Zuraidawani, Saidatul, Nurharlina, Habsah, Josep, Nurul Khairunnisa and Raudah.
I also wish to express my appreciation to my husband; Ismail Abdul Talib, my son;
Muhammad Ikram and my daughters: Nur Hanis, Nur Nadiah, Nur Husnina and Nur Insyirah for their understanding and inspiring me to complete this project succesfully.
Finally, thanks to FRGS grant provided the facilities and powders for my experiments.
Thanks. Wassalam.
NORMAH BINTI CHEMAN.
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CONTENTS
CONTENTS Page
CHAPTER 1 1.1
1.2
1.3
1.4
1.5 1.
CHAPTER 2 2.1
2.1.1
ACKNOWLEDGEMENT CONTENTS
LIST OF FIGURES LIST OF TABLES
LIST OF ABBREVIATIONS LIST OF SYMBOLS
LIST OF APPENDIXS LIST OF PUBLICATION ABSTRAK
ABSTRACT
INTRODUCTION Research introduction
The effect of magnesium addition in MMCs on corrosion
The effect of alumina Saffil™ short fibres reinforcement on corrosion
The effect of powder metallurgy method on corrosion behaviour
Objective of research Approach of research
LITERATURE REVIEW Metal matrix composite Matrices Matrices
ii iii ix xiii xiv xv xv xvi xvii xviii
1 3
4
5
7 7
10 10
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2.1.1.1
2.1.1.1.1 2.1.1.1.2 2.1.1.2
2.1.2
2.1.2.1 2.1.3
2.2 2.2.1 2.2.2
2.2.2.1 2.2.2.2 2.2.2.3 2.2.3
2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.4
2.4.1 2.4.2
CHAPTER 3
Aluminium
Aluminium powder
Aluminium matrix composite (AMCs) Magnesium
Reinforcements
Types of reinforcement Wetting process
Fabrication method Powder metallurgy
The process of PM fabrication Mixing
Compaction Sintering
Powder metallurgy product Introduction to corrosion Principles of corrosion Types of corrosion General corrosion
Corrosion in Al and Al alloys
Studies of corrosion behaviour on metal matrix composite
Studies of corrosion on PM metal matrix composite (MMCs)
Studies of corrosion behaviour on PM aluminium matrix composite (AMCs)
METHODOLOGY
11 13 13 15 18 19 22 23 23 25 27 27 27 29 31 32 35 35 35 37
41
43
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3.1 3.2
3.2.1 3.2.2 3.2.3 3.3
3.3.1 3.3.2 3.3.3 3.4
3.4.1 3.4.2
3.4.2.1 3.4.2.2 3.4.2.3 3.4.2.4 3.4.3
3.4.4 3.4.5 3.5
3.5.1
3.5.1.1 3.5.1.2 3.5.1.3 3.5.2
Introduction Raw material Aluminium powder Magnesium powder
Alumina Saffil™ short fibre
Fabrication of PM Al-Mg alloy and PM Al-Mg composite
Mixing
Compaction Sintering
Samples analysis
Density and porosity analysis Microstructural studies Sample preparation Optical image analyzer
Scanning electron microscope (SEM) EDX analysis
Hardness testing
X-Ray Diffraction analysis (XRD) Linear shrinkage analysis
Corrosion study Immersion testing Sample preparation
Procedure of immersion testing Corrosion rate
Potentiodynamic polarization
47 49 49 50 50 51
51 52 52 53 54 54 54 55 55 56 56 56 57 57 57 57 58 59 59
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3.5.2.1 3.5.2.2 3.5.2.3 3.6
3.6.1 3.6.2
CHAPTER 4 4.1
4.2
4.2.1
4.2.1.1 4.2.1.2 4.2.1.3 4.2.2
4.2.2.1 4.2.2.2 4.2.2.3 4.2.2.4 4.2.3
4.2.3.1 4.2.3.2 4.2.3.3
Sample preparation
Procedure of potentiodynamic polarization Corrosion rate
Testing and equipment Particle size analyzer Surface roughness
RESULTS AND DISCUSSION Introduction
Part A: Fabrication of PM Al-Mg alloy and PM Al-Mg composite reinforced with alumina Saffil™ short fibres
Raw material Aluminium powder Magnesium powder
Alumina Saffil™ short fibre
A study of magnesium content on PM Al-Mg alloy
Density and porosity test Vickers hardness test Linear shrinkage
Microstructural studies Fabrication of PM Al-Mg composites
reinforced with alumina Saffil™ short fibres Density and porosity test
Linear shrinkage Vickers hardness tester
59 60 61 62 62 63
66 66
66 66 67 69 70
70 72 72 73 75
75 76 77 78
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4.2.3.4 4.2.4
4.2.4.1 4.2.4.2 4.2.4.3 4.3
4.3.1
4.3.1.1 4.3.1.2 4.3.1.3 4.3.1.4 4.3.1.5 4.3.2
4.3.2.1 4.3.2.2 4.3.3
4.3.4
4.3.5
4.3.6
CHAPTER 5 5.1
5.2
Microstructural studies
The effect of magnesium powders on PM Al composites reinforced with alumina Saffil™
short fibres
Density and porosity
Vickers hardness test Microstructural studies Part B: Corrosion test Immersion test
Microstructural studies Pitting and EDX analysis XRD analysis
Effect of duration of exposure to corrodent Surface roughness
Electrochemical test Tafel plots
Potentiodynamic polarization
The reaction of corrosion process in sample The effects of magnesium addition on corrosion behaviour of PM Al-Mg composites
The effects of alumina Saffil™ short fibres reinforcement on corrosion behaviour of PM Al composites
The effects of powder metallurgy process on corrosion behaviour
CONCLUSION
Conclusion
Recommendation for future research
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80 81 82 85 85 85 89 93 94 97 103 103 106 107 108
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111
113 114 115
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REFERENCES APPENDICES
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LIST OF FIGURES Figure
2.1
2.2
2.3
2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13
2.14
2.15
2.16
2.17
2.18
Hardness of EN AW-2124 aluminum alloy matrix composite reinforced with different particles Schematic presentation of various types of metal matrix composites
Schematic illustration of contact angle in a (a) non-wetting system, and (b) wetting system
Basic steps in making parts by powder metallurgy Two sphere point contact sintering model
Gears and piston ring PM parts market
Factor affecting corrosion resistance of a metal Mechanical analogy of free energy change
Electrochemical reactions for aluminium in hydrochloric acid Pourbaix diagram for aluminium in pure water
Corrosion attack at the Al 6013/ SiC matrix interface
Transmission electron micrograph showing dislocation at the Al6013/SiC interface
Corrosion at the fibre/matrix interface progresses, continual attack at the interface leads to debonding of the fibre and metal matrix
(f) normal plane of Al2O3/AS41-Mg composite, polarised up to “ 900mV”, showed the resistance of matrix around the fibers “F” to attack
Microstructure of stainless steel produced by powder
metallurgy exposed to 50% wt H2SO4 solution during 48 hour, a) CAST AISI 316L b) AISI 316 L P/M
Optical micrografs of P/M 316 SS sintered at a)1250 C b) 1400 C
The microstructure of b) cast material and c) P/M material
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19
22 25 29 30 31 32 33 34 36 38 38
39
40
42
42
45
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2.19
3.1 3.2 3.3 3.4 3.5
3.6
3.7 3.8
3.9
4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10
4.11
Cross-sectional micrograph (perpendicular to the extrusion direction) of (a) AA6061-T6 Al alloy and (b) PM AlScZr alloy after immersion in aqueous chloride-peroxide solution Methodology of study
An experiment metal mould
The sintering profile for samples fabrication Schematic diagram for immersion test
Schematic diagram of potentiodynamic polarization experiment
Theoretical Tafel plots illustrating the Tafel polarization method
The profile shows the deviations (Yi) from the mean line
The profile shows the deviations (Yp) and (Yv) from the mean line
The profile shows the deviations (Ypi) and (Yvi) from the mean line
SEM micrograph of aluminium powder Particle size distribution of Al powder Mophology SEM of magnesium powder Particle size distribution of Mg powder
Mophology SEM of alumina Saffil short fibre The influence of magnesium content on bulk density The influence of magnesium content on apparent porosity The influence of magnesium content on hardness
The influence of magnesium content on linear shrinkage SEM micrographs of A, B, C, D, E and F with 0.5 wt%, 1.0 wt
%, 1.5 wt%, 2.0 wt%, 2.5 wt% and 3.0 wt% of Mg with1000x magnification
The influence of alumina Saffil™ short fibres content on bulk density
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48 52 53 58 60
61
63 64
65
67 67 68 68 69 70 71 72 73 74
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4.12
4.13
4.14
4.15
4.16 4.17 4.18 4.19
4.20 4.21
4.22
4.23
4.24
4.25
4.26
4.27
The influence of alumina Saffil™ short fibres content on apparent porosity
The influence alumina Saffil™ short fibres content on shrinkage
The influence alumina Saffil™ short fibres content on hardness
SEM micrographs of P, Q, R,T and S with 5wt%, 10wt%, 15wt%, 20wt% and 25 wt% of alumina Saffil™ short fibre with1000x magnification
The influence of Mg powders on density
The influence of Mg powders on apparent porosity The influence of Mg powders on hardness
SEM micrograph (a) composite without Mg(b) composite with 2 wt% of Mg with 10000x magnification
EDX analysis of PM Al-Mg composite
SEM photomicrographs of PM Al after exposure (a) 28 days (b) 56 days (c) 84 days with 1000x magnification 98
SEM morphology showing fine cracks observed on the surface of PM Al-Mg alloy after exposure (a) 28 days (b) 56 days and (c) 84 days with 1000x magnification
SEM morphology showing cracks developed on the surface of Al-Mg alloy after exposure (a) 28 days (b) 56 days and (c) 84 days with 5000x magnification
SEM photomicrograph showing cracks developed on the surface of PM Al composite after exposure (a) 28 days (b) 56 days (d) 84 days with 1000x magnification
SEM photomicrographs of PM Al-Mg composite after exposure (a) 28 days (b) 56 days (c) 84 days with 1000x magnification
SEM photomicrographs of PM Al-Mg composite after exposure (a) 28 days (b) 56 days and (c) 84 day with 5000x magnification
SEM morphology shows a decreasing in pitting size after exposure (a) 28 days (b) 56 days and (c) 84 days with 250x
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80 81 82 83
84 85
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4.28
4.29
4.30
4.31
4.32
4.33
4.34
4.35 4.36
magnification
SEM morphology shows a decreasing in pitting size after exposure (a) 28 days (b) 56 days and (c) 84 days with 150x magnification
XRD analysis of PM Al-Mg alloy (a) before immersion test (b) after 84 days exposure in 3.5 wt% NaCl solution
XRD analysis of PM Al-Mg composite (a) before immersion test (b) after 84 days exposure in 3.5 wt% NaCl solution XRD analysis of PM Al composite (a) before immersion test (b) after 84 days exposure in 3.5 wt% NaCl solution
Variation of corrosion rate ofPM Al, PM Al-Mg alloy, PM Al composite and PM Al-Mg composite in 3.5 wt% NaCl
Mass change of PM Al, PM Al-Mg alloy, PM Al composite and PM Al-Mg Composite
Tafel plots for (a) Al, (b) Al-Mg alloy, (c)Al composite and (d)Al-Mg composite in 3.5 wt% of NaCl
Potentiodynamic polarization plots for (a)PM Al, (b) PM Al- Mg alloy, (c)PM Al composite and (d) PM Al-Mg composite in 3.5 wt% of NaCl
Corrosion reactionof aluminium in 3.5 wt% NaCl solution
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100
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102
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LIST OF TABLES Table
1.1 2.1 2.2 2.3 2.4 2.5
3.1 3.2 3.3 3.4 3.5
4.1
4.2
4.3
4.4
4.5
Standard emf series of metals
Numerical designation of wrought aluminium alloy groups Numerical designation of cast aluminium alloy groups
Effects of various alloying elements on aluminium alloys
Properties of investigate composite material
Properties of the Saffil alumina fibre (RF grade) The properties of aluminium powder
Physical and chemical properties of magnesium powder Properties of the alumina SaffilTM short fibres grade RF Ratio percentage for each Al- Mg alloy composition Value of constant, K
Variation of oxygen intensity for PM Al sample after immersion test in 3.5 wt% NaCl
Variation of Mg and Al content in PM Al-Mg alloy after immersion test in 3.5 wt% NaCl
Variation of Mg and Al content in PM Al-Mg composite after immersion test in 3.5 wt% NaCl
The roughness parameter values Ra, Ry, Rq and Rz for PM Al, PM Al-Mg alloy, PM Al composite and PM Al-Mg composite before and after immersion
Electrochemical parameter of PM Al, PM Al-Mg, PM Al- composites and PM Al-Mg composite in 3.5 wt % NaCl
4 12 12 16 19 21 49 50 50 51 59 90
90
92
98
103
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List of abbreviations Al
AIN Ag Al2O3
Al Al1.67O4
Al (OH)3 AMC ASM ASTM Ca Cu EDX H2O HV Ltd Li Mg Mn MMC Nd NaCl P PM Sc SiN4
Si3Ni SCE SEM SiO2
SiC TiH2
TiC TEM XRD WC Zr
Aluminium
Aluminium nitride Silver
Alumina
Aluminum oxide Aluminium hydroxide Aluminium matrix composite American Society of Materials
American Society of Testing and Materials Calcium
Copper
Energy dispersive X-ray spectroscopy Water
Vickers hardness value Limited
Lithium Magnesium Manganese
Metal matrix composite Neodymium
Sodium Chloride Phosphorus
Powder metallurgy Scandium
Silicon nitride Silicon nickel
Saturated colamel electrode Scanning electron microscope Silica
Silica carbide Titanium hydride Titanium carbide
Transmission electron micrograph X-Ray Diffraction
Tungsten carbide Zirconium
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List of symbols A/cm2
wt%
∆G Ecorr
Icorr
mV MPa Rp
Rpm mg/cm2 µm/year
Ampere per centimetre square Weight percent
Change in free energy Corrosion potential Corrosion current density Micro Volt
Mega Pascal
Resistance polarization Rotation per minute
Milligram per centimetre square Micrometer per year
List of appendices
Appendix A EDX analysis for the %mass of Mg in PM Al-Mg composite Appendix B EDX spectrum of PM Al under nitrogen gas after exposure of 28, 56 and 84 days
Appendix C EDX spectrum of PM Al-Mg alloy under nitrogen gas exposed for 28, 56 and 84 days
Appendix D EDX spectrum of PM Al-Mg composite under nitrogen gas after exposure of 28, 56 and 84 days
Appendix E EDX spectrum of PM Al composite under nitrogen gas exposed for 28, 56 and 84 days
Appendix F The calculation to determine equivalent weight of sample
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LIST OF PUBLICATIONS
1. Normah Cheman, Mohd Nazree Derman, Nur Maizatul Shima Adzli, Mohd Zaherudin Kasmuin, Shamsul Baharin Jamaludin and Noraziana Parimin (2007), The effect of magnesium powders on PM Al-composites reinforced with short fibre alumina, National Metallurgical Conference, 26th – 27th November 2007, Puteri Pacific Hotel, Johor.
2. Normah Cheman, Mohd Nazree Derman, Nur Maizatul Shima Adzli, Mohd Zaherudin Kasmuin, Shamsul Baharin Jamaludin and Noraziana Parimin (2007), The fabrication and characterization of powder metallurgy aluminium – magnesium, 6th Asean Microscopy Conference, 10th – 12th December 2007, Cherating, Pahang.
3. Normah Cheman, Mohd Nazree Derman, Nur Maizatul Shima Adzli, Shamsul Baharin Jamaludin (2008), The fabrication and properties of PM Al-Mg composites reinforced with short fibre alumina, Malaysian Universities Conferences Engineering and Technology, 15th - 17th March 2008, Putra Palace, Perlis.
4. Normah Cheman, Mohd Nazree Derman, Nur Maizatul Shima Adzli (2008), Effects of alumina Saffil™ short fibres reinforcement on corrosion behaviour of Al composites prepared by powder metallurgy, 2nd International Conference for Young Chemists, 18th – 20th June 2008, Universiti Sains Malaysia.
5. Normah Cheman, Mohd Nazree Derman, Shamsul Baharin Jamaludin, Nur Maizatul Shima Adzli (2008), Effects of magnesium addition on corrosion behaviour of PM Al- Mg composites reinforced with alumina Saffil™ short fibres, Malaysian metallurgical Conference, 3th – 4th December 2008, Universiti Kebangsaan Malaysia.
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TINGKAH LAKU KAKISAN KOMPOSIT MATRIK ALUMINIUM YANG DIPERKUAT DENGAN GENTIAN PENDEK ALUMINA SAFFIL™
ABSTRAK
Kehadiran elemen pengaloian dan bahan penguat akan mempengaruhi tingkah laku kakisan komposit matrik aluminum. Tumpuan kajian ialah pengaruh penambahan Mg dan gentian pendek alumina Saffil™
dalam AMCs yang diperbuat melalui kaedah metalurgi serbuk terhadap tingkah laku kakisannya. Sampel untuk ujian kakisan iaitu komposit PM Al dan komposit PM Al-Mg telah diperbuat daripada serbuk Al berbentuk kepingan , serbuk Mg berbentuk butiran dan gentian pendek alumina Saffil™ yang mempunyai purata ukuran butir masing-masing ialah 70.58 µm, 273.41 µm and 16.71 μm. Parameter kajian semasa menghasilkan komposit PM Al dan komposit PM Al-Mg ialah kandungan Mg (dari 0.5% berat hingga 3.0% berat) dan kandungan gentian pendek alumina Saffil™ (dari 5% berat hingga 25% berat). Sampel diperbuat dengan menggunakan tekanan searah sebanyak 200 MPa dan disinter dalam gas argon selama 6 jam. Sampel yang telah disinter dipercirikan dengan ujian mikrostruktur, ujian ketumpatan dan keliangan dan ujian kekerasan Vickers. Dua jenis ujian kakisan iaitu ujian rendaman dan ujian elektrokimia telah dijalankan ke atas komposit yang optima. Kedua-dua ujian tersebut menggunakan larutan 3.5% berat natrium klorida (NaCl) sebagai persekitaran kakisan. Lengkung pengutuban telah dilukis berdasarkan data dari ujian elektrokimia dengan menggunakan potentiostat jenis Gamry G300 yang telah dijalankan.
Ujian redaman dijalankan selama 28, 56 dan 84 hari. Graf kehilangan berat sampel ujian rendaman dilukis berdasarkan data dari ujian rendaman. Photomikrograf sampel sebelum dan selepas ujian rendaman juga dinilai dengan menggunakan mikroskop biasan elektron (SEM). Elemen komposisi yang wujud dalam sampel dianalisa dengan menggunakan tenaga penyebaran sinar-X (EDX). Kehadiran fasa oxid dalam lapisan oxid di kenalpasti denagn analisis fasa pembelauan sinar-X (XRD). Daripada penyelidikan ini, menunjukkan penambahan magnesium dalam komposit PM Al telah dapat memperbaiki kebasahan di antara gentian pendek alumina Saffil™ dan matrik Al. Komposit yang optima ialah PM Al-15% berat Al2O3 dan PM Al-2%berat Mg-15% berat Al2O3. Kadar kakisan komposit PM Al pula menurun dari 67.69 µm/tahun ke 0.66 µm/tahun apabila 2% berat magnesium dimasukkan ke dalam komposit PM Al-Mg. Micrograf SEM pula menunjukkan kakisan bopeng berlaku di kawasan tumpuan juzuk Mg yang terdapat di permukaan komposit PM Al-Mg. Fasa aluminum oxid (δ-Al Al1.67O4), aluminium hydroxid (Al (OH)3) and aluminium oxid (α-Al2O3) telah dikenalpasti wujud dalam lapisan oxid. Ketumpatan arus kritikal (Icrit) komposit PM Al (2.0 x 10-1 A/cm2) dimana ianya lebih kecil berbanding dengan ketumpatan arus kritikal (Icrit) komposit PM Al-Mg ( 4.0 x10-1 A/cm2). Magnesium bertindak sebagai perencat kepada tingkah laku kakisan dalam komposit PM Al-Mg dengan menukar dan melindungi komposit dari pembentukan lapisan oksid yang stabil. Rintangan kakisan komposit PM Al berkurang kerana pembentukan lapisan oksid dipermukaan tidak sekata dengan kehadiran gentian pendek alumina Saffil™ dalam komposit dan ini menyediakan tempat permulaan kakisan berlaku pada komposit.
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CORROSION BEHAVIOUR OF POWDER METALLURGY ALUMINIUM MATRIX COMPOSITE REINFORCED WITH ALUMINA SAFFIL™ SHORT FIBRES
ABSTRACT
The corrosion behaviour of PM Al composite will be affected by the presence of alloying element and reinforcing phase in the composite. This studies were focused on the influence of magnesium addition and alumina Saffil™ short fibres reinforcement on corrosion behaviour of PM AMCs. PM Al composite and PM Al-Mg composite were fabricated from flaky aluminium powder, granular magnesium and alumina Saffil™ short fibres with an average particle size of 70.58 µm, 273.41 µm and 16.71 μm, respectively. The experimental parameter were maintained the content of magnesium powder (ranging from 0.5 wt% to 3.0 wt%) and alumina Saffil™ short fibres (ranging from 5 wt% to 25 wt%) . The samples were fabricated by using 200 MPa uniaxial pressing and sintered in argon atmosphera for 6 hours. Sintered samples were characterized by using SEM, density and porosity analysis and Vickers hardness testing. The optimum samples were tested in corrosion properties by using electrochemical test and immersion test. Both experiment using 3.5 wt% NaCl as corrosion environment. Electrochemical test was carried out with apply potentiostat Gamry G300 to create Tafel plot and polarization curve. The immersion test was carried out for 28, 56 and 84 days. Weight loss curves were acquired from immersion test results. Photomicrographs of samples were examined using scanning electron microscopy (SEM).
The elemental composition of sample was analyzed by Energy dispersive X-ray (EDX) analysis. The presence of an oxide phases were verified by X-ray diffraction analysis (XRD). Experimental results showed the wetability between alumina Saffil™ short fibres and Al matrix have been improved by adding magnesium in the PM Al composite. PM Al – 15 wt% Al2O3 and PM Al – 2 wt% Mg - 15 wt% Al2O3 were identified as the optimum sample. The corrosion rate of PM Al composite was decreased from 67.69 µm/year to 0.66 µm/year when 2 wt% of magnesium was added in PM Al-Mg composite. SEM photomicrograph observed that pitting was localized surrounding the magnesium constituents which occur randomly throughout the surface of PM Al-Mg composite. XRD analysis revealed that aluminum oxide (δ-Al Al1.67O4), aluminium hydroxide (Al (OH)3) and aluminium oxide (α-Al2O3) phases were detected in oxide film. The critical current density (Icrit) of PM Al composite is lower than PM Al-Mg composite which is 2.0 x 10-1 A/cm2 and 4.0 x10-1 A/cm2, respectively. Magnesium is used as inhibitor in corrosion behaviour of PM Al-Mg composite have been changed and protected with a stable formation of oxide film. The corrosion resistance of PM Al-composite was decreased with the presence of alumina Saffil™ short fibres in composite due to discontinuities in oxide film provided the corrosion initiated in sample.
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CHAPTER 1
INTRODUCTION
1.1 Research introduction
Metal matrix composites (MMCs) are under attention for many applications in aerospace and automobile industries. MMCs are classified by the type of metal which is involved matrix and reinforcement. Aluminium, titanium and magnesium are widely used as matrix materials. Meanwhile, silica carbide (SiC) and alumina (Al2O3) are the most commonly used as reinforcement. Among MMCs, aluminium matrix composites (AMCs) are being the main interest due to the superior properties such as light weight, high strength, high specific modulus, low coefficient of thermal expansion, good wear resistance properties as compared with monolithic alloys.
Major fabrication methods used for aluminium matrix composites are powder metallurgy, casting and spray deposition. The powder metallurgy (PM) technique for manufacturing AMCs offer some advantages compared with casting. The main advantage is the low manufacturing temperature that avoids strong interfacial reaction and minimizing the undesired reactions between the matrix and the reinforcement (Torralba et al., 2003; William and Harrigan, 1998). PM technique also efficient in energy operation, less anisotropy, easy for automation and economically for certain type of products for example small components which would ordinary require a prohibitive a mount of machining and material waste.
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AMCs via powder metallurgy technique is produced the complex, near net-shape and lightweight component. The demands for lightweight metals are increased tremendously especially in automotive industry. Lightweight materials are used as automotive components for reducing fuel consumption. Consequently, annual production of aluminium PM blended is expected will increase from 1200 tonnages in 1998 to 25000 tonnages within 10 years (Bishop et al., 2000).
AMCs have developed rapidly since the first appeared in 1960s. A number of researchers have attempted to enhance AMCs by adding various of alloying elements and reinforcements. The alloying elements such as magnesium, copper, silicon and manganese are usually incorporated into aluminium alloys. Meanwhile, ceramic particles and short fibres reinforcements such as SiC and Al2O3 are mostly used in AMCs. Indeed, short fibres reinforcements are used as engineering materials because the composites have good properties at low cost as compared to particles reinforcements.
In this research, powder metallurgy technique has been chosen to fabricate Al-Mg composite reinforced with short fibre alumina Saffil™. PM Al-Mg composite have a good properties such as near net-shape, light weight, have a low coefficient of thermal expansion and excellent mechanical properties as compared with aluminium alloys.
However, the corrosion resistance of PM Al-Mg composites are lower than pure aluminium due to magnesium alloying and short fibres alumina Saffil™ reinforcement.
The previous researchers such as Tiwari et al. (2006), Bakkar and Neurbert (2007) have found that corrosion problems likely occurred in AMCs is galvanic coupling between
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the reinforcing phase and matrix and the selective corrosion at the reinforcement and matrix interface. Thus, the addition of magnesium and short fibre alumina Saffil™ in Al-Mg composites will be influenced the corrosion behaviour. Moreover, the porosity in composites will be arising from powder metallurgy technique also can alter the corrosion behaviour of Al-Mg composites.
1.2The effect of magnesium addition in Al-Mg composites on corrosion behaviour
Magnesium is one of the lightest metallic materials available and has the lowest density.
Magnesium has natural affinity for wetting or bonding to reinforcing materials. On the contrary magnesium has low stiffness, low strength, low wear resistance and high coefficient of thermal expansion.
The addition of magnesium in Al-composite and Al alloy can improve wettability.
Wetting is an important aspect in AMCs synthesis for formatting strong chemical bonds at the interface between reinforcement and matrix. Asavavisithchai and Kennedy (2006) and Schaffer et al. (2001) were supported that the wettability of PM Al composite reinforced with alumina (Al2 O3) particles and sintered Al-Mg alloy have been improved by adding the highly reactive magnesium.
However, magnesium addition in Al-Mg composites can cause galvanic corrosion because two dissimilar metal in contact with each other. Magnesium has been chosen as alloying elements in this study for reducing galvanic corrosion potential because magnesium and aluminium are closer in EMF Series as shown in Table 1.1. Magnesium also perform as good as aluminium and better than mild steel in rural and mild industrial
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environments but magnesium frequently suffer severe damage in wet and salt- containing environments (Eliezer and Alves, 2002).
Table 1.1: Standard Electrode Potential (EMF) series of metals (Fontana, 1987)
Metal-metal ion
Equilibrium (unit activity)
Electrode potential Vs. normal hydrogen Electrode at 25oC, volts
(V vs SHE) Au-Au +3
Pt-Pt+2 Pd-Pd+2 Ag-Ag+ Hg-Hg2+2
Cu-Cu+2 H2-H+ Pb-Pb+2 Sn-Sn+2 Ni-Ni+2 Co-Co+2 Cd-Cd+2 Fe-Fe+2 Cr-Cr+3 Zn-Zn+2 Al-Al+3 Mg-Mg+2 Na-Na+ K-K+
+1.498 +1.2 +0.987 +0.799 +0.788 +0.337 0.000 -0.126 -0.136 -0.250 -0.277 -0.403 -0.440 -0.744 -0.763 -1.662 -2.363 -2.714 -2.925
1.3The effect of short fibres alumina Saffil™ reinforcement on corrosion behaviour.
The incorporation of second reinforcing phase into metal matrix composite not also enhances the physical and mechanical properties of a material, but it also can alter the material’s corrosion behaviour. The presence of the short fibres alumina Saffil™
reinforcement will be accelerated the corrosion in Al-Mg composites as compared to
4 Noble or
Cathodic
Active or Anodic
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unreinforced Al alloy. Accelerated corrosion is originated from electrochemical and chemical interaction between Al-Mg composite constituents and microstructural effects.
According to Winkler and Flower (2004), corrosion will be affected by reinforcement addition in MMCs. A reaction between the reinforcement and metal matrix occurs during the processing of metal matrix composite and formed an interfacial phase that binds the two materials together. Thus, preferential corrosion will be occurred at the interfacial phase which is strongly anodic or cathodic.
Pardo et al. (2004), also reported that thermal stress arise at the interfaces between the matrix and the reinforcement due to considerable mismatch the thermal expansion coefficient when a metal matrix composite is subjected to temperature changes. As a result a dislocation at interface is generated. The dislocation could lead to further discontinuities in the oxide film, increasing the number of sites where corrosion can be initiated and rendering the composite liable to severe attack. Meanwhile, Zhang et al.
(2007), Dobrzanski et al. (2005) and Datta et al. (2004) were founds that the corrosion attacked adjacent to the fibers in MMCs.
1.4 The effect of the powder metallurgy process on corrosion behaviour The corrosion behaviour will be affected by changing a microstructure of composites and the presence of defects from manufacturing processes. Yoshiaki et al. (1995) have found that manufacturing parameter can affect the corrosion behaviour. Bakkar and Neubert (2007), also have been reported that microstructure changes and process contaminants resulted from manufacture of the MMCs.
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Powder metallurgy is one of a method to manufacture Al-Mg composites. However, processing materials through conventional powder metallurgy method will be resulted the presence of such micro pores (Chandramouli et al., 2007) and defects in Al-Mg composite were occurred during compaction and sintering process (Updhyaya, 2000).
The tenacious oxide film over the aluminium particles will be ruptured during compaction process, thus resulting in metal-to-metal contact. Meanwhile, during sintering, aluminium expands more than aluminium oxide layer over particles. As a result, the hair-line cracks will be developed on the oxide layer.
The micro pores and defects will be influenced the corrosion behaviour of Al-Mg composites. Silva-Maia et al. (1999) and Hosni et al. (2008) has been reported that corrosions are closely associated to the presence of heterogeneities such as porosity, voids, or precipitates. Based on the study from Shankar et al. (2004), PM stainless steel applications were limited due to the relatively poor mechanical and corrosion properties when compared to wrought products.
According to Pardo (2004), a research on corrosion properties of aluminium matrix composites is still at development stage. Thus, the purpose of this research is to study the corrosion behaviour of PM Al-Composites reinforced with short fibres alumina SaffilTM. The study will be focused on the effects of alloying magnesium and alumina Saffil™ reinforcement on corrosion behaviour of Al-Mg composites.
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