influence of microstructure on corrosion behaviour of magnesium alloys

306 Pages·2011·11.48 MB·English

INFLUENCE OF MICROSTRUCTURE ON THE
CORROSION BEHAVIOUR OF MAGNESIUM ALLOYS

A thesis submitted to The University of Manchester for the degree of
Doctor of Philosophy (PhD)
in the Faculty of Engineering and Physical Sciences

SURAJKUMAR GANPAT PAWAR

2011
School of Materials
CONTENTS

List of contents………………………………………………………………….......2
List of figures………………………………………………………………….…....8
List of tables………………………………………………………………….……21
List of abbreviations………………………………………………………….…...23
Abstract……………………………………………………………………………24
Declaration………………………………………………………………………...25
Copyright Statement……………………………………………………………...26
Acknowledgements………………………………………………………………..27

Chapter 1 Introduction
1.1 Need of alloy development………………………………………………………...31
1.2 Focus of research…………………………………………………………………..32
1.3 Layout of the thesis………………………………………………………………...34

Chapter 2 Literature Survey
2.1 Introduction………………………………………...………………………………40
2.1.1 Physical metallurgy of magnesium………………………………………………...41
2.1.2 Solid solution hardening of magnesium…………………………………………...41
2.1.3 Physical and chemical properties of magnesium…………………………………..42
2.2 Magnesium-aluminium system…………………………………………………….42
2.2.1 Alloy designations and tempers……………………………………………………43
2.2.2 Alloying behaviour………………………………………………………………...43
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2.2.2.1 Effect of addition of aluminium…………………………………………...44
2.2.2.2 Effect of addition of zinc…………………………………………………..45
2.2.2.3 Effect of addition of manganese…………………………………………...45
2.3 Casting of magnesium alloys………………………………………………………46
2.4 Solidification behaviour and microstructure of (Mg-Al) magnesium alloys………46
2.5 Twin roll casting (TRC) process…………………………………………………...49
2.5.1 Introduction………………………………………………………………………...49
2.5.2 Principle of the TRC process………………………………………………………50
2.5.3 Defects produced during TRC processing…………………………………………50
2.5.3.1 Introduction………………………………………………………………...50
2.5.3.2 Centre-line segregation…………………………………………………….51
2.5.3.3 Edge cracking………………………………………………………………51
2.6 Melt-conditioned twin roll casting (MCTRC) process…………………………….51
2.6.1 Introduction………………………………………………………………………...51
2.6.2 Principle of the MCTRC process…………………………………………………..52
2.6.3 MCAST process……………………………………………………………………52
2.6.4 Grain refinement through enhanced heterogeneous nucleation……………………53
2.7 Welding…………………………………………………………………………….55
2.7.1 Introduction………………………………………………………………………...55
2.7.2 Friction stir welding (FSW) process……………………………………………….55
2.7.3 Advantages of FSW process……………………………………………………….56
2.7.4 Friction stir weld microstructure…………………………………………………...57
2.7.5 Friction sir welding parameters…………………………………………………….59
2.7.5.1 Tool rotation rate (W) and welding speed (V)……………………………..59
2.7.5.2 Welding pressure…………………………………………………………..60
2.7.5.3 Tilt angle…………………………………………………………………...60
2.7.5.4 Insertion depth……………………………………………………………..61
2.7.5.5 Preheating or cooling………………………………………………………61
2.7.6 Material flow in FSW process……………………………………………………..61
2.7.7 Flaws in friction stir welding………………………………………………………62
2.7.7.1 Voids……………………………………………………………………….62
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2.7.7.2 Joint line remnant…………………………………………………………..63
2.7.7.3 Lack of penetration…………………………………………………….......63
2.7.7.4 Other flaw types…………………………………………………………....64
2.8 Corrosion of magnesium and its alloys…………………………………………….65
2.8.1 Corrosion (pure magnesium)………………………………………………………65
2.8.2 The negative difference effect (NDE)……………………………………………...67
2.8.3 Kinetics of film formation…………………………………………………………69
2.8.4 Impurity elements………………………………………………………………….70
2.8.5 Corrosion of magnesium alloys……………………………………………………70
2.8.6 Types of corrosion…………………………………………………………………72
2.8.6.1 Galvanic corrosion…………………………………………………………72
2.8.6.2 Localized corrosion………………………………………………………...74
2.8.6.3 Intergranular corrosion……………………………………………………..75
2.8.6.4 Stress corrosion cracking…………………………………………………..76
2.8.6.5 Corrosion fatigue…………………………………………………………..76
2.8.7 Factors influencing the corrosion behaviour……………………………………….77
2.8.7.1 Influence of principal alloying elements…………………………………...77
2.8.7.2 Influence of impurity elements…………………………………………….78
2.8.7.3 Influence of second phases………………………………………………...80
2.8.7.4 Influence of microstructure………………………………………………...81
2.8.7.5 Influence of environment…………………………………………………..83

Chapter 3 Experimental Procedure
3.1 General introduction……………………………………………………………...105
3.2 Specimen preparation……………………………………………………………..106
3.3 Optical microscopy (OM)………………………………………………………...107
3.4 Scanning electron microscopy (SEM)……………………………………………107
3.4.1 Introduction……………………………………………………………………….107
3.4.2 High resolution SEM……………………………………………………………..108
3.4.3 Electron backscattered diffraction (EBSD)……………………………………….109
3.5 Transmission electron microscopy (TEM)……………………………………….109
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3.5.1 Introduction……………………………………………………………………….109
3.5.2 High resolution TEM……………………………………………………………..110
3.6 Precision Ion Beam Polishing (PIPS)…………………………………………….111
3.7 Scanning Kelvin probe force microscopy (SKPFM)……………………………..112
3.8 Corrosion performance assessment……………………………………………….115
3.9 Electrochemical measurements…………………………………………………...115

Chapter 4 Microstructures of AZ Series Magnesium Alloys
4.1 Introduction……………………………………………………………………….120
4.2 Microstructure investigation of TRC and MCTRC magnesium alloys…………..121
4.2.1 Microstructure characterization of AZ31 magnesium alloys……………………..121
4.2.2 Microstructure characterization of AZ61 magnesium alloys……………………..125
4.2.3 Microstructure characterization of AZ91 magnesium alloys……………………..127
4.3 Defects produced during twin roll casting………………………………………..131
4.3.1 Centre-line segregation…………………………………………………………...131
4.3.2 Localized plastic deformation/shear bands……………………………………….132
4.3.3 Hot cracking/tear………………………………………………………………….133
4.4 Microstructure of downstream processed AZ31 magnesium alloys……………...133
4.5 Discussion………………………………………………………………………...137
4.5.1 Solidification behaviour of MCTRC magnesium alloys…………………………137
4.5.2 Influence of casting processes in the as-cast microstructure……………………..139
4.5.2.1 Grain size…………………………………………………………………139
4.5.2.2 Eutectic morphology, composition and precipitation…………………….140
4.5.2.3 Microconstituents and intermetallic morphology………………………...143
4.5.3 Influence of downstream processing on AZ31 magnesium alloys……………….144
4.5.3.1 Microstructure…………………………………………………………….145
4.5.3.2 Texture……………………………………………………………………146
4.6 Summary………………………………………………………………………….146

Chapter 5 Corrosion Behaviour of AZ Series Magnesium Alloys
5.1 Introduction……………………………………………………………………….177
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5.2 Scanning Kelvin probe force microscopy (SKPFM)……………………………..179
5.2.1 AZ31 magnesium alloys………………………………………………………….180
5.2.2 AZ61 magnesium alloys………………………………………………………….182
5.2.3 AZ91 magnesium alloys………………………………………………………….183
5.3 Corrosion of as-cast TRC and MCTRC magnesium alloys………………………184
5.3.1 AZ31 magnesium alloys………………………………………………………….184
5.3.2 AZ61 magnesium alloys………………………………………………………….187
5.3.3 AZ91 magnesium alloys………………………………………………………….188
5.4 Electrochemical measurements of as-cast TRC and MCTRC magnesium alloys.190
5.5 Corrosion of downstream processed AZ31 magnesium alloys……….......………191
5.6 Electrochemical measurements of downstream processed AZ31 alloys. ..............192
5.7 Discussion...............................................................................................................193
5.7.1 Influence of microstructure on the corrosion behaviour.........................................194
5.7.2 Influence of microconstituents on corrosion behaviour..........................................198
5.8 Corrosion mechanism.............................................................................................200
5.9 Summary.................................................................................................................205

Chapter 6 Friction Stir Welding of Magnesium Alloys
6.1 Introduction……………………………………………………………………….235
6.2 Microstructure of FSW AM60 magnesium alloy………………………………...236
6.2.1 Parent alloy microstructure: Die-cast AM60 magnesium alloy…………………..236
6.2.2 Specimen A: FSW parameter (W/V) ratio 1.5……………………………………237
6.2.3 Specimen B: FSW parameter (W/V) ratio 1.2……………………………………239
6.2.4 Specimen C: FSW parameter (W/V) ratio 1.0……………………………………240
6.2.5 Grain size variations in different weld zones……………………………………..241
6.3 Microstructure of FSW AZ91 magnesium alloy………………………………….242
6.3.1 Parent alloy microstructure: Die-cast AZ91 magnesium alloy…………………...242
6.3.2 Specimen D: FSW parameter (W/V) ratio 1.2……………………………………243
6.3.3 Specimen E: FSW parameter (W/V) ratio 1.0………………………....................244
6.3.4 Specimen F: FSW parameter (W/V) ratio 0.85…………………………………...245
6.3.5 Grain size variations in different weld zones……………………………………..246
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6.4 Corrosion studies of FSW AM60 magnesium alloy………………………...……246
6.4.1 Electrochemical measurements of FSW AM60 magnesium alloys………………246
6.4.2 SKPFM investigation of FSW AM60 magnesium alloys……………………...…248
6.4.3 Corrosion testing of FSW AM60 magnesium alloys……………………………..249
6.5 Corrosion studies of FSW AZ91 magnesium alloy………………………………250
6.5.1 Electrochemical measurements of FSW AZ91 magnesium alloys……………….250
6.5.2 SKPFM investigation of FSW AZ91 magnesium alloys…………………………251
6.5.3 Corrosion testing of FSW AZ91 magnesium alloys……………………………...252
6.6 Discussion………………………………………………………………………...252
6.7 Summary…………………………………………………………………….........259

Chapter 7 General Conclusions and Future Work
7.1 General summary and conclusions……………………………………………….287
7.1.1 Microstructures of AZ series magnesium alloys………………………………….278
7.1.2 Corrosion behaviour of AZ series magnesium alloys…………………………….289
7.1.3 Friction stir welding of magnesium alloys………………………………………..290
7.2 Suggestions for future work………………………………………………………292

References………………………………………………………………………………294

Final word count: 64,073 words

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LIST OF FIGURES

Chapter 1
Fig.1.1 Breakdown of the usage of magnesium in the western world in 1999 [6].
Fig.1.2 Some industrial applications of magnesium alloys.
Fig.1.3 Directions of alloy development to improve the performance of magnesium
components [174].
Fig.1.4 AZ91magnesium alloy specimens showing the development in microstructure from
conventional casting to MCTRC processing.

Chapter 2
Fig.2.1 Microstructure of magnesium [current research].
Fig.2.2 Principal planes (I, II) and directions (III) in the magnesium hcp unit cell. [Polmear
2006]
Fig.2.3 A single hcp lattice with (A) Miller indices notation for the coordinate system, and
examples of (B) basal slip, (C) prismatic slip, and (D) pyramidal slip [5]
Fig.2.4 Section of binary phase diagram for Mg-Al binary alloy [7].
Fig.2.5 The Mg-rich portion of Mg–Al phase diagram with marked chemistries and
preheating temperatures of alloys investigated.
Fig.2.6 Effect of Al and Zn contents (wt.%) on die castability of Mg–Al–Zn alloys [19].
Fig.2.7 The effect of aluminium content, zinc content and cooling rate on eutectic
morphology in permanent mould cast hypoeutectic Mg-Al alloys [7].
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Fig.2.8 Grain size vs. manganese addition for high purity Mg–3%Al, Mg–6%Al & Mg–
9%Al alloys [12].
Fig.2.9 Microstructures of Mg–Al alloys as-cast and after solution heat-treatment [11].
Fig.2.10 Metallographic photos of Mg–Zn alloys as-cast and after heat-treatment [11].
Fig.2.11 Micrographs of magnesium-aluminum alloys with increasing aluminum content.
The transition from a globular dendritic structure to a fully developed dendritic
structure with increasing aluminium content is readily noticeable [7].
Fig.2.12 Possible morphologies of Mg-Al Eutectic [17].
Fig. 2.13 Schematic diagram for solidification process in terms of microstructure of AZ91
magnesium alloy [Fan et. al. 2009, Ohno et. al. 2006, Dahle et. al. 2001].
Fig.2.14 Schematic diagram for the conventional eutectic (I) and the non-conventional
eutectic (II) formation in the AZ91 magnesium alloy.
Fig.2.15 Schematic diagram of the TRC machine used for strip casting of Mg alloys.
Fig.2.16 A schematic diagram of the ‗Melt Conditioned Twin-roll casting‘ (MC-TRC) as a
novel process used for production of high quality Mg alloys strips, [BCAST, Brunel
University].
Fig.2.17 Schematic diagram of the ‗MCAST‘ machine used in MC-TRC process [BCAST,
Brunel University]
Fig.2.18 Schematic diagram of FSW process
Fig.2.19 Schematic cross-section of a typical FSW weld showing four distinct zones: (A)
base metal, (B) heat-affected, (C) thermomechanically affected and (D) stirred
(nugget) zone.
Fig.2.20 (a) Metal flow patterns and (b) metallurgical processing zones developed during
friction stir welding (after Arbegast [210]).
Fig.2.21 Characteristic defect types observed at variable FSW parameters in friction stir
welds [252].
Fig.2.22 Potential-pH diagram for the Mg-H O system at 25°C [88].
2
Fig.2.23 (a) Potential-pH diagram for the Mg-Al alloy system at 25°C
(b) pH range showing corrosion and passivation zones for Mg-Al alloys [253].
Fig.2.24 The negative difference effect (NDE [67].
Fig.2.25 a) External galvanic corrosion. b) Internal galvanic corrosion [67].
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Fig.2.26 Schematic representation of overall corrosion where the β-phase acts as a
corrosion barrier; (A) Initial surface, (B) Final surface [67].
Fig.2.27 Schematic diagram presenting the undermining of the β-phase, eventually after the
preferential corrosion occurring in the primary α and eutectic α–phase [67].
Fig.2.28 Corrosion rate for binary alloys exposed for 16 weeks to alternate immersion in
3% NaCl (30 s in solution; 2 min in air) [92].
Fig.2.29 Generalized curve showing the influence of element X on the corrosion rate of
magnesium: X = Fe, Ni, Cu [67].

Chapter 3
Fig. 3.1 Schematic representation of sectioning the specimens for microstructure and
texture characterization; A-Surface examination, B-Transverse section, C–Texture
study (EBSD)
Fig.3.2 Schematic representation of a typical SEM (11)
Fig. 3.3 Signal detection in SEM.
Fig.3.4 Schematic representation of electron beam interactions in HRSEM.
Fig.3.5 Schematic representation of a typical TEM (15)
Fig. 3.6 Schematic illustration of the PIPS™ work chamber, showing the Penning ion gun,
the specimen exchange mechanism, the Faraday cup, the airlock and the specimen
post.
Fig. 3.7 (A) The cantilever in the AFM, (B) Atomic forces between the scanning tip of the
AFM and the specimen surface.

Chapter 4
Fig. 4.1Plane polarized light optical micrographs of transverse sections of AZ31
magnesium alloy; (image taken at Brunel University); (A) MCTRC processed alloy,
(B) TRC processed alloy showing (I & V) chill zone microstructure, evidence of hot
tearing shown by arrows (II & IV) Columnar zone microstructure with coarse
columnar grains, (III) Equiaxed zone, Evidence of centre-line segregation, indicated
by arrow
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