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Influence of Microstructure on the Corrosion Behaviour of Magnesium Alloys

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Influence of Microstructure on the Corrosion Behaviour of Magnesium Alloys 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 - 2 - 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 - 3 - 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 - 4 - 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 - 5 - 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 - 6 - 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
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