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Formability of Magnesium AZ80 Muhammad Waseem Soomro A Formability of Magnesium AZ80 Muhammad Waseem Soomro A thesis submitted to Auckland University of Technology in fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) 2016 School of Engineering, Computer and Mathematical Sciences Declaration I hereby declare that this submission is my own work and that to the best of my knowledge and belief, it contains no material previously published or written by another person (except where explicit has been defined in the acknowledgements), nor material which to a substantial extent has been submitted for the award of any other degree or diploma of a university or other institution of higher learning. Auckland 2016 Muhammad Waseem Soomro ii Abstract Economical and fuel-efficient vehicles are a current topic of interest worldwide, due to increasing fuel costs and environmental concerns. The most strident demand from various auto-sector stakeholders is to reduce emissions from vehicles and keep the environment clean. To solve this issue we need to reduce vehicle weight to lower fuel consumption. In this context, magnesium alloys can be considered as an alternative for sheet metal components, as they are 35 percent lighter than aluminium alloys and 78 percent lighter than steel. The AZ magnesium alloy series is widely used by the modern light metal industry. In this research, AZ80 magnesium alloy was selected. AZ80 is an important structural wrought-magnesium alloy with a high aluminium content of about 8 wt. %. AZ80 is the main subject of this study because it offers higher strength and greater hardness than the more widely used AZ31. In this research, magnesium AZ80 alloys with two different grain sizes were considered, that is fine grain AZ80 (grain size ≈ 10 µm) and coarse grain AZ80 (grain size ≈ 60 µm). These grain sizes were chosen for investigations of the effect of grain size at higher temperatures and to observe changes in mechanical and forming capabilities with respect to grain size. To investigate the formability characteristics of magnesium alloys, various tensile tests and deep drawing tests were performed at different temperatures, test speeds and grain sizes to understand the nature of the material. Anisotropy of material, strain sensitivity index and flow stress were also determined by varying all of these parameters. Moreover, load-displacement diagrams, forming-limit diagrams and the effects of variations in various pre- and-post processing parameters were also examined. A further part of this research was to examine microstructure changes through the use of microscopic images at high magnification by utilising optical and scanning electron microscopes. It was observed that the microstructure of magnesium alloys was extremely sensitive to processing parameters in tensile tests and deep-drawing tests. i The final part of the research was the verification of mathematical modelling and simulation of metal forming processes by using the commercial FEA software package Abaqus. ii Contents Abstract……………….....................................................................................................i List of figures……………………………………………………………………...........x List of tables…………………………………………………...…………………......xvii List of symbols……………………………………………...…………………….…xviii Acknowledgement………………………………………………………………….....xxi Chapter 1 ......................................................................................................................... 1 Introduction ..................................................................................................................... 1 1.1 Definition of the problem ........................................................................................ 1 1.2 Selection of material ............................................................................................... 4 1.3 Testing equipment, calibration and standards ......................................................... 6 1.4 Forming issues at elevated temperatures................................................................. 7 1.4.1 Experimental setup development ......................................................................... 8 1.4.2 Examination of the basic forming parameters and results from literature ........... 8 1.4.3 Construction of forming limit curves (FLC) ........................................................ 9 1.5 Research limitations .............................................................................................. 11 1.6 Scope and aspirations ............................................................................................ 11 Chapter 2 ....................................................................................................................... 15 Characteristics and Properties of Magnesium Alloys................................................ 15 2.1 Magnesium Alloys ................................................................................................ 15 2.2 Classification and designations ............................................................................. 16 2.2.1 Cast magnesium alloys ....................................................................................... 18 2.2.2 Wrought magnesium alloys ............................................................................... 22 2.3 Role and influences of alloying constituents ........................................................ 25 2.3.1 Aluminium ......................................................................................................... 25 iii 2.3.2 Zinc .................................................................................................................... 26 2.3.3 Manganese.......................................................................................................... 28 2.4 Deformation theories for HCP materials .............................................................. 29 2.4.1 Slip ..................................................................................................................... 29 2.4.2 Twinning ............................................................................................................ 33 2.5Texture development in magnesium alloys ........................................................... 35 2.6 Superplasticity ....................................................................................................... 37 2.7 Diffusion creep ...................................................................................................... 40 2.8 Grain boundary sliding .......................................................................................... 42 2.9 Recrystallisation in magnesium alloys .................................................................. 45 2.10 Grain growth ....................................................................................................... 45 2.11 Cavitation ............................................................................................................ 46 2.12 Conclusion .......................................................................................................... 47 Chapter 3 ....................................................................................................................... 48 Formability of magnesium alloys ................................................................................. 48 3.1 Introduction ........................................................................................................... 48 3.2 Problems of elevated temperature processing ....................................................... 48 3.2.1 Sensitivity of elevated temperature forming towards strain rate ....................... 49 3.2.2 Grain size effects on SPF of magnesium alloys ................................................. 50 3.2.3 Influence of temperature on SPF of Mg alloys .................................................. 53 3.3 Formability of magnesium alloys.......................................................................... 54 3.3.1 Experimental setup development ....................................................................... 55 3.3.2 Examination of the basic forming parameters and results ................................. 56 3.3.2.1 Influence of punch speed in forming of magnesium alloys ........................ 58 3.3.2.2 Influence of the punch force in forming magnesium alloys ....................... 59 3.3.2.3 Influence of the blank holder force in forming magnesium alloys ............. 60 3.3.2.4 Influence of the texture (anisotropy) in forming of magnesium alloys ...... 60 iv 3.3.2.5 Influence of temperature in forming magnesium alloys ............................. 61 3.3.2.6 Temperature distribution during forming of magnesium alloys ................. 62 3.3.2.7 Thickness distribution during forming of magnesium alloys ..................... 63 3.3.2.7 Spring-back properties in forming of magnesium alloys ............................ 64 3.3.2.8 Tooling geometries influences in forming magnesium alloys .................... 65 3.3.2.9 Other influences .......................................................................................... 66 3.4 Construction of forming limit curves (FLC) ......................................................... 66 3.5 Strain measurement ........................................................................................... 71 3.6 Constitutive analysis ............................................................................................. 71 3.6.1 Phenomenological models
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