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|Title:||Material Processing and Forming Approaches for Enhancing Room Temperature Formability of Automotive Mg Sheet|
|Keywords:||AZ31, formability, annealing, pre-straining, deformation, twins|
|Abstract:||Automotive magnesium sheets typically exhibit poor room temperature ductility which makes them unsuitable for room temperature sheet stamping applications. This research involved aspects of re-processing and forming of AZ31 automotive magnesium sheet to improve its room temperature ductility and bendability (and, more generally, formability). The sheet re-processing studies for formability improvement were carried out by two different methods, (i) cyclic bending-unbending and annealing (or CBUA) and (ii) wire brushing and annealing (or WBA). These two processing methods led to a complex stress and strain distribution through the thickness and a multi-layered microstructure after annealing. The grain structure, micro-texture, and micro-hardness of each of the layers were studied by optical microscopy, electron back-scattered diffraction (EBSD) and indentation measurements, respectively. The through-thickness grain structure study indicated grain refinement and texture randomization in the surface layers for both CBUA and WBA processed materials. In addition, the as-received (and fully annealed) sheet as well as processed materials were subsequently deformed in uniaxial tension and bending by a process referred to in the literature as pre-strain annealing (or PSA). The PSA process was studied as a single step as well as multi-step process to assess its effect on formability improvement, underlying changes in microstructural and mechanical behavior, and to explore practical limitations and advantages of the process. The results from single-step PSA process were also used to develop a microstructure-based constitutive material model to capture and predict the observed mechanical and microstructural response of AZ31 sheet to PSA variables. This model explicitly considered the effect of recovery on recrystallization kinetics, and non-constant nucleation and growth rate. The model was extended to predict the grain size at the end of recrystallization and within the grain growth stage as well as post-PSA yield and work hardening characteristics. The mechanical property prediction was based on considering the microstructure as a composite of un-recrystallized, recrystallized and coarsened grain structure and by employing a rule of mixture. The processing and forming methods led to significantly increased cumulative uniaxial tensile ductility and plane strain cumulative bendability of AZ31 sheet at room temperature depending upon PSA process parameters. The experimental and modeling studies collectively helped correlate mechanical properties from various processing conditions and forming methods with microstructural parameters, and to explain the improvement in room temperature formability based on microstructural and textural considerations.|
|Appears in Collections:||Open Access Dissertations and Theses|
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|Finalized thesis-PhD-Mahdi Habibnejad korayem-Sep 24-2015.pdf||10.03 MB||Adobe PDF||View/Open|
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