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. / Dissertation / Doctor of Philosophy (PhD)

Le grenaillage de précontrainte est un procédé couramment utilisé dans l’industrie (automobile, aréonautique, …) pour augmenter la durée de vie des pièces mécaniques et de structure : des contraintes de compression sont générées par déformation plastique de la surface. Dans le cas des aciers TRIP (TRansformation Induced Plasticity), qui possèdent une microstructure complexe, l’austénite métastable est susceptible de se transformer en martensite lors du grenaillage. L’état de contraintes obtenu est donc complexe : il résulte de l’effet combiné de la déformation plastique induite par le procédé et de la transformation martensitique qui conduit à une redistribution des contraintes entre l’austénite et la martensite. Ce travail a pour objectif de caractériser expérimentalement l’état mécanique, à l’échelle des phases, de différents aciers TRIP (AISI 301LN, TRIP 780 et 23MnCrMo5) ainsi que leurs fractions de phase respectives après grenaillage et d’en proposer une modélisation par éléments finis pouvant être, à terme, utilisée en bureaux d’études. Le modèle élastoplastique à transformation de phase, développé dans cette thèse, permet de prédire l’évolution des différents champs mécaniques, de manière macroscopique mais également à l’échelle des phases, ainsi que l’évolution de la fraction d’austénite résiduelle. / Shot-peening is commonly used in mechanical industries to increase life duration of mechanical and structural parts: residual compressive stresses are developed at the sub-surface of the material by plastic stretching of the surface. In the case of TRIP-effect steels (TRansformation Induced Plasticity), the metastable austenite can transform into martensite during shot-peening. The final distribution of stress is then more complex than for “standard steels” as it results from the mechanical strain imposed by the process and the martensitic transformation leading to a stress redistribution between austenite, martensite and the other phases. This work aims to characterize experimentally the mechanical state, at phase scale, of different TRIP steels (AISI 301LN, TRIP 780 and 23MnCrMo5) as well as the fraction of each phase after shot-peening and to propose a numerical model by finite elements which could be used in the future by engineering offices. An elastoplastic model with phase transformation was developed in this thesis which permits to predict the evolution of mechanical variables, macroscopically and at the phase scale, as well as the evolution of austenite volume fraction

In a Sheet Metal Forming (SMF) operation, complex geometries in multi-stage forming processes are mostly common. Forming a blank, major and minor straining willoccur. Follow the straining of the blank elements over the forming process will provide its strain paths. The strain paths can be visualized in a Forming Limit Diagram(FLD); with a Forming Limit Curve (FLC) corresponding to the strained material.In the diagram, the determination whether an element is critical due to fracture ornecking is determined. Utilizing the FLD, the formability of a material is defined;the elements and their paths are however linear. Manufacture a sheet metal usinga multi-stage forming process will contribute to Non-Linear Strain Paths (NLSP).Thus, the FLD is no longer valid. Providing a tool from the company RISE IVF AB to be used for pre-strainingoperations, the objective of this thesis work is to enhance and investigate the possibility of generating the three main strain paths - uniaxial tension, plane strain,equibiaxial tension - of the dual-phased steel DP800. This study is in collaborationwith Volvo Cars Body Components (VCBC) in Olofström, where the pre-strainingwill be used in a future study of the SMF non-linear behaviour. Utilizing the finiteelement software AutoForm - specialized on SMF operations - this numerical basedstudy can be conducted. The ability of generating the three main strain paths will be achieved by modifying the blank geometries and provided tooling. By changing the dimensions ofthe punch and draw beads, critical regions and forced concentrated straining weresupposed to be achieved. These changes are implemented with the intention to fulfillthe criterion of the straining in terms of magnitude and gradient. The result from the simulations shows that the modifications have different effecton both the straining level and gradient. The modifications of both the draw beadand the punch were not of any significant use, while the blank dimension was mostvital when generating sufficient strain paths. Hence, the tooling modifications withinthis thesis work did not enhance the prediction of the three strain paths.

Clinching is a joining process that is becoming more and more important in industry due to the increasing use of multimaterial designs. Despite the already widespread use of the process, there is still a need for research to understand the mechanisms and design of clinched joints. In contrast to the tool parameters, process and material disturbances have not yet been investigated to a relatively large extent. However, these also have a great influence on the properties and applicability of clinching. The effect of process disturbances on the clinched joint are investigated with numerical and experimental methods. The investigated process variations are the history of the sheets using the pre-hardening of the material, different sheet thicknesses, sheet arrangements and punch strokes. For the consideration of the material history, a specimen geometry for pre-stretching specimens in uniaxial tension is used, from which the pre-stretched secondary specimens are taken. A finite element model is set up for the numerical investigations. Suitable clinching tools are selected. With the simulation, selected process influences can be examined. The effort of the numerical investigations is considerably reduced with the help of a statistical experimental design according to Taguchi. To confirm the simulation results, experimental investigations of the clinch point geometry by using micrographs and the shear strength of the clinched joint are performed. The analysis of the influence of difference disturbance factors on the clinching process demonstrate the importance of the holistic view of the clinching process.