The horizontal twin-roll casting (TRC) process is an energy saving and cost-efficient method for producing near-net-shape sheets of castable metals for light-weight production. In order to investigate the TRC process numerically, a code is generated in OpenFOAM and the commercial software STAR-CCM+ is used. Both are validated with the Stefan problem, the gallium melting test case, and a continuous casting experiment for magnesium AZ31. Different solidification models are tested that are similar to solution domain definitions and solid-fraction temperature relations. The comparison with temperature measurements of the MgF GmbH Freiberg pilot plant and the final microstructure exhibits good correlation. Sensitivity studies are carried out for thermophysical properties of AZ31 as well as pilot plant parameters. Furthermore, the rolls are incorporated into the simulation to determine the effect of a location-dependent heat-transfer coefficient. Finally, the results are compared to a second pilot plant situated at the Helmholtz-Centre Geesthacht in order to explore differences and similarities. / Das horizontales Gießwalzen ist eine energiesparende und kostengünstige Methode zur Erzeugung von Flachprodukten, die im Leichtbau verwendet werden. Um dieses Verfahren numerisch zu untersuchen wurde ein Programmcode in OpenFOAM entwickelt und die kommerzielle Software STAR-CCM+ verwendet, wobei beide mit dem Stefan Problem, dem Schmelzen von Gallium und Messdaten des Stranggusses von Magnesium AZ31 validiert wurden. Verschiedene Erstarrungsmodelle werden ebenso getestet wie Variationen des Simulationsbereiches und Feststoff-Temperatur-Verläufe. Vergleiche mit Temperaturmessdaten der Pilotanlage MgF GmbH Freiberg und der finalen Mikrostruktur zeigen gute Übereinstimmungen. Sensitivitätsanalysen werden durchgeführt, um die Einflüsse von thermophysikalischen Eigenschaften und Anlagenparametern abzuschätzen. Des Weiteren werden die Walzen in die Simulation mit einbezogen, um den Effekt eines lokal veränderlichen Wärmeübergangskoeffizienten zu beurteilen. Schließlich werden die Ergebnisse mit denen einer zweiten Pilotanlage am Helmholtz-Zentrum Geesthacht verglichen. / Le laminage de coulée continue horizontal possède une faible consommation d’énergie et est bon marché pour la production des feuilles de métaux coulables utilisés dans la construction légère. Afin d’examiner ce processus numériquement, un code est généré dans OpenFOAM et le logiciel commercial STAR-CCM+ est utilisé, tous les deux sont validés en utilisant le problème de Stefan, la fusion du gallium et la coulée continue verticale de magnésium AZ31. Plusieurs modèles de solidification sont testés, ainsi que la variation du domaine de simulation, et des rélations entre la teneur en matière solide et la température. Des comparaisons avec des résultats de mesures de la température à l’installation pilote de MgF GmbH Freiberg ainsi que la microstructure donnent des bons résultats. Des analyses de sensibilité sont effectuées afin d’évaluer l’influence des propriétés thermophysiques et des paramètres de l’installation. De plus, les cylindres sont intégrés dans la simulation pour estimer l’impact du coefficient de transfert de chaleur dépendant du lieu. Finalement, les résultats sont comparés avec ceux du Helmholtz-Centre Geesthacht.

In der vorliegenden Arbeit wird ein prozessübergreifendes Simulationsmodell für die Temperatur- und Gefügeentwicklung im Band und Coil beim reversierenden Warmwalzen entwickelt. In der Software werden die erstmals aufgestellten Modelle der Bandab- und aufwicklung implementiert. Die Temperatur- und Gefügeveränderungen im gewalzten Warmband werden lokal und prozessübergreifend betrachtet. Die für das gesamte Modell notwendigen Koeffizienten zur Beschreibung der Wärmeübertragung wurden mittels der inversen Methode bestimmt. Die Bestimmung der radialen Wärmeübertragung im Coil erfolgte mit Hilfe von Laboruntersuchungen in Abhängigkeit von der Temperatur, der Banddicke und des radialen Druckes. Die Validierung des Modells für die Temperatur- und Gefügeentwicklung erfolgte am Beispiel des reversierenden Warmwalzens der Magnesiumlegierung AZ31. Zu diesem Zweck wurden Versuche zu Temperaturmessungen in den einzelnen Phasen der Prozesskette durchgeführt. Die Ermittlung der Einflüsse der Umformbedingungen auf die Temperatur- und Gefügeentwicklung während des reversierenden Warmbandwalzens erfolgte abschließend mit Hilfe des entwickelten Modells. Die Ergebnisse zeigen auf, wie sich die Walzbedingungen auf die Entwicklung der Temperatur und des Gefüges auswirken.

The horizontal twin-roll casting (TRC) process is an energy saving and cost-efficient method for producing near-net-shape sheets of castable metals for light-weight production. In order to investigate the TRC process numerically, a code is generated in OpenFOAM and the commercial software STAR-CCM+ is used. Both are validated with the Stefan problem, the gallium melting test case, and a continuous casting experiment for magnesium AZ31. Different solidification models are tested that are similar to solution domain definitions and solid-fraction temperature relations. The comparison with temperature measurements of the MgF GmbH Freiberg pilot plant and the final microstructure exhibits good correlation. Sensitivity studies are carried out for thermophysical properties of AZ31 as well as pilot plant parameters. Furthermore, the rolls are incorporated into the simulation to determine the effect of a location-dependent heat-transfer coefficient. Finally, the results are compared to a second pilot plant situated at the Helmholtz-Centre Geesthacht in order to explore differences and similarities. / Das horizontales Gießwalzen ist eine energiesparende und kostengünstige Methode zur Erzeugung von Flachprodukten, die im Leichtbau verwendet werden. Um dieses Verfahren numerisch zu untersuchen wurde ein Programmcode in OpenFOAM entwickelt und die kommerzielle Software STAR-CCM+ verwendet, wobei beide mit dem Stefan Problem, dem Schmelzen von Gallium und Messdaten des Stranggusses von Magnesium AZ31 validiert wurden. Verschiedene Erstarrungsmodelle werden ebenso getestet wie Variationen des Simulationsbereiches und Feststoff-Temperatur-Verläufe. Vergleiche mit Temperaturmessdaten der Pilotanlage MgF GmbH Freiberg und der finalen Mikrostruktur zeigen gute Übereinstimmungen. Sensitivitätsanalysen werden durchgeführt, um die Einflüsse von thermophysikalischen Eigenschaften und Anlagenparametern abzuschätzen. Des Weiteren werden die Walzen in die Simulation mit einbezogen, um den Effekt eines lokal veränderlichen Wärmeübergangskoeffizienten zu beurteilen. Schließlich werden die Ergebnisse mit denen einer zweiten Pilotanlage am Helmholtz-Zentrum Geesthacht verglichen. / Le laminage de coulée continue horizontal possède une faible consommation d’énergie et est bon marché pour la production des feuilles de métaux coulables utilisés dans la construction légère. Afin d’examiner ce processus numériquement, un code est généré dans OpenFOAM et le logiciel commercial STAR-CCM+ est utilisé, tous les deux sont validés en utilisant le problème de Stefan, la fusion du gallium et la coulée continue verticale de magnésium AZ31. Plusieurs modèles de solidification sont testés, ainsi que la variation du domaine de simulation, et des rélations entre la teneur en matière solide et la température. Des comparaisons avec des résultats de mesures de la température à l’installation pilote de MgF GmbH Freiberg ainsi que la microstructure donnent des bons résultats. Des analyses de sensibilité sont effectuées afin d’évaluer l’influence des propriétés thermophysiques et des paramètres de l’installation. De plus, les cylindres sont intégrés dans la simulation pour estimer l’impact du coefficient de transfert de chaleur dépendant du lieu. Finalement, les résultats sont comparés avec ceux du Helmholtz-Centre Geesthacht.

The desire for energy and resource savings brings magnesium alloys as lightweight materials with high specific strength more and more into the focus. Most structural components are subjected to cyclic loading. In the course of computer aided product development, a numerical prediction of the fatigue life under these conditions must be provided. For this reason, the mechanical properties of the considered material must be examined in detail. Wrought magnesium semifinished products, e.g. magnesium sheet metals, typically reveal strong basal textures and thus, the mechanical behavior considerably differs from that of the well-established magnesium die castings. Magnesium sheet metals reveal a distinct difference in the tensile and compressive yield stress, leading to non-symmetric sigmoidal hysteresis loops within the elasto-plastic load range. These unusual hysteresis shapes are caused by cyclic twinning and detwinning. Furthermore, wrought magnesium alloys reveal pseudoelastic behavior, leading to nonlinear unloading curves. Another interesting effect is the formation of local twin bands during compressive loading. Nevertheless, only little information can be found on the numerical fatigue analysis of wrought magnesium alloys up to now. The aim of this thesis is the investigation of the mechanical properties of wrought magnesium alloys and the development of an appropriate fatigue model. For this purpose, twin roll cast AM50 as well as AZ31B sheet metals and extruded ME21 sheet metals were used. Mechanical tests were carried out to present a comprehensive overview of the quasi-static and cyclic material behavior. The microstructure was captured on sheet metals before and after loading to evaluate the correlation between the microstructure, the texture, and the mechanical properties. Stress- and strain-controlled loading ratios and strain-controlled experiments with variable amplitudes were performed. Tests were carried out along and transverse to the manufacturing direction to consider the influence of the anisotropy. Special focus was given to sigmoidal hysteresis loops and their influence on the fatigue life. A detailed numerical description of hysteresis loops is necessary for numerical fatigue analyses. For this, a one-dimensional phenomenological model was developed for elasto-plastic strain-controlled constant and variable amplitude loading. This model consists of a three-component equation, which considers elastic, plastic, and pseudoelastic strain components. Considering different magnesium alloys, good correlation is reached between numerically and experimentally determined hysteresis loops by means of different constant and variable amplitude load-time functions. For a numerical fatigue life analysis, an energy based fatigue parameter has been developed. It is denoted by “combined strain energy density per cycle” and consists of a summation of the plastic strain energy density per cycle and the 25 % weighted tensile elastic strain energy density per cycle. The weighting represents the material specific mean stress sensitivity. Applying the energy based fatigue parameter on modeled hysteresis loops, the fatigue life is predicted adequately for constant and variable amplitude loading including mean strain and mean stress effects. The combined strain energy density per cycle achieves significantly better results in comparison to conventional fatigue models such as the Smith-Watson-Topper model. The developed phenomenological model in combination with the combined strain energy density per cycle is able to carry out numerical fatigue life analyses on magnesium sheet metals.

The proposed dissertation thesis deals with the characterization of HVOF and APS-thermally sprayed coatings prepared on the AZ31 and AZ91 magnesium substrates. The theoretical part of the thesis describes in-detail Mg substrates used in the experimental part of the thesis. There are also characterized materials and coatings based on NiCrAlY and FeCrNiMoSiC metals, WC-CoCr cermets, and YSZ ceramic materials. At the end of the theoretical part, the literary research summarizing the characterization and analysis performed on thermally sprayed coatings on Mg alloys. Based on the theoretical knowledge, the characterization of Mg substrates and deposited coatings was performed in terms of the surface morphology, microstructure, and the chemical composition using the light microscopy (LM) and scanning electron microscopy with energy-dispersive spectroscopy (SEM+EDS). The phase composition of the coatings was analyzed using the X-ray diffraction (XRD). The diffractions corresponding to the sprayed coatings were compared with the feedstock powders, i.e. materials used for the spraying of the coatings. The characterization of the prepared coatings in terms of the mechanical and tribological properties was performed. The hardness and microhardness of the coatings as well as the coefficient of friction, and the wear rate were measured. The last chapter of the experimental part deals with the evaluation of the electrochemical corrosion properties by the potentiodynamic measurements in a 3.5% NaCl solution. In the case of exposed samples, the evaluation of the surface and coating/substrate interface was performed using LM and SEM with EDS. The mechanism of the corrosion attack and degradation was determined from the acquired knowledge and base on the results of the short-term measurements. Based on the measured results, it can be stated that the deposited coatings were successfully applied on the surface of both Mg alloys. All the coatings increase the surface hardness of the Mg alloys and significantly improve their tribological properties. However, except for FeCrNiMoSiC coatings, the corrosion properties of Mg alloys deteriorate due to the fact that the corrosion environment can pass through the coating to the less noble Mg substrate and the corrosion microcells are created.

Additive manufacturing processes have been investigated for some years, and are commonly used industrially in the field of plastics for small- and medium-sized series. The use of metallic deposition material has been intensively studied on the laboratory scale, but the numerical prediction is not yet state of the art. This paper examines numerical approaches for predicting temperature fields, distortions, and mechanical properties using the Finite Element (FE) software MSC Marc. For process mapping, the filler materials G4Si1 (1.5130) for steel, and AZ31 for magnesium, were first characterized in terms of thermo-physical and thermo-mechanical properties with process-relevant cast microstructure. These material parameters are necessary for a detailed thermo-mechanical coupled Finite Element Method (FEM). The focus of the investigations was on the numerical analysis of the influence of the wire feed (2.5–5.0 m/min) and the weld path orientation (unidirectional or continuous) on the temperature evolution for multi-layered walls of miscellaneous materials. For the calibration of the numerical model, the real welding experiments were carried out using the gas-metal arc-welding process—cold metal transfer (CMT) technology. A uniform wall geometry can be produced with a continuous welding path, because a more homogeneous temperature distribution results.

Indiana University-Purdue University Indianapolis (IUPUI) / In recent years, magnesium alloys, due to their high strength and biocompatibility, have attracted significant interest in medical applications, such as cardiovascular stents, orthopedic implants, and devices. To overcome the high corrosion rate of magnesium alloys, coatings have been developed on the alloy surface. Most coating methods, such as anodic oxidation, polymer coating and chemical conversion coating, cannot produce satisfactory coating to be used in human body environment. Recent studies demonstrate that micro-arc oxidation (MAO) technique can produce hard, dense, wear-resistant and well-adherent oxide coatings for light metals such as aluminum, magnesium, and titanium. Though there are many previous studies, the understanding of processing conditions on coating performance remains elusive. Moreover, previous tests were done in simulated body fluid. No test has been done in a cell culture medium, which is much closer to human body environment than simulated body fluid. In this study, the effect of MAO processing time (1 minute, 5 minutes, 15 minutes, and 20 minutes) on the electrochemical behaviors of the coating in both conventional simulated body fluid and a cell culture medium has been investigated. Additionally a new electrolyte (12 g/L Na2SiO3, 4 g/L NaF and 4 ml/L C3H8O3) has been used in the MAO coating process. Electrochemical behaviors were measured by performing potentiodynamic polarization and electrochemical impedance spectroscopy tests. In addition to the tests in simulated body fluid, the MAO-coated and uncoated samples were immersed in a cell culture medium to investigate the corrosion behaviors and compare the difference in these two kinds of media. The results show that in the immersion tests in conventional simulated body fluid, the 20-minute MAO coated sample has the best resistance to corrosion due to the largest coating thickness. In contrast, in the cell culture medium, all MAO coated samples demonstrate a similar high corrosion resistance behavior, independent of MAO processing time. This is probably due to the organic passive layers formed on the coating surfaces. Additionally, a preliminary finite element model has been developed to simulate the immersion test of magnesium alloy in simulated body fluid. Comparison between the predicted corrosion current density and experimental data is discussed.