Metal foams have found increasing applications in a wide range of structural and functional products, due to their exceptional mechanical, thermal, acoustic and electrical properties and offer great potential for lightweight structures for energy absorption in packaging during impact at high velocities. Metal foam structures have densities only fractions of that of a solid structure and have high specific strength and higher stiffness than other contemporary packaging materials. Therefore, the metal foam in particular “Aluminium Foam” has an important application as packaging material for transportation of Reactor fuels and Radioactive samples. Nuclear materials are transported in packages which should meet stringent safety standards like impact resistance, thermal shock etc. Therefore, in the transportation of the above materials, aluminium foam can play a key role in providing a cushion for absorption of shock and impact. The aim of this work is to develop a process for fabrication of aluminum foam. Two methods are experimented to manufacture metal foams. The first method involves mixing of a foaming agent in a liquid aluminium pool and the subsequent cooling of the melt while hydrogen is released from the foaming agent. The second method of metal foaming process is based on a procedure consisting of a base metal and a foaming agent, which are milled for homogeneous mixing and then pre-compacted by cold isostatic pressing. This is followed by cold/warm extrusion. The extruded piece is then heated up to a certain foaming temperature. The heating process leads to partial metal melting as well as to the release of the hydrogen gas and consequently to the formation of metal foam in the semi-solid state. In this thesis, the technology for fabrication of Al foam having a density of around 0.2-0.3 g/cm3 has been made & discussed in detail. The effects of various fabrication parameters like compaction pressure, extrusion ratio and foaming temperature on the formation of the Al foam are discussed. The quality of fabricated Al foams is characterized in terms of density, microstructure, porosity content etc. The various mechanical properties like yield strength, tensile strength and impact energy of the Al foams are evaluated in order to understand their behavior under different conditions. The typical values of Young’s modulus, plateau stress, densification strain and energy absorbed for the foam tested are tabulated. The observations, which are made from the data, can be briefly quoted as under: a) As the length of the specimen increases, plateau stress increases which increases the energy absorption. b) The energy absorption for Al-20% Mg alloy has been found to be minimum. The foam exhibited brittle behaviour and crumbled under load application. c) Young’s modulus varies in the range of 0.057 – 0.13 GPa for the foam. d) As density increases, Young’s modulus also increases and correspondingly the energy absorption value increases for Al-foam. It is found that the variation in the plateau stress with density is marginal. But the strain value was found to be dependent on the alloy composition and the density. The strain obtained for all cases was found to be very near to 75-90%. Al-20%Mg alloy showed an inferior behaviour compared to pure Al. It showed a lesser plateau stress and crumbled while testing. This shows that this alloy is highly brittle in nature. Also, the Al-Mg foam obtained did not exhibited good luster.

Title: Study of cooperative dislocation phenomena in solids by the acoustic emis- sion technique Author: Michal KNAPEK Department: Department of Physics of Materials Supervisor: doc. RNDr. František Chmelík, CSc., Department of Physics of Materials Abstract: Plastic deformation of micron-scale crystalline materials differs con- siderably from bulk specimens, as it is characterized by random strain bursts. Three categories of metallic samples were investigated in this thesis: micron- scale copper micropillars with varied geometries, submillimeter-scale aluminum microwires, and aluminum and aluminum-magnesium salt-replicated foams. Very precise fabrication methods and sensitive measurement set-ups consisting of uni- axial compression and tensile tests with concurrent acoustic emission (AE) record- ing were developed. These fine methods allowed for investigations of effects re- lated to plastic deformation at micrometer scales, i.e. the dislocation dynamics associated with the stress drops. Size effects in plastic deformation, as well as clear correlations between the stress drops and the AE events, were found in mi- crosamples, confirming that dislocation avalanches are indeed responsible for the stochastic character of deformation processes also at microscales. Open-cell pure aluminum and aluminum-magnesium...

Experiments on the production of porous metallic materials were performed on Fe-base and Al-base alloys. The method involves dissolution of gases in the liquid state and solidification at various cooling rates. The alloy compositions were selected to induce solidification of primary particles intended to control the pore distribution. For the Fe-base alloys, nitrogen was introduced into the melt by dissolution of chromium nitride powder. Fe-Cr-Mn-Si-C alloys featuring M7C3 carbide particles were selected. For the Al-base alloys, hydrogen gas was dissolved into the melt by decomposition of water vapor. Al-Ti and Al-Fe alloys featuring primary Al3Ti and Al3Fe intermetallic particles, respectively, were considered. In the Fe-base alloys, a homogeneous distribution of gas pores through the specimens’ volume was obtained at high cooling rate (water quenching) and after introduction of external nucleating agents. In the case of the Al-base alloys, a good pore distribution was observed at all cooling rates and without addition of nucleating agents. Calculations of the variation of nitrogen (respectively hydrogen) solubility based on Wagner interaction parameters suggest that pore nucleation and growth occur during precipitation of the primary particles (M7C3 carbides, Al3Ti or Al3Fe intermetallics), due to composition changes in the melt and resultant supersaturation with gas atoms. Microscopic analyses revealed that the primary particles control the pore growth in the melt and act as barriers between adjacent pores, thereby preventing pore coalescence and promoting a fine pore distribution. Uniaxial compression testing of the porous Al-Ti and Al-Fe materials showed the typical compressive behavior of cellular metals. Further work is needed to improve the quality and reproducibility of the porous structures which can possibly be used in energy absorption or load-bearing applications. As a corollary result of the quenching of hypereutectic Fe-Cr-Mn-Si-C alloys in the experiments of synthesis of porous metals, a homogeneous featureless structure was observed in some parts of the samples, instead of the equilibrium structure of M7C3 and eutectic phases. Subsequent investigations on rapid solidification of Fe-base alloys at various alloy compositions and cooling rates led to the formation of a single-phase structure for the composition Fe-8Cr-6Mn-5Mo-5Si-3.2C (wt.%), at relatively low cooling rates (≈103 K/s) and for large sample dimensions (2.85 mm). The single phase, which is likely to be the hcp ɛ-phase, was found to decompose into a finely distributed structure of bainite and carbides at ≈600 °C. The annealed structure showed very high hardness values (850 to 1200 HV), which could be exploited in the development of high-strength Fe-base materials. / QC 20100809

L'objectif de cette thèse de doctorat est d'étudier expérimentalement et numériquement le comportement thermique des mousses d'aluminium et des matériaux à changement de phase (MCP), présentés sous la forme d’un composite, afin de connaître le phénomène de stockage d’énergie thermique dans ces matériaux. Le procédé de fabrication de la mousse d'aluminium à cellules ouvertes est d’abord analysé numériquement dans le but de réduire les défauts formés durant la fabrication. Les caractéristiques de transfert de chaleur du MCP dans les mousses d'aluminium comportant différentes porosités sont ensuite étudiées en analysant les processus de fusion et la variation de températures dans ces composites. Deux modèles numériques pour la mousse d'aluminium à faible et à haute porosité sont établis afin d’évaluer la performance de stockage d'énergie des composites. Les résultats montrent que la mousse d'aluminium peut améliorer considérablement la performance de transfert de chaleur du MCP en raison de sa conductivité thermique élevée. La performance de stockage d'énergie dépend fortement de la porosité des mousses d'aluminium. Une porosité optimisée met en évidence cette performance et l’amélioration du comportement thermique. La dernière partie de la thèse porte sur une structure améliorée de la mousse par rapport à la structure uniforme: Association de l’ailette métallique et du gradient de porosité de la mousse. Cette nouvelle structure donne ainsi une performance de stockage d'énergie encore meilleure surtout dans le cas d’une source de chaleur isotherme / The objective of this Ph.D. thesis is to study the thermal behavior of the aluminum foam and phase change material (PCM) composite by both experimental and numerical methods in order to know the phenomena of storage of thermal energy in these materials. The manufacturing process of open-cell aluminum foam is firstly analyzed numerically to reduce the manufacturing defects in the samples. The heat transfer characteristics of PCM embedded in aluminum foams with different porosities are then investigated by analyzing the melting processes and the temperature variations in the composites. Two numerical models for low and high porosity aluminum foam are established to evaluate the energy storage performance of the composites. The results show that the aluminum foam can greatly improve the heat transfer performance in PCM due to its high thermal conductivity. The energy storage performance depends strongly on the porosity of the aluminum foam/PCM composite. An optimized porosity highlights this performance and improves the thermal behavior. The last part of this thesis proposes an improved structure of aluminum foam with respect to the uniform structure: Association of the metal fin and the foam with graded porosity. This new structure possesses a better energy storage performance especially in the case of the isothermal heat source

L'objectif de cette thèse est d'étudier et d'optimiser le processus de fabrication des mousses métalliques et le comportement thermique du matériau de la mousse d'aluminium/matériau de changement de phase (MCP) par des méthodes expérimentales et numériques. Le processus d’élaboration de la mousse d’aluminium à pore ouvert est développé et optimisé pour contrôler précisément les paramètres de fabrication. Deux modèles de mousse d'aluminium à haute porosité (MAHP)/MCP composite et à faible porosité (MALP)/MCP composite sont établis pour la simulation numérique. En simulant le processus de fusion d'un système de stockage d'énergie, les composites MAHP/MCP et MALP/MCP sont comparés numériquement afin d'évaluer la performance de stockage d'énergie thermique. Les résultats montrent que la mousse d'aluminium améliore nettement le processus de transfert de chaleur dans MCP en raison de sa haute conductivité thermique. La porosité des mousses d'aluminium influence non seulement le processus de fusion du composite mais aussi la performance de stockage d'énergie thermique. Grâce à la collaboration avec EPF, une nouvelle méthode d’élaboration des mousses périodiques d'aluminium à pore ouvert est développée dans cette thèse sur la base d’impression 3D. Le comportement thermique des mousses d'aluminium périodiques à pore ouvert/MCP est analysé expérimentalement et numériquement / The objective of this thesis is to study and optimize the manufacturing process of metal foams and the thermal behavior of the aluminum foam/phase change material (PCM) composite by experimental and numerical methods. The manufacturing process of open-cell aluminum foam is developed and optimized to precisely control the parameters of mufacturing. Two pore-scale models of high porosity aluminum foams (HPAF)/PCM composite and low porosity aluminum foams (LPAF)/PCM composite are established for numerical simulation. By simulating the melting process of a layer energy storage system, the HPAF/PCM and LPAFS/PCM composite are compared numerically in order to evaluate the energy storage performance. The results show that aluminum foam improves greatly the heat transfer process in PCM due to its high thermal conductivity. The porosity of aluminum foams could not only influence the melting process of composite but also the energy storage performance. Thanks to the collaboration with EPF, a new manufacturing method of periodic open-cell aluminum foams is developed based on 3D rapid tooling. The thermal behavior of the periodic open-cell aluminum foams/PCM composite is experimentally and numerically analyzed

Zellulare metallische Strukturen zeichnen sich durch ihre hohe Porosität und ihre komplexe geometrische Struktur aus. Dadurch ist die Bestimmung ihrer Wärmeleitfähigkeit eine komplizierte und aufwändige Messaufgabe. Für die vorliegende Arbeit wurden Materialien ausgewählt, die sich im Grundmaterial, in der Herstellungsmethode und im strukturellen Aufbau unterscheiden. Dabei wurden Materialien mit unterschiedlicher Porosität und Porengröße untersucht. Die effektive Wärmeleitfähigkeit wurde mit drei unterschiedlichen Messverfahren bestimmt. Die Messungen wurden bei normalem Druck bei Raumtemperatur und für ausgewählte Materialien bis maximal 800 °C durchgeführt. Dabei wurden die Bedingungen für die optimale Durchführung der Messungen und die Probleme jedes Verfahrens herausgearbeitet. Daraus wurden Empfehlungen für die optimale Messungsdurchführung abgeleitet. Die gewonnenen Messergebnisse wurden miteinander, mit Werten aus der Literatur und mit vorhandenen mathematischen Modellen verglichen.

Zellulare metallische Strukturen zeichnen sich durch ihre hohe Porosität und ihre komplexe geometrische Struktur aus. Dadurch ist die Bestimmung ihrer Wärmeleitfähigkeit eine komplizierte und aufwändige Messaufgabe. Für die vorliegende Arbeit wurden Materialien ausgewählt, die sich im Grundmaterial, in der Herstellungsmethode und im strukturellen Aufbau unterscheiden. Dabei wurden Materialien mit unterschiedlicher Porosität und Porengröße untersucht. Die effektive Wärmeleitfähigkeit wurde mit drei unterschiedlichen Messverfahren bestimmt. Die Messungen wurden bei normalem Druck bei Raumtemperatur und für ausgewählte Materialien bis maximal 800 °C durchgeführt. Dabei wurden die Bedingungen für die optimale Durchführung der Messungen und die Probleme jedes Verfahrens herausgearbeitet. Daraus wurden Empfehlungen für die optimale Messungsdurchführung abgeleitet. Die gewonnenen Messergebnisse wurden miteinander, mit Werten aus der Literatur und mit vorhandenen mathematischen Modellen verglichen.