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Experimental and numerical simulation studies were carried out to enhance the understanding of the compaction behaviour of powder materials and to study the breakage behaviour of tablets after compaction. In order to simulate powder compaction and post compaction behaviour an appropriate constitutive model is required. To calibrate the constitutive model (e.g. a Drucker-Prager Cap model) a series of experiments were carried out including closed die compaction, uniaxial and diametrical compression tests. A newly developed apparatus consisting of a die instrumented with radial stress sensors was used to determine constitutive parameters as well as friction properties between the powder and die wall. The calibration of constitutive models requires accurate stress-strain curves. During die compaction the deformation of the powder material is determined by considering the elastic deformation (or compliance) of the system. The effect of different compliance correction methods was evaluated with regards to the accuracy of models predicting the pressing forces. A method for accounting for non-homogeneous stress states in instrumented die compaction was also developed. A complete data extraction procedure was presented. The breakage behaviour of flat and curved faced tablets was investigated and the breakage patterns of tablets were examined by X-Ray computed tomography. An empirical equation that relates the material strength to the break force was proposed. The constitutive model was implemented into the finite element package Abaqus/Standard to simulate powder compaction and breakage. A range of failure criteria have been evaluated for predicting break force of flat and curved faced tablets under diametrical compression.

This project sought to design, implement and evaluate a process for the manufacture of porous, spherical salt beads, in order to enhance the reproducibility in mechanical properties of open cell aluminium foams made by a replication-based manufacturing technique. Porous beads were favoured in order to increase the dissolution rate of the salt from the preform, thereby making the manufacture of large foam parts practical. Salt beads were made by a novel method using fine NaCI powder, flour and water to make a paste that was subsequently disintegrated into large beads by mechanical stirring in oil. The NaCI paste viscosity was found to be important to the production of spherical beads and by varying the intensity of mechanical disintegration of the paste, control of the bead size was possible. The salt beads with sizes from 0.5 to 3 mm diameter were compacted into preforms and made into moulds for infiltration with molten pure aluminium by pressure- assisted investment casting. The heat treatment used to "cure" the plaster mould containing the preform was sufficient to remove the flour from the beads, sinter the preform and increase its strength. The effect of preform compaction conditions on the size, shape and volume fraction of porosity was quantified using a number of techniques, including mercury porosimetry, which was used to model the infiltration process. At the highest infiltration pressure 0.25 MPa (2.5 bar) the resulting foam densities were higher, but led to extensive penetration of molten aluminium into the porous beads, slowing down salt removal. In general, the compression strength increased with increasing foam density, and was highly reproducible, but where metal infiltration into the beads was extensive, the foam density increased but with little improvement in the compressive strength.

This dissertation reports a study about vibration analysis of spinning discs in centrifugal atomization, which are known as atomizing discs. Centrifugal atomization is an important process to produce metal powders which have vast applications in powder metallurgy. The main focuses of this study are to establish a dynamic model to investigate the transient vibration of atomizing discs during centrifugal atomization process and evaluate the influence of disc vibration on powder formation. In centrifugal atomization, a lot of studies investigating powder morphology, powder sizes and the flow behaviour of the molten metal on spinning discs have been reported. Vibration of spinning discs in other applications such as CDs, circular saws and turbine rotors has been extensively studied. However, vibration of atomizing discs is lack of investigation and is normally regarded as undesirable because of its negative effects such as possible generation of loud noise and potential contribution to device failure. But, in real practice, the disc vibration definitely has an influence on the molten metal flow, which may benefit the disintegration of molten metal and therefore makes an impact on powder formation in centrifugal atomization. In this thesis, a dynamic model to represent atomizing discs as a spinning Kirchhoff plate subjected to moving weight and moving mass is developed for better understanding of generation and characteristics of vibration of spinning discs in centrifugal atomization. The transient vibration of two cases of atomizing discs with uniform and non-uniform thickness is numerically analyzed. Three stages in disc vibration have been found. The investigation also reveals that the amplitude of the vibration increases greatly when the hydraulic jump takes place in the active area of the disc, which suggests the clamping ratio should be larger than the critical jump radius ratio in atomizing discs. A possible case considering asymmetric loads and air damping which may lead to unstable vibration is also investigated. The mechanisms of powder formation in centrifugal atomization is then investigated on the basis of vibration analysis. A theoretical model to predict powder size is developed and validated by numerical simulations and by experimental comparisons. The correlations between the size of powders generated by atomizing discs and geometry and process parameters in centrifugal atomization, which can provide a guidance for atomizer designs, are proposed.

Metal foams have attracted wide range of interest from researchers and industries because of their unique combinations of properties. Of particular interest, open cell metallic foams have good weight-specific mechanical properties, and improvements could make these materials highly desirable for lightweight structural and energy absorption applications. These properties could potentially be increased for open cell foams by treatments affecting their large surface areas. The effect could be very significant, especially when the dominant deformation mode is bending of the foam struts, as the coating will be located away from the neutral bending axis of these struts maximizing its effect. This has been previously found after the application of surface treatments, such as electroplating. The technique of Plasma Electrolytic Oxidation (PEO) is another process that shows an even greater effect on foam specific mechanical properties. In this work, Plasma Electrolytic Oxidation (PEO) coating treatment is applied to open celled aluminium foams with different structures, aiming to improve the mechanical and weight-specific properties of the hybrid material. Open cell aluminium foams of different types, both investment cast (Duocel foam) and replicated (produced in the laboratory) have been produced and PEO coated using a range of different processing parameters. Two pore sizes of Duocel aluminium foam (measured as 2.2 mm and 2.5 mm average pore diameter) with porosity of 90–91%, and a single pore size (1.6 mm diameter) of the pure aluminium replicated foam with porosity around 60–64% have been examined. The PEO treatment of foams was carried out in the pulsed bipolar current mode, with a range of processing times (20, 40, 60 and 80 minutes), pulse frequencies (50 to 6250 Hz) and duty cycles (different ON/OFF waveform ratios). These processing parameters were explored in the present work in four different stages of investigation, as will be explained in detail later. The mechanical properties (yield stress, specific strength, Young’s modulus and energy absorption) of the coated foams produced are assessed experimentally, both in tension and compression, and simple models developed to describe the elastic behaviour, based on either the Gibson-Ashby model of foams as a regular cellular array, or the Markaki-Clyne model of randomly intersecting fibres are used to make predictions to compare to these results. Complimentary characterisation was carried out using SEM, EDX, XRD and nanoindentation techniques to understand the nature of PEO coatings on foams (including coating thickness, growth rate, mechanical properties, porosity, elemental and phase compositions), and the effect this has on mechanical properties. Thereby, the process can be optimised to improve the mechanical performance of the foams. It was demonstrated that PEO coatings can be successfully applied to open cell foams (of low and high level of porosity) and the coating penetrates completely into the structure up to several millimetres depth, with thickness diminishing with depth. The presence of this coating is of benefit for uniaxial mechanical properties as well as specific foam properties. PEO pulse frequency influences coating thickness, porosity and the measured mechanical properties. The major effect on coating hardness and elastic modulus as well as on the strength and stiffness of the coated foams is associated with the volume fraction of porosity within the coating. The effect of using different duty cycles (associated with the ON and OFF times in each cycle in the current pulse frequency used) results in different coating morphology, thickness, distribution and deposition rate. Very fast coating growth rate has been shown to be not always beneficial, whereas low coating growth rate may be useful for the formation of good quality coatings (containing fewer microcracks and possibly lower intrinsic stresses), with potential for a very even distribution into the foam internal structure. An assessment based on strength increase (∆σ) and density increase (∆ρ) of the coated foams shows that the benefits of the application of PEO coatings to metal foams are greater than those shown in other metal foams coated by different techniques. The primary reason for this is that the oxide ceramic coatings formed on foams have low density, excellent mechanical properties and good adhesion to the substrate. These properties have been improved for foams by the PEO optimization process carried out in the present work.

Laser Melting (LM) is an Additive Layer Manufacturing (ALM) process used to produce three-dimensional parts from metal powders by fusing the material in a layerby- layer manner controlled by a CAD model. During LM, rapid temperature cycles and steep temperature gradients occur in the scanned layers. Temperature gradients induce thermal stresses which remain in the part upon completion of the process (i.e. residual stresses). These residual stresses can be detrimental to the functionality and structural integrity of the built parts. The work presented in this thesis developed a finite element model for the purpose of investigating the development of the thermal and residual stresses in the laser melting of metal powders. ANSYS Mechanical software was utilised in performing coupled thermal-structural field analyses. The temperature history was predicted by modelling the interaction of the moving laser heat source with the metal powders and base platform. An innovative ‘element birth and death’ technique was employed to simulate the addition of layers with time. Temperature dependent material properties and strain hardening effects were also considered. The temperature field results were then used for the structural field analysis to predict the residual stresses and displacements. Experiments involving laser melting Ti-6Al-4V powder on a steel platform were performed. Surface topography analyses using a laser scanning confocal microscope were carried out to validate the numerically predicted displacements against surface measurements. The results showed that the material strain hardening model had a direct effect on the accuracy of the predicted displacement results. Using the numerical model, parametric studies were carried out to investigate the effects of a number of process variables on the magnitude of the residual stresses in the built layers. The studies showed that: (i) the average residual stresses increased with the number of melted powder layers, (ii) increasing the chamber temperature to 300°C halved the longitudinal stresses. At 300°C, compressive stresses appeared on the Ti64 surface layer, (iii) reducing the raster length from 1 mm to 0.5 mm reduced the average longitudinal stress in the top layer by 51 MPa (0.04σy), (iv) reducing the laser scan speed from 1200 mm/s to 800 mm/s increased the longitudinal stress by 57 MPa (0.05σy) but reduced the transverse stress by 46 MPa (0.04σy).

Le sujet de thèse s’inscrit dans une problématique d’allègement de structures des moyens de transport civils et militaires. Actuellement, les pièces de structures métalliques de ces moyens sont principalement composées d’alliages d’aluminium et d’aciers. Avec une densité inférieure et des propriétés mécaniques spécifiques similaires à ces métaux, l’alliage de magnésium AZ91 représente une solution de substitution prometteuse. En dépit de son durcissement structural par précipitation, ses propriétés mécaniques relativement faibles limitent son emploi actuel comme matériau de structure. Une amélioration de ces propriétés pourrait être permise au travers d’un affinement de la microstructure et d’un renforcement par l’ajout de particules céramiques. La métallurgie des poudres, en particulier le procédé Spark Plasma Sintering (SPS), permettrait d’allier ces deux voies d’amélioration en produisant un composite à fine microstructure avec un renforcement particulaire contrôlé. Cette thèse a pour objectifs le développement par SPS d’alliages AZ91, l’optimisation de leurs propriétés mécaniques par un contrôle de leur microstructure et l’étude de leur renforcement potentiel par l’ajout de particules de SiC. Le contrôle microstructural a été réalisé par l’intermédiaire des paramètres du procédé SPS et a porté principalement sur la densification de l’alliage, sa taille de grains et sa teneur en précipités. La caractérisation mécanique des matériaux produits a été composée d’essais de dureté, d’essais de compression en conditions quasi-statiques et dynamiques, ainsi que d’essais de traction. Les essais de traction ont été réalisés à l’issue d’un changement d’échelle de production, passant de pièces cylindriques Ø30 mm à des pièces Ø80 mm. Outre la réalisation d’essais de traction, le changement d’échelle a permis d’étudier la reproductibilité des conditions de production. Alors que l’optimisation mécanique des matériaux frittés a porté sur leurs propriétés en compression, les essais de traction ont permis d’évaluer leur cohésion et leur ductilité. Afin de montrer les améliorations permises par l’affinement microstructural et le renforcement particulaire, les matériaux élaborés par SPS ont été comparés à des alliages AZ91 produits par fonderie / The present PhD thesis falls within a structure lightweighting issue in the transport field for civil and military applications. Today, the metallic structural parts in transports are mainly composed of aluminum alloys and steels. With an inferior density and a similar specific mechanical strength to these metals, the AZ91 alloy appears to be a promising alternative. Despite its precipitation strengthening, its relative low mechanical properties limit its current use as engineering material. An improvement could be reached via microstructure refinement and ceramic particle strengthening. Powder metallurgy, involving Spark Plasma Sintering (SPS), will be used as an effective way to improve the AZ91 properties using these two approaches. AZ91 alloys were produced by SPS and reinforced by SiC particles. Their mechanical properties were optimized by microstructure control. This control was carried out by adjusting the SPS processing parameters to optimize the alloy densification, its grain size and its precipitate content. The mechanical properties of the materials were evaluated via hardness testing, compression tests in quasi-static and dynamic conditions as well as quasi-static tensile tests. The tensile tests were carried out after an up-scaling of the production process from Ø30 mm cylindrical pieces to Ø80 mm pieces. In addition to the tensile tests, the up-scaling step allowed to study the repeatability of the process conditions. While the mechanical optimization of the SPS processed materials was paid on their compressive properties, their tensile properties gave information on their cohesion and ductility. In order to highlight the mechanical improvement got by microstructure refinement and particle strengthening, the SPS processed materials were compared with cast AZ91 alloys

L’allègement des structures des véhicules est l’une des problématiques actuelles majeures car il permet d’atteindre de meilleures performances, une autonomie plus importante et une consommation plus faible. Ceci est d’autant plus vrai dans le domaine de la défense où la nécessité de se protéger face aux menaces balistiques implique un ajout de masse conséquent. Les alliages d’aluminium sont pour l’instant l’un des meilleurs compromis, mais augmenter leurs performances permettrait un nouveau gain de masse. Dans ce contexte, la métallurgie des poudres (MdP) se présente comme une alternative de choix aux procédés de mise en forme traditionnels car elle permet de combiner différents modes de renforcement des propriétés mécaniques, à savoir la nanostructuration, l’écrouissage, les solutions solides et les renforts particulaires. Dans un premier temps, l’étude s’est focalisée sur la possibilité de combiner haute densité et durcissement structural d’un alliage Al-Zn-Mg. La précipitation confère au matériau brut fritté des propriétés mécaniques un peu plus faibles que celles d’un alliage AA7020 de coulée traité à l’état T651. L’approche a ensuite été appliquée à la poudre broyée, le but étant alors d’associer densité et précipitation tout en préservant les grains ultrafins obtenus par broyage. Enfin, la problématique de la consolidation de composites à matrice métallique à grains ultrafins et à renforts B4C a été abordée / Lightweight materials are very attractive in the global industry, and more specifically in the field of automotive and aeronautics fields. For army vehicles the reduction of the weight has increased the need for lightweight metal and ceramic armor systems ; the combination of these materials being a key element in modern packages. Nowadays, aluminum alloys are widely introduced in such systems. Increasing the mechanical properties of these alloys will automatically imply a decrease of the mass of ballistic protections. In this context, the powder metallurgy route appears promising as it allows simultaneously to nanostructure the matrix by strain hardening and to scatter properly particles reinforcements. First, the choice has been made to focus on the combination of high density and hardening precipitation of an Al-Zn-Mg alloy. Strengthening precipitates give the consolidated raw powder mechanical properties close to the ones of a commercial wrought aluminum alloy AA7020 in a T651 temper. Then the same process was optimized on a milled powder in order to preserve the fine grains obtained by high energy ball milling. Finally, B4C particles were introduced as reinforcements in the aluminum matrix to develop an ultrafine-grained metal matrix composite