Yes / Obtaining closed porosity, i.e. densities >7.4 g.cm-3, is a major target in PM development. To increase density, strength and surface hardness of low-carbon PM steels: cold and warm compaction, sintering and slow cooling through the ferrite transformation region, followed by cold repressing and surface hardening were investigated. The slow cooling resulted in soft, ferritic, microstructure amenable to cold resizing. Repressing at 700-900 MPa densified the samples to ~7.6 g.cm-3. Mechanical properties, after repressing and surface hardening, are characterised by appreciable plasticity following macroscopic yielding at stresses of 400-1200 MPa. Reference is made to possible further increases in strength by incorporation of small additions of clean, fine Mn containing master alloy into the powder mix. Results were verified industrially on hollow cylinders made from Fe-0.5Mo or Fe-1.5Cr-0.2Mo base powders.

Copper is a material of interest to Additive Manufacturing (AM) owing to its outstanding material properties, which finds use in enhanced heat transfer and electronics applications. Its high thermal conductivity and reflectivity cause challenges in the use of Powder Bed Fusion AM systems that involve supplying high-energy lasers or electron beams. This makes Binder Jetting a better alternative as it separates part creation (binding together of powders) from energy supply (post-process sintering). However, it is challenging to fabricate parts of high density using this method due to low packing density of powder while printing. This work aims to investigate the effects of Hot Isostatic Pressing (HIP) as a secondary post-processing step on the densification of Binder Jet copper parts. By understanding the effects of HIP, the author attempts to create parts of near-full density, and subsequently to quantify the effects of the developed process chain on the material properties of resultant copper parts. The goal is to be able to print parts of desired properties suited to particular applications through control of the processing conditions, and hence the porosity. First, 99.47% dense copper was fabricated using optimized powder configurations and process parameters. Further, the HIP of parts sintered to three densities using different powder configurations was shown to result in an improvement in strength and ductility with porosity in spite of grain coarsening. The strength, ductility, thermal and electrical conductivity were then compared to various physical and empirical models in the literature to develop an understanding of the process-property-performance relationship. / Master of Science

Cette étude s’intéresse à l’amélioration et l’allégement des protections balistiques transparentes. L’utilisation conventionnelle de verre assure une haute efficacité face à un projectile, cependant associée à une masse élevée et à une forte épaisseur du blindage. Le remplacement de la face avant par une céramique polycristalline, telle que le spinelle MgAl2O4, assure un gain de performance et une réduction du volume de l’assemblage. L’élaboration de ce matériau requiert la combinaison d’une haute qualité optique dans le domaine du visible et de propriétés mécaniques élevées. Dans ce travail, l’application d’un frittage naturel sous vide d’une poudre commerciale de haute pureté a permis de limiter l’introduction d’impuretés néfastes à la transparence et la croissance granulaire. Une étape supplémentaire de pressage isostatique à chaud s’est montrée nécessaire à l’élimination des pores résiduels et à l’obtention de spinelles transparents de haute qualité optique (80% à 400-800 nm, e = 2 mm, Ø21 mm). Une étude de la microstructure avant et après post-traitement a permis de mettre en relation la taille des grains et des pores avant post-frittage avec la croissance granulaire observée pendant ce traitement. Une optimisation du procédé a ainsi pu être mise en place afin de limiter l’augmentation de la taille des grains et obtenir une microstructure homogène (~ 12 μm). Après un changement des dimensions des échantillons réussi (e = 4 mm, Ø60 mm), différents spinelles à propriétés microstructurales et mécaniques distinctes ont été sélectionnés pour une évaluation en conditions balistiques. / This work focuses on the improvement and the lightening of transparent ballistic armours. The conventional use of glass provides high efficiency against a projectile, however associated with a heavy and thick armour. The replacement of the strike face by a polycrystalline ceramic, such as MgAl2O4 spinel, leads to a performance gain and a decrease of the protection volume. The development of this material requires the combination of a high optical quality in the visible domain and high mechanical properties. In this work, pressureless sintering under vacuum of a high purity commercial powder allowed to minimize the addition of impurities, detrimental to the transparency, and the grain growth phenomenon. An additional step of hot isostatic pressing was necessary to eliminate residual porosity and to obtain transparent spinel with high optical quality (80% at 400-800 nm, t = 2 mm, Ø21 mm). An analysis of the microstructure before and after the post-treatment made it possible to determine the link between the grain and pore sizes before post-sintering and the observed grain growth during this treatment. An optimisation of the process was established in order to restrain the grain size increase, and thus to obtain a homogeneous microstructure (~ 12 μm). After a successful up-scaling of the samples (t = 4 mm, Ø60 mm), several spinel samples with distinctive microstructural and mechanical properties were selected in order to evaluate their performances through ballistic tests.

Ce travail de recherche porte sur l'élaboration de céramiques transparentes de CaLa₂S₄ pour la réalisation d'optiques MWIR et LWIR. Différentes méthodes de synthèse de poudres ont été explorées afin d'élaborer des poudres céramiques de grande pureté, cubiques et de morphologie adaptée à la densification. Il en a résulté le développement d'une méthode combustion comme nouvelle voie de synthèse de ce matériau. Différentes techniques de frittage ont été explorées : le SPS, le pressage à chaud (HP) et le frittage sous sulfure d'hydrogène (H₂S) suivi d'un post-frittage HIP. Le pressage à chaud permet d'élaborer des céramiques transparentes dans l'IR. Cependant leurs propriétés optiques sont dégradées par la présence de bandes d'absorption et par un noircissement important dû à l'interaction du matériau avec le graphite. Ces deux problématiques ont été résolues en combinant frittage naturel et compaction isostatique à chaud. La transmission des céramiques obtenues par ce procédé atteint à 13µm la transmission théorique de 68%. / This work focuses on the elaboration of CaLa₂S₄ transparent ceramics for IR optical applications in the MWIR and LWIR atmospheric windows. Various synthesis methods were explored for the elaboration of high purity cubic ceramic powders with adequate morphology for densification. As a result, we developed an innovative combustion method of this material. Different sintering techniques were investigated: Spark Plasma Sintering (SPS), Hot Pressing (HP) and Sintering in sulfurizing atmosphere (H₂S) combined to Hot Isostatic Pressing (HIP). Hot Pressing produces IR transparent ceramics. However, their optical properties are affected by the presence of absorption bands and significant blackening due to the interaction of the material with the graphite. These two issues have been solved by using sintering coupled to post-HIP. Transmission of the optics obtained by this process reaches the theoretical transmission of 68 % at 13μm.

Group IV transition metal carbides are promising materials for high temperature structural application, due to their unique sets of properties such as high melting temperature, high temperature strength, hardness, elastic modulus, wear and corrosion resistance, metal-like thermal and electrical conductivity and thermal shock resistance. This group includes zir-conium carbide, which, along with its composites, are potential candidates for applications such as nose cones for re-entry vehicle, engines, wear resistant parts and in nuclear fuel cladding. Such structural applications demand high strength material with minimal flaws, in order to achieve the required reliability. Attainment of high strength calls for fully dense material with as small a grain size as possible. Producing fully dense zirconium carbide requires very high temperature, which is a direct consequence of its high melting point. Higher processing temperatures increase grain size, thereby also causing a loss in strength, along with the increased cost. Therefore, there is always a driving force to produce such a material in fully densified form at as low a temperature as possible. There have been a number of studies on processing and densification of zirconium carbide. Pressureless sintering of zirconium carbide requires temperature of 2400oC-3000oC to reach reasonably high density. At such high temperatures, abnormal grain growth limits the final density, as pores get entrapped inside the grains. Hot pressing of zirconium carbide also requires upwards of 2000oC to reach high density and is the primary route to produce densified zirconium carbide product. Reactive hot pressing (RHP), is a relatively new processing approach. Here, the reaction between zirconium and carbon to produce zirconium carbide and the densification of the porous mass, occurs simultaneously. Study on reactive hot pressing of zirconium carbide have shown that, it is possible to achieve very high density at much lower temperatures 1600oC. Clearly, reactive processing is an exciting new technique to process zirconium carbide. However, there has been a lack of studies to understand why it provides better densification than conventional hot pressing. Such understanding is of paramount importance, as it can lead to better optimization of RHP and perhaps even lower the process temperature further. The objective of the present study is to understand the densification process in RHP of zirconium carbide through systematic and carefully designed experiments. A model of reactive hot pressing is also constructed to get more insight into the phenomenon. 0.1 Pressureless Reaction Sintering of Zirconium Car-bide Pressureless reaction sintering (RS) of zirconium carbide is studied to understand the role of stoichiometry and zirconium metal in densification. ZrC of four different stoichiometries are chosen for these sets of experiments which are conducted in vacuum at 1200oC and 1600oC for 1 hour to understand the role of stoichiometry. One sample of pure Zr is also sintered to elucidate the role of zirconium in densification. After reaction sintering, all the samples are characterized by density measurement, x-ray diffraction and microstructure, using scanning electron microscopy. After pressureless sintering at 1600oC, zirconium metal reaches the highest relative density of ~ 95%. Densification decreases monotonically with increasing stoichiometry. Zr+0.5C composition reaches the next best relative density (of 90%), while Zr+0.67C composition shows much lower densification. The other two compositions, Zr+0.8C and Zr+C, in contrast, display de-densification rather than densification. Since the pure zirconium sample reaches high density, it can, in principle, help in densification of the mixed powders before getting fully reacted. Non-stoichiometric carbides also exhibit higher diffusivity of carbon, which aids the densification and the greater the deviation from stoichiometry, the smaller the deleterious effects of de-densification from reaction. This troika of factors is responsible for the substantially better densification in non-stoichiometric carbide, compared to stoichiometric carbide. 0.2 Reactive Hot Pressing of Zirconium and Carbon Reactive hot pressing of zirconium carbide is explored with the emphasis on finding the underlying densification mechanism. The earlier proposed densification mechanism for RHP is the plastic flow of transient non-stoichiometric carbide. To differentiate the effect of transient phases from that of zirconium, RHP is carried out at 800oC. At this low temperature, transient phases cannot take part in plastic flow and subsequent densification. Thus, any densi cation at this temperature can be totally attributed to zirconium and the role of zirconium thus can be separated from that of transient phases. A combination of RHP and RS experiments are carried out at 1200oC to better understand the phenomenon. Again, ZrC carbide of four different stoichiometries are investigated in this RHP study. After RHP at 800oC, all the four different ZrC compositions reached more than 90% RD through plastic flow of the Zr leading to a continuous matrix with embedded graphite particles. Since the reaction remains incomplete at this temperature, it is clear that Zirconium alone is responsible for enabling densification at such a low temperature. It is therefore argued that any unreacted Zr would, at higher temperature, be able to drive densification even more. Thus, zirconium does not only participate in densification; it is a dominant factor enabling low temperature densification. Pressureless reaction sintering at 1200oC following the RHP at 800oC, results in de-densi fication, as the reaction between zirconium and carbon occurs with significant volume shrinkage. Since such shrinkage increases with stoichiometry of the carbide, the higher stoichiometry carbides are more susceptible to de-densification. RHP at 1200oC, mostly completes the reaction, but only ZrC0:5 reaches near theoretical density. Thus, the final density of the fully reacted mixture is arrived at through a combination of processes in which the more stoichiometric carbides suffer from not only the smaller metal content but also a greater volume shrinkage during reaction. Thus, ZrC0:5 reaches 99% RD whereas ZrC reaches only 85% RD. The interplay between these two processes may be controlled by a two step RHP begin-ning at 800oC followed by a ramp up to 1200oC. The higher RD achieved at 800 C results in a higher final density for all the four compositions. Thus, two step RHP is a novel way to get better densification in RHP of zirconium carbide. 0.3 Hot Pressing of Zirconium Carbide Powders of Different Stoichiometry In the literature, densification in RHP is mostly attributed to the presence of transient non-stoichiometric carbides. To examine this hypothesis, ZrC of three different stoichiometries are prepared and then subjected to hot pressing at the same temperature and pressure as the previous RHP experiments (i.e. 1200oC and 40MPa for 30 min). After the hot pressing experiments, ZrC0:5 composition shows significant densification (95% RD), whereas ZrC0:67 composition shows very limited densification (70% RD) and ZrC composition shows little or no densification (50% RD). Evidently, the transient phase formed with stoichiometry close to ZrC0:5 can certainly contribute substantially to densification. But for the more carbon-rich compositions, the transient phases do not appear to play a significant role and the benefit of RHP, wherein ZrC can reach 90% RD, must come from the contribution of metal plasticity. 0.4 Reactive Hot Pressing of Zirconium and Zirconium Carbide Two limiting factors for densification during RHP are, de-densification (courtesy of the reaction) and the gradual increase in volume fraction of a rigid, non-sintering phase. To investigate the role of these factors further, two compositions of mixed metal and carbide powders, namely Zr+ZrC and 0.5Zr+ZrC, are subjected to RHP. When reaction is complete, the compositions after RHP will correspond to ZrC0:5 and ZrC0:67, respectively, but with the following difference with respect to the metal-carbon mixtures investigated earlier: these new compositions do not experience de-densification due to reaction and they contain significantly more amount of hard phase (53 and 69%) in the starting composition than their zirconium and carbon mixture counterparts i.e. Zr+0.5C and Zr+0.67C (16 and 20%). These two compositions are subjected to the same process schedules, i.e., RHP at 800oC, pressureless reaction sintering at 1200oC following RHP at 800oC and two step 800oC and 1200oC RHP. After 800oC RHP, Zr+ZrC and 0.5Zr+ZrC compositions reach much lower density than Zr+0.5C and Zr+0.67C compositions as a direct consequence of the larger amount of hard phase hindering densification at the lower temperature. After the 1200oC pressureless sintering following the RHP at 1200oC, the RD of Zr+ZrC and 0.5Zr+ZrC compositions increase (which is opposite to the behaviour of Zr+0.5C and Zr+0.67C com-positions) as they do not su er from reaction derived de-densification. After two step RHP, Zr+ZrC and 0.5Zr+ZrC compositions reach a final RD that is higher than the Zr+0.5C and Zr+0.67C compositions, even though after the first RHP at 800oC, they were much less densified. Thus, the absence of de-densification during reaction is able to more than compensate for the increase in hard phase content. 0.5 Reactive Hot Pressing: Low temperature process-ing route Based on the major factors of densification identified earlier, it was investigated whether RHP temperatures could be brought down further while being supplemented by a free sintering step to complete the reaction without de-densification. From a practical standpoint, such a process would allow dense products to be made by hot pressing with low temperature dies and fixtures while carrying out a more economical pressureless sintering at higher temperatures Therefore, Metal-carbide mixtures, Zr+ZrC and Ti+ZrC, are chosen, along with a temperature of 900 C which is above the allotropic phase transformation temperature for Zr around 880oC, thereby utilizing a zirconium phase that is softer than the hexagonal Zr. For completion of reaction, pressureless reaction sintering is done at 1300oC and 1400oC. It is found that after 1400oC reaction sintering, both the compositions reach almost full density and the Ti+ZrC composition also shows a higher hardness (13 vs 10 GPa) than the Zr+ZrC composition, due to the formation of a binary carbide with consequent solid solution hardening. 0.6 Effect of Particle Size on Reactive Hot Pressing During RHP, premature exhaustion of zirconium by reaction can limit densification. One way to have better densification is to slow down the reaction, so that significant amount of densification takes place before the metal zirconium is exhausted. One way to reduce reaction rates is to increase particle size. Larger particles are expected to slow down the reaction without affecting sintering, as densification is controlled by power law creep of Zr which is grain size independent. Because of lack of availability of Zr with different particle sizes, two different graphite particle sizes, i.e. 7-10 m and 50-60 m, were studied and it was shown that after 1200oC RHP, indeed the larger particle size improves densification. 0.7 Modelling of Reactive Hot Pressing Reactive hot pressing is a complicated phenomenon, and to get an insight and also to optimize the parameters, the availability of a computational model is of paramount importance. Keeping that in mind, a model of RHP has been constructed based on four different parts, namely: 1. Densification of zirconium under pressure 2. Reaction of zirconium and carbon 3. The constraint on sintering from a rigid phase and, finally, 4. The volume contraction during reaction. The model uses published data for the 4 steps and shows reasonable qualitative and quantitative agreement with the experimental results. Further experiments are done with the model to optimize the processing parameters. Results from the virtual experiments consolidates our earlier conviction gained from experimental results, by showing zirconium is the principal factor in densification and exhaustion of zirconium coupled with reaction derived de-densification prevent the higher stoichiometric carbide from achieving full densification. It also shows, RHP gives best densification when reaction is 70-80% complete. So two step RHP where the first RHP will only complete the reaction 70-80%, and a final RHP at temperature which will complete the reaction, will possibly be the way to achieve best densification. 0.8 Conclusions The study on RHP of zirconium carbide led to the following conclusions: • Zirconium plays the most crucial role in densification. • Transient phases only play a role when the final stoichiometry of RHPed carbide is close to that of ZrC0:5. • De-densification from reaction prevents higher stoichiometric carbide from reaching full densification. • Two step RHP, with one RHP at lower temperature at which reaction will remain incomplete, and the other at higher temperature to complete the reaction, yields best densification. • For lower stoichiometric carbide (ZrC0:5,ZrC0:67), full densification can be achieved at 1200oC. For higher stoichiometric carbide, even though large amount of densification upward of 90% RD is achieved at 1200oC, full densification will be out of reach. • RHP shows better densification than conventional hot pressing for all stoichiometries.

Purpose of this thesis is to make constructional modification of single-purpose pressing machine for Huisman Konstrukce, s.r.o. company. Present machine is non-operational and not enough powerful for present requests of workshop. Intended modifications are to describe present condition of machine, find present requirements and design simple modifications. All modifications are checked by calculations and sketched in shop drawings. There were also made alternative options of modification to show machine more versatile and automatic.

Diploma thesis focuses on preparation and characterization of bulk material prepared from pure magnesium powder. The topic includes all the parts of process of bulk material preparation, sintering at hot pressing, diffusion processes and structurally-mechanical properties with respect to final material porosity of the produced material. Thesis solving focuses on study and control of processes during bulk material preparation and description of the processes from physically-chemical point of view of the structure creation and final material properties.

Detta examensarbete handlar om konceptframtagning av ett tilläggssystem för QFM-pressen som är tillverkad av Quintus Technologies. I dagsläget kan pressen endast utföra kallformspressning och efter ett avslutat projekt är målet att kunna kombinera både kall- och varmformspressning med hjälp av tilläggssystemet. Olika konceptidéer genererades med hjälp av brainstorming och sedan valdes det mest lämpliga konceptet utifrån en beslutsmatris. Genom ett iterativt arbete med det valda konceptet kunde ett resultat presenteras med 3D-modellering i PTC Creo Parametric. Efter ett avslutat projekt uppfylldes syftet och målet med detta examensarbete. / The purpose of this thesis project is to create a concept for an additional system that may be used by the QFM-press. Currently, the press can only execute cold-form pressing rather than a combination of both cold- and hot-form pressing. Different conceptual ideas were generated using brainstorming method, and the most suitable concept was chosen based on a decision matrix. After investigating possible solutions, results associated with the 3D-modelling in PTC Creo Parametric were present. By the end of this study, the purpose and goals of this thesis were fulfilled.

L’élaboration de combustible nucléaire UO2-30%PuO2 pour les nouveaux réacteurs nucléaires de Génération IV repose sur l’utilisation de plutonium issu du recyclage des combustibles MOX (Mélange d’OXydes UO2-PuO2) des réacteurs actuels. Par rapport au Pu initial, ce plutonium présente une proportion d’isotopes fissiles beaucoup plus faible et une quantité plus importante (x30) en 238Pu dont l’activité spécifique alpha et la puissance thermique sont importantes. Le procédé d’élaboration du combustible qui consiste à mettre en forme les poudres d’oxydes par pressage met en jeu un lubrifiant organique, le stéarate de zinc. L’objectif de la thèse est d’étudier le comportement en température et sous irradiation de ce composé. Un effet de la montée en température et du vieillissement en température a été observé sur les propriétés cristallographiques du stéarate de zinc, avec une amorphisation partielle du matériau qui engendre une détérioration de ses propriétés de lubrification dès 110°C. La dégradation radiolytique du stéarate de zinc a été étudiée à travers l’analyse des gaz produits par irradiation alpha au contact de poudres de PuO2 ou par irradiation externe aux hélions, complétée par des analyses physico-chimiques du stéarate irradié. Les rendements de production de gaz sont calculés et permettent d’établir un mécanisme de radiolyse. Il a été montré que l’impact de la radiolyse sur les propriétés de lubrification est moindre que l’effet de la température. Le couplage des dégradations a un effet synergique, avec une détérioration des propriétés de lubrification observée à des températures plus faibles que sur le matériau non irradié. A partir de ces résultats, des recommandations d’utilisation du stéarate de zinc ont été proposées. / The manufacturing of nuclear fuels UO2-30%PuO2 for the Gen IV nuclear reactors is based on the use of plutonium coming from MOX (Mixed OXides) fuel recycling from actual reactor. This plutonium would contain a few quantities of fissionable isotopes and a significant amount (x30) of 238Pu compared to initial Pu. This isotope possesses a strong alpha activity and a great thermal power. The manufacturing process which consists in powders pressing will use zinc stearate, an additive used as lubricant. The aim of this PhD is to study the behaviour in temperature and under irradiation of this compound. An effect of temperature increasing and thermal ageing has been observed on crystallographic properties with a material amorphisation and a deterioration of lubricant properties from 110°C. Radiolytic degradation of zinc stearate has been studied through the analysis of gases produced by alpha radiation at the contact of PuO2 powders or by external radiation by helions, with the support of chemical analysis of irradiated solid. Gaz production yields are calculated and enable establishment of a radiolysis mechanism. It has been showed that impact of radiolysis on lubricant properties is less important than temperature effect. The coupling of degradations has a synergic effect, with a deterioration of lubricant properties observed at lower temperature compared to non-irradiated material. From these results, recommendations for use of zinc stearate have been proposed.

A presente Tese de Doutorado analisa as modificações induzidas por prensagem isostática a quente (HIP) após a sinterização na resistência à fadiga de compósitos de metal duro com partículas de carboneto de tamanho médio na faixa de 0,6 a 2,0 ?m. A correlação entre a redução de porosidade no processo de prensagem isostática a quente e o aumento de resistência à ruptura transversal é bem conhecida. No entanto, existem questões em relação à eficácia deste processo no aumento da resistência à fadiga, devido à existência de outras imperfeições na microestrutura do material. Um aumento maior na resistência transversal é reportado na literatura para temperaturas de HIP acima do ponto eutético da liga. Os experimentos foram realizados em dois níveis diferentes de pressão de HIP, simulando condições de processo que existem em fornos de média e alta pressão (5,4 MPa e 150 MPa). Também são analisadas duas condições de temperatura de sinterização, 1.350ºC e 1.430ºC. A análise das diferentes condições de processamento mostra que a microestrutura mais homogênea, praticamente sem presença de lagos de cobalto, foi obtida a 1.430°C de temperatura de sinterização e de HIP. Esta condição de processamento resultou em melhores valores de resistência mecânica (estática e dinâmica). As diferenças de microestrutura e propriedades mecânicas obtidas nas duas condições de pressão de HIP (5,4 MPa e 150 MPa) são pouco significativas, tendo maior impacto a correlação entre a microestrutura e propriedades mecânicas em função dos diferentes ciclos de temperatura e tempo de sinterização. A análise das superfícies de fratura do material em fadiga indica uma importante influência de defeitos microestruturais, como regiões alinhadas da fase ? entre grãos de carboneto de tungstênio na iniciação e na propagação da trinca de fratura. / The present Thesis evaluates modifications induced by hot isostatic pressing (HIP) after sintering on fatigue strength of WC-11Co (in weight %) composites with sintered tungsten carbide grains of 0.6 to 2 ?m. The correlation between decreasing porosity due to hot isostatic pressing process and subsequent increase on transversal rupture strength is well known. Nevertheless, there are questions related to the efficiency of HIP process to increase fatigue strength, mainly due to the existence of microstructure defects besides porosity. An increase on transversal rupture strength is reported in the literature for HIP temperatures above the eutectic point of the composite. Experiments were carried out at two different levels of HIP pressure (5.4 and 150 MPa) covering conditions existing on medium and high pressure furnaces. Two temperatures were also evaluated (1,350 and 1,430°C). The analysis of different processing conditions shows that a more uniform microstructure, without the presence of cobalt lakes, was obtained at a sintering and HIP temperature of 1,430°C. This processing condition also resulted in better statical and dynamical mechanical properties. Microstructure and mechanical properties obtained at two different HIP pressure conditions (5.4 and 150 MPa) were rather similar. Major changes in microstructure and mechanical properties were found as a function of different cycles of temperature and sintering time. Fatigue fracture surfaces were evaluated revealing major influence of microstructural defects such as regions of aligned ? phase where crack initiation and fracture propagation are favored.