Vacuum-Arc Remelting (VAR) is an important process for manufacturing Ti- and Ni-based superalloys. Currently, the sources and mechanisms behind microstructural anomalies produced in VAR superalloy ingots are not well understood. In order to help understand formation processes, a method of characterizing specific anomalies in VAR ingots is desired. This paper presents a method of characterizing the composition and morphology of anomalies in VAR alloy ingots using a combination of serial sectioning and X-ray fluorescence (XRF) energy dispersive spectroscopy (EDS) techniques. This process is demonstrated on a dirty white spot from an Alloy 718 sample. The white spot of interest was serial polished and 2-D XRF EDS maps were acquired at each polish depth. The EDS maps were then stacked to form a 3-D representation of the white spot. In addition, SEM and optical microscopy techniques were used to further characterize the composition and morphology of the dirty white spot. The dirty white spot is composed of both Ti-enriched and Nb-depleted regions. The 2-D EDS maps acquired with the XRF equipment provided adequate contrast for creating a 3-D representation of the Ti-rich region of the dirty white spot. However, contrast was not sufficient to create a 3-D representation of the Nb-depleted region. The XRF EDS equipment combined with SEM and optical microscopy techniques provided valuable information about the morphology and composition of the Alloy 718 dirty white spot. It is concluded that this dirty white spot was produced by fall-in from either the crown or shelf regions during the VAR process. / text

A soldagem tem grande importância no setor metroferroviário, pois é empregada na fabricação de componentes estruturais e no acabamento de vagões de passageiros, que em sua maior parte são de aço inoxidável. As juntas soldadas podem apresentar descontinuidades que são interrupções que afetam as propriedades mecânicas e metalúrgicas da junta soldada. A presença destas descontinuidades, dependendo do seu tamanho, natureza ou efeito combinado, pode ocasionar a reprovação da junta soldada, quer pela redução de propriedades mecânicas ou pela não aceitação, segundo critérios estabelecidos em normas. Uma estrutura que tenha uma solda reprovada durante sua qualificação ou inspeção, pela presença de descontinuidades pode ser recuperada, por meio de um retrabalho a ser realizado nesta junta. A refusão do cordão de solda por meio do processo TIG (Tungsten Inert Gas), sem a utilização do material de adição, é uma técnica de retrabalho que pode ser empregada, em especial pela viabilidade técnica e econômica do processo. Neste estudo analisou-se a influência que o processo de reparo por soldagem TIG exerceu no comportamento mecânico e microestrutural das juntas soldadas pelo processo MIG, por meio de: ensaios mecânicos (tração, fadiga e microdureza Vickers), ensaios não destrutivos (inspeção visual e líquidos penetrantes) e caracterização microestrutural do cordão de solda. Resultados das amostras de ensaio de tração e fadiga indicaram que o reparo dos cordões de solda não alterou o comportamento mecânico das juntas. As juntas submetidas ao reparo nas quais foram retirados os reforços dos cordões apresentaram modificações nas propriedades mecânicas, mas também apresentaram resultados satisfatórios. / The welding has great importance in manufacture of subway cars because it is used in the manufacture of structural components and finishing in passenger cars which are mostly stainless steel. Joints may show the presence of discontinuities. The discontinuities are interruptions that affect the mechanical and metallurgical properties of the weld. The presence of these discontinuities, depending on their size, nature or the combined effect may lead to failure of the weld either by reduction of mechanical properties or non-acceptance criteria established in standards. A structure that has a weld failed during its qualification or inspection by the presence of discontinuities can be retrieved through a rework to be done in this joint. The remelting of the weld using the TIG process without the use of additional material, rework is a technique that can be employed in particular the technical and economic feasibility of the process. This study analyzes the influence that the process of TIG welding rework has on mechanical behavior and microstructure of welded joints by means of: mechanical tests (tensile and Vickers hardness), non-destructive testing (visual inspection and liquid penetrant) and characterization microstructure of the weld. Results of tensile test specimens and fatigue indicated that the repair of the weld beads did not change the mechanical behavior of joints. The joints subjected to repair in which they were removed the ribs of the beads showed changes in the mechanical properties, but also had satisfactory results.

<p> The dominant slag-metal reaction site in AC electro-slag remelting (ESR) has been directly determined by estimating the rate of sulphur transfer at the electrode tip/slag and metal pool/slag interfaces. In this investigation, negligible sulphur transfer has been assumed at the droplet/slag interface, and the basis for such assumption is discussed. Using a 5.825" diameter mould and CaF2-CaO slags, it has been established that the electrode tip/slag interface is the dominant reaction site. Calculations indicate that the pool sulphur is close to equilibrium with the slag. The importance of all six reaction sites in AC ESR are discussed. The results indicate the importance of the slag/atmosphere interface, especially when using slags of low lime content. Conventonal overall sulphur balances have been carried out and the results are in agreement with the rates obtained at the different interfaces, thus indicating internal consistency of the results. </p> / Thesis / Master of Engineering (MEngr)

Nästan 80 procent av den globala energitillförseln kommer från fossila källor, och behöver drastiskt minska. Industriell spillvärme är en energitillgång som blir allt attraktivare men är ofta lågtempererad, vanligtvis mellan 30°C och 100°C, genom att tillsätta en värmepump och låta värmen gå till fjärrvärmenätet blir ofta återbetalningstiden låg. Uddeholms AB använder idag fjärrvärme och en naturgasbaserad spetsvärme för att möta sitt energibehov. Uddeholm AB har stora mängder lågtempererad spillenergi i form av kylvatten från processtegen Electro slag remelting (ESR) och ljusbågsugnen (LBU) som i nuläget inte utnyttjas. Syftet är därför att skapa ett underlag för rekommendation om hur energi som genereras inom industrin kan utvinnas för att möta energibehovet och för att minska användandet av naturgasbaserad spetsvärme. Genom simuleringsprogrammet Simulink har en dynamisk modell byggts upp för att simulera 6 olika system över ett år med en timmes tidssteg. Systemen består av olika kombinationer av värmepumpar, värmeväxlare och ackumulatortank och använder kylvattenflödet från ESR och LBU. Referenssystemet är systemet som används idag, och det första systemet består av en värmepump som använder ESRs kylvattenflöde på värmepumpens kalla sida för att leverera energi till fjärrvärmereturen (VPESR). Det andra systemet består av en värmeväxlare på ljusbågsugnens kylvatten som återvinner energin från ljusbågsugnens kylvatten till fjärrvärmereturen (VVX). Det tredje systemet är en kombination av de två första systemen (VVX + VPESR). I det fjärde systemet tillsätts en extra värmepump efter värmeväxlaren på det tredje systemet (VVX + VPLBU + VPESR). Det femte systemet innebär en värmepump på ljusbågsugnens kylvatten och en värmepump på ESRs kylvatten (VPLBU + VPESR). Det sjätte systemet är samma som det femte men med undantaget att en ackumulatortank tillsätts på ljusbågsugnens kylvatten innan det går in i värmepumpen (VPLBU + Ack + VPESR). Studiens resultat visar att behovet av den naturgasbaserade spetsvärmen försvinner för samtliga fall och miljöpåverkan är lägre än vid referensfallet för samtliga system eftersom den naturgasbaserade spetsvärmen inte längre behöver användas. Vilken miljöpåverkan systemen har beror till stor del på vilken indata som används, och behöver därför tas i beaktning. Ekonomiskt så är samtliga system lönsamma över 20 år med en återbetalningstid på under 5 år. I framtiden bör möjligheten till att sälja energi som produceras till fjärrvärmeföretaget för att maximera lönsamheten ytterligare. Resultaten följer trenden att fler och större värmepumpar resulterar i en högre elförbrukning, lägre fjärrvärmebehov och högre miljöpåverkan. En ackumulatortank bidrar till ett jämnare COP och ett högre årsmedel COP men anses vara en mindre fördelaktig investering eftersom den inte genererar tillräckligt höga resultat för att täcka sin höga initiala kostnad. Det är av största grad viktigt att höja temperaturen på ljusbågsugnens kylvatten eftersom en höjd temperatur minskar energibehovet med ungefär 20%, och miljöpåverkan i form av koldioxidalstring sänks då med ungefär 20%. Systemet uppnår även ungefär 90% större lönsamhet med ökade temperaturer och ett år kortare återbetalningstid utan ackumulatortank och 5 år kortare med ackumulatortank. Det fjärde systemet (VVX + VPLBU + VPESR) har lägst energibehov på ungefär 2 000MWh och uppnår störst lönsamhet över 20 år på nästan 400 Mkr. System två (VVX) har lägst miljöpåverkan i form av koldioxidalstring med en minskning på ungefär 90% jämfört med referenssystemet och kortast återbetalningstid på under 2 år. Därav rekommenderas det andra systemet (VVX) om miljö och återbetalningstid prioriteras och det fjärde systemet (VVX + VPLBU + VPESR) om lönsamhet och ett minskat energibehov prioriteras. / Almost 80% of global energy supply comes from fossil sources and needs to be drastically reduced. Industrial waste heat is an increasingly attractive energy resource but is often low temperature, usually between 30°C and 100°C, and by adding a heat pump and letting the heat go to the district heating network, the payback time is often low. Uddeholm AB currently uses district heating and a natural gas-based peak heating to meet its energy needs. Uddeholm AB has large amounts of low-temperature waste energy in the form of cooling water from the process steps Electro slag remelting (ESR) and the electric arc furnace (LBU) that are currently not utilized. The purpose is therefore to create a basis for recommendations on how energy generated in industry can be recovered to meet the energy demand and to reduce the use of natural gas-based peak heat. Through the simulation program Simulink, a dynamic model has been built to simulate 6 different systems over a year with one hour time steps. The systems consist of different combinations of heat pumps, heat exchangers and storage tanks and use the cooling water flow from ESR and LBU. The reference system is the system used today, and the first system consists of a heat pump that uses the ESR cooling water flow on the cold side of the heat pump to supply energy to the district heating network (VPESR). The second system consists of a heat exchanger on the arc furnace cooling water that recovers the energy from the arc furnace cooling water to the district heating network (VVX). The third system is a combination of the first two systems (VVX + VPESR). The fourth system adds an additional heat pump after the heat exchanger of the third system (VVX + VPLBU + VPESR). The fifth system involves a heat pump on the arc furnace cooling water and a heat pump on the ESR cooling water (VPLBU + VPESR). The sixth system is the same as the fifth but with the exception that an accumulator tank is added to the arc furnace cooling water before it enters the heat pump (VPLBU + Ack + VPESR). The results of the study show that the need for the natural gas-based peak heat disappears for all cases and the environmental impact is lower than in the reference case for all systems because the natural gas-based peak heat no longer needs to be used. The environmental impact of the systems depends largely on the input data used, and therefore needs to be taken into account. Economically, all systems are profitable over 20 years with a payback period of less than 5 years. In the future, the possibility of selling the energy produced to the district heating company should be considered to further maximize profitability. The results follow the trend that more and larger heat pumps result in higher electricity consumption, lower district heating demand and higher environmental impact. An accumulator tank contributes to a more even COP and a higher annual average COP but is considered a less beneficial investment as it does not generate high enough results to cover its high initial cost. It is of utmost importance to increase the temperature of the cooling water of the electric arc furnace because an increased temperature reduces the energy demand by about 20%, and the environmental impact in terms of carbon dioxide generation is then reduced by about 20%. The system also achieves about 90% greater profitability with increased temperatures and a one year shorter payback period without a storage tank and 5 years shorter with a storage tank. The fourth system (VVX + VPLBU + VPESR) has the lowest energy demand of about 2 000 MWh and achieves the highest profitability over 20 years of almost 400 MSEK. System two (VVX) has the lowest environmental impact in terms of CO2 generation with a reduction of about 90% compared to the reference system and the shortest payback period of less than 2 years. Hence, the second system (VVX) is recommended if environment and payback time are prioritized and the fourth system (VVX + VPLBU + VPESR) if profitability and reduced energy demand are prioritized.

La projection hybride combinant la projection plasma et la refusion laser est une solution alternative permettant de diminuer ou même d'éliminer la présence de pores et de fissures au sein d'un dépôt brut de projection. Lors d'un procédé de projection plasma ou de refusion laser, le système substrat/dépôt subissant l'élévation de température,la fusion, la solidification et le refroidissement rapide est ainsi soumis à des gradients de température élevés, un niveau de contraintes élevé, et même un risque de formation de fissures. Il est alors important de pouvoir maîtriserles variations de température et la distribution des contraintes résiduelles au sein même du système. Concernantles travaux effectués dans cette thèse, des modèles thermiques et mécaniques ont été développés à l'aide dulogiciel ANSYS, de sorte à améliorer notre compréhension des comportements thermique et mécanique des revêtements élaborés par projection plasma avec refusion laser.Tout d'abord, la simulation du procédé de projection plasma a été développée afin de prédire les champs transitoires de température, la déformation finale de l'échantillon et les contraintes résiduelles dans des dépôts céramiques (alumine) et métalliques (NiCrBSi). Les contraintes résiduelles résultent de l'équilibre entre contraintes de trempe (toujours en tension) et contraintes thermiques lors du refroidissement final (en compression ou en tension suivant le cas). En raison de la faible limite d'élasticité du dépôt, les contraintes thermiques dominent les contraintes résiduelles dans le cas du dépôt d'alumine. En revanche, les contraintes de trempe dans le dépôt de NiCrBSi sont plus importantes. Par ailleurs, l'augmentation de l'efficacité du système de refroidissement mis en oeuvre permet de diminuer le niveau des contraintes résiduelles dans le dépôt d'alumine. Inversement les contraintes résiduelles au sein du dépôt de NiCrBSi augmentent lorsque le refroidissement en cours de projection est accru. L'analyse thermique permettant de simuler le procédé de refusion laser à posteriori d'un revêtement de NiCrBSi a été effectuée de sorte à évaluer les effets des paramètres du laser sur le champ de température engendré et laforme du bain refondu. Un coefficient d'absorption de 0.5 a été estimé par comparaison de l'épaisseur du bain refondu obtenue par les méthodes numérique et expérimentale. De plus, les morphologies de revêtements refondus ont été caractérisées par méthodes expérimentales. Selon l'analyse mécanique de la refusion laser à posteriori, les contraintes résiduelles sont en tension dans le dépôt refondu, mais en compression vers la zone non-refondue.Concernant le modèle portant sur l'élaboration de couches multiples, permettant de simuler le procédé de refusion laser in-situ, une diminution des contraintes transitoires au sein des couches préalablement élaborées est induite par l'accumulation progressive de chaleur au cours du processus d'élaboration. Après refroidissement final, les contraintes résiduelles suivant la direction de déplacement présentent une amplitude plus importante que suivant les autres directions. De plus, des solutions permettant de diminuer le niveau des contraintes dans le dépôt après refusion ont été recherchées, en faisant notamment varier l'efficacité du système de refroidissement ou le préchauffage du substrat. Il s'avère que l'amplitude des contraintes résiduelles dans le dépôt après refusion est plus sensible à la variation du préchauffage du substrat qu'à la modification de l'efficacité du système de refroidissement. / The hybrid spraying process consisting in plasma spraying and laser remelting is an alternative method to minimize or even eliminate the potential defects within the as-sprayed coatings.During the treatment of plasma spraying or laser remelting, the substrate/coating system undergoing heating, melting, solidification and fast cooling processes is submitted to high temperature gradients, high stress levels and even risks of crack formation. It is therefore important to control the temperature variation and stress level within the substrate/coating system. In this study, thermal and mechanical models established with ANSYS were developed to provide a fundamental understanding of thermal and mechanical behaviors of deposited coatings during plasma spraying and laser remelting processes.3D simulation models were first developed to predict the temperature field, the final deformation of the specimen, and the residual stresses within ceramic (alumina) and metallic (NiCrBSi) coatings. The final residual stresses result from the balance between quenching stresses (tensile) and thermal stresses (compressive or tensile). Due to the low value of the yield stress, the thermal stresses (compressive for that case) dominate the final stress level in the case of the ceramic coating (alumina). On the contrary, the final residual stresses within the NiCrBSi coating are tensile. It is also predicted that an increase of the cooling efficiency induces a lower stress level for the alumina coating, while improving the cooling efficiency slightly increases the residual stresses for the case of the metallic coating (NiCrBSi).Thermal analysis to predict laser post-remelting of a NiCrBSi coating was then carried out to investigate the effects of the laser parameters on the temperature field as well as on the remelted pool shape. An absorption coefficient of 0.5 was estimated by comparison of the remelted pool depths obtained by experimental and numerical methods. In addition, the morphologies of theremelted coating were characterized by experimental methods. According to the mechanical analysis for laser post-remelting of NiCrBSi coatings, the predicted residual stresses were tensile within the remelted coating, whereas those within the non- emelted zone were reversed from tensile before remelting to compressive after laser post-remelting. According to the numerical analysis of in situ laser remelting by a multi-layer model, transient stresses in the former deposited layers were decreased progressively due to the heat accumulation during the following deposition process. For the residual stresses after final cooling, the stress component along the displacement direction presents a higher level in comparison with the transverse andthrough thickness components. Solutions allowing decreasing the stress level within the remelted coating were studied by changing the cooling efficiency and the initial temperature of the substrate. It was noted that the generated residual stress (tensile) was more sensitive to the initial temperature of the substrate, than to the efficiency of the cooling system. In particular, increasing the initial temperature of the substrate could significantly decrease the stress magnitude within the remelted coating, and even reverse the residual stresses within the nonremelted part of the coating to compressive values.

Made available in DSpace on 2016-12-08T17:19:23Z (GMT). No. of bitstreams: 1 Marcio de Rossi.pdf: 25090067 bytes, checksum: ab7c04de669e27612ef763eb16850faf (MD5) Previous issue date: 2014-01-31 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O presente trabalho descreve o tratamento térmico superficial a laser, caracterização dos microconstituintes e perfis de dureza obtidos no ferro fundido cinzento classe NBR FC 250. Foram variados os parâmetros de potência (300w, 400w, 500w e 600w) e velocidade de varredura (100mm/s, 20mm/s, 10mm/s, 5mm/s, 3,5mm/s e 2mm/s). Os parâmetros constantes utilizados foram diâmetro feixe do laser 8,18mm e sem sobreposição das trilhas do laser. As amostras foram caracterizadas após tratamento térmico superficial através de microscopia óptica, microscopia eletrônica de varredura e microdureza Vickers. A profundidade da camada refundida das amostras apresentou valores crescentes com a energia, para a profundidade da ZTA (camada temperada) relação com a energia incidida não foi observada. Dos resultados obtidos verificou-se que a têmpera superficial sem refusão no ferro fundido cinzento FC250 ocorre em uma faixa bem restrita, sendo que através dos parâmetros de tratamento térmico utilizados neste trabalho foi possível determinar linhas de tendência para obtenção de tal condição. Os parâmetros utilizados no tratamento térmico superficial a laser das amostras K (500w, 5mm/s), L (600w, 5mm/s), N (400w, 2mm/s) , O (500w, 2mm/s), P (600w, 2mm/s), X (500w, 3,5mm/s), Z (600w, 3,5mm/s), foram os que se obtiveram transformação de fase e consequentemente endurecimento superficial. Os parâmetros de tratamento térmico utilizados nas amostras K e N foram suficientes para promover têmpera superficial no ferro fundido cinzento, sendo que se observou 6 microestrutura martensítica refinada nestas condições. Nas amostras K e N obteve-se microdurezas superficiais de 800HV e 600HV respectivamente, sendo estes valores de microdureza superficial superior as amostras refundidas. Os parâmetros de tratamento térmico utilizados nas amostras L, X, Z, O, P promoveram refusão no ferro fundido cinzento, sendo que nestas condições foi observado microestrutura ledeburítica na região refundida, com presença de austenita retida entre a região refundida e a zona termicamente afetada (região temperada). A presença de ledeburita e austenita retida na superfície após tratamento térmico das amostras L, X, Z, O, P implicaram em menores durezas superficiais, porém nestas condições obteve-se maior profundidade de endurecimento quando comparada as amostras K e N.

Le procédé de refusion sous laitier électroconducteur (ESR =Electro Slag Remelting) permet de produire des alliages à haute valeur ajoutée utilisés pour des applications critiques. Les mesures in-situ sur les fours industriels étant coûteuses, la simulation numérique est essentielle à la maitrise et à la compréhension de ce procédé complexe. L’Institut Jean Lamour a développé depuis plusieurs années un modèle 2D axisymétrique qui permet de simuler la refusion d’une électrode consommable au sein d’une lingotière considérée comme totalement isolée électriquement du reste du système. Sont alors décrit en régime transitoire les transferts couplés de chaleur et de quantité de mouvement, ainsi que le passage du courant électrique dans le laitier et le lingot lors de la croissance et de la solidification de ce dernier. Les connaissances acquises au cours des dernières années sur le procédé ESR remettent en cause l’hypothèse d’isolation électrique du moule au cours de la refusion. L’objet de cette thèse est d’intégrer et d’étudier la possibilité de passage de courant entre le laitier et la lingotière au cours de la refusion. Un premier modèle a été développé. Il consiste en un calcul électromagnétique complet dans l'ensemble du système pour une géométrie simplifiée. Il a permis de simplifier la mise au point du modèle global, tout en fournissant de premiers résultats. Par la suite, un modèle totalement couplé a été finalisé puis les modifications du code ont fait l’objet de validations avec des mesures expérimentales. Des études de sensibilité ont été menées pour tester l’influence des propriétés du laitier et des paramètres opératoires sur la qualité du lingot final. / The ElectroSlag Remelting process (ESR) is widely used to produce high added value alloys for critical applications (aerospace industry, nuclear plants, etc.). Trial-and-error based approaches being expensive, numerical simulation is fundamental to improve the knowledge and the understanding of this complex process. The Institut Jean Lamour has been developing for several years a numerical code to simulate the melting of a consumable electrode, supposedly perfectly cylindrical, within a mold assumed to be perfectly electrically insulated from the electrode-slag-ingot system. Based on these assumptions, the 2-D axisymmetrical transient-state numerical model accounts for electromagnetic phenomena and coupled heat and momentum transfers, to simulate the continuous growth of the electroslag remelted ingot and the solidification of the metal and slag. Recent studies on the ESR process are challenging the insulated mold hypothesis. Therefore, the main objective of the thesis is to acknowledge and study the existence of a mold current during an ESR remelting. A first model has been set-up, aimed to simulate the electromagnetic phenomena in the whole system for a simplified geometry. The possibility of the existence of such a mold current was confirmed. Based on this work, a fully-coupled model has then been developed and the results have been compared with experimental data to check the validity of the modifications. The influence of slag properties and operating parameters on the final quality of the ingot has been tested.

Made available in DSpace on 2017-07-21T20:42:47Z (GMT). No. of bitstreams: 1 Katieli Tives Micene.pdf: 4623642 bytes, checksum: 36b44944e6ba4a03ed1e2905ac60f954 (MD5) Previous issue date: 2014-08-31 / The technique of remelting superficial laser (RSL) has been set up as a tool of increasing interest in recent years because of its ability to improve the corrosion performance of aluminum alloys, as a result of the formation of thin melting layers, with refined microstructures and practically free of intermetallic precipitates and inclusions. The laser treated samples are much more chemically homogeneous than the base material. This technique is promising versatile and it can be used to modify the surface region by melting and rapid solidification processes without affecting the property of the base material. In this study we used the RSL technique in a media without gas protection and with 2 kW Yb-fiber laser (IPG YLR-2000S), it was applied to the hypereutectic alloy Al-2.0 wt%Fe in order to analyze the changes caused by the treatment, therefore the resultant microstructure was evaluated, as well as, the surface quality (roughness), hardness and corrosion resistance in the aerated media 25ºC, solution of H2SO4 0.1 mol/L. The study of corrosion was analyzed by different techniques such as corrosion potential measurement, cyclic voltammetry and potentiodynamic polarization tests (micro and macro polarization) and the technique of electrochemical impedance spectroscopy (EIS). By EIS technique, the characterization of the electrochemical behavior of the treated layer was performed and the values of the equivalent circuit, composed of resistors, capacitors and inductors. Therefore equivalent electrical circuits were generated according to the results presented by EIS using computational mathematical tools and Matlab software (tools toolbox) was used to perform this step. These results showed that the laser surface treatment performed on the alloy Al-2.0 wt%Fe resulted to the formation of a more compact structure, homogeneous and finer microstructure, producing an increase in microhardness at about 60.7% compared to the base material, reducing roughness, and corrosion rates was of approximately 11 lower smaller when compared to the untreated alloy, the laser treated layer has behaved passively in a wide range of potential, the polarization resistance of the treated layer was higher in relation to the untreated sample, by the EIS technique at high frequencies the samples had capacitive behavior, however, at low frequencies the samples had an inductive behavior. The treated layer thus evidence the effectiveness of treatment RSL to improve the surface quality of the samples and wear and corrosion resistance. / A técnica de Refusão Superficial a Laser (RSL) tem se configurado como uma ferramenta de crescente interesse nos últimos anos, por sua capacidade de melhorar o desempenho à corrosão das ligas de alumínio, tendo como resultado a formação de camadas fundidas finas com microestruturas refinadas que são praticamente isentos de precipitados intermetálicos e inclusões. As amostras tratadas com laser são muito mais homogêneas quimicamente do que o material base. Esta técnica é versátil e promissora e pode ser utilizado para modificar a região superficial por processos de fusão e solidificação rápida sem afetar a propriedade da amostra base. No presente trabalho utilizou-se a técnica de RSL sem proteção a gás com 2 kW Yb-fibra laser (IPG YLR-2000S) que foi aplicado na liga hipereutética Al-2%Fe a fim de analisar as modificações causadas pelo tratamento, para tanto foram avaliadas a microestrutura resultante, qualidade superficial (rugosidade), dureza e a resistência à corrosão em meio aerado à 25ºC em solução de H2SO4 0,1 mol/L. O estudo da corrosão foi analisado por diferentes técnicas tal como o monitoramento do potencial de corrosão, voltametria cíclica e ensaios de polarização potenciodinâmicas (micro e macropolarização), além da técnica de espectroscopia da impedância eletroquímica (EIE). Através da EIE, pôde-se caracterizar o comportamento eletroquímico da camada tratada e estipular os valores do circuito equivalente formado por resistores e capacitores e indutores. Para tanto foram gerados circuitos elétricos equivalentes aos dados apresentados pela EIE utilizando ferramentas matemáticas computacionais, sendo o software Matlab com ferramentas (toolbox) escolhido para realização desta etapa. Os resultados desta pesquisa indicaram que o tratamento superficial a laser efetuado na liga Al-2%Fe conduziu à formação de uma estrutura com microestrutura mais compacta, homogênea e refinada, levando ao aumento de dureza superficial em torno de 60,7% em relação ao material base, redução de rugosidade e taxas de corrosão de aproximadamente 11 vezes menor quando comparada com a liga não tratada, evidenciando assim a eficiência do tratamento RSL para melhorar a qualidade superficial das peças em termos de resistência ao desgaste à corrosão.

Afin de protéger les composants hydrauliques des phénomènes d’érosion par cavitation, les pièces sont très souvent revêtues par projection thermique. Parmi les matériaux qui présentent de bonnes performances contre cette forme d’érosion, certaines recherches, trop rares encore, relèvent les excellentes propriétés de la zircone stabilisée à 8 wt% yttrium (YSZ). C’est pourquoi, dans cette étude, des revêtements YSZ à 8 wt% ont été fabriqués avec différents procédés de projection thermique et post-traités par refusion laser puis ils ont été soumis aux tests de cavitation référencés selon la norme ASTMG32.Le revêtement YSZ a tout d’abord été élaboré par projection plasma atmosphérique (APS). Différentes tailles de poudre YSZ et différentes températures de préchauffage du substrat ont été étudiées pour observer leurs effets sur la tenue en cavitation des échantillons. La simulation de cavitation acoustique de type Caflisch a permis d’estimer la vitesse du jet d'eau et les contraintes exercées par l'effondrement des bulles à la surface des échantillons. Les résultats en cavitation des deux revêtements YSZ et 304SS ont été étudiés et leurs mécanismes discutés.Ensuite et afin densifier ces échantillons, des revêtements composites YSZ-NiCrBSi contenant de 5 à 25 wt% NiCrBSi dans le mélange ont été fabriqués avec le même procédé APS. Les résultats de l'érosion par cavitation montrent qu'une faible adhérence des particules de NiCrBSi vient contrebalancer l'effet positif de la diminution de la porosité dans ces revêtements. Le revêtement YSZ a été finalement post-traité par refusion laser et différents paramètres laser ont été testés pour optimiser leur densification. Il s’avère que la refusion au laser produit une couche refondue dense en surface mais aussi des fissures à l'intérieur des revêtements. Exposés au test d'érosion par cavitation, il a été observé des arrachements importants de la partie refondue du revêtement et un décollement à son interface, d’où une perte de masse rapide. C’est pourquoi, de l'époxy a été utilisée pour infiltrer ces fissures. Le revêtement refondu par laser et infiltré par époxy présente une amélioration significative de la résistance à la cavitation. Enfin, le procédé de projection de plasma sous vide (VPS) qui permet d’obtenir des revêtements très denses a été utilisé pour fabriquer le revêtement YSZ. Un revêtement de porosité inférieure à 1% a été obtenu. En raison de la température élevée du substrat pendant la projection VPS, les liaisons inter lamellaires sont améliorées, contribuant à augmenter significativement la résistance à la cavitation. / In order to protect the hydraulic components from cavitation erosion phenomena, the parts are often coated by thermal spraying. Buck YSZ shows an excellent performance against cavitation erosion. However, the cavitation erosion resistance of YSZ coatings have vaguely been studied. Therefore, in this study, YSZ were manufactured with different thermal spraying processes and post-treated by laser remelting, then they were subjected to cavitation tests according to ASTM G32.The YSZ coating was first manufactured by atmospheric plasma spraying (APS). Various sizes of YSZ powder and different preheating temperatures of the substrate were studied to observe their effect on the cavitation behavior of the samples. The acoustic cavitation simulation of the Caflisch type has been used to estimate the speed of the water jet and the stresses exerted by the collapse of the bubbles on the surface of the samples. The cavitation results of the two YSZ and 304SS coatings were aslo studied to discuss cavitation damage of a thermally sprayed coating.Then in order to densify these samples, YSZ-NiCrBSi composite coatings containing from 5 to 25 wt% NiCrBSi in the mixture were manufactured with the APS process. The results of cavitation erosion show that low adhesion of NiCrBSi particles counterbalances the positive effect of decreasing porosity in these coatings. The YSZ coating was then post-treated by laser remelting and various laser parameters were tested to optimize their densification. It turns out that laser remelting produces a dense surface layer but also cracks inside the coatings. Exposed to the cavitation erosion test, it was observed significant removal of the remelted parts of the coating and, resulting in rapid mass loss. This is why epoxy has been used to infiltrate these cracks. The laser remelted and epoxy infiltrated coating exhibits a significant improvement in cavitation resistance. Finally, the vacuum plasma spraying method (VPS), which provides very dense coatings, was used to make the YSZ coating. A porosity coating of less than 1% was obtained. Due to the high temperature of the substrate during VPS projection, the interlamellar bonds are improved, contributing to a significant increase of cavitation resistance.

Melt convection and solid phase movement play an important role in solidification processes, which significantly influence the formation of grain structures and solute segregations. In general, the melt convection and grain movement are a result of buoyancy forces. The densities within melt are different due to the variation of temperature and concentration, leading to thermally and solutally driven melt convection. Similarly, the density differences between the grains and the bulk melt cause the grain movement, leading to solid sedimentation or grain floating, as the case may be. Free, unattached solid grains are produced by partial remelting and fragmentation of dendrites, by mechanical disturbances such as stirring or vibration and by heterogeneous nucleation of grains in solidification of grain-refined alloys. In this way, movement of solid crystals during solidification can be ascertained in the following two cases. In the first case, during columnar solidification of non-grain-refined alloys, solid movement is possible in the form of dendrite fragments detached from the columnar stalks by the process of remelting and fragmentation. Movement of grains during columnar solidification gives rise to altogether different microstructure from columnar to equiaxed. In the second case, during equiaxed solidification of grain-refined alloys, the movement of solid crystals is possible in the form of equiaxed dendrite crystals nucleated due to presence of grain refiners. The rate and manner by which the free solids settle (or float) will influence macrosegregation in metal castings. Control of the solidification process is possible through an understanding of the solid movement and its effect on macrosegregation and microstructure. With this viewpoint, the overall objective of the present thesis is to study, experimentally and numerically, the phenomenon of solid phase movement during solidification. Through this study, deeper insights of the role of solid phase movement in solidification are developed which can be used for possible control of quality in castings. Both columnar and equiaxed solidification are considered. Models for transport phenomena associated with columnar solidification with solid phase movement are rarely found in the literature, because of inherent difficulty associated with consideration of microscopic features such as remelting and fragmentation. To tackle this problem, solidification modules for remelting and fragmentation are developed first, followed by integration of these molecules in a macroscopic solidification model. A Rayleigh number based fragmentation criterion is developed for detachment of dendrite fragments from the developing mushy zone, which determines the conditions favorable for fragmentation of dendrites. The criterion developed is a function of net concentration difference, liquid fraction, permeability, growth rate of mushy layer, and thermophysical properties of the material. The effect of various solidification parameters on fragmentation is highlighted. The integrated continuum model developed is applied to stimulate the solidification of aqua-ammonia system in a side-cooled rectangular cavity. The numerical results are in good qualitative agreement with those of experiments reported in literature. A gentle ramp of the mushy zone due to settling of solid crystals, as also noticed in experimental literature, is observed towards the bottom of the cavity. The influence of various modeling parameters on solid phase movement and resulting macrosegregation is investigated through a parametric study. Movement of grains during columnar solidification gives rise to altogether different microstructure and sometimes may initiate a morphological transition of the microstructure from columnar to equiaxed if the number and size of equiaxed grains ahead of the columnar front become sufficient to arrest the columnar growth. The generalised model developed, considering solid phase movement during columnar solidification is used to predict columnar-to-equiaxed transition (CET) based on a prescribed cooling rate criterion. It is found that presence of convection significantly affects the solidification behaviour. Moreover, the movement of dendrite fragments and their accumulation at the columnar front further trigger the occurrence of CET. Cooling configuration, too significantly affects the nature of CET. In unidirectional solidification cases, the locations of CET are found to be in a plane parallel to the chill face. However, for the case of the non-unidirectional solidification (as in side-cooled cavity), the locations of CET need not be in a plane parallel to the chill face. In contrast to fixed columnar solidification, equiaxed solidification is poorly understood; in particular, the phenomena associated with solid crystal movement. Movement of unattached solid crystals, formed due to heterogeneous nucleation on grain-refiners, is induced by the convective currents as well as by buoyancy effects, causing the solid to sediment or to float, depending on density of solid compared to that of the bulk melt. While moving in the bulk melt these crystals can also remelt or grow. A series of casting experiments with AI-based alloys are performed to investigate the role and influence of movement of solid crystals on macrosegregation and microstructure evolution during equiaxed solidification. Controlled experiments are designed for studying, separately, settling and floatation of equiaxed crystals for different cooling conditions and configurations. Further, these experiments are carried out in convective and non-convective cases to understand the effect of convection on solid phase movement. Temperature measurements are performed at various locations in the mould during the experiments. After the cavity is solidified, microstructural and chemical analyses of the experimental samples are carried out, several notable features are observed in temperature histories, macrosegregation pattern, and microstructures due to settling/flotation phenomenon of solid crystals. It is found that the flow behavior of solid grains has a profound influence on the progress of solidification (in terms of grain size distribution and fraction eutectic) and macrosegregation distribution. In some cases, the induced flow due to solid phase movement can cause a flow reversal. The observations and quantitative data obtained from experiments, with the help of detailed solidification conditions provided, can be used for future validations of models for equiaxed solidification. Subsequently, numerical studies are carried out, using a modified version of the macroscopic model developed for columnar solidification with motion of solid crystals, to predict the transport phenomena during equiaxed solidification. The model is applied to simulate the solidification processes corresponding to each of the experimental cases performed in this study. For a better understanding of the phenomenon of movement of solid crystals, the following two special cases of solidification are also presented: 1) without movement of solid crystals and 2) movement of solid crystals without any relative velocity between solid and liquid phases. The numerical predictions showing nature of flow field and progress of solidification are substantiated by the experimental data for the thermal analysis, qualitative microstructural Images and quantitative microstructural analysis. It is concluded, with the help of various experiments and simulations, that movement of solid crystals influences the casting quality appreciably, in terms of macrosegregation and microstructures. It is expected that the improved understanding of the role and influence of solid phase movement during solidification processes (both columnar and equiaxed) obtained through this thesis will be useful for possible control of quality of as-cast products.