A soldagem altera as propriedades mecânicas dos aços ligados ao níquel, em especial seu desempenho em aplicações criogênicas. Assim, este trabalho apresenta um estudo sobre a soldabilidade metalúrgica do aço com 9% níquel e tem como objetivos: identificar se a fragilização em temperatura abaixo da temperatura Ac3 ocorre em ZACs com dois e três ciclos térmicos simulados fisicamente; analisar as características da junta soldada pelo processo de soldagem por atrito linear com mistura e os efeitos da soldagem multipasse; e analisar a influência dos passes de enchimento e acabamento sobre o comportamento mecânico da ZAC da raiz de uma junta soldada pelo processo de soldagem MIG/MAG. Destacam-se entre os resultados da simulação física da ZAC: as amostras que tiveram a temperatura máxima abaixo da temperatura Ac1, no terceiro ciclo térmico, não apresentaram o efeito de redução da energia absorvida no ensaio Charpy V; a fração volumétrica de austenita retida não aumenta a quantidade de energia absorvida no ensaio Charpy V para as amostras que tiveram a temperatura máxima do segundo ciclo térmico abaixo da temperatura Ac3 (723°C); e a correlação linear múltipla sugere um modelo empírico, baseado nos dados deste trabalho, onde os fatores de fração volumétrica do microconstituinte martensita-austenita, fração volumétrica de austenita retida e tamanho de grão são mais relevantes para a quantidade de energia absorvida no ensaio Charpy V. Destacam-se entre os resultados da soldagem por atrito linear: a energia absorvida no ensaio Charpy V da zona misturada do primeiro cordão (CP1) é menor que o metal de base; o segundo cordão gera duas regiões na zona misturada do primeiro cordão, que tendem a aumentar a energia absorvida no ensaio Charpy V; os valores de energia absorvida no ensaio Charpy V apresentam correlações lineares simples com a microdureza, a fração volumétrica do microconstituinte martensita-austenita e com o tamanho de grão. Destacam-se entre os resultados da soldagem com MIG/MAG: A soldagem do aço com 9% de níquel com a liga Inconel 625 gera uma zona não misturada entre o metal de solda e a ZAC; a amostra com todos os passes de solda (CP3) apresenta a menor energia absorvida no ensaio Charpy V entre todos os experimentos; e a trinca, no ensaio Charpy V, propaga na zona não misturada no CP1 e no CP2, que tiveram as maiores energias absorvidas no ensaio Charpy. O CP3 apresenta propagação de trinca na linha de fusão e possui a menor energia absorvida no ensaio Charpy V. / Welding modify the mechanical properties of nickel steels, especially their performance in cryogenic applications. Thus, this work presents a study on the metallurgical weldability of 9% nickel steel and its objectives are: identify if the embrittlement in temperature below the Ac3 temperature occurs in HAZs with two and three thermal cycles simulated physically; analyze the characteristics of the joint welded by friction stir welding process and the effects of multipass welding; and analyze the influence of the filling and finishing passes on the mechanical behavior of HAZ from the root of a joint welded by the GMAW welding process. The results of the physical simulation of the HAZ were: the samples that had the maximum temperature below the temperature Ac1, in the third thermal cycle, did not present the effect of reduction of the energy absorbed in the Charpy V test; the retained austenite volumetric fraction does not increase the amount of energy absorbed in the Charpy V test for the samples having the maximum temperature of the second thermal cycle below the Ac3 temperature (723 °C); and the multiple linear correlation suggests an empirical model, based on the data of this work, where the volumetric fraction factors of the martensite-austenite microconstituent, retained austenite volumetric fraction and grain size are more relevant for the amount of energy absorbed in the Charpy V test. The FSW welding highlights results: the energy absorbed in the Charpy V test of the mixed zone of the first pass (CP1) is smaller than the base metal; the second pass generates two regions in the mixed zone of the first pass, which tend to increase the energy absorbed in the Charpy V test; the values of energy absorbed in the Charpy V test show simple linear correlations with the microhardness, the volumetric fraction of the martensite-austenite microconstituent and with the grain size. The GMAW welding highlights results: welding of the steel with 9% nickel with the Inconel 625 alloy makes an unmixed zone between the weld metal and the HAZ; the sample with all weld passes (CP3) shows the lowest energy absorbed in the Charpy V test among all the experiments; and the crack, in the Charpy V test, propagates in the unmixed zone in CP1 and CP2, which had the highest energies absorbed in the Charpy test. The CP3 shows crack propagation in the melting line and has the lowest energy absorbed in the Charpy V test.

The current US fleet of nuclear reactors has been in service for three decades. Over this period, existing welds in stainless steel (SS) shrouds have sustained stress corrosion cracking (SCC) and are in need of repair. Additionally, helium has formed interstitially as a byproduct of proton bombardment. Current repair technology, such as TIG welding, puts extreme amounts of heat into the material and allows for interstitial helium atoms to aggregate and form bubbles/voids at grain boundaries. This significantly weakens the material, proving to be a very counterproductive and ineffective repair technique. Much study has been done on friction stir processing (FSP), but none has explored it as an enabling repair technology for use in nuclear applications. Because of its relatively low energy input as a solid state joining technology, it is proposed that FSP could effectively heal SCCs in these existing welds without the negative side effect of helium bubble formation. A spread of speeds and feeds were initially tested using a PCBN-W-Re tool on 304L SS. Six of these parameter sets were selected as representations of high, medium, and low temperature-per-power outputs for this research: 2 IPM-80 RPM, 2 IPM-150 RPM, 4 IPM-150 RPM, 4 IPM-250 RPM, 6 IPM-125 RPM, and 6 IPM-175 RPM. These varied parameter sets were tested for their tensile, micro-hardness, and corrosion resistant properties. In general, the lower IPM and RPM values resulted in higher ultimate tensile strengths (UTS). Higher IPM and RPM values resulted in tunnel, pin hole, and surface void defects. These defects caused premature failure in tensile tests and could often be identified through microscopy. Micro-hardness testing demonstrated a strong correlation per the Hall-Petch relationship – finer grain sizes resulted in higher yield strength (hardness values) of the material. The tool temperature during FSP was a good indicator of the expected hardness – lower temperatures resulted in higher hardness values. Corrosion testing was performed with a 1000-hour alternate immersion test in a room temperature 3.5% NaCl solution. With these testing parameters, the results demonstrated that FSP had no effect on the corrosion resistance of 304L SS under these conditions.

Continuing an investigation on using FSP to heal stress corrosion cracks (SCC) in welds on nuclear reactors, this study seeks to use AM in addition to FSP to aid crack repair. Previous studies address that current repair technology on nuclear reactors involves the use of TIG welding which can allow helium atoms to aggregate and form voids at the grain boundaries. This weakens the material and renders the repair ineffective. Another previous study evaluated the effectiveness of FSP alone in repairing SCC which did have defects depending on the parameters used during FSP. This study evaluated the use of AM in addition to FSP. Literature is available on FSP and AM separately and literature is available on technologies that used both them together. However, the current processes that are available that use both AM and FSP can be expensive and may be impractical for some purposes. This study shows a new process that is both less expensive and more practical in SCC repair. Initial proof of concept trials was performed on 1018 mild steel using both wire fed additive and insert additive technologies. A slot would be removed and filled in with an additive process and processed using FSP. Because of poor repeatability, substantial distortion, and voids present this study went forward using insert technologies in further experiments rather than wire wed additive technologies. In addition, the depth and width of the insert or area where the added material would be placed was varied in initial trails. Tensile testing was performed on initial steel trials and the stainless steel experiments and it demonstrated a correlation between depth of the added material and the tensile strength. Micro-hardness mapping performed on initial steel trials also showed hardening in the FSP stir zone. Three-point bend tests were performed to show that an existing crack underneath the FSP zone would not propagate through the nugget. All evaluations supported a substantial increase in yield strength increased after FSP.

Methods for predicting weld properties based on welding parameters are needed in friction stir welding (FSW). FSW is a joining process in which the resulting properties depend on the thermal cycle of the weld. Buckingham's Pi theorem and heat transfer analysis was used to identify dimensionless parameters relevant to the FSW process. Experimental data from Al 7075 and HSLA-65 on five different backing plate materials and a wide range of travel speeds and weld powers was used to create a dimensionless, empirical model relating critical weld parameters to the peak temperature rise and cooling rate of the weld. The models created have R-squared values greater than 0.99 for both dimensionless peak temperature rise and cooling rate correlations. The model can be used to identify weld parameters needed to produce a desired peak temperature rise or cooling rate. The model can also be used to explore the relative effects of welding parameters on the weld thermal response.

Neste trabalho os efeitos da soldagem por fricção e mistura na resistência à corrosão da liga 2098-T351 foram avaliados e correlacionados com sua microestrutura. Neste sentido, técnicas eletroquímicas convencionais e locais (EIS, curvas de polarização, SVET, SECM e LEIS) em conjunto com técnicas de caracterização microestrutural (XPS, SEM, TEM, DSC, microdureza, perfilometria ótica) foram utilizadas. Devido a pouca informação encontrada na literatura sobre este material, uma caracterização prévia da liga na condição como recebida pelo fabricante foi realizada. Com isso, observou-se a existência da influência da preparação da amostra na resistência à corrosão da liga. Amostras na condição como recebida apresentaram maior resistência à corrosão do que as amostras polidas. Em seguida, a liga 2098-T351 foi caracterizada e sua resistência à corrosão comparada com a da liga convencional 2024-T3, amplamente usada na indústria aeroespacial. Os resultados mostraram diferentes mecanismos de propagação da corrosão para as duas ligas. Em termos de resistência à corrosão, a liga 2098-T351 apresentou potencial para substituir a liga 2024-T3 em termos de resistência a penetração do ataque localizado. Por fim, a reatividade das diferentes zonas de soldagem foi correlacionada com as respectivas características microestruturais. Os resultados mostraram que a região da junta soldada, que compreende a região da zona misturada e zona termomecânicamente afetada, foi mais resistente à corrosão do que a zona termicamente afetada e o metal base. A alta densidade da fase T1 (Al2CuLi) nestas ultimas regiões, comparadas à junta soldada foi considerada responsável por esse comportamento. / In this work, the effects of friction stir welding on the corrosion resistance of the 2098-T351 alloy were evaluated and correlated with its microstructure. In this sense, conventional and local eletrochemical techniques (EIS, polarization curves, SVET, SECM e LEIS) together with microstructural characterization techniques (XPS, SEM, TEM, DSC, microhardness, optical profilometry) were used. Due to the little information found in the literature on the 2098-T351 alloy a prior characterization of the alloy in the as received condition by the manufacturer was carried out. Thus, the influence of sample preparation on the corrosion resistance of the alloy was observed. Samples in the as-received condition showed higher corrosion resistance than the polished ones. Next, the the 2098-T351 alloy was characterized and its corrosion resistance compared to that of the conventional 2024-T3, widely used in the aerospace industry. The results showed different mechanisms of corrosion propagation for the two alloys. In terms of corrosion resistance, the 2098-T351 alloy presents potential to replace the conventional 2024-T3 alloy due to its resistance to corrosion attack penetration. Finally, the reactivity of the different FSW welding zones was correlated with their microstructural features. The results showed that the weld joint, which comprises the stir zone and the thermomechanically affected zone, was more resistant to corrosion than the heat affected zone and base metal. The high density of T1 phase (Al2CuLi) in these last zones, compared to the weld joint was considered responsible for this behavior.

In positron emission tomography (PET) the current trend is to use the fully 3D capabilities of the scanner to increase sensitivity, hence improve the quality of data or reduce the scanning time. However, some difficulties have to be resolved. In 3D PET, the largest contributor to image degradation is Compton scatter since the scattered photons may comprise more than 50% out of all coincidences in the whole body studies. Much progress has been achieved the last few years by the use of scatter correction algorithms, such as the single scatter simulation (SSS). In this work, a model-based scatter simulation (MBSS) algorithm has been implemented in a software library called STIR (i.e. Software for Tomographic Image Reconstruction) initially based on SSS. The aim of the current work is to validate the MBSS implementation; investigate the influence of several parameters; and, if possible extend the existing algorithm. The results are compared with both SimSET Monte Carlo simulation package and measured data. The comparison shows that SSS is in excellent agreement with the single scatter distribution produced by SimSET and in several cases can also approximate accurately the total scatter. However, SSS is just an attempt to estimate the total Compton scatter effect, as it is possible that both photons may scatter, and potentially more than once. As shown, the single scatter distribution may have different shape from the total scatter distribution. How accurate this approximation is, it depends on how many detected photons are scattered multiple times. Multiple scatter is more likely to happen if the attenuation medium has large volume, hence it is more severe in 3D studies of the torso than the brain. In this work, the methodology used for the single scatter simulation algorithm is extended to handle twice-scattered events. Detailed description on how to implement the double scatter simulation (DSS) together with a preliminary evaluation is included. The results are promising even if the required computational time for DSS is much higher than for SSS, though not being prohibited. Finally, at the end of the thesis, an efficient recursive formula is proposed to estimate the rest multiple scatter distribution. / Κατά την τομογραφική απεικόνιση εκπομπόμενων ποζιτρονίων είναι αρκετά διαδεδομένη η χρήση της τρισδιάστατης ανίχνευσης, ώστε να βελτιωθεί η ευαισθησία και η ποιότητα των δεδομένων, αλλά και να μειωθεί ο συνολικός χρόνος εξέτασης. Για να είναι αυτά εφικτά πρέπει πρωτίστως να αντιμετωπιστούν αποτελεσματικά κάποιες δυσκολίες. Συγκεκριμένα, ένας από τους σημαντικότερους παράγοντες που υποβαθμίζουν την ποιότητα της εικόνας είναι η σκέδαση Compton, διότι, εξαιτίας αυτής, τα σκεδαζόμενα φωτόνιων που ανιχνεύονται μπορούν να ξεπεράσουν το 50% των συνολικών ανιχνεύσεων σε αρκετές μελέτες του ανθρώπινου κορμού. Σημαντική πρόοδος έχει επιτευχθεί τα τελευταία χρόνια με τη χρήση αλγόριθμων διόρθωσης σκέδασης και, κυρίως, με τη χρήση του αλγόριθμου προσομοίωσης μίας και μόνο σκέδασης (ΠΜΣ). Στην παρούσα μελέτη, ένας αλγόριθμος βασισμένος σε αυτό το μοντέλο δημιουργήθηκε σε μια βιβλιοθήκη λογισμικού για ανακατασκευή τομογραφικής εικόνας. Ο στόχος αυτής της εργασίας είναι να πιστοποιήσει τη σωστή λειτουργία του αλγόριθμου, να μελετήσει την επίδραση διαφόρων παραμέτρων και, εάν είναι εφικτό, να τη βελτιώσει. Η σύγκριση των αποτελεσμάτων έδειξε πως ο ΠΜΣ επιβεβαιώνεται με Μόντε Κάρλο προσομοιώσεις. Ωστόσο, ο αλγόριθμος ΠΜΣ είναι μια προσέγγιση του συνολικού ποσοστού φωτονίων σκέδασης Compton. Υπάρχει πάντα πιθανότητα και τα δύο φωτόνια να σκεδαστούν μία ή και περισσότερες φορές. Όπως αποδεικνύεται στην παρούσα μελέτη, η κατανομή μίας και μόνο σκέδασης έχει διαφορετική μορφή σε σύγκριση με τη συνολική κατανομή της. Πόσο ακριβής είναι αυτή η προσέγγιση εξαρτάται από τον αριθμό των πολλαπλά σκεδαζόμενων φωτονίων που έχουν ανιχνευτεί. Το φαινόμενο πολλαπλής σκέδασης είναι πιθανότερο εάν το μέσον απορρόφησης ακτινοβολίας καταλαμβάνει μεγάλον όγκο και συνεπώς κατά τις τρισδιάστατες μελέτες του κορμού, παρά του εγκεφάλου. Στην παρούσα εργασία η μεθοδολογία που χρησιμοποιήθηκε για τον αλγόριθμο προσομοίωσης μίας και μόνο σκέδασης επεκτάθηκε, ώστε να συμπεριλάβει και γεγονότα διπλής σκέδασης. Μια αναλυτική περιγραφή παρουσιάζεται για το πώς μπορεί να υλοποιηθεί η προσομοίωση διπλής σκέδασης (ΠΔΣ), που ακολουθείται από μία προκαταρκτική αξιολόγηση. Τα αποτελέσματα είναι αρκετά ενθαρρυντικά ακόμη και αν ο απαιτούμενος υπολογιστικός χρόνος για την ΠΔΣ είναι αρκετά μεγαλύτερος από την ΠΜΣ, χωρίς να την καθιστά απαγορευτική. Στο τέλος της διπλωματικής εργασίας προτείνεται ένας ολοκληρωμένος αναδρομικός αλγόριθμος για τον αποδοτικό υπολογισμό του συνολικού ποσοστού σκεδάσεων.

A procura cada vez mais intensa por redução de peso e consequente diminuição do consumo de combustível dos veículos automotores e aeronaves, tem proporcionado um crescimento constante na utilização do alumínio e suas ligas nos diversos setores da indústria de manufatura. No entanto, a dificuldade de união em juntas de alumínio que atendam requisitos de alta resistência à fratura e à fadiga, tem intensificado as pesquisas por novas tecnologias em seu processo de soldagem. Neste contexto, o processo de soldagem denominado Friction Stir Welding (FSW), vem ganhando espaço nas pesquisas e também na indústria. Neste processo, uma ferramenta não consumível é projetada especialmente para ser introduzida nas juntas das chapas a serem soldadas, gerando calor e misturando mecanicamente o material da junta, consolidando a solda. Este trabalho teve como objetivo avaliar os efeitos dos parâmetros de soldagem FSW, na consolidação das propriedades mecânicas das soldas produzidas, na microestrutura, além de buscar a otimização do processo para a liga em estudo. Para tanto, a geometria da ferramenta foi projetada, fabricada e testada, de forma a definir os parâmetros ideais para obtenção de juntas soldadas sem defeitos. Uma máquina fresadora CNC foi utilizada para a execução dos testes. As soldas foram produzidas em chapas de alumínio da liga Al 5182-O com espessura de 3,25mm, com velocidade rotacional da ferramenta mantida constante em 500 rpm e velocidades de soldagem de 50, 100, 150, 200 e 250 mm/min. A ferramenta foi inclinada em um ângulo de 1º, todas as soldas foram realizadas com sentido horário de rotação, após a realização dos testes as juntas soldadas foram submetidas a uma série de ensaios destrutivos e não destrutivos caracterizando cada teste realizado. Juntas de topo com penetração completa e livre de defeitos foram produzidas com velocidades de soldagem de 100, 150, 200 e 250 mm/min, enquanto com a velocidade de soldagem de 50 mm/min resultou em defeito do tipo vazio na zona da mistura no lado do avanço, defeito este caracterizado através de inspeção por microtomografia, o que fez com que as soldas produzidas com esta velocidade de soldagem rompessem na junta soldada. Em todas as velocidades de soldagem a caracterização através de microscopia óptica na seção transversal das soldas revelou um refino microestrutural, obtido pelo fenômeno de recristalização dinâmica, que resultou em um aumento nos valores de dureza na zona da mistura. Os resultados obtidos demostram que soldas de boa qualidade podem ser produzidas com este processo. Além disso, foi estabelecida uma correlação entre a história térmica associada ao processo de FSW, a microestrutura produzida e o desempenho mecânico das juntas soldadas. / The increasingly intense demand for weight reduction and consequent reduction of the fuel consumption of motor vehicles and aircraft , has provided a steady growth in the use of aluminum and its alloys in various sectors of the manufacturing industry , but the difficulty in joining together aluminum that meet high fracture resistance and fatigue , has intensified the research for new technologies in the process of welding. In this context , the welding process called Friction Stir Welding ( FSW ) has been gaining ground in research and also in industry . In this process , non-consumable tool is specially designed to be inserted in the joints of the plates to be welded, generating heat and mechanically mixing the gasket material, consolidating the weld. This study aimed to evaluate the effects of welding parameters FSW , the consolidation of the mechanical properties of the welds produced, microstructure, and seek to optimize the process for the alloy under study. For this, the geometry of the tool is designed, manufactured and tested in order to define the optimal parameters for obtaining welded joints without defects. A milling machine CNC was used for the tests. The welds were produced in sheets of aluminum alloy Al 5182-O with a thickness of 3,25mm with rotational speed of the tool maintained constant at 500 rpm and welding speeds of 50, 100, 150, 200 and 250 mm / min, the tool was tilted at an angle of 1°, all welds were performed with clockwise direction of rotation, after the tests the welded joints were subjected to a series of destructive testing and nondestructive characterizing each test performed. Butt welding with full penetration and free defects were produced with welding speeds of 100, 150, 200 and 250 mm / min, while with the welding speed 50 mm / min resulted in failure of the void type in the zone of the mixture side of the advance, this defect characterized by inspection by microtomography, which has meant that the welds produced with this welding speed would break the joint weld. In all welding speeds characterization by optical microscopy in cross section of welds revealed a microstructure refining, obtained by dynamic recrystallization phenomenon, for which resulted in an increase in the hardness values in the mixing zone. The results demonstrate that good quality welds can be produced with this process. Moreover, a correlation between the thermal history associated with the process produced the microstructure and mechanical performance of welded joints was established.

Le soudage par friction (Friction Stir Welding - FSW) est un processus efficace de se joindre des alliages d'aluminium à haute résistance en évitant les défauts que l'on trouve habituellement lorsque les techniques de soudage classiques sont utilisés. L'industrie de l'aviation a montré un grand intérêt pour cette méthode de soudage, tant pour l'union des alliages semblables comme pour dissemblables. Cependant, ce processus entraîne des changements de microstructure dépendantes des conditions de traitement thermique ou thermomécanique. Le contact électrique entre les différentes zones de microstructures, à son tour, peut conduire à un couplage galvanique entre les différentes zones. Dans la présente étude, le soudage FSW a été utilisé pour joindre deux alliages d'aluminium dissemblables,AA2024-T3 et AA7475-T651. La résistance à la corrosion des joints soudés a été évaluée par des tests électrochimiques, en particulier, les mesures de potentiel en circuit ouvert(OCP), les courbes de polarisation, et des mesures de spectroscopie d'impédance électrochimique, soit globale (EIS) ou local (LEIS) en solution de 0,1 M Na2SO4 ou 0,1MNa2SO4 + 1 mM NaCl. Les tests électrochimiques ont montré l'effet de couplage galvanique dans les joints soudés. La résistance à la corrosion intergranulaires et la résistance à corrosion par exfoliation des joints soudés ont également été évaluées et comparées à celles du AA2024-T3 et les alliages AA7475-T651 non soudées. Les résultats ont montré une réactivité accrue des joints soudés en comparaison avec les alliages non soudés en notant l'attaque plus intense sur l'alliage AA7475-T651. L'identification des zones cathodiques et anodiques dans les joints soudées ont été évaluée par un test consistant à déposer un gel(agar) avec l'indicateur universel et il a été observé que l'alliage AA2024-T3 a agi en tant que cathode, tandis que la AA7475- T651 comme anode. Par ailleurs, le dégagement d'hydrogène a été observée dans la région d'interface entre la zone affectée thermomécaniquement et l'alliage AA7475-T651 affectée par la chaleur. Les résultats des essais de LEIS effectuées dans différentes zones des deux alliages soudés par FSW ont montré l'engagement galvanique à l'interface entre eux pour de courtes durées d'analyse etde déplacement de la région la plus active dans le temps de test pour l'alliage AA7475-T651,plus précisément à l'interface entre la zone affectée thermomécaniquement et la thérmique affectée de cette alliage. / Friction Stir Welding (FSW) is an efficient process of joining high strength aluminum alloys avoiding defects that are usually created when conventional welding techniques are used. The aircraft industry has shown great interest in this welding method, both for welding of similar or dissimilar alloys. However, this process causes microstructural changes that are dependent on the thermal or thermomechanical conditions applied. Electrical contact between zones of different microstructures, in turn, can result in galvanic coupling. In the present study, FSW was used to join two dissimilar aluminum alloys, AA2024-T3 and AA7475-T651 and the effect of this processon the corrosion resistance of the welded joints and on the microstructure of the alloys was evaluated. For corrosion resistance evaluation, electrochemical tests were used, specifically,open circuit potential measurements (OCP) as a function of time of exposure time to the corrosive environment, polarization tests, and electrochemical impedance spectroscopy, global (EIS) orlocal (LEIS), in two solutions, either 0.1 M Na2SO4 or 0.1M Na2SO4 + 1 mM NaCl. The electrochemical tests showed galvanic coupling effects in the welded joints. Microstructural characterization was carried out by optical microscopy, scanning electron microscopy,transmission electron microscopy and differencial scanning calorimetry. The welded affected zones showed significant microstructural changes indicated by precipitation and dissolution of precipitates that affect the localized corrosion resistance. Intergranular and exfoliation corrosion resistance of the welded joints were also evaluated and compared to those of unwelded AA2024-T3 and AA7475-T651 alloys. The results showed increased susceptibility of welded joints to these forms of corrosion in comparison with the unwelded alloys with more severe attack associated tothe AA7475-T651 alloy. Identification of anodic and cathodic areas due to galvanic coupling in the welded joints was evaluated by a test consisting in depositing a gel layer (ágar-ágar) with universal indicator on the surface of the welded alloys. The AA2024-T3 alloy worked as cathode,while the AA7475-T651 as anode in the galvanic coupling. Furthermore, hydrogen evolution was observed at the interface region between the thermomechanically affected zone and the heat affected alloy AA7475-T651 showing that cathodic reactions also occurred on this last alloy. LEIS results obtained in different zones of the two FSW welded alloys showed galvanic coupling at the interface between them for short test times and displacement of the most active region to theAA7475-T651 alloy, at longer periods of test, specifically to the interface between the thermomechanically affected and the heat affected zones of this last alloy. / A soldagem por fricção (Friction Stir Welding - FSW) é um processo eficiente de unir ligas dealumínio de alta resistência evitando defeitos que são usualmente criados quando técnicasconvencionais de soldagem são utilizadas. A indústria aeronáutica tem mostrado grande interesseneste método de soldagem, tanto para a união de ligas similares como dissimilares. Entretanto,este processo causa modificações microestruturais dependentes das condições de tratamentotérmico ou termomecânico. Contato elétrico entre zonas de microestruturas diferentes, por sua vez,pode resultar em acoplamento galvânico. No presente estudo, a soldagem por FSW foi usada paraunir duas ligas de alumínio dissimilares, AA2024-T3 e AA7475-T651 e o efeito desta soldagem naresistência à corrosão das juntas soldadas e na microestrutura das ligas foi avaliada. Nainvestigação da resistência à corrosão foram utilizados ensaios eletroquímicos, especificamente,medidas de potencial de circuito aberto (OCP) em função do tempo de exposição ao meio corrosivo,ensaios de polarização e de espectroscopia de impedância eletroquímica, global (EIS) ou local(LEIS), em duas soluções, seja 0,1 M Na2SO4 ou 0,1 M Na2SO4 + 1 mM NaCl. Os ensaioseletroquímicos evidenciaram efeito de acoplamento galvânico nas juntas soldadas. Acaracterização microestrutural foi realizada por microscopia ótica, microscopia eletrônica devarredura, microscopia eletrônica de transmissão e por calorimetria diferencial. As zonas afetadaspela solda tiveram importantes modificações na microestrutura indicadas pela precipitação edissolução de precipitados que afetam a resistência à corrosão localizada. A resistência à corrosãointergranular e a resistência à esfoliação das juntas soldadas também foram avaliadas ecomparadas com as das ligas AA2024-T3 e AA7475-T651 não soldadas. Os resultados mostraramaumento da suscetibilidade das juntas soldadas a estas formas de corrosão em comparação comas ligas não soldadas sendo observado ataque mais severo na liga AA7475-T651. A identificaçãodas áreas anódicas e catódicas resultantes do acoplamento galvânico nas juntas soldadas foirealizada por teste que consistiu na deposição de camada de gel (ágar-ágar) com indicadoruniversal na superfície das ligas soldadas. A liga AA2024-T3 atuou como cátodo, enquanto aAA7475-T651, como ânodo no par galvânico. Além disso, evolução de hidrogênio foi observada naregião de interface entre a zona termomecanicamente afetada e a termicamente afetada da ligaAA7475-T651 mostrando que reações catódicas também ocorreram localmente nesta última liga.Resultados de LEIS obtidos nas diferentes zonas das duas ligas soldadas por FSW mostraramacoplamento galvânico na interface entre elas para tempos curtos de ensaio e deslocamento daregião mais ativa com o tempo de ensaio para a liga AA7475-T651, mais precisamente para ainterface entre a zona termomecanicamente afetada e a térmicamente afetada desta liga.

Friction Stir Welding (FSW) is a rapidly growing solid state welding process and has been a proven method for welding high strength aluminium alloys which were formerly not recommended for joining by conventional fusion welding methods. Based on the information acquired from previous studies, to obtain a defect free Friction Stir (FS) weld with suitable strength, three basic requirements need to be fulfilled (i) Filling of the cavity created behind the tool pin during its traverse and ensuring satisfactory contact of filled material with newly generated surface (on advancing side trailing edge of the pin) (ii) Disrupting and distributing the oxide layer at the initial weld interface (iii) Adequate level of mixing of both side material (Adjacent and Retreating side) in similar welding. In the case of dissimilar welding mixing is desired in controlled amount (to prevent or curtail formation of intermetallics) depending on material combination. Failure to achieve the first precondition results in void. Second and third precondition are interconnected for similar FSW as adequate mixing in weld helps in disruption and distribution of oxide layer at initial weld interface. Failure to achieve this, results in Joint Line Remnant (JLR). Metal to metal contact cannot be established due to the presence of JLR (aligned oxide particles) and subsequently initial interface is left unwelded which deteriorate the static and dynamic strength of friction stir welds. The problem aggravates while friction stir welding materials with tenacious contaminant layer. Therefore, appropriate stirring (which entails large deformation and mixing) of initial weld interface is essential for successful FS welds. Hence, process aspects assisting mixing of adjacent (Advancing and Retreating side) materials need to identified and studied, which are missing in former studies. Experiments are conducted with classical FS tool (possessing frustum shaped/tapered circular pin) to analyse the effect of welding parameters (tool rotation speed, traverse speed, plunge depth, tool tilt and tool position w.r.t initial interface) and tool runout by changing these parameters over a range. Tool rotation speed, traverse speed, plunge depth and tool position with initial interface are changed continuously and tool tilt and tool runout are changed in discrete steps. Tool geometry is considered to be a prime parameter controlling the magnitude of mixing, as interaction of rotating tool with initial abutting base metal interface makes the process mechanism complex, unlike other solid state welding process, namely forge welding, diffusion welding, friction welding, explosive welding, ultrasonic welding and roll bonding. Furthermore, due to asymmetric nature of material flow in FSW process, the material located in different locations with respect to the tool is subjected to different levels of deformation. For this purpose experiments have been carried out to analyse the effect of different tool geometrical aspects on level of mixing and material flow. On the other hand, visualizing flow and mixing in metals is debatable as insertion of marker material in the weld line can alter the nature of material flow in the weld due to different material flow characteristics of the base and marker materials and introduction of additional interfaces. Further, using dissimilar materials for flow studies cannot be considered for comparison with similar friction stir welds as their flow properties are different. Therefore, an alternate experimental strategy is devised in these studies using physical modelling approach which is effective and helps in identifying and quantifying mixing observed under different tooling and process conditions. In the present investigation, plasticine of primary colours is adopted and the hue attribute of colour is used to study and quantify intermixing. Yellow and Blue plasticine are placed on advancing and retreating sides respectively. The degree of mixing is indicated by the intensity of generated green. Digital images of the cross section in weld nugget region are taken. To obtain hue component of these digital images the RGB color-maps are converted to HSV color-maps. Overall, these studies help in formulating the guidelines which are useful during tool design, and administering the process to obtain a defect free well mixed welds. Based on the experimental results following conclusions are derived. 1. Following process aspects: tool geometry, interface offset, tool rotation and tool runout demonstrate a significant impact on material mixing and breaking and dispersion of initial interface in weld nugget. Tool tilt, plunge depth, tool traverse exhibit negligible effect on degree of mixing. 2. Increase in tool rotation speed (with other parameters fixed) improves mixing substantially but can be increased to a certain limit after which voids emerge due to loss of weld nugget material in the form of flash. 3. Reducing the weld pitch (i.e. increasing tool rotation speed for a given tool traverse speed) reduces the size of the weld nugget and vice versa. Tool traverse speed largely affects advancing side material and rotation speed affects retreating side material. Therefore, for higher weld pitch advancing side material (yellow plasticine) dominates the weld nugget, whereas for lower weld pitch retreating side material (blue plasticine) dominates the weld nugget. 4. The extended macro-structural feature commonly observed in FS welds occurs under influence of plunge depth. Consequently, this macro-structural feature serves as the demarcation point between shoulder affected and pin induced material flow in FS weld. 5. The degree of mixing and subsequent elimination of JLR, improves, when original interface is offset on the advancing side w.r.t tool axis for all the tools investigated in the present study. Triangular and square pin generate larger pin induced mixing which intensifies further with interface on advancing side, indicating tools with such profiles to possess larger safe zone with better mixing characteristics 6. At zero interface offset with all the process parameters fixed, tapered triangular and square pin profile tools produce welds with maximum mixing. For pins with faces, material is transported in lumps around the pin. The size of lump increases with lesser number of faces on pin. Material in the vicinity of the pin experiences spinning/whirling movement. The volume of material experiencing spinning in a single tool revolution depends on (a) weld-pitch (lesser volume of material for smaller weld pitch and vice versa) and (b) number of faces on the pin (lesser volume of material for greater number of faces and vice versa). Therefore, circular pin which can be considered to be made of infinite faces, spinning of material occurs at micro level for relatively smaller weld pitch. 7. For classical FS tool (tapered circular/frustum shape), there exists an optimum ratio (shoulder diameter/pin diameter) situated between 2.7 to 3.6 to produce void free well mixed welds. Tools with ratio of 2.7 and below possess a tendency to produce welds with void but with better mixing in weld region. Tools with ratio of 3.6 and above possess a tendency to produce void free welds but with poor mixing in weld region. Voids appear and grow under following circumstances (a) with increase in pin diameter (for a fixed shoulder diameter), (b) with decrease in shoulder diameter (for a fixed pin diameter), (c) with decrease in pin taper (for a fixed shoulder diameter and top diameter of pin). Pin length has no effect on void formation. However, it is obvious, length of root defect increases with decrease in pin length. The tooling guidelines established in this study through plasticine work can be extended to metallic friction stir welds of various thickness plates by proportionately increasing or decreasing the tool dimensions as long as they fall in the recommended range. 8. Smaller pin diameter tools exhibit higher optimum weld pitch (but with lower degree of mixing) when compared to larger pins (but with higher degree of mixing). Optimum weld pitch represents weld pitch resulting in void free welds. Consequently, tools with higher optimum weld pitch help in welding at a better rate. 9. Tool runout is replicated through tools with eccentric pins. It is interesting to note that, all the tools with pin eccentricities do not assist in mixing but tools with only certain eccentricities (0.3 and 0.6mm assisted in mixing in the present investigation). It implies that tool runout of certain values facilitate mixing in weld. On the other hand density of void increases with eccentricity of pin/tool runout. 10. In dissimilar FSW investigated with plasticine A, B, C and D possessing different flow stresses (flow stresses ascending in the order of A, B, C and D) and strain rate sensitivity of 0.24, 0.22, 0.19 and 0.18 respectively, following inferences are drawn (i) For combination A and B, weldability improves when plasticine B is on Advancing Side (AS) and A is on Retreating Side (RS). The level of mixing also improves when interface is on AS (w.r.t tool axis) for this handedness. On the contrary, severe discontinuities emerge when plasticine B is on RS and A is on AS, especially when interface is closer to the tool pin axis. (ii) For combination A and C, weldability improves when C is on AS and A is on RS. The level of mixing also enhances when interface is on AS (w.r.t tool axis) for this handedness. (iii) For combination A and D, joining is poor for both the handedness. However, nature of defect is different in both the combinations. Cracks are observed when A is located on AS and voids emerge when D is located on AS. On the other hand, placing A on AS results in weld thinning. (iv) For combination B and C, there is no appreciable change in terms of weldability and level of mixing. Both the handedness in this combination yielded fairly similar results. (v) For combination B and D, though discontinuities do not emerge with change in handedness, mixing in weld improves when B is on AS unlike to its location on RS. (vi) For combination C and D, there is no appreciable change in terms of defect formation and level of mixing with change in handedness. Both the handedness in this combination yield fairly similar results.

A procura cada vez mais intensa por redução de peso e consequente diminuição do consumo de combustível dos veículos automotores e aeronaves, tem proporcionado um crescimento constante na utilização do alumínio e suas ligas nos diversos setores da indústria de manufatura. No entanto, a dificuldade de união em juntas de alumínio que atendam requisitos de alta resistência à fratura e à fadiga, tem intensificado as pesquisas por novas tecnologias em seu processo de soldagem. Neste contexto, o processo de soldagem denominado Friction Stir Welding (FSW), vem ganhando espaço nas pesquisas e também na indústria. Neste processo, uma ferramenta não consumível é projetada especialmente para ser introduzida nas juntas das chapas a serem soldadas, gerando calor e misturando mecanicamente o material da junta, consolidando a solda. Este trabalho teve como objetivo avaliar os efeitos dos parâmetros de soldagem FSW, na consolidação das propriedades mecânicas das soldas produzidas, na microestrutura, além de buscar a otimização do processo para a liga em estudo. Para tanto, a geometria da ferramenta foi projetada, fabricada e testada, de forma a definir os parâmetros ideais para obtenção de juntas soldadas sem defeitos. Uma máquina fresadora CNC foi utilizada para a execução dos testes. As soldas foram produzidas em chapas de alumínio da liga Al 5182-O com espessura de 3,25mm, com velocidade rotacional da ferramenta mantida constante em 500 rpm e velocidades de soldagem de 50, 100, 150, 200 e 250 mm/min. A ferramenta foi inclinada em um ângulo de 1º, todas as soldas foram realizadas com sentido horário de rotação, após a realização dos testes as juntas soldadas foram submetidas a uma série de ensaios destrutivos e não destrutivos caracterizando cada teste realizado. Juntas de topo com penetração completa e livre de defeitos foram produzidas com velocidades de soldagem de 100, 150, 200 e 250 mm/min, enquanto com a velocidade de soldagem de 50 mm/min resultou em defeito do tipo vazio na zona da mistura no lado do avanço, defeito este caracterizado através de inspeção por microtomografia, o que fez com que as soldas produzidas com esta velocidade de soldagem rompessem na junta soldada. Em todas as velocidades de soldagem a caracterização através de microscopia óptica na seção transversal das soldas revelou um refino microestrutural, obtido pelo fenômeno de recristalização dinâmica, que resultou em um aumento nos valores de dureza na zona da mistura. Os resultados obtidos demostram que soldas de boa qualidade podem ser produzidas com este processo. Além disso, foi estabelecida uma correlação entre a história térmica associada ao processo de FSW, a microestrutura produzida e o desempenho mecânico das juntas soldadas. / The increasingly intense demand for weight reduction and consequent reduction of the fuel consumption of motor vehicles and aircraft , has provided a steady growth in the use of aluminum and its alloys in various sectors of the manufacturing industry , but the difficulty in joining together aluminum that meet high fracture resistance and fatigue , has intensified the research for new technologies in the process of welding. In this context , the welding process called Friction Stir Welding ( FSW ) has been gaining ground in research and also in industry . In this process , non-consumable tool is specially designed to be inserted in the joints of the plates to be welded, generating heat and mechanically mixing the gasket material, consolidating the weld. This study aimed to evaluate the effects of welding parameters FSW , the consolidation of the mechanical properties of the welds produced, microstructure, and seek to optimize the process for the alloy under study. For this, the geometry of the tool is designed, manufactured and tested in order to define the optimal parameters for obtaining welded joints without defects. A milling machine CNC was used for the tests. The welds were produced in sheets of aluminum alloy Al 5182-O with a thickness of 3,25mm with rotational speed of the tool maintained constant at 500 rpm and welding speeds of 50, 100, 150, 200 and 250 mm / min, the tool was tilted at an angle of 1°, all welds were performed with clockwise direction of rotation, after the tests the welded joints were subjected to a series of destructive testing and nondestructive characterizing each test performed. Butt welding with full penetration and free defects were produced with welding speeds of 100, 150, 200 and 250 mm / min, while with the welding speed 50 mm / min resulted in failure of the void type in the zone of the mixture side of the advance, this defect characterized by inspection by microtomography, which has meant that the welds produced with this welding speed would break the joint weld. In all welding speeds characterization by optical microscopy in cross section of welds revealed a microstructure refining, obtained by dynamic recrystallization phenomenon, for which resulted in an increase in the hardness values in the mixing zone. The results demonstrate that good quality welds can be produced with this process. Moreover, a correlation between the thermal history associated with the process produced the microstructure and mechanical performance of welded joints was established.