Approved for public release, distribution is unlimited / The effect of friction-stir processing (FSP) on the microstructure of a cast nickel-aluminum bronze (NAB) material has been characterized by various micro-analytical methods including orientation imaging microscopy (OIM). Cast NAB is widely utilized in the production of propellers for the surface ships and submarines of the U.S. Navy due to excellent corrosion-resistance. New applications require improved mechanical properties that may be attainable using FSP to achieve localized microstructure modification. Friction between a rotating tool and the surface of the material results in a *stirring* action that, in turn, produces adiabatic heating and local softening of the material. The tool rotation results in very large shear deformations in the softened regions and thus microstructure refinement and homogenization; in effect FSP may convert an as-cast microstructure to a wrought condition in the absence of macroscopic shape change. In as-cast material, results of optical and scanning electron microscopy (using energy dispersive analysis) show an ` (FCC) matrix containing globular and particulate dispersions that correspond to the *I, *II and *IV second phases; these represent various morphologies of the Fe3Al intermetallic compound, which has a D03 structure. Also present are lamellar particles of *III, which is NiAl and has a B2 structure. The grain size in the ` matrix is ~ 1 mm. In OIM, the microtexture and microstructure in the ` (FCC) matrix may be readily obtained and analyzed. However, interatom distances in the Fe3Al and NiAl phases differ by only about one percent and so these phases are not distinguishable from one another during OIM. Altogether, microstructure and microtexture analysis showed that there are several regions in the thermomechanically affected zone (TMAZ) of a material subjected to FSP. From base material inward toward the TMAZ, these include: annealing effects in undeformed base material; a region just inside the TMAZ in which grain deformation and C-type shear deformation textures are observed; regions of highly refined and recrystallized grains further inside the TMAZ, wherein the grain size is < 5æm; and, finally, regions of elongated, banded and twinned grain structures that suggest grain growth following recrystalliztion. / Lieutenant, United States Navy

Approved for public release, distribution is unlimited / The extreme strain, strain rate and temperature gradients during Friction Stir Processing (FSP) render measurement of key parameters in the stir zone infeasible with common methods. The objective of this research was to separate the effects that temperature and deformation in an experimental study of the microstructure and mechanical properties of Ni-AL bronze (NAB). This was accomplished by subjecting as-cast NAB material to several isothermal annealing and quenching treatments as well as isothermal hot rolling processes. Sufficient material was generated to provide results and data for subsequent optical microscopy, tensile, & hardness tests. All results were then compared to similar data collected from previous works completed here at Naval Postgraduate School and with other DARPA FSP program participants. During the course of this work correlations were drawn between FSP material and the material subjected to isothermal hotworking, which may enhance our understanding of the roles that various FSP process parameters have on the microstructural transformation sequence within this material. The hot-rolling study conducted here suggests that FSP process parameters leading to severe deformation at temperatures between 950-1000 C in the NAB material provides high ductility (elongation approximately 28%) with moderate strengths. / Lieutenant, United States Coast Guard

Pour réduire le poids des structures aéronautiques, plusieurs voies ont été explorées. Parmi elles, l'utilisation des alliages d'aluminium légers et le remplacement des structures rivetées par des structures soudées par Friction Stir Welding (FSW) sont envisagées. La question de la durée de vie de ces structures préoccupe les industriels. Dans ce cadre, cette étude porte sur la sensibilité à la corrosion sous contrainte intergranulaire (CSC-IG) du noyau de soudure par FSW de l'alliage Al-Cu-Li 2050. Ce matériau est composé de grains équiaxes dont la taille diminue de 17 à 4 µm à mesure que l'on s'éloigne de la surface de soudage. Une variation de texture est révélée grâce à des cartographies EBSD formant la microstructure des « onion rings ». La périodicité de ces « onions rings » est égale à l'avancée du pion FSW sur un tour (500 µm pour notre matériau). Ces hétérogénéités microstructurales entraînent des gradients de champs mécaniques locaux quantifiés par corrélation d'images lors des essais mécaniques. Ces hétérogénéités microstructurales et mécaniques favorisent les phénomènes de corrosion localisée lorsque le matériau est soumis à un environnement agressif. Les effets des contraintes et de la microstructure sur la CSC-IG sont mis en évidence par des essais de corrosion et des essais de corrosion sous contrainte (CSC). Les essais de corrosion montrent une sensibilité du matériau à la piqûration alors que les essais de CSC révèlent l'amorçage de fissures intergranulaires. Les plus grosses fissures s'amorcent préférentiellement à la frontière des « onion rings ». Un modèle par éléments finis a été développé dans le but de simuler la propagation des fissures intergranulaires sur des agrégats réels générés par des cartographies EBSD. / To reduce the aircraft components weight, several solutions were explored. Among them, the using of light aluminum alloys and the substitution of riveting by friction stir welding (FSW) are investigated. Industry is concerned by the question of the life of such structures. For this, this study is focused on intergranular stress corrosion cracking (IGSCC) sensitivity of the 2050 Al-Li-Cu alloy friction stir weld nugget. This material consists of equiaxed grains whose size is decreasing with the distance from the weld surface between 17 µm at the top and 4 µm at the bottom. The “onion rings” microstructure is revealed by EBSD cartographies as a texture variation. They appear with a periodicity of 500 µm corresponding to the advance per revolution of the tool. Those microstructural heterogeneities cause local mechanical field gradients quantified by digital image correlation measurements during mechanical tests. Those microstructural and mechanical heterogeneities promote localized corrosion when the material is submitted to an aggressive environnement. Microstructure and stress effects on IGSCC are shown by corrosion tests and stress corrosion tests. The first ones show a sensitivity to pitting corrosion and a stress application reveal initiation of intergranular cracks. The biggest ones preferentially initiate at “onion rings” boundaries. A finite element model was developed in order to simulate intergranular cracks propagation on real aggregates obtained by EBSD cartographies.

O processo de soldagem por fricção e mistura (FSW) tem despertado grande interesse nos últimos anos e tornou-se uma alternativa para unir materiais de baixa soldabilidade, como as ligas de alumínio das séries 2XXX e 7XXX, as quais são empregadas na estrutura das aeronaves, por possuírem elevada relação resistência/peso. O processo FSW, todavia, causa mudanças microestruturais nos materiais soldados, particularmente na zona misturada (ZM) e nas zonas termicamente (ZTA) ou termomecanicamente (ZTMA) afetadas. Estas mudanças geralmente interferem no desempenho frente à corrosão das ligas soldadas. No presente estudo, a resistência à corrosão das ligas de alumínio 2024-T3 e 7475-T761, unidas pelo processo FSW foi investigada em solução 10 mM de NaCl. Ensaios de visualização em gel ágar-ágar e de imersão associados a técnicas microscópicas foram realizados para investigar o efeito do acoplamento galvânico na corrosão das diferentes regiões da junta soldada. Os resultados do ensaio de visualização em gel mostraram que, quando acopladas, a liga 2024 atua como cátodo e a 7475 como ânodo. Os ensaios de imersão revelaram acoplamento galvânico entre as ligas na zona misturada (ZM). A região mais afetada pela corrosão foi a ZTMA da liga 7475, com corrosão intergranular desde as primeiras horas de imersão. A influência do processo de soldagem na resistência à corrosão das duas ligas de alumínio foi investigada por ensaios eletroquímicos. Os ensaios eletroquímicos adotados foram medidas de potencial de circuito aberto (PCA) em função do tempo de exposição ao meio corrosivo, espectroscopia de impedância eletroquímica (EIE) e curvas de polarização potenciodinâmica. Os ensaios de polarização mostraram elevada atividade eletroquímica na zona de mistura indicada pelos altos valores de densidade de corrente em comparação com as demais zonas testadas. Os resultados de EIE globais mostraram que nas primeiras horas de exposição ao eletrólito o processo de corrosão foi predominantemente controlado pela liga 7475; todavia, com o tempo de exposição ao eletrólito, a corrosão passou a ser controlada pela liga 2024. / Friction stir welding (FSW) has roused great interest in recent years and it is now an alternative for joining materials of low weldability, such as the aluminum alloys of the 2XXX and 7XXX series, used in the aircrafts structure due to their high strength /weight ratio. However, FSW causes material microstructural changes, mainly in the stir zone (SZ), the heat affected zone (HAZ) or thermomechanically (TMAZ) affected zones of the materials welded. These generally interfere with the corrosive performance of the welded joint. In the present study, the corrosion resistance of the 2024-T3 and 7475-T761aluminum alloys, joined by FSW was investigated in 10 mM NaCl electrolyte. Agar-agar gel and immersion tests associated with microscopic techniques were performed to investigate the effect of galvanic coupling between the welded materials. Results from this test showed that, when galvanically coupled, the 2024 alloy acts as cathode and the 7475 as anode. Immersion tests revealed galvanic coupling between the alloys in the SZ. The zone most susceptible to corrosion was the TMAZ of the 7475. Intergranular corrosion was observed in this zone since the first hours of immersion. The influence of the welding process on the corrosion resistance of the alloys was also evaluated by electrochemical tests. The electrochemical tests adopted were open circuit potential measurements (OCP) as a function of time of exposure to the electrolyte, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. The polarization tests showed high electrochemical activity in the stir zone indicated by the high current densities measured comparatively to the other tested zones. The global EIS results indicated that in the first few hours of exposure to the electrolyte the corrosion process was predominantly controlled by the 7475 alloy; however, with time of exposure to the electrolyte, the corrosion was controlled by alloy 2024.

Este trabalho teve como principal objetivo a determinação dos parâmetros de soldagem a ponto pelo processo de fricção-mistura (FSSW) em uma liga de alumínio AA2024-T3, pela comparação de resultados obtidos em ensaios de cisalhamento e arrancamento em tração e ensaios de fadiga em juntas sobrepostas. Juntas rebitadas da mesma liga também foram ensaiadas, nas mesmas condições, para comparação de propriedades mecânicas. Os principais parâmetros do processo FSSW são: velocidades de avanço e de rotação da ferramenta, profundidade de penetração e tempo de patamar. Foram explorados principalmente os parâmetros: profundidade de penetração e rotação da ferramenta. Os resultados dos ensaios de cisalhamento em tração possibilitaram a determinação da resistência ao cisalhamento das juntas soldadas a ponto, que apresentaram valores inferiores, mas próximos aos obtidos nas juntas rebitadas. Análises microestruturais e de microdureza foram realizadas para elucidar alguns resultados dos ensaios executados. Os ensaios de fadiga foram executados sob controle de carga, com razão de carga R=0,1, em corpos de prova confeccionados com os parâmetros que geraram os melhores resultados em ensaios de cisalhamento em tração. Para as juntas soldadas, uma técnica de monitoramento de defeitos (MCV monitoramento comparativo de vácuo), baseada na diferença de pressão de vácuo, foi utilizada apresentando bons resultados. Apesar de resultados similares em ensaios de cisalhamento em tração, as juntas soldadas apresentaram vida bastante inferior nos ensaios de vida à fadiga, quando comparadas com as juntas rebitadas. / The main aim of this work was the determination of the parameters governing the Friction Stir Spot Welding (FSSW) of a AA2024-T3 aluminum alloy, by the obtained results comparison in pull-out and shear tensile tests and fatigue tests in lap-joints. Riveted joints of the same alloy were also tested in the same conditions for mechanical properties comparison. The main FSSW process parameters were: plunge rate, dwell time, tool penetration and tool rotational speed. The tool penetration and the tool rotational speed parameters were focused. The shear tensile tests results allowed the lap-joint shear resistance determination, that was lower, but close, of those obtained with riveted lap-joints. Microstructure and micro-hardness analysis was carried out to elucidate some tests perfomed results. The fatigue tests was performed in load control, with load ratio R of 0,1, in a lap joint produced with the same parameters of the specimen which presented the best tensile results. For the welded joints, a monitoring defect technique (CVM comparative vacuum monitoring), difference vacuum pressure based, was used showing good results. Although similar results in shear tensile tests, the welded joints showed so lower life in the fatigue tests, when compared with riveted joints.

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.

O processo de soldagem por atrito linear com mistura (FSW) é uma técnica recente para a soldagem no estado sólido de materiais, em particular para o alumínio e suas ligas. O processo foi inventado na Inglaterra em 1991. Neste processo, as partes a serem soldadas são fixadas e uma ferramenta especial realiza a soldagem de forma contínua. A ferramenta possui uma velocidade de rotação e, durante a sua translação, o material é misturado no estado sólido e, conseqüentemente, soldando as duas partes. A junta soldada por FSW de alumínio AA 6063-T6, com espessura de 3 mm foi caracterizada. A soldagem foi realizada com uma rotação da ferramenta de 710 rpm e com uma velocidade de translação de 5,3 mm/s. A ferramenta empregada é do tipo three flats, com diâmetro do ombro de 14 mm, diâmetro do pino de 3 mm e com ângulo de 90° com relação à horizontal. Os corpos de prova foram soldados em três condições: um passe, dois passes e dois passes com inversão de rotação do pino. Após a soldagem foram realizados os seguintes tratamentos térmicos: solubilização, envelhecimento e recozimento. A junta soldada foi caracterizada por macrografias, micrografias, microdureza, ensaios de calorimetria diferencial e EBSD. Os resultados mostraram que existem ZTMAs diferentes conforme a condição dos de passes. O número de passes tem influência nas componentes da textura alterando de Cubo para Latão e para Goss + Cobre. Os tratamentos térmicos de envelhecimento e recozimento produziram as maiores e menores durezas do cordão, respectivamente. Foi observada a sinergia entre os fatores número de passes e região do cordão no tamanho de grão do cordão. O lado de retrocesso, após o tratamento térmico, apresentou os grãos mais finos. / Friction stir welding (FSW) is a recent process for aluminium welding in solid state. This process was invented in England in 1991. The welding process is done with a special rotating tool that travels along the joint while the parts are fixed. The tool has a speed and a rotation and during its translation the material mixtures in solid state and the joint occurs. The objective of this paper is to show the metallurgical and mechanical characteristics of a 3 mm thick Aluminum AA 6063 T6 plate welded joint. The tool rotation speed was 710 rpm and the translation speed was 5.3 mm/s. The type of the tool used was three flats, with a shoulder diameter of 14mm and pin diameter of 3mm and perpendicular to the plate. The samples were welded in three conditions: one pass, two passes and two passes with pin rotation inversion in the second pass. The welded samples were also submitted to solution heat treatment, solution heat treatment followed by aging and annealing heat treatments. The welded joint was studied with these main experimental techniques: optical and scanning electron microscopy, microhardness, differential scanning calorimetry and electron backscatter diffraction for texture analysis. The results showed different TAZs according to the welding conditions. The number of passes has influence over the texture components changing from Cube to Brass and to Goss + Copper. The aging and solution heat treatments showed the highest and the lowest hardness, respectively. Synergy between the welding conditions and weld region was observed for the grain size results. The retreating side produced the finest grains after heat treating.

Le procédé de soudage par friction malaxage (FSW) entraîne, d’une manière générale, une importante chute de dureté à travers le joint soudé. Dans le but de concevoir des structures aéronautiques soudées par FSW en fatigue, il est nécessaire de connaître l’impact de cette chute de dureté dans le comportement mécanique global de la soudure. Dans ces travaux, l’alliage d’aluminium à durcissement structural 2198-T8 est considéré. Une chaîne de calcul de durée de vie en fatigue d’une structure soudée par FSW est mise en place. Elle intègre un couplage de calculs et d’expériences grâce auxquels le comportement mécanique de la structure est modélisé. Dans un premier temps, le gradient de comportement mécanique de la soudure est étudié. Des essais mécaniques de traction et cycliques sont réalisés à température ambiante. La méthode de corrélation d’images numériques (DIC) est utilisée dans le but de mesurer les champs de déplacements localement dans et au voisinage du joint soudé. À partir des résultats expérimentaux, les paramètres mécaniques d’un modèle de comportement sont identifiés à partir d’un élément de volume, zone par zone à travers le joint soudé. En parallèle, une quantification des précipités durcissants T1 (Al2CuLi) est menée dans différentes zones du joint soudé à l’aide d’un Microscope Electronique en Transmission (MET). Un lien entre l’évolution de la microstructure à travers la soudure et l’évolution des paramètres mécaniques est recherché. Le modèle de comportement mécanique est utilisé sur des calculs de structure utilisant la méthode des éléments finis pour simuler le joint soudé. En parallèle, des essais de fatigue sont réalisés sur des éprouvettes uniaxiales et cruciformes soumises à des chargements uniaxiaux et multiaxiaux. À l’aide des simulations du gradient de comportement mécanique du joint soudé ainsi que des résultats mesurés en fatigue, les paramètres d’un modèle d’endommagement sont identifiés. Ce modèle est utilisé pour prédire les durées de vie en fatigue et les zones d’amorçages de fissure pour une structure soudée soumise à des chargements multiaxiaux. / The Friction Stir Welding (FSW) process generally induces a critical hardness decrease inside the welded joint. To design aeronautical structure welded by FSW in fatigue, it is then necessary to know the impact of this hardness drop on the constitutive behaviour of the junction. In this study, the hardening structural aluminium alloy 2198-T8 is considered.A fatigue lifetime assessment loop of a welded structure is implemented. It integrates a calculations and experiments coupling which is used to model the structure’s mechanical behaviour. The gradient mechanical behaviour of the weldment is initially studied.Monotonic and cyclic mechanical tests are carried out to room temperature. Digital Image Correlation (DIC) is used to measure local displacement fields around the junction. Based on this experimental data, mechanical parameters for a constitutive model are identified on a volume element, zone by zone across the welded joint. In parallel, a quantification of the T1 (Al2CuLi) strengthening precipitates is realized in different region of the joint with a Transmission Electron Microscope. A connection between the microstructure evolution and the mechanical parameters is researched. The gradient mechanical behaviour of the joint is assessed on a 3D structure by Finite Element Analysis. Furthermore, fatigue tests are carried out on uniaxial and multiaxial loadings welded specimen. Thanks to the mechanical behaviour model and the fatigue lifetime measured, a damage model is used to predict the fatigue lifetime and the crack initiation zone for a welded structure which is subjected to higher multiaxial loads.

Refill Friction Stir Spot Welding (RFSSW) is a new solid-state joining technology, which is suitable for joining similar and dissimilar overlap sheets connections, particularly in aluminium and magnesium alloys. This welding method is expected to have wide applications in joining of body parts in the automotive industry. In the present study, RFSSW has been used to join 1.0 mm gauge sheets of two material combinations: similar AA6111-T4 automotive aluminium alloy joints and a dissimilar aluminium AA6111-T4 to magnesium AZ31-H24 alloy combinations. The performance of the joints was investigated in terms of the effect of the welding parameters (including tool rotation rate, sleeve plunge depth, and welding time etc.) to improve current understanding and allow optimisation of the process for short welding-cycles when joining similar and dissimilar light alloys. The results of the investigations on similar AA6111 welds showed the ability to use a wide window of process parameters that resulted in joints with a successfully refilled keyhole and flat weld surface, even when using a welding time as short as 0.5 s. The joints in the as-welded condition showed strengths as high as 4.2 kN, when using welding parameters of 1500 rpm, 1.0 mm with a range of welding times from 0.55 to 2.0 s. All joints showed a nugget pull-out failure mode when using a sleeve plunge depth of 0.8 mm or more, as a result of increasing the joint area. The strength of the joints further improved and reached peak loads of 5.15 and 6.43 kN after natural and artificial ageing, respectively, for welds produced using optimised welding parameters of a 2500 rpm tool rotation rate, a 1.5 s welding time and a 1.0 mm plunge. This improvement in strength resulted from the improvement in the local mechanical properties in the HAZ and other regions, which results from a minimal HAZ due to the rapid weld cycle and the re-precipitation of GPZs and clustering on natural ageing, or β on artificial ageing. A modification to the RFSSW process was developed in this project to solve the problems faced when dissimilar welding Mg to Al. This modified process involved adding a final brief pin plunge stage to consolidate refill defects and it was successful in producing nearly defect-free joints with improved mechanical properties, using a wide range of the process parameters. The average peak load of the joints increased with increasing tool rotation rate, to reach a maximum value at 2500 rpm due to eliminating the weld defects by increasing the material plasticity. However, increasing the tool rotation rate further to 2800 rpm led to a decrease in the average peak failure load due to eutectic melting at the weld interface. The optimum welding condition was thus found to be: 2500 rpm, 1.0 s, and 1.0 mm, which gave an average peak failure load of 2.4 kN and average fracture energy of 1.3 kN.mm. These values represent an improvement of about 10 % and 27 %, respectively, compared to welds produced with the conventional RFSSW process, and about 112 % and 78 % of the Mg-Mg similar joints produced using the same welding conditions. A FE model developed in this project was successful in increasing understanding of the behaviour of the RFSSW joints when subjected to lap tensile-shear loading. The stress and strain distribution in the modelled samples showed that the highest concentration occurring in the region of the confluence of the SZ with the two sheets. With increasing extension, these regions of highest stress and strain propagated to the outer surfaces of the two sheets and then annularly around the weld nugget. This annular ring of high strain concentration agreed well with the failure path and results in the full plug pull-out fracture mode shown by the experimentally tested samples. The predicted force-extension curves showed high agreement with the experimental results, especially when including the effect of the hook defect and correction of compliance in the experimental results.

Le soudage par friction malaxage (FSW) est un procédé de soudage innovant pour les matériaux à bas point de fusion (aluminium, cuivre…). Il a été breveté en 1992 par l’organisme anglais The Welding Institute (TWI). Depuis plusieurs années, celui-ci se développe dans l’industrie en cherchant à réduire son coût d’investissement. Le principe du FSW est de réaliser un cordon de soudure grâce à un outil animé d’un mouvement de rotation et d’avance. Les niveaux d’efforts et de précision requis contraignent à l’utilisation de machines cartésiennes de grande envergure. L’utilisation des robots industriels est un moyen de réduire les coûts, mais ils ne sont pas conçus pour ce genre d’applications et leur inconvénient majeur réside dans leur manque de rigidité. Ainsi, lorsque l’outil entre en contact avec les pièces à assembler, celui-ci peut dévier de plusieurs millimètres dans différentes directions de l’espace, rendant la mise en oeuvre d’une compensation de la trajectoire du robot obligatoire afin d’obtenir des soudures sans défauts. Le but de cette thèse a été de développer un procédé robotisé robuste. Le premier objectif est la mise en oeuvre d’une commande en effort robuste. En effet, en FSW, le maintien d’un effort axial constant est obligatoire. Le contrôle de cet effort permet de compenser la déviation axiale de l’outil et les défauts de mise en position des pièces à souder. Ainsi, une démarche d’identification et de modélisation afin de créer une commande en effort a été mise en oeuvre. La commande est définie de manière robuste afin d’éviter les réglages de l’asservissement lorsque les outils, les paramètres de soudage ou les trajectoires du robot changent. Une validation expérimentale complète a été réalisée dans le contexte du FSW. Le second objectif de cette thèse a été de développer une compensation de la déviation latérale de l’outil. Contrairement à l’objectif précédent, il n’y a pas d’effort à maintenir pour compenser cette déviation latérale. Dans l’industrie, cette déviation peut être compensée à l’aide d’un système de vision, mais ce dernier comporte de nombreux inconvénients en FSW (réflexion de l’aluminium, non visibilité du joint, coût, mise en oeuvre complexe). Ainsi, dans cette partie, un algorithme de compensation temps réel de la déviation latérale de l’outil a été mis en oeuvre. Celui-ci repose sur l’identification d’un modèle élasto-statique du robot. L’algorithme de compensation de la déviation latérale de l’outil a été couplé à la commande en effort et validé expérimentalement en FSW. La différence avec la majorité des travaux de recherche dans ce domaine est que les procédures d’identification n’utilisent pas de système de mesure 3D (photogrammétrie CCD ou laser de poursuite) dont le coût est un frein indéniable pour beaucoup d’industriels. La démarche est simple à mettre en oeuvre sur un robot industriel du marché actuel, et applicable pour d’autres procédés à contact comme l’usinage ou le polissage. / Friction Stir Welding (FSW) is an innovative welding process for materials with a low melting point (aluminium, copper…). It was patented in 1992 by the English organization The Welding Institute (TWI). For many years, an effort is done to reduce the investment cost for industrial applications. FSW process involves a rotating tool advancing along a path. Currently, gantry-type CNC systems are using for FSW manufacturing. These machines offer a high stiffness and can tolerate the forces during FSW in order to carry out a good weld quality. Industrials robots can reduce the investment cost; however they are not design for these applications. The main limitation is the low stiffness of the robot structure. Consequently, the robot deformation under the high process forces causes tool deviations about several millimeters. The robot path has to be compensated in order to obtain a good weld quality. The aim of this thesis is to develop a robust robotized process. The first goal is to realize a robust force control. During FSW, a constant axial forging force should be applied. Axial tool deviation is compensated with the force control approach. In this way, a modeling and identification method is done in order to design a force controller. The force controller is robust because no tuning is required, even if welding parameters or robot paths change. An experimental validation in FSW is done. The second goal is to realize a compensation of the lateral tool deviation. Unlike the axial deformation, there is no force to maintain for compensate this deviation. In industry, the lateral tool deviation could be compensated with a camera or laser sensor in order to track the weld seam path during welding. However, the cost of a seam tracking device, the aluminium reflexion and the lack of visibility in lap joint configuration are significant drawbacks. In this chapter, a compensation algorithm is designed. An elastostatic model of the robot is used to estimate in real time the deflection of the robot TCP. The compensation algorithm is coupled with the force controller defined previously. Compare with others research works about this topic, identification methods don’t need a 3D measurement system (CCD camera or laser tracker). The cost of such system is a main drawback for industrial applications. In this thesis, identification methods are easy to implement in an industrial robot and available for others processes like machining or polishing.