Les composites à matrice métallique ont été développés dans les années 60 initialement pour les besoins de l’industrie aérospatiale. De nos jours, les alliages d’Aluminium à durcissement structural sont souvent combinés à des particules céramiques afin d’atteindre des propriétés de résistance élevées, maintenues à haute température, tout en conservant la légèreté de l’alliage, pour un coût faible. Cependant, l’utilisation de ces alliages nécessite une bonne connaissance des transformations microstructurales ayant lieu lorsqu’ils sont soumis à certaines conditions de température puisque des phénomènes de précipitation ont lieu et impactent les propriétés de résistance mécanique du matériau. De plus, la présence du renfort, induit des modifications microstructurales majeures et notamment lors des phénomènes de précipitation de la matrice. Ainsi, ces travaux de thèse portent sur un alliage d’Aluminium à durcissement structural de la série 6xxx qui, durant son utilisation, peut être confronté à des températures comprises entre 100°C et 350°C, et ayant une séquence de précipitation complexifiée par la présence de Silicium en excès et de particules de renfort céramiques. Les objectifs visés par ces travaux sont alors : 1) De décrire l’évolution microstructurale de l’alliage d’AlMgSi à excès de Silicium étudié, avec et sans particules céramiques. Ensuite, l’évolution de la résistance mécanique a été caractérisée à partir d’un état T6 lors de traitements isothermes, 2) De prédire ces évolutions microstructurales et les propriétés mécaniques qui en découlent par des modèles à base physique. La précipitation des principales phases de l'alliage étudié a pu être prédite grâce un modèle à champ moyen basé sur des lois classiques de germination-croissance et par une approche par classes de type KWN, en tenant compte : 1) de la compétition entre la phase cohérente et semi-cohérente, 2) d'une morphologie en bâtonnet avec un rapport de forme variable et propre à la phase modélisée. Ce modèle a permis, de tracer le diagramme Temps-Température-Transformation de l’alliage et du composite tout en tenant compte de l'accélération des cinétiques de précipitation en présence de renfort et attribuée à la forte densité de dislocations. Pour finir, 2 méthodes de prédiction des propriétés mécaniques ont été confrontées : 1) Une approche empirique de type JMAK (Johnson-Mehl-Avrami-Kolmogorov), 2) une approche à base physique. Malgré une prédiction proche des résultats expérimentaux par l'approche JMAK, cette méthode ne permet pas de remonter aux mécanismes physiques à l’origine des variations observées. Ainsi, l'approche à base physique basée sur le modèle de prédiction de la microstructure constitue une alternative prometteuse pour une prédiction plus précise des évolutions de propriétés de résistance mécaniques de ces matériaux même si de nombreuses adaptations restent à faire dans le cas du composite. / Precipitation-strengthened alloys as Al-Mg-Si alloys reinforced with ceramic particles are an appropriate alternative for industrial applications. The precipitation sequence in Al-Mg-Si alloys is particularly complex when Silicon is in excess with respect to the Mg2Si composition and it is expected to be modified by the presence of the ceramic reinforcement. This is why, for industrial applications, under certain use conditions, it is fundamental to be able to predict the evolution of the microstructure in the alloy and the consequences on mechanical properties. The present work is devoted to the study of an age-hardenable Al-Mg-Si aluminium alloy which, can be facing temperatures between 100°C and 350°C in use conditions. This material is characterized by a complex precipitation sequence due to Silicon-excess and ceramic particles. The main objectives of the work are the following: 1) To describe how the microstructure evolves in the Al-Mg-Si alloy with silicon excess studied, with or without reinforcement, during a long storage period at a temperature between 100°C and 350°C. Then, it appeared necessary to describe the evolution of the mechanical properties in the same conditions but starting from a T6 state (corresponding to peak aged conditions). 2) To predict these evolutions (microstructure and strength) using an appropriate model. It was highlighted that the high silicon excess in the studied alloy leads to a simultaneous precipitation of several semi-coherent phases. Their precipitation has been predicted thanks to a KWN-type model based on classical nucleation-growth theories, validated by the experiments, and implemented considering: 1) the competitive precipitation between coherent et semi-coherent phases, 2) the rod-shape morphology of precipitates with a variable aspect ratio. This model has been used for the prediction of the Time-Temperature-Transformation diagram of the alloys and its composite considering the acceleration of the precipitation kinetics observed and attributed to the high dislocation density resulting from the presence of ceramic particles. Finally, 2 methods for the mechanical properties prediction have been compared: 1) a JMAK-type empirical approach 2) a physically based approach. The JMAK approach allowed us a quicker and easier prediction of the loss of hardness from the T6 state, for alloy and composite, during isothermal and non-isothermal treatment. Despite a prediction close to the experimental results, this approach cannot give us information about the physical mechanisms responsible for the observed mechanical variations. Then, a physically based approach taking into account the predictions of the precipitation model was used for the yield stress estimation during aging with a micromechanical model. This approach gave encouraging results and could be a powerful tool for the prediction of the strength during industrial use conditions.

The continuing quest for higher performance levels of modern gas turbine engines has been accompanied by the demand for higher engine operating temperatures. The use of Thermal Barrier Coatings (TBCs) enabled gas turbines to operate at higher temperatures by protecting the blade material (nickel superalloy) while operating in extreme environments. The TBC system typically consists of a bond coat for protection of the nickel–based superalloy against oxidation followed by a top coat consisting of a thermally insulating zirconia-yttria. In addition to the complex gradation in phases, the coatings are subjected to continuous oxidation with service exposure, mechanical loading on rotating parts, fatigue, thermal mis-match and temperature gradients. Hence, the study of failure mechanisms of TBCs become important in deciding operational reliability and service life of the coating. As there are many systems in which the operating temperatures are not high enough to warrant the use of the top coat (ceramic layer), the study of failure mechanisms in superalloys coated with only the bond coat continue to be of great interest. The present work concentrates on the fracture behavior of NiAl bond coats on nickel superalloy and seeks to evaluate the fracture toughness of the coating through the use of micro-machined samples. A review of the relevant literature indicated that while a considerable body of work exists on bulk tests of failure (spalling, splitting, etc.), not much has been reported in the open literature on the evaluation of basic quantities such as the toughness of the coating itself. The present thesis seeks to establish a protocol for the evaluation of toughness and crack propagation mechanisms in coatings through a combination of micro-sample testing that allows fracture to be correlated with location in the film and the use of an analytical model to quantitatively evaluate stress intensity factors in a bi-material system. A system of NiAl coating produced by pack aluminizing is studied for the fracture properties of the coating. Specimen geometries are optimized to enable micro-cracks to be machined and propagated in a low load testing system, such as a depth sensing indenter, so that crack lengths (and position relative to the interface) can be correlated with load. To enable linear elastic theory to be used, dimensions are determined that allow fracture before general yielding. A three point bending test using miniaturized micro-beam specimens of ~ 4 X 0.3 X 0.3 mm is found to be suitable for the above purpose. The technique is a challenging one that requires focused ion beam machining (FIB) along with careful handling and alignment of small samples. The coatings are characterized for their microstructure by electron microscopy to identify compositional variation across the thickness and to determine the thickness of the coating and inter diffusion zone (IDZ). The crack advancement is monitored with increments of loading and the stress intensity factor is evaluated using a program written in “MAPLE” for an edge crack subjected to bending in a bilayered material. Surprisingly, fracture in this system is found to be stable owing to a gradual increase in toughness from the coating surface to the interface. Such an increase from less than 2 to more than 9 MPa m0.5 may be due to the increasing Ni/Al ratio across the thickness of the bond coat. Crack branching is observed as the crack approaches the IDZ and the reasons for such behaviour are not fully understood. This work establishes the viability of this technique to determine fracture properties in highly graded coated systems and may be readily extended to more complex coating architectures and other forms of loading such as cyclic, mixed mode, etc.

Η σφυρηλάτηση με βολή σωματιδίων (shot peening) είναι μία επιφανειακή κατεργασία που πραγματοποιείται με σκοπό την αύξηση της αντοχής μεταλλικών υλικών και εφαρμόζεται στο τελευταίο στάδιο της γραμμής παραγωγής. Η αύξηση της αντοχής επιτυγχάνεται με την ανάπτυξη θλιπτικών παραμενουσών τάσεων κοντά στην επιφάνεια του υλικού έπειτα από την κρούση σωματιδίων με υψηλές ταχύτητες. Η ανάπτυξη θλιπτικών παραμενουσών τάσεων αυξάνει την αντοχή σε κόπωση, σε εργοδιάβρωση, καθώς και σε άλλες μηχανικές καταπονήσεις και επιτρέπει την μείωση του βάρους σχεδιάζοντας διατομές με μικρότερο πάχος. Στην παρούσα Διδακτορική Διατριβή παρουσιάζεται μια ολοκληρωμένη αριθμητική προσομοίωση της κατεργασίας και εξετάζεται η μηχανική συμπεριφορά των υπό κατεργασία υλικών σε υψηλούς ρυθμούς καταπόνησης. Συγκεκριμένα η μεθοδολογία που αναπτύσσεται περιλαμβάνει την ανάπτυξη ενός αριθμητικού προτύπου για την προσομοίωση της κατεργασίας της σφυρηλάτησης με βολή σωματιδίων και τον υπολογισμό των αποτελεσμάτων της στο υλικό. Τα βήματα που ακολουθηθήκαν για την ανάπτυξη του αριθμητικού προτύπου είναι, α) ο χαρακτηρισμός του κράματος αλουμινίου 7449-Τ7651 σε υψηλούς ρυθμούς καταπόνησης μέσω της πειραματικής διάταξης Split Hopkinson Bar που σχεδιάστηκε και κατασκευάστηκε στο Εργαστήριο Τεχνολογίας και Αντοχής Υλικών, β) η ανάπτυξη βοηθητικών επιμέρους αριθμητικών μοντέλων, γ) η ανάπτυξη κινηματικών μοντέλων προσομοίωσης της ροής των σωματιδίων, δ) η ανάπτυξη κριτηρίων και η εφαρμογή τους για τον υπολογισμό του ελαχίστου απαιτούμενου αριθμού σωματιδίων για την προσομοίωση, καθώς και των θέσεων κρούσης, ε) η ανάπτυξη ενός αριθμητικού προτύπου πλήρους γεωμετρίας της πλάκας για την κρούση του απαιτούμενου αριθμού σωματιδίων και στ) η πειραματική επαλήθευση του αριθμητικού προτύπου. Με το αριθμητικό πρότυπο που αναπτύχτηκε υπολογίστηκαν τα αποτελέσματα της κατεργασίας της σφυρηλάτησης με βολή σωματιδίων στο υλικό και επιβεβαιώθηκαν μέσω συγκρίσεων με αντίστοιχα πειραματικά αποτελέσματα. Αποτελέσματα της κατεργασίας εκτός από τις παραμένουσες τάσεις αποτελούν και η πλαστική παραμόρφωση, η σκληρότητα, η επιφανειακή τραχύτητα και κατ' επέκταση ο συντελεστής έντασης τάσης. Στη συνέχεια, πραγματοποιήθηκε μια παραμετρική μελέτη για την επίδραση της διαμέτρου, της ταχύτητας και της γωνίας κρούσης στην ανάπτυξη των παραμενουσών τάσεων. Επίσης το αριθμητικό πρότυπο επαληθεύτηκε και για άλλα μεταλλικά υλικά. / Shot peening is a surface treatment process that is performed to increase the strength of metallic materials and is applied to the last stage of the production line (post manufacturing process). The increase in strength is achieved by the developed compressive residual stresses near the surface and the subsurface of the treated material after the impact of small diameter particles with high speeds. The developed compressive residual stresses increases the fatigue strength, the mechanical performance of the component under stress corrosion cracking (SCC), under higher stresses and allows lighter structure design. This PhD thesis presents a comprehensive numerical simulation of the Shot peening process and includes a comprehensive study of the mechanical behaviour of treated materials under high strain rates of deformation. Specifically, the methodology developed includes the development of a comprehensive numerical model to simulate Shot peening treatment and calculate the results on the treated material. The steps followed for the development of the numerical model are: a) the characterization of the Aluminium alloy 7449-T7651 at high strain rates using a Split Hopkinson Bar apparatus designed and built at the Laboratory of Technology and Strength of Materials, b) the development of auxiliary partial numerical models, c) the development of a kinematic simulation model for the analysis of the flow particles, d) the development and the application of two criteria for the successful calculation of the minimum number of particles that required for the simulation, and the impact positions e) the development of a numerical model describing the full plate geometry for the impact of the minimum number of particles required and f) the experimental verification of the numerical model. The process outcomes and results on the treated material were calculated by the numerical model developed. The numerical results that were calculated for the threaded material were confirmed by comparison with experimental results. Treatment results include the residual stresses, the plastic deformation, hardness, surface roughness, and hence the stress concentration factor. A parametric study on the effect of the diameter, speed and angle of impact to the development of residual stresses was performed. The numerical model was also verified for a number of other metallic materials.

Ce travail est dédié á l´étude de l´influence de nano-particules de alumina (Al2O3) et de di-borure de titane (TiB2) sur la solidification, la microstructure et les propriétés thermiques et mécaniques de l´alliage d´aluminium renforcés, Al-Si9Cu3Fe1. Les matériaux ont été obtenus par un procédé de fonderie à haute pression en coulant les alliages dans les mêmes conditions que les alliages non renforcés correspondants.On a constaté que les particules de Al2O3 et de TiB2 ont une influence directe sur les caractéristiques de l´alliage telles que la microstructure, la précipitation des phases pendant la solidification et les propriétés mécaniques et électriques. On a ainsi montré que les particules de Al2O3 et de TiB2 peuvent être utilisées pour ajuster les caractéristiques des alliages et obtenir des propriétés spécifiques pour des applications dans les secteurs de matériaux légers. / The work has been focused on the study of the influence of TiB2 and Al2O3 nano-particles (up to 1 wt. %) on the properties and physical features of an aluminium casting alloy, Al-Si9Cu3Fe1.Samples have been obtained through the High Pressure Die Casting (HPDC) process and compared with unreinforced samples obtained at the same conditions. It has been observed that the Al2O3 and TiB2 particles have a direct influence on several features of the alloy such as the microstructure and precipitating phases as well as in the improvement of the soundness and mechanical and electrical properties. Al2O3 and TiB2 particles can be used to tailor the properties of the alloy and to match the specifications of light weight applications

L’alliage d’aluminium 6061-T6 a été choisi comme matériau de structure du casier et du caisson du cœur de réacteur Jules Horowitz (RJH). Transparent aux neutrons, il doit ses bonnes propriétés mécaniques à la précipitation de fines aiguilles nanométriques appelées béta'' contenant Mg et Si et à la présence de dispersoïdes Al(Cr,Fe,Mn)Si jouant un rôle important dans la résistance à la recristallisation. Le caisson et le casier seront soumis à de forts flux neutroniques à une température avoisinant les 50°C. L’objectif de cette thèse est d’étudier les évolutions microstructurales de l’alliage sous irradiation et plus particulièrement la stabilité des précipités. Pour cela, des études analytiques par irradiations in-situ et ex-situ aux électrons et aux ions à température ambiante et forte dose ont été réalisées ainsi qu’une étude du comportement des précipités sous irradiations aux neutrons à faible dose. La caractérisation fine des précipités par Microscopie Electronique en Transmission a montré que les dispersoïdes sont stables sous irradiation, cependant ils présentent une structure cœur/coquille avec un cœur riche en (Fe, Mn) et une coquille riche en Cr qui s’accentue sous irradiation par accélération de la diffusion. En revanche, les nano-phases type béta’’ sont déstabilisées par l’irradiation. Elles sont dissoutes par irradiation aux ions au profit de l’apparition d’amas riches en Mg, Si, Al, Cu et Cr participant à l’augmentation du durcissement de l’alliage, tandis qu’elles tendent à se transformer en précipités cubiques sous irradiation aux neutrons. / The 6061-T6 Aluminium alloy, whose microstructure contains Al(Fe,Mn,Cr)Si dispersoids and hardening needle-shaped beta” precipitates (Mg, Si), has been chosen as the structural material for the core vessel of the Material Testing Jules Horowitz Nuclear Reactor. Because it will be submitted to high neutron fluxes at a temperature around 50°C, it is necessary to study microstructural evolutions induced by irradiation and especially the stability of the second phase particles. In this work, analytical studies by in-situ and ex-situ electron and ion irradiations have been performed, as well as a study under neutron irradiation. The precipitates characterization by Transmission Electron Microscopy demonstrates that Al(Fe,Mn,Cr)Si dispersoids are driven under irradiation towards their equilibrium configuration, consisting of a core/shell structure, enhanced by irradiation, with a (Fe, Mn) enriched core surrounded by a Cr-enriched shell. In contrast, the (Mg,Si) beta” precipitates are destabilized by irradiation. They dissolve under ion irradiation in favor of a new precipitation of (Mg,Si,Cu,Cr,Al) rich clusters resulting in an increase of the alloy’s hardness. beta’’ precipitates tend towards a transformation to cubic precipitates under neutron irradiation.

Submitted by Bruna Rodrigues (bruna92rodrigues@yahoo.com.br) on 2016-09-21T14:11:42Z No. of bitstreams: 1 DissRDC.pdf: 7324446 bytes, checksum: 5e04b539c5f14da2f1e2fb0184498eb9 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-21T18:26:41Z (GMT) No. of bitstreams: 1 DissRDC.pdf: 7324446 bytes, checksum: 5e04b539c5f14da2f1e2fb0184498eb9 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-21T18:26:47Z (GMT) No. of bitstreams: 1 DissRDC.pdf: 7324446 bytes, checksum: 5e04b539c5f14da2f1e2fb0184498eb9 (MD5) / Made available in DSpace on 2016-09-21T18:26:54Z (GMT). No. of bitstreams: 1 DissRDC.pdf: 7324446 bytes, checksum: 5e04b539c5f14da2f1e2fb0184498eb9 (MD5) Previous issue date: 2014-07-28 / Não recebi financiamento / The homogenizing heat treatment of aluminium billets has direct influence on extrusion process productivity, surface quality and mechanical properties of extruded shapes. The homogenizing process parameters that impacts on metallurgical properties of 6063 alloys are: temperature, time and cooling rate. The maximum recommended homogenizing temperature for 6063 alloy is 590ºC. The minimum homogenizing time is 2,25h. The minimum cooling rate recommended is 400ºC/h. As for extrusion logs homogenizing process of 6063 6” diameter at Sapa Aluminium Brasil the furnace is a process bottleneck for logs production, this work has investigated about the possibility of time reducing without negative impacts on billet metallurgical properties and without detrimental impacts on productivity of extrusion process and quality of its products. This investigation was done by billet metallographic analysis, cold and hot mechanical tests and extrusion/anodizing trials for different homogenizing time conditions. The results have shown that the time reducing did not cause negative impacts for metallurgical neither for mechanical properties and because of that no detrimental effects for extrusion performance and surface finish and anodized quality was observed. Hence, was possible to decrease 20 minutes on homogenizing time and gain 14,5% of productivity on heat treatment process. / O tratamento térmico de homogeneização de tarugos de alumínio tem influência direta na produtividade do processo de extrusão, na qualidade superficial e nas propriedades mecânicas dos perfis extrudados. As variáveis do tratamento térmico da liga 6063 que influem na qualidade metalúrgica são: temperatura, tempo e taxa de resfriamento. Para liga 6063, a máxima temperatura recomendada de homogeneização é de 590ºC. O tempo mínimo para a homogeneização é de 2,25h. A taxa de resfriamento mínima recomendada é 400ºC/h. Como para o tratamento térmico de tarugos de 6” há um gargalo no forno de homogeneização da Sapa Aluminium Brasil, investigouse neste trabalho possibilidades de redução de tempo no processo de homogeneização evitando impactos negativos na qualidade metalúrgica dos tarugos e/ou na performance dos tarugos no processo de extrusão. Tal investigação foi feita através de análises metalográficas, ensaios mecânicos a frio (dureza e tração) e a quente (torção) e testes práticos no processo de extrusão/anodização para amostras retiradas de tarugos homogeneizados em diferentes condições de tempo de homogeneização. Os resultados obtidos mostraram que a redução no tempo de homogeneização não impactou negativamente na qualidade metalúrgica dos tarugos e nem em suas propriedades mecânicas a frio e que as pequenas variações ocorridas mantiveram-se dentro dos limites de especificação e não reduziram a performance do processo de extrusão bem como as propriedades mecânicas e qualidade superficial dos perfis extrudados. Com isso, foi possível reduzir 20 minutos no tempo de tratamento térmico de homogeneização e gerar um ganho de produtividade de 14,5% no processo.

This thesis is focused on fatigue behaviour of aluminium alloy 2024-T351 in low cycle and extremely low cycle fatigue regime. Test specimens were firstly subjected to quasi-static tensile and compression tests to establish basic mechanical properties of the experimental material. Fatigue tests were conducted in strain-control mode, when cyclic plastic response and S-N curves were determined. All of the experiments were conducted at room temperature. Shapes of mechanical hysteresis loops are dependent on the strain amplitude and clearly exhibit cyclic plasticity of the alloy. Cyclic deformation curve was fitted by power regression function and subsequently compared with the tensile test. Experimental data of the S-N curves were fitted by Manson-Coffin and Wöhler-Basquin law. The discrepancy of the fatigue data was observed in the extremely low cycle fatigue regime. Based on this phenomenon, new regression function was used to overcome shortening of fatigue life in the extremely low cycle regime.

This diploma thesis deals with various procedures of surface pretreatment of steel and aluminum alloy for subsequent application of powder coating substance. The work begins with an overview of the general characteristics of the materials used. The core part of the work is then a summary of the individual methods of pretreatment that can be used to modify the material surface. A special chapter is devoted to methodologies for the application of powder substances. Before separate application, the sample must first be carefully pretreated to ensure removal of organic compounds (e.g. oils from primary processing - grinding, cutting), or dust impurities to ensure the correct barrier effect. After dividing the individual methods of surface preparation, a powder paint system with a total thickness of approximately 50 µm is applied to the test material. Samples with different surface pretreatments will be compared both in terms of mechanical properties and in terms of electrochemical properties.

Práce se zabývá tvárným lomem, který je výsledkem víceosého kvazi-statického monotónního namáhání doprovázeného rozsáhlými plastickými deformacemi, přičemž pro degradaci materiálu je uvažován lokální přístup. Ve výpočtech o rozvoji poškození rozhodují použité mezní podmínky tvárného lomu. Tyto byly teoreticky studovány v úvodu práce a po výběru vhodné mezní podmínky byl stanoven postup kalibrace. Dále byl rozpracován plán měření a realizovány zkoušky při pokojové teplotě na slitině hliníku 2024-T351, zahrnující tah, krut a tlak, pro studium rozvoje poškození a věrohodnou kalibraci vybraného fenomenologického modelu tvárného porušování, vyjádřeného pomocí lomového přetvoření a závislého na hydrostatickém tlaku a deviátoru tenzoru napětí. Mezní podmínka tvárného lomu byla posléze svázána s podmínkou plasticity. Plasticita byla pro zkoumaný materiál uvažována ve tvaru zohledňujícím i stav třetího invariantu deviátoru tenzoru napětí. Celý navržený přístup, plně aplikovatelný na víceosé úlohy, byl implementován pomocí uživatelské rutiny do komerčního programu založeného na explicitní variantě metody konečných prvků. V závěru práce je předložena aplikace navrženého přístupu k modelování tvárného porušování v podobě verifikace na vybraných zkušebních testech, z níž plynou závěry a doporučení pro další práci.

L'automobile nécessite l'utilisation d'échangeurs thermiques permettant d'assurer au moteur des conditions de fonctionnement en température acceptables (autour de 90°C). La fiabilité de ces échangeurs ne peut être négligée car ils peuvent être à l'origine de complications mécaniques importantes en cas de dysfonctionnement. La maîtrise des divers modes d'endommagement des échangeurs thermiques liés aux conditions d’utilisation devient dès lors un enjeu incontestable pour les industriels en charge de leur production, tant du point de vue matériaux, que du comportement général de la structure en service (influence du procédé d'assemblage, design,...). Les échangeurs thermiques présentent aujourd’hui une sensibilité accrue aux sollicitations thermomécaniques cycliques induites en service, du fait, essentiellement, d’une constante diminution des épaisseurs des composants. Celle-ci est responsable d’une augmentation significative des contraintes internes pour des conditions en service identiques, pouvant avoir pour conséquence directe et irréversible la rupture d’un tube, témoin d’une incompatibilité matière/design/process. Deux configurations matières industrielles ont ici été étudiées. Il s’agit de structures tri-couches colaminées de type tube, constituées respectivement de deux et trois alliages d’aluminium distincts (4xxx/3xxx/4xxx ou 4xxx/3xxx/7xxx), pour une épaisseur totale de 270µm. Ces dernières ont été développées pour permettre l’utilisation du procédé de brasage comme procédé d’assemblage (alliage 4xxx). Toutefois, une telle architecture, combinée à un procédé thermique d’assemblage sévère (600°C), est responsable d’une modification profonde de la microstructure avec l’apparition de structures de solidification, responsables entre autres de nombreuses irrégularités de surface (appelées Gouttes de Placage Résiduelles - GPR) ainsi que d’importants gradients de propriétés mécaniques dans l’épaisseur. Ces travaux de recherche s’appuient sur une approche expérimentale et numérique développée pour étudier les mécanismes d'endommagement en fatigue relatifs à de telles structures fines hétérogènes. Associant diverses techniques expérimentales telles que la corrélation d’images numériques (2D-3D) ou la tomographie à rayons X (de laboratoire ou à l’ESRF), elle permet une analyse précise des mécanismes d’amorçage et de propagation des fissures de fatigue (sur éprouvette de fatigue classique ou de type échangeur thermique). Le rôle des différents placages dans chacune des phases de l’endommagement a ainsi été mis en évidence (4xxx : amorçage, 7xxx : propagation des fissures). Des simulations par la méthode des éléments finis nous ont permis de compléter ces observations en proposant une quantification précise de l’influence de l’état de surface (GPR) sur la tenue en fatigue des éprouvettes testées. Enfin, des essais de fatigue réalisés directement sur échangeurs thermiques ont permis de corroborer les résultats obtenus sur éprouvettes modèles. / The automotive industry, like many other industrial fields, requires the use of heat thermal exchangers to allow optimal thermal service conditions of the engine (around 90°C for a car engine). The exchangers’ reliability has to be guaranteed to avoid a decrease of the engine efficiency or detrimental mechanical damage resulting from too high service temperatures. It is therefore necessary to control the different damage modes of such thermal heat exchangers according to the conditions of use. Thanks to their good thermal, corrosion and mechanical properties, aluminium alloys have steadily replaced copper alloys and brass for manufacturing heat exchangers in cars or trucks. Such components have been constantly optimized in terms of exchange surface area and, nowadays, this has led to Al components in heat exchangers with a typical thickness of the order of 0.2 to 1.5 mm. With such small thicknesses, the load levels experienced by heat exchangers components has drastically increased leading to an important research effort in order to improve the resistance to damage development during service life. Two industrial materials made of 3 co-rolled aluminium alloys (total thickness 0.27 mm) have been studied. In spite of their small thickness, the materials exhibit a composite structure comprising a core material (3xxx alloy) and 2 clads (4xxx and/or 7xxx alloys according to material configuration: 4xxx/3xxx/4xxx or 4xxx/3xxx/7xxx). The lower melting point 4xxx alloy is used for producing the heat exchanger assembly during a brazing process while the 7xxx alloy improves internal corrosion resistance. Such complex architecture, combined to the severe brazing thermal treatment, leads to important microstructural modifications, mainly characterized by the formation of brazing joints or Clad Solidification Drops (CSD) on the surface. Both of them are responsible for significant gradients of the mechanical properties on the thickness. The present study is based on an original experimental and numerical approach developed to characterise the different fatigue damage mechanisms operating in such thin heterogeneous structures. Digital image correlation (2D-3D) and X-rays tomography (at different resolutions) have been used to analyze the crack initiation and propagation mechanisms, highlighting the impact of each clad on each damage step. While the 4xxx clad corresponds to preferential crack initiation zones, the 7xxx clad seems to affect significantly the crack propagation phase. Finite Elements simulations have been carried out to complete these experimental observations, putting forward an accurate quantification of the surface state influence (through the CSD). All the different results and observations made on fatigue samples with a simplified geometry have been finally confirmed by fatigue tests on thermal exchanger configurations.