Natural ageing is known to have a negative effect on the formability and bake-hardening response of Al-Mg-Si alloys. This is attributed to the formation of Mg and Si clusters during natural ageing. The clustering process was the subject of many studies in the literature, however, the formation mechanism and kinetics of it, continues to be poorly understood. The aim of this project is to shed some light on the cluster formation mechanism and measure clustering kinetics at low temperatures. A series of electrical resistivity measurements, positron annihilation lifetime spectroscopy and hardness tests were performed on samples aged over the temperature range of -20 to 50°C following solution treatment at temperatures of 525 and 560°C. A very good correlation between the results of various techniques was observed. In addition, three different stages in the clustering process were detected. Not surprisingly it was found that the excess quenched-in vacancies are the key players in the cluster formation process. In the first stage, annihilation of near-sink vacancies occurs while other vacancies start to bind with solute atoms and form clusters. In the second stage, clustering continues to take place but its rate slows down since the effective diffusion coefficient of vacancies decreases as they bind with more solute. Finally, the clustering process enters the third stage with much slower kinetics. Interestingly, positron annihilation lifetime also reaches a constant value at the beginning of stage III which suggests the stabilization of vacancies. Two hypothesis were then developed to explain the existence of stage III: one based on the immobilization of vacancies due to the increased binding with solute atoms and another one which considers the overlapping of solute diffusion profiles around the clusters. Finally it was shown that the resistivity change in stage II can be used to find the activation energy of clustering which is calculated to be approximately 46 kJ/mol. This is very close to the migration energy of vacancies and Mg atoms. Thus it was concluded that migration of these species is the major rate controlling parameter for the clustering process. / Thesis / Master of Applied Science (MASc)

The current, commercially available, press and sinter Al-Mg-Si-Cu alloys are based on wrought or cast alloy compositions and have not been tailored for the press and sinter process. The limited development of the Al-Mg-Si-Cu alloys for the press and sinter process can be partly attributed to a poor understanding of the effects of processing conditions on the sintering behaviour. The primary objective of this work was to investigate and understand the effects of processing conditions on the sintering behaviour of Al-Mg-Si-Cu-(Sn) alloys. Dilatometry was used in conjunction with other experimental techniques to elucidate and understand the expansion and shrinkage events that occur during the liquid-phase sintering of Al-Mg-Si-Cu-(Sn) powder metallurgy alloys. Samples were uni-axially pressed from elemental metal powder blends, de-waxed, and then sintered in a horizontal push-rod dilatometer to record the dimensional changes in the pressing direction. The processing conditions examined included the alloy composition, temperature, green density and atmosphere. A liquid forms during heating due to reactions between the alloying elements and the aluminium. This liquid is initially non-wetting on the oxide layer of the aluminium particles, resulting in separation of the particles, which is manifested by expansion of the sample. The oxide is reduced as sintering progresses, alleviating the non-wetting conditions. When more liquid forms, further expansion occurs, despite the improved wetting conditions. It is proposed that atmospheric oxygen and/or nitrogen can react with the liquid, forming a solid phase (‘shell’) at the liquid-vapour interfaces. These shells prevent the liquid from wetting the particles, resulting in further expansion and preventing shrinkage. Unbalanced diffusivities (the Kirkendall effect) between the aluminium and silicon contribute to the expansion. A mechanism is proposed to account for the transition to shrinkage, whereby the shells at the liquid-vapour interface rupture when there is a rapid increase in the volume of contained liquid. The liquid then flows out and over the shells, onto the aluminium substrate, causing shrinkage. Magnesium and nitrogen delay the transition to shrinkage by facilitating nitride shell formation at the solid-liquid interface. Silicon and tin cause an earlier transition to shrinkage by increasing the liquid volume. In addition, tin promotes shrinkage by segregating to the liquid-vapour interfaces, limiting the thickness of the shells at the liquid-vapour interfaces. The two dominant liquid-phase shrinkage mechanisms during the sintering of Al-Mg-Si-Cu-(Sn) alloys are rearrangement and pore-filling. Contact-flattening is not a dominant shrinkage mechanism, but may occur concurrently with the other mechanisms. If contact flattening occurs, a decrease in the pressure of isolated pores increases the total shrinkage rate. Nitrogen increases the shrinkage rate during rearrangement by restricting grain-growth. Magnesium increases the shrinkage rate during rearrangement by reducing the solid-liquid interface energy. Magnesium and nitrogen are essential for the formation of nitride within isolated pores, which decreases the pore pressure and increases the contribution of contact-flattening on the total shrinkage rate. Silicon reduces the beneficial influence of magnesium during rearrangement by diluting the magnesium content in the liquid. Silicon increases the pore-filling rate due to an increase in the liquid volume. Magnesium increases the pore-filling rate by facilitating aluminium nitride formation within isolated pores and by increasing the pore-filling. Tin additions can decrease the pore-filling rate due to its segregation to the liquid-vapour interface, limiting the consumption of nitrogen within isolated pores.

La mise en forme à froid des alliages d’aluminium peut être limitée par les transferts de matière sur la surface des outillages pouvant mener au grippage. L’objectif de ce travail de thèse est de contribuer à une meilleure compréhension de ces mécanismes pour permettre à terme d’optimiser les procédés de formage d’un alliage Al-Mg-Si. Une approche couplée expérimentale et numérique est développée afin d’alimenter une base de données d’indicateurs de dégradations des surfaces. L’approche expérimentale développée au cours de ces travaux repose sur l’utilisation de l’essai de compression-translation. Ce dispositif permet de reproduire une large gamme de conditions de contact rencontrées en mise en forme à froid. Une méthode d’analyse des surfaces reposant sur des mesures par profilométrie optique et des analyses au microscope électronique à balayage est développée afin d’évaluer le volume de matière adhérée sur les outils de chaque configuration d’essai. Le comportement rhéologique de l’alliage Al-Mg-Si est caractérisé à l’aide du simulateur thermomécanique GLEEBLE 3500 sur la gamme de températures de 298 à 423 K et à deux vitesses de déformation de 0,1 et 1 s-1. Un modèle de comportement physique reposant sur l’analyse du taux d’écrouissage et l’utilisation du modèle de la contrainte mécanique seuil est développé puis implémenté dans un logiciel éléments finis. Un modèle numérique de l’essai de caractérisation tribologique est développé. Il s’agit d’une analyse thermomécanique utilisant la formulation arbitrairement lagrangienne-eulérienne. Les données locales telles que la pression de contact et la vitesse de glissement calculées par ce modèle permettent de mieux comprendre les mécanismes de collage observés expérimentalement. Enfin, un programme de traitement des résultats numériques est développé afin de réaliser les modifications géométriques de la surface de l’outil en fonction du dépôt relevé expérimentalement. Un modèle d’usure basé sur le travail des forces de frottement est identifié. Ce dernier est un bon indicateur de la tendance au collage et permet une première approximation de ce mécanisme. / Cold forming of aluminium alloys can be limited by a severe material transfer to the die surfaces, compromising the process viability. The purpose of this research work is to contribute to a broader understanding of the galling mechanisms which would further allow the optimisation of the forming processes of an Al-Mg-Si alloy. A methodology combining experimental and numerical approaches is introduced in order to build up a database of surface degradations indicators. The experimental approach is carried out by means of upsetting-sliding tests which reproduce a wide range of contact conditions experienced at the tool/workpiece interface during cold forming operations. To evaluate the amounts of adhered material on the tools of each test configuration, surface topography acquisitions are performed by optical profilometry and coupled to scanning electron microscope analyses. Axisymmetric compression tests are carried out by means of the GLEEBLE 3500 thermomechanical simulator to determine the deformation behaviour of the Al-Mg-Si alloy at high strains, in the temperature range of 298 to 423 K and strain rates of 0.1 and 1 s¡1. A constitutive model based on both the Mechanical Threshold Stress model and the analysis of the work-hardening rate is proposed and then implemented in a finite element code. A finite element simulation of the upsetting-sliding test, which is a thermomechanical analysis using the arbitrary lagrangian-eulerian formulation, is introduced. The local contact variables such as the contact pressure and the sliding velocity are evaluated through this simulation and allow a better understanding of the galling mechanisms observed experimentally. Finally, a post-processing program, which analyses the results of the finite element simulation and updates the tool shape according to the amount of adhered material determined experimentally, is developed. A wear model based on the friction work is introduced. This model is a good indicator of the galling tendency and allows a first approximation of this mechanism.

Aluminium is increasingly being used in the automotive industry to reduce the weight of vehicles. It is the additions of transition elements such as Mn and Cr that can be picked up during recycling, that can form dispersoid particles during homogenisation. Dispersoids play a significant role in the recrystallization and texture development for wrought Al-Mg-Si alloys by inhibiting grain boundary motion. It is therefore important to understand the precipitation kinetics of such particles. The Mn+Cr dispersoid phases are currently thought to nucleate on β'-Mg1.8Si particles via an intermediate semi-coherent precipitate denoted the u-phase. In this study, Al-Mg-Si alloys with additions of Fe and varying levels of Cr were cast to study the effect of different homogenisation regimes on the dispersoid precipitation mechanisms and final characteristics. Electron Probe Micro Analysis (EPMA) was conducted to study the inhomogeneity of elements in the cast structure and through heating to the homogenisation temperature. It was found that Mg, Si and Fe segregate towards the dendrite edges during solidification while Cr segregates towards the dendrite centre. During heating, the matrix composition of both Mg and Si decrease and increase due to precipitation of Mg+Si phases. Cr and Fe stay segregated during the heating process due to the slower diffusion rates in the face centred cubic Al matrix. Dispersoid free regions have also been observed in the microstructure correlating to the elemental segregation in the as-cast condition. Optical, scanning and transmission electron microscopy was utilised in order to study the change in dispersoid characteristics with varying homogenisation regimes and as a function of distance through a grain. With an increase in homogenisation temperature, the mean size of dispersoids increased but number density decreased. For a longer dwell time, the dispersoids remained approximately the same size but increased in volume fraction and density. Increasing the heating rate did not significantly change the dispersoid size, volume fraction or density. The dispersoids size and number density was also studied as a function of distance through a number of grains with the interplay of nucleation, growth and coarsening discussed. Both α-Al(FeCr)Si and α'-AlCrSi dispersoids were found to exist with a variety of morphologies while the α'-AlCrSi dispersoids were found to have a larger effective diameter.

Microstructural evolution in a naturally-aged and cold-rolled AA6451 aluminum alloy during a non-isothermal annealing process, which leads to significant grain refinement, is investigated through: (a) conducting a comprehensive experimental analysis and (b) developing a computational modeling technique. The underlying mechanisms of annealing have been investigated through analysing interactive phenomena between precipitation and concurrent recovery and recrystallization. It is shown that the interactions between solute elements, clusters, and fine precipitates with dislocations restrict dynamic and static recovery during deformation and subsequent annealing. Inhibition of recovery favours recrystallization that initiates at 300oC and progresses through a nucleation and growth mechanism. Despite localized inhomogeneities, nucleation mainly occurs in non-recovered high energy sites which are uniformly distributed within the entire structure. Growth of the recrystallized nuclei is restricted by pinning precipitates that undergo a concurrent coarsening process. The fine, uniform distribution of recrystallized nuclei and their limited growth result in the formation of a fine-grained microstructure, after completion of recrystallization. The acquired knowledge has been used to develop a computational modeling technique for simulating microstructural evolution of the alloy. Microstructural states are simulated by integrating analytical approaches in a Monte Carlo algorithm. The effects of deformation-induced and pre-existing inhomogeneities, as well as precipitate coarsening and grain boundary pinning on the competitive recovery-recrystallization process are included in the simulation algorithm. The developed technique is implemented to predict the microstructural evolution during isothermal and non-isothermal annealing of AA6xxx sheets. A good quantitative agreement is found between the model predictions and the results from the experimental investigations.

In this work, we have investigated the internal stress contribution to the flow stress for a commercial 6xxx aluminium alloy (AA6111). In contrast to stresses from forest and precipitation hardening, the internal stress cannot be assessed properly with a uniaxial tensile test. Instead, tension-compression tests have been used to measure the Bauschinger stress and produce a comprehensive study which examines its evolution with i) the precipitation structure and ii) a wide range of applied strain. A large set of ageing conditions was investigated to explore the effect of the precipitation state on the development of internal stress within the material. It is shown that the Bauschinger stress generally increases with the applied strain and critically depends on the precipitate average radius and is thus linked to the shearable/non shearable transition. Further work in the case of non-shearable particles shows that higher strain eventually lead to particle fracture and the Bauschinger stress then rapidly decreases. Following the seminal work of Brown et al, a physically based approach including plastic relaxation and particle fracture is developed to predict the evolution of the internal stress as a function of the applied strain. Knowing the precipitation structure main characteristics –such as the average precipitate radius, length and volume fraction– allows one to estimate accurately the internal stress contribution to the flow stress with this model.

Microstructural evolution in a naturally-aged and cold-rolled AA6451 aluminum alloy during a non-isothermal annealing process, which leads to significant grain refinement, is investigated through: (a) conducting a comprehensive experimental analysis and (b) developing a computational modeling technique. The underlying mechanisms of annealing have been investigated through analysing interactive phenomena between precipitation and concurrent recovery and recrystallization. It is shown that the interactions between solute elements, clusters, and fine precipitates with dislocations restrict dynamic and static recovery during deformation and subsequent annealing. Inhibition of recovery favours recrystallization that initiates at 300oC and progresses through a nucleation and growth mechanism. Despite localized inhomogeneities, nucleation mainly occurs in non-recovered high energy sites which are uniformly distributed within the entire structure. Growth of the recrystallized nuclei is restricted by pinning precipitates that undergo a concurrent coarsening process. The fine, uniform distribution of recrystallized nuclei and their limited growth result in the formation of a fine-grained microstructure, after completion of recrystallization. The acquired knowledge has been used to develop a computational modeling technique for simulating microstructural evolution of the alloy. Microstructural states are simulated by integrating analytical approaches in a Monte Carlo algorithm. The effects of deformation-induced and pre-existing inhomogeneities, as well as precipitate coarsening and grain boundary pinning on the competitive recovery-recrystallization process are included in the simulation algorithm. The developed technique is implemented to predict the microstructural evolution during isothermal and non-isothermal annealing of AA6xxx sheets. A good quantitative agreement is found between the model predictions and the results from the experimental investigations.

Dans le cadre d'un programme de recherche visant à l'allègement de la structure des véhicules, l'origine de lignage dans des tôles en aluminium AA6016 a été étudiée. Ce phénomène, qui peut apparaître à la suite d'une déformation plastique, est apparenté à de la rugosité de surface alignée dans la direction de laminage (DL). Sa présence est néfaste à une bonne finition de surface, et son intensité est appréciée visuellement par les fabricants.Une méthode de quantification rationnelle a été développée. La caractérisation de la distribution morphologique des motifs de rugosité a été rendue possible par l'utilisation de fonctions fréquentielles telle la densité de puissance spectrale. La note globale, construite à partir de la quantification individuelle des composantes de lignage pur et de rugosité globulaire, s'est montrée en bon accord avec les estimations visuelles, et notamment avec le niveau de lignage intermédiaire regroupant plusieurs aspects de surface différents.La microstructure des matériaux à l'état T4 a été expérimentalement mesurée couche de grains par couche de grain à l'aide d'un couplage entre polissage contrôle et acquisition par EBSD. Les 4 à 5 premières couches sous la surface (-120μm) semblent jouer un rôle mécanique prépondérant dans la formation du lignage car elles offrent à la fois une grande taille de grains moyenne, une importante ségrégation d'orientations cristallines, et une forte similitude de longueurs d'onde entre la rugosité de surface et les motifs de la microtexture.Des simulations numériques ont permis de vérifier que les couples de texture identifiés (Cube/Goss, Cube/Aléatoire et Cube/CT18DN) possédaient des différences d'amincissements hors-plans suffisantes pour générer l'ondulation d'une couche d'éléments. En revanche, l'influence mécanique de cette même couche décroit très rapidement avec son enfouissement dans la profondeur et devient négligeable sous plus de 4 couches d'éléments. / As part of a project on aluminium alloys for vehicle weight reduction, the origins of roping in AA6016 aluminium sheets have been studied. This strain-induced phenomenon is related to surface roughness but involves narrow alignments along rolling direction (RD). Its lowers the surface quality, and its intensity is visually evaluated by vehicle manufacturers.An original quantification method is proposed. The morphological characterization of roughness features has been measured by using frequency functions such as the areal power spectral density. The overall roping quality mark, determined from quantifications of both the isotropic and unidirectional components, shows good agreement with the visual assessment, especially for the intermediate roping levels which exhibit several different surface appearances.The material microtexture has been experimentally measured through grain to grain layers by using serial sectioning and EBSD scans. The first 4 to 5 layers under the surface (-120μm) seem to play a leading role in the micromechanics of roping developpment since they simultaneously exhibit a high average grain size, significant segregation of crystallographic orientations, and a close similitude between surface roughness and microstructural feature wavelenghts.Numerical simulations verified that the identified texture pairs (Cube/Goss, Cube/Random and Cube/CT18DN) have sufficient out-of-plane strain difference to promote one element thick layer undulations. But, the mechanical influence of this layer decreases gradually with depth, and becomes negligible below 4 other layers.

Les alliages Al-Mg-Si-(Cu) de la série 6016 sont utilisés sous forme de tôles pour<br />la réalisation d'ouvrants dans l'industrie automobile. Leur particularité est de pouvoir<br />être durcis durant la cuisson des peintures (typiquement 30 minutes à 185°C). Ce phénomène<br />s'explique par un durcissement structural : le traitement thermique de cuisson des<br />peintures appliqué à une solution solide sursaturée active la diffusion des solutés et la précipitation<br />de phases métastables plus ou moins cohérentes avec la matrice d'aluminium<br />qui vont ralentir le mouvement des dislocations.<br />Ces différentes phases métastables ont des tailles extrêmement petites (parfois inférieures<br />à 1nm), ce qui les rend difficiles à analyser autrement qu'en sonde atomique tomographique.<br />Une attention toute particulière a été apportée au développement de nouvelles<br />techniques d'interprétation et de traitement des données afin de faire reculer les limites de<br />l'instrument.<br />Grâce à la sonde atomique, parfois couplée au microscope électronique en transmission,<br />les mécanismes de précipitation de la phase beta" ont été mieux compris. En particulier,<br />nous avons montré qu'un traitement de prérevenu à 90°C appliqué avant le revenu rendait<br />ce dernier plus efficace par la formation d'amas diffus de Mg et de Si qui augmentent la<br />densité numérique d'objets durcissants (amas +beta"). La présence d'aluminium dans les<br />précipités a été démontrée.<br />L'effet du prérevenu sur la corrélation entre les atomes durant les tout premiers stades<br />de décomposition a été étudié directement par le calcul de fonctions partielles de corrélation<br />de paires. Cette méthode a également permis de suggérer un scénario expliquant le<br />durcissement pendant le stockage à l'ambiante par la formation d'atmosphères de solutés<br />autour des dislocations, ralentissant ainsi leur mouvement.

Dans le cadre d'un programme de recherche visant à l'allègement de la structure des véhicules, l'origine de lignage dans des tôles en aluminium AA6016 a été étudiée. Ce phénomène, qui peut apparaître à la suite d'une déformation plastique, est apparenté à de la rugosité de surface alignée dans la direction de laminage (DL). Sa présence est néfaste à une bonne finition de surface, et son intensité est appréciée visuellement par les fabricants.Une méthode de quantification rationnelle a été développée. La caractérisation de la distribution morphologique des motifs de rugosité a été rendue possible par l'utilisation de fonctions fréquentielles telle la densité de puissance spectrale. La note globale, construite à partir de la quantification individuelle des composantes de lignage pur et de rugosité globulaire, s'est montrée en bon accord avec les estimations visuelles, et notamment avec le niveau de lignage intermédiaire regroupant plusieurs aspects de surface différents.La microstructure des matériaux à l'état T4 a été expérimentalement mesurée couche de grains par couche de grain à l'aide d'un couplage entre polissage contrôle et acquisition par EBSD. Les 4 à 5 premières couches sous la surface (-120μm) semblent jouer un rôle mécanique prépondérant dans la formation du lignage car elles offrent à la fois une grande taille de grains moyenne, une importante ségrégation d'orientations cristallines, et une forte similitude de longueurs d'onde entre la rugosité de surface et les motifs de la microtexture.Des simulations numériques ont permis de vérifier que les couples de texture identifiés (Cube/Goss, Cube/Aléatoire et Cube/CT18DN) possédaient des différences d'amincissements hors-plans suffisantes pour générer l'ondulation d'une couche d'éléments. En revanche, l'influence mécanique de cette même couche décroit très rapidement avec son enfouissement dans la profondeur et devient négligeable sous plus de 4 couches d'éléments.