Submitted by PPG Engenharia e Tecnologia de Materiais (engenharia.pg.materiais@pucrs.br) on 2017-10-10T12:03:59Z No. of bitstreams: 1 Tese_Bernardo_2017_R18_3_Versao_PGETEMA.pdf: 6957919 bytes, checksum: e8811a19bda3fe37ea9ed6f6247594ac (MD5) / Approved for entry into archive by Caroline Xavier (caroline.xavier@pucrs.br) on 2017-10-10T13:43:30Z (GMT) No. of bitstreams: 1 Tese_Bernardo_2017_R18_3_Versao_PGETEMA.pdf: 6957919 bytes, checksum: e8811a19bda3fe37ea9ed6f6247594ac (MD5) / Made available in DSpace on 2017-10-10T13:48:22Z (GMT). No. of bitstreams: 1 Tese_Bernardo_2017_R18_3_Versao_PGETEMA.pdf: 6957919 bytes, checksum: e8811a19bda3fe37ea9ed6f6247594ac (MD5) Previous issue date: 2017-08-31 / CAPES/PROSUP / The objective of the present work is to develop two modified ternary Al-Zn-Mg alloys, with Zn content about 5wt% and 8wt%, and to evaluate the precipitation hardening (solutioning and artificial ageing ? T6) as a function of solidification conditions. The 752 (Al-5%Zn-2%Mg) and 782 (Al-8%Zn-2%Mg) alloys were obtained to analyze the influence of Zn content, as well as the relationship between solidification conditions, precipitation hardening, and mechanical properties before and after heat treatments. Unidirectional vertical upward solidification was performed in a metallic mold cooled by water in the bottom and instrumented using thermocouples to obtain the ingots. Samples extracted from ingots were metallography characterized by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and the mechanical properties were determined by hardness, microhardness, tensile and wear testing. The results showed that the increase of 3% of zinc in the chemical composition improved the hardness of 44%, the superficial wear of 27.7% and the tensile strength of 31.5% (in T6 condition). When comparing the results against data from some similar wrought alloys, it is concluded that the as-cast and heat treated 782 alloy can replace those, according to design specifications. / O objetivo do presente trabalho foi desenvolver duas ligas tern?rias fundidas modificadas de Al-Zn-Mg, com varia??o de 5% e 8% em Zn (em massa), e investigar o endurecimento por precipita??o (solubiliza??o e envelhecimento artificial - T6) em fun??o das condi??es de solidifica??o. Foram produzidas as ligas 752 (Al-5%Zn-2%Mg) e 782 (Al-8%Zn-2%Mg) para analisar, al?m da influ?ncia do teor de zinco, a correla??o entre as condi??es de solidifica??o, o endurecimento por precipita??o e as propriedades mec?nicas iniciais e finais. Para obten??o dos lingotes, a solidifica??o unidirecional vertical ascendente foi realizada em molde met?lico resfriado com ?gua na base e instrumentado com termopares. Amostras foram caracterizadas por an?lises metalogr?ficas em microscopia ?ptica (MO), microscopia eletr?nica de varredura (MEV), espectroscopia de energia dispersiva (EDS), e ensaios mec?nicos para determina??o das propriedades mec?nicas como dureza, microdureza, tra??o e desgaste, antes e ap?s os tratamentos t?rmicos. Os resultados mostraram que o aumento em 3% de zinco na composi??o qu?mica melhorou a dureza em 44,4%, o desgaste superficial em 27,7% e a resist?ncia ? tra??o em 31,5% (em condi??o T6). Comparando estes resultados com dados de algumas ligas conformadas similares, conclui-se que a Liga 782, fundida e tratada termicamente, pode substituir as ligas conformadas comerciais com adi??o de cobre.

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Nous étudions la fragilisation par l'hydrogène de l'alliage d'aluminium 7108. Une technique expérimentale spécifique a été développée : Un pré-chargement en hydrogène des échantillons, à travers un dépôt de nickel de quelques dizaines de microns, qui empêche la dissolution du substrat d'aluminium, est utilisé. Il permet la comparaison de la résistance à la fragilisation de différentes microstructures modèles. Nous étudions l'effet du traitement thermique et de la précipitation sur la sensibilité à l'hydrogène pour des vitesses de déformation macroscopiques imposées variables. Différents modes de rupture sont observés ainsi que des transitions entre eux. Au moyen de simulations numériques à l'échelle mésoscopique, l'effet de taille des précipités intergranulaires pré-fragilisés sur la ténacité des joints de grains est estimé, en utilisant un modèle de zone cohésive. Nous analysons la compétition entre la diffusion de l'hydrogène vers la pointe de la fissure et la vitesse de fissuration par un couplage mécanique - diffusion basé sur la diffusion de l'hydrogène assistée par la contrainte hydrostatique. Une vitesse critique au-delà de laquelle l'hydrogène ne peut plus suivre la fissure, est mise en évidence. L'influence de la microstructure du joint de grains sur cette vitesse est analysée. La valeur est comparée à une estimation des vitesses de propagation expérimentales obtenues pour différentes vitesses de déformation macroscopiques. Nous analysons l'effet du piégeage de l'hydrogène par les précipités intergranulaires et la désorption sur la répartition de l'hydrogène le long du joint de grains en imposant un flux au niveau de l'interface précipités - matrice. / We study the hydrogen embrittlement of the 7108 aluminum alloy. A specific experimental technique was developed : A hydrogen pre-charging, through few tens of microns of deposit of nickel, which prevents the dissolution of the aluminum substrate is used. It allows a comparison of the resistance to embrittlement of different model microstructures. We study the effect of heat treatment and intergranular precipitation on the susceptibility to hydrogen embrittlement for several macroscopic strain rates. Different failure modes and transitions between them are observed. Through numerical simulations, at the mesoscopic scale, the effect of the size of pre-weakened intergranular precipitates on the grain boundary toughness is estimated using a cohesive zone model. We further analyze the competition between the hydrogen diffusion toward the crack tip and crack velocity. For this purpose, a mechanical – diffusion coupling based on the hydrogen diffusion assisted by hydrostatic stress is elaborated. A critical crack velocity, beyond which hydrogen can no longer follow the crack, is highlighted. The influence of the grain boundary microstructure on this critical crack velocity is evaluated and its value is compared with an estimate of velocities obtained for different experimental macroscopic strain rates. We analyze the effect of hydrogen trapping by intergranular precipitates and hydrogen desorption by imposing a flux at the precipitates – matrix interfaces.

Le soudage des alliages Al-Zn-Mg est de plus en plus utilisé dans l'industrie du transport. Les variations brutales de température induites par le soudage à l'arc provoquent des modifications importantes de la microstructure dans la zone dite affectée thermiquement.<br />Cette étude porte sur l'évolution de la précipitation suite à ce procédé de l'alliage 7108.50 présentant différentes microstructures initiales. Une caractérisation fine des précipités durcissants dans la zone affectée thermiquement a été menée par SAXS, MET et DSC juste après soudage et après traitements thermiques post-soudage. Afin de mieux cerner les mécanismes influençant la précipitation au cours de ce procédé anisotherme, des traitements thermiques contrôlés, de type réversion ou rampes de montée en température, ont également été étudiés.<br />Un modèle basé sur l'évolution d'une distribution en taille de particules a été développé afin de prédire l'état de précipitation après divers traitements thermiques, dont le soudage.

Nous étudions la fragilisation par l'hydrogène de l'alliage d'aluminium 7108. Une technique expérimentale spécifique a été développée : Un pré-chargement en hydrogène des échantillons, à travers un dépôt de nickel de quelques dizaines de microns, qui empêche la dissolution du substrat d'aluminium, est utilisé. Il permet la comparaison de la résistance à la fragilisation de différentes microstructures modèles. Nous étudions l'effet du traitement thermique et de la précipitation sur la sensibilité à l'hydrogène pour des vitesses de déformation macroscopiques imposées variables. Différents modes de rupture sont observés ainsi que des transitions entre eux. Au moyen de simulations numériques à l'échelle mésoscopique, l'effet de taille des précipités intergranulaires pré-fragilisés sur la ténacité des joints de grains est estimé, en utilisant un modèle de zone cohésive. Nous analysons la compétition entre la diffusion de l'hydrogène vers la pointe de la fissure et la vitesse de fissuration par un couplage mécanique - diffusion basé sur la diffusion de l'hydrogène assistée par la contrainte hydrostatique. Une vitesse critique au-delà de laquelle l'hydrogène ne peut plus suivre la fissure, est mise en évidence. L'influence de la microstructure du joint de grains sur cette vitesse est analysée. La valeur est comparée à une estimation des vitesses de propagation expérimentales obtenues pour différentes vitesses de déformation macroscopiques. Nous analysons l'effet du piégeage de l'hydrogène par les précipités intergranulaires et la désorption sur la répartition de l'hydrogène le long du joint de grains en imposant un flux au niveau de l'interface précipités - matrice.

Les alliages d'aluminium de la série 7xxx sont des alliages à durcissement structural, principalement utilisés dans des applications aéronautiques en raison de leurs propriétés mécaniques spécifiques élevées. Le composant principal de leur résistance mécanique est leur microstructure de précipitation, obtenue par le biais de traitements thermomécaniques complexes. Ceux-ci induisent cependant des contraintes internes à longue distance, susceptibles de générer des distorsions sévères dans les pièces aéronautiques. Certains traitements thermomécaniques comme le grenaillage (shot peening), le formage laser (laser forming) et le formage revenu (age-forming), des procédés respectivement purement mécanique, purement thermique et thermomécanique, permettent de réduire les distorsions de pièces usinées. Ils modifient cependant la microstructure, et, par conséquent, les propriétés mécaniques. La présente étude, portant sur les couplages entre ces traitements thermomécaniques, la microstructure et les propriétés mécaniques d'un alliage 7xxx, comprend un travail de caractérisation expérimentale et de modélisation à base physique. L'effet de la précipitation sur l'écrouissage du matériau est ainsi examiné, et un modèle d'écrouissage destiné à être implémenté dans un modèle éléments finis de simulation du grenaillage est proposé. L'étude du formage laser aboutit quant à elle à un modèle prédictif de l'état de précipitation et des propriétés mécaniques locales induits par ce procédé. La dernière partie est consacrée à l'étude de l'effet du formage revenu – procédé combinant déformation plastique et traitement thermique simultanés – sur la précipitation.

Casting, Solidification, Aluminum, Physical Metallurgy, Mechanical Metallurgy, Strengthening Model, Aluminum Wrought Alloys, Aluminum 7xxx Series, Al-Zn-Mg-Cu / Over the past decades, researchers in casting fields, especially in semi-solid metal state, have endeavored to find new ways to enable the Al wrought alloys of casting using the conventional casting processes; mainly in order to improve the product properties and decrease the product cost. The thixoforming and rheocasting processes have been presented as ways by which the microstructure of Al-base wrought alloys can be changed into non-dendritic, which in turn can lead to improvement to the mechanical properties. This can be because the effect of the non-dendritic microstructure on the mechanical properties of the material. Unfortunately, these processes have proved to be cost prohibitive and be a bit complicated for commercial applications. Further, conventional casting of Al-base wrought alloys along with their superior properties and performance have been a challenge for foundry industry due to the main disadvantage of hot tearing or hot cracking during solidification process. This can render the cast component ineffective. To overcome the disadvantages of thixoforming and rheocasting processes, Controlled Diffusion Solidification (CDS) process was innovated mainly to enable casting of aluminum wrought alloys with a non-dendritic morphology of the primary Al phase in the resultant cast microstructure and thus alleviating the problem of hot tearing and obtaining a cost effective product with improved mechanical properties. The CDS is a simple process involving mixing of two precursor alloys of different thermal masses (temperature and solute) and subsequently casting the resultant mixture of the desired solute composition and temperature as a near net shaped cast product. The process can easily be commercialized with a marginal capital cost required for set up such as the addition of an extra holding furnace. Further, the CDS process would prove itself to be unique in its ability to cast Al-based wrought alloys into near net shaped components without additional processes and cost. The originality of this study is to present a viable casting process for the Al-7xxx wrought alloys (Al-Zn-Mg-Cu); by which the Al-7xxx family alloys are presented in cast condition to have an acceptable uniaxial property range that is comparable with their wrought counterparts. This study presents the process and alloy parameters necessary for the casting of Al-7xxx wrought alloys (Al-Zn-Mg-Cu), by using the CDS process coupling with tilt pour gravity casting (TPGC) machine. The uniaxial tensile mechanical properties of several Al-7xxx CDS castings under various heat treatment conditions, namely, solutionizing (T4), peak aged (T6) and annealing (O), necessary for development of an ageing process on the material were investigated and presented. The tilt pour gravity casting process coupled with the CDS technology was employed to demonstrate the ability to cast Al-7xxx wrought alloys into high integrity components with high strength and ductility. The microstructure characterization was carried out by Electron Microscopy (TEM, SEM and EDS) and DSC test experiments for all the as cast (F), T4, T6 and anneals (O) conditions of the CDS cast components. Also, the predictive capabilities for the yield strength of Al 7xxx alloys CDS cast components was investigated using structural-properties modeling for the various strengthening effects that are recently proposed specifically for the Al-7xxx wrought counterparts. The study has successfully led to a more in-depth understanding of the innovative CDS casting process by applying it to several compositions of Al-7xxx wrought alloys in an industrial scale CDS casting experiments, using tilt pour gravity casting (TPGC) machine. This will hopefully lead us to a clearer path towards commercializing the CDS process and obtaining a viable casting process for Al-base wrought alloys into near net shape components without much change to economics of the casting process. / Dissertation / Doctor of Philosophy (PhD) / Casting, Solidification, Aluminum, Physical Metallurgy, Mechanical Metallurgy, Strengthening Model, Aluminum Wrought Alloys, Aluminum 7xxx Series, Al-Zn-Mg-Cu

This project was funded by Automotive Partnership Canada (APC), an initiative created by the Government of Canada in an attempt to support significant, collaborative R&D activities in order to benefit the entire Canadian automotive industry. / High strength Al-Zn-Mg-Cu alloys have been increasingly employed in the transportation industry due to the increased demands for light structural components. However, their applications have been limited to relatively expensive wrought products. Application of the shape cast Al-Zn-Mg-Cu parts has never been the focus of attention due to their poor castability and mechanical properties. Improving the casting quality is expected to increase their utilization within the automotive industry. The poor castability and mechanical properties of some alloys in this family may be effectively improved through optimized chemistry control and melt treatment including grain refinement. The primary objective of this project is to optimize the chemistry and heat treatment of the Al-Zn-Mg-Cu alloy family that results in improved strength with acceptable level of ductility and casting quality relative to other shape cast Al alloys. The Taguchi experimental design method was used to narrow down the number of required casting experiments required to meet the research objective. Three levels across four elements yielded a total of 9 Al-Zn-Mg-Cu alloys, which were cast using a tilt pour permanent mold process. The effect of each major alloying element on the microstructure, and mechanical properties was investigated. Tensile measurements were made on the 9 alloys subjected to two steps solution treatments. Mechanical properties such as yield strength (YS), ultimate tensile strength (UTS), and elongation at fracture (El.%) were experimentally measured and statistically analyzed. An ANOVA analysis was employed to quantify the percentage contribution of the alloying elements on the material properties. Grain refinement was found to play a significant role in improving the hot tearing resistance and, thereby ameliorating quality. The alloying element that affected the YS and UTS to the greatest extent was Cu, followed by Zn. In contrast, the effect of Mg and Ti on YS and UTS was insignificant. Moreover, a decrease in Mg content had the greatest effect in enhancing the El.%. A regression analysis was used to obtain statistical relationships (models) correlating the material properties with the variations in the content of the major alloying elements. The R-square values of YS, UTS, and El.% were 99.7 %, 98 %, and 90 %, respectively, showing that the models replicated the experimental results. Verification measurements made on shape cast Al-6Zn-2Mg-2Cu alloy revealed that the material property model predictions were in agreement with the experimentally measured values. The results show that secondary and over ageing treatments of the shape cast Al-Zn-Mg-Cu alloys lead to superior combination of YS and El.%. The ongoing advances in shape casting of Al-Zn-Mg-Cu alloys with high will make them suitable choices for commercial load-bearing automotive components, when it comes to the selection of a material meeting the minimum requirements for strength, damage tolerance, cost and weight. / Thesis / Master of Applied Science (MASc)

Al-Zn-Mg-Cu-Zr based AA7010 aluminum alloy belongs to the class of heat treatable alloys and the semi-finished products are generally produced by hot rolling, forging or extrusion processes. It is well known that the thermo-mechanical processing parameters strongly influence both the evolution of texture as well as microstructure in the material. As a result, the semi-finished products exhibit anisotropy in mechanical properties causing legitimate concerns on the applicability of the alloys. In the present thesis, a systematic study on the evolution of texture and microstructure and its implications on the mechanical properties anisotropy of AA7010 alloy has been attempted. A brief introduction on the development of texture and its influence on the anisotropy of the mechanical properties of 7xxx series aluminum alloys is presented first with a view to explore the scopes for further investigation. An overview of the relevant literature is described subsequently. The development of texture and microstructure in an Al-Zn-Mg-Cu-Zr based 7010 aluminum alloy during uneven, hot cross-rolling is presented. Materials processing involves three different types of uneven cross-rolling. The variations in relative intensity of the β-fibre components as a function of cross rolling modes have been investigated. It has been shown that the main attributes to the texture evolution in the present study are (a) cross-rolling and inter-pass annealing that reduce the intensity of Cu component following each successive pass, (b) recrystallization resistance of Bs oriented grains, (c) stability of Bs texture under cross-rolling, and (d) Zener pinning by Al3Zr dispersoids. The stability of the unique single, rotated Brass-{110}(556) component developed in the present alloy, during long term thermal annealing and cold rolling deformation has been systematically investigated further. Subsequently, the influence of development of microstructure and texture on the in-plane anisotropy (AIP) of yield strength, work hardening behavior and yield locus anisotropy has been presented. The AIP and work hardening behavior are evaluated by tensile testing at 0o, 45o and 90o to the rolling direction, whilst yield loci have been generated by Knoop hardness method. It has been observed that in spite of having strong rotated Brass texture, the specimens show low AIP especially in peak aged temper. The in-plane anisotropy (AIP) of yield strength, and work hardening behavior of a heat treated 7010 aluminum alloy sheet having strong, rotated Brass-{110}556 component with different texture intensity and volume fraction of recrystallization has been further evaluated. It is observed that the AIP increases with increase in texture intensity and volume fraction of recrystallization. In the subsequent chapter, the tensile flow and work hardening behavior are described using constitutive equations. Room temperature tensile properties have been evaluated as a function of tensile axis orientations in as-hot rolled as well as peak aged conditions. It has been found that both the Ludwigson and a generalized Voce-Bergström relation adequately describe the tensile flow behavior in all conditions compared to the Hollomon relation. The Voce-Bergström parameters define the slope of - plots in the stage-III regime when the specimens show a classical linear decrease in hardening. Further analysis of work hardening behavior throws light on the effect of texture on the dislocation storage and dynamic recovery. An overall summary of the experimental results and the scopes for future studies have been presented at the end.