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.

Les alliages d'aluminium de la série 7000 à hautes caractéristiques mécaniques, constitués principalement des éléments d'addition Zn, Mg, et Cu, sont notamment utilisés dans l'industrie aéronautique civile. Néanmoins, les traitements thermiques et/ou thermomécaniques appliqués pour maximiser les propriétés mécaniques de ces alliages, peuvent les sensibiliser à divers modes de corrosion structurale dont la corrosion feuilletante, dont les mécanismes sont encore mal compris. En outre, les nouvelles générations d'alliages, développées en vue d'augmenter les propriétés mécaniques, contiennent toujours plus d'éléments d'addition, ce qui est susceptible de modifier leur sensibilité à ce phénomène. <br />Ce travail s'attache à faire progresser la compréhension des mécanismes de corrosion feuilletante des alliages 7000, en cherchant à identifier le rôle des principaux éléments d'alliage. Pour cela nous avons caractérisé finement les états de précipitation d'un grand nombre de microstructures, comme la composition des précipités durcissants nanométriques et de la matrice, par ASAXS et 3DAP. En parallèle, nous avons développé de nouvelles techniques électrochimiques permettant de quantifier la sensibilité de ces mêmes microstructures à la corrosion feuilletante. <br />Les résultats de l'étude mettent en évidence l'existence de deux mécanismes de corrosion : endommagement par dissolution intergranulaire et par rupture intergranulaire, dont la prédominance dépend de la composition de l'alliage et du traitement thermique. Nous proposons une explication au comportement en corrosion des différentes microstructures, faisant intervenir la composition chimique des différentes entités microstructurales.

Nous avons étudié l'influence du traitement thermomécanique des alliages Al-Zn-Mg-Cu sur leur microstructure et leurs propriétés mécaniques. Plus spécifiquement, nous avons étudié comment les mécanismes de précipitation et de durcissement structural sont modifiés lorsqu'on introduit des dislocations par détensionnement préalablement au traitement de revenu.<br />La première approche de ce problème est expérimentale. Le but était d'obtenir des relations entre les microstructures développées suivant les différents traitements thermomécaniques et les propriétés mécaniques qui en résultent. Pour cela, nous avons utilisé de nombreuses techniques d'investigation complémentaires: microscopie optique et électronique en transmission, calorimétrie différentielle, diffusion centrale des rayons X, tomographie atomique, résistivité, microdureté et essais de traction.<br />Dans un deuxième temps, nous avons développé un modèle de précipitation compétitive homogène / hétérogène sur les dislocations. Ce modèle intègre continûment les différentes étapes de la précipitation (germination, croissance et coalescence) et du durcissement structural (cisaillement, contournement). Nous avons pu retrouver les principaux résultats expérimentaux à l'aide de ce modèle. <br />Enfin, nous avons étudié les propriétés plastiques de nos matériaux, suivant l'état de précipitation. Plus particulièrement, nous avons pu mettre en évidence la présence d'une précipitation dynamique lors d'essais de traction dans des états sursaturés, apportant un écrouissge très élevé.

In this work, ultrafine-grained (UFG) Al-5wt%Zn alloy was produced by equal channel angular extrusion (ECAE). The microstructure evolution during ECAE and the mechanical properties of the UFG Al-Zn alloy were investigated. In order to identify the effect of Zn in the Al-Zn alloy, pure aluminum (4N, 99.99%) was also studied for comparison. The grains of the Al-Zn alloy could be refined effectively by increasing the ECAE passes. However, as the ECAE passes increased, the microhardness increased initially but maintained constant after 4 ECAE passes. The dislocation density within grain interior was decreased gradually with increasing ECAE passes. After being processed to twelve ECAE passes, the UFG Al-Zn alloy exhibited 53.7% of the grain boundaries being high angle grain boundaries (HAGBs). The UFG Al-5wt%Zn alloy exhibits superior tensile strength and elongation as compared with pure aluminum fabricated by the same ECAE process. Experimental results indicated that adding Zn in aluminum alloy could provide solid-solution strengthening and considerable enhancement in tensile ductility which might be related to an improved post-uniform elongation (PUE). The strain rate sensitivity (SRS) of the UFG Al-Zn alloy also increased with increasing the ECAE passes, which might be related to the fine grain size and the contribution of grain boundary sliding. The activation volume of the UFG Al-Zn alloy was in the range of 32b3~76b3, and the pure aluminum was in the range of 57b3~122b3. Because of the small value of the activation volume, it is suggested that the controlling mechanism for dislocation glide in the UFG Al-Zn alloy might be related to the generation and absorption of dislocations in grain boundary, as well as the interaction between dislocations and solute Zn atoms in the grain boundary.

No description available.

Aluminium alloys, which are the most widely used materials in the aircraft and aerospace industries, find their applications due to high strength–to-density ratio, resistance to catastrophic fracture, high degree of toughness, fabricability including good weldability and availability. High strength aluminum alloys are used in different forms like sheets, forgings and extruded rods, welded and machined components in the aerospace industry. One major application of the aluminium alloys in the space sector is in the launch vehicle and satellite sub-systems. The Indian Space Research Organization has met major challenges of indigenization of suitable aluminium alloys, for example, Al-Cu alloys (like AA2219) and Al–Zn-Mg alloys (like AA7075 and AFNOR 7020). Many failures of the metallic sub-systems made of different grades of aluminum alloys have confirmed that high levels of residual stresses and unacceptable microstructures have played a role. Crystallographic texture in these materials has a very significant role to play in the performance of these materials in service. The anisotropy in the mechanical properties caused by crystallographic texture would add to the woes of the existing problems of residual stresses and directionality in the microstructure. In this context, a detailed study of crystallographic texture and residual stresses of high strength aluminium alloys is mandatory. It is also important to study the influence of texture on the anisotropy in mechanical properties. The present research programme aims at addressing some of these aspects. The entire work has been divided in three major sections, namely macro and micro texture analysis, non-destructive measurement of residual stresses using X-ray Diffraction (XRD) and the Ultrasonic Testing (UST) and the study of anisotropy in the mechanical properties arising due to the above two factors. The thesis composition is as follows. In Chapter I, a detailed survey of the literature has been presented wherein basic physical metallurgy for different aluminum alloys of interest has been given. Thereafter, details of texture measurement by the X-ray diffraction and Electron Back Scatter Diffraction (EBSD) are presented. This is followed by a detailed review on the texture studies carried out in aluminium alloys under various conditions. Literature review on the two non-destructive methods, namely the X-ray diffraction and ultrasonic method has been carried out in detail. In order to account for microstructural changes, Differential Scanning Calorimetry (DSC) was carried out. Recent work on the mechanical property anisotropy arising due to high degree of mechanical working in aluminium alloys has been reviewed. Chapter II includes the experimental details involved in the course of the present investigation. The procedural details of cold rolling and associated microstructural changes are given in this chapter. This is followed by the texture measurement methods. Experimental details of the bulk texture measurement using the X-ray diffraction and micro texture measurements by the Electron Back Scatter Diffraction (EBSD) in the SEM are described. Details of the texture computation procedure as well as micro texture analysis methods are also presented. Basic principles of the non-destructive methods of measuring residual stresses, viz., the X-ray diffraction and the Ultrasonic testing, including the theory of measurements, are dealt with. Finally, the details of measurements of anisotropy in mechanical properties, including simulation carried out, for the three alloys are delineated. Chapter III deals with the results of the crystallographic texture measurements carried out on the cold rolled and artificially aged aluminium alloys. Results obtained from the pole figure analysis, Orientation Distribution Function (ODF) method and estimation of the various fibres present in the cold rolled material and the volume fraction of the texture components are discussed in detail for the three aluminium alloys. Results of the micro texture measurements using the EBSD are presented, explained and analyzed in detail. A comparison of the inverse pole figures (IPFs), Image Quality (IQ) maps, Misorientation angle, Grain Orientation Spread (GOS), Kernal Average Misorientation (KAM), CSL boundaries, Grain size and Grain boundary character distribution (GBCD) for materials cold rolled to different reduction for each of the alloys are done and analyzed. Conclusions are drawn regarding the evolution of texture from the above analysis. Deformation texture components Cu, Bs and S increase from the starting material as the rolling percentage increases. On the other hand, recrystallization texture components of Goss and Cube are observed to be weak. AFNOR 7020 developed the strongest texture followed by the AA7075 and AA2219 alloys. The Bs component is stronger in AFNOR 7020 alloy. This is attributed to the shear banding. Average KAM value increases as the cold working in the material increases confirming that the material contains high dislocation density at higher working percentages. Chapter IV deals with residual stresses in the aluminium alloys. Measurement of residual stresses has been carried out on the same sheets and plates, wherever it was possible, using the two methods. The residual stresses have been measured in two mutually perpendicular directions of the aluminium alloy sheets. Residual stress measurements by the ultrasonic method using the Critically Refracted Longitudinal (LCR) wave technique is also used to measure the subsurface stresses non-destructively. Acousto Elastic Coefficients (AEC) is determined for the alloys, in uniaxial tension. Using the AEC for the alloys, the RS at a depth of 3mm are evaluated using a 2MHz probe. Results of the stresses measured by the two methods have been discussed. The trends and anisotropy in the stress values due to texture are discussed and compared with the literature available. Surface residual stresses by the XRD method show compressive stresses at a majority of the locations. Residual stresses measured by the ultrasonic technique, which has a depth of penetration of about 3mm, have shown tensile stresses on many locations. Residual stresses are influenced by the crystallographic texture. Anisotropy in stress values in the longitudinal and transverse directions is demonstrated. In Chapter V, the anisotropy in mechanical properties for the three alloys is discussed in detail. The anisotropy in the three directions, namely the parallel, transverse and 45 deg orientation to the rolling directions is evaluated. The Lankford parameter, otherwise known as Plastic Anisotropy Ratio “r”, has been measured from the tensile tests of the alloy samples in the cold rolled conditions. These have been compared with the computed “r” from the XRD ODF data using the VPSC simulations and found to be qualitatively matching. These trends are discussed with the available literature on the anisotropy of the mechanical properties for aluminium alloys. Samples subjected to high cold rolling show anisotropy of UTS, YS and ‘n’ values. Experimentally measured “r” values in all the deformation conditions match the trend qualitatively with the simulated ones. The maximum anisotropy was observed at 45o orientation to the rolling direction in all the three alloys. Chapter VI gives the summary of the results from the study and the suggestions for future work.