Effects of Grain Refining on Columnar-to-Equiaxed Transition in Aluminum Alloys

The effects of grain refining in ultra-pure aluminum, commercially pure aluminum (1050), and Al-7%Si binary alloy were investigated, using different additions of Al-10%Ti, Al-5%Ti-1%B, and Al-4%B master alloys. Thermal analysis and metallography were used to assess the variations in microstructure resulting from these additions, at solidification rates of 0.8°C/s and ~10°C/s. The results revealed that addition of Al-4%B to ultra-pure aluminum forms AlB12 and AlB2 which have no grain-refining effect. Without grain refiner addition, the pure aluminum microstructure exhibits a mixture of columnar and equiaxed grains. Addition of 30ppm Ti is sufficient to promote equiaxed grains at ~10°C/s but requires addition of 1000 ppm B to obtain similar results at 0.8°C/s. Increasing the Si content to 7% reduces the initial grain size of pure aluminum from 2800 μm to ~1850 μm, and further to 450 μm with ddition of ~500ppm B. In commercial aluminum, the B reacts with traces of Ti forming Al3Ti and TiB2 phases which are active grain-refiners. In Al-7%Si, Ti reacts with Si forming (Al,Si)2Ti phase, which is a poor refining agent. This phenomenon is termed poisoning. No interaction between B and Si is observed in the commercial aluminum or Al-7%Si alloy when B is added

Effect of Microalloying Elements on the Heat Treatment Response and Tensile Properties of Al-Si-Mg Alloys

This study was carried out on a series of heat-treatable Al-Si-Mg alloys to determine the effects of Fe, Mg, Sr and Be addition on their microstructural characteristics and tensile properties. The results showed that the eutectic temperature was reduced by 10°C with 0.8 wt% Mg addition. The solidification curves and first derivatives of Sr-free alloys with high Fe and Mg contents revealed a peak at 611°C consequent to the formation of a script-like Be-Fe (Al8Fe2BeSi) phase, which was very close to the peak for α-Al. The morphology of the β-iron platelets underwent changes due to their dissolution, thinning, necking, and fragmentation with increase in solutionizing time. Increased Mg contents are beneficial to the tensile properties unlike the detrimental effect of increasing Fe contents. Additions of Be and Sr noticeably improved the properties at the same Fe and/or Mg contents, the enhancements being markedly observed at higher Mg contents and reduced Fe levels. At high Fe levels, addition of Be is preferable as it neutralizes the deleterious effects of Fe in these alloys; however, addition of 500 ppm Be is inadequate for interacting with other alloying elements

Generation and Relaxation of Residual Stresses in Automotive Cylinder Blocks

There is direct proportionality between ultimate tensile stress (UTS) and residual stresses (RS). Residual stresses gradually decrease with decreasing cooling/quenching rates. Quenching in cold water develops highest, whereas air cooling produces lowest, residual stresses. Significant increase in RS is observed in specimens with low dendrite arm spacing (high solidification rate), while lower residual stresses are measured in specimens with high dendrite arm spacing (low solidification rate). For I-4 and V-6 engine blocks, there is refinement in microstructure due to the increase in solidification rate along the cylinder length. The developed residual stresses are normally tensile in both engine types. Air cooling following solution heat treatment produces higher RS compared to warm water and cold water quenching. Solution heat treatment and freezing lead to maximum RS relaxation where 50% of the stresses are reduced after the solution heat treatment step. Aging time and temperature are directly proportional to the residual stresses relaxation. Relaxation of RS also depends on the geometry and size of the workpiece. It should be mentioned here that the I-4 and V-6 cylinder blocks were provided by Nemak-Canada (Windsor-Ontario-Canada). Residual stress measurements technique and procedure are typical of those used by the automotive industry in order to provide reliable data for industrial applications supported by intensive experiments

Phase-field modeling in eutectic alloys and dichalcogenides

This thesis develops two phase-field models for the microstructural modelingof two different systems with the purpose of in the future, joining them in a singlemodel. The first phase-field model was developed in the past to describe eutecticsystems undergoing solidification, this model is extended to a situation where analloy is generated and afterwards very rapidly cooled. The second phase-field modelis an extension of a previous model that describes bulk diffusion limited void growth,the model is modified to describe non-linear growth of a void on the surface of T iSe2as well as modeling the anisotropic growth of the void. We focus mainly on thedynamics and microstructural evolution of the models. It is shown that the voidgrowth phase-field model captures the correct physics of vacancy generation at theinterface of a void by comparison with experimental data provided by ChenggangTao and co-workersCette thèse développe deux modèles de champ de phase pour la modélisation microstructurale de deux différents systèmes avec l'objectif futur de les joindre dans un seul modèle. Le premier modèle de champ phase a été développé dans le passé pour décrire des systèmes eutectiques en en processus de solidification, ce modèle est ici étendu à une situation où un alliage est généré et ensuite très rapidement refroidi. Le deuxième modèle de champ de phase est une extension d'un modèle précédent décrivant la diffusion en volume de la croissance limitée des lacunes, ce modèle est ici modifié pour décrire la croissance non-linéaire des lacunes sur la surface de TiSe2 ainsi que pour modéliser la croissance anisotrope de lacunes. Nous nous concentrons principalement sur la dynamique et l'évolution microstructurale des modèles. Il est déjà montré que le modèle de champ de phase de la croissance des lacunes saisit correctement la physique de la génération de lacunes à l'interface d'un vide lorsque comparé aux données expérimentales fournies par Chenggang Tao et ses collègues

Estudios de solidificación de monoblques de aluminio

Estudio que presenta los resultados del estudio de la cinética de solidificación en diversas zonas de monobloques de aluminio llenados a baja presión por medio de la inserción de termopares, analizando también el efecto del calentamiento de las camisas por medio de inducción

Aprovechamiento tradicional de una especie protegida (Litsea glaucescens kunth) en "Sierra del Laurel", Aguascalientes, México

Whenever a natural area is proposed to be a protected area, the information that supports the proposal is usually based on the physical environmental, rather than on the social environment and on the management of natural resource. This study was conducted from May 2004 to June 2005 whit the objective to analyze the process of exploitation of Mexican Bay Leaf (Litsea glaucescens Kunth) in rural communities of Sierra del Laurel, a priority area in the state of Aguascalientes, México. Thirteen semi-structured interview were applied for local collectors and seven more were applied to inhabitants of the region to understand the trends in the use of resource. Three interviews were also applied to environmental officials from federal and local institutions to know about their position regarding the use of the Mexican Bay Leaf. A participative workshop with producers and collectors was conducted to document their opinion on the status of the population of this specie. Trend in utilization of the Mexican Bay Leaf have led it to be in danger of extinction: there is not organization nor legal utilization schemes among collectors that allow regeneration; the lack of institutional coordination, along whit inefficient implementation of laws and other legal instruments to regulate the management of natural resources, has contributed so that users have not adequate risk management procedures.Cuando se propone una zona como área natural protegida, por lo general la información que fundamenta la propuesta se basa más en el entorno biofísico, que en el contexto social y de gestión de sus recursos naturales. El presente trabajo se realizó durante mayo 2004 a junio 2005, con el propósito de analizar el proceso de aprovechamiento del laurel silvestre (Litsea glaucescens Kunth), una especie protegida, en Sierra del Laurel, considerada como una de las áreas prioritarias para la conservación en el estado de Aguascalientes. Se aplicaron entrevistas semi-estructuradas a recolectores locales y habitantes de la región, con el fin de conocer las tendencias de aprovechamiento del recurso; se entrevistó a funcionarios de instituciones ambientales estatales y federales para conocer su postura frente al aprovechamiento de la especie, y se realizó un taller participativo con productores y recolectores para documentar su opinión sobre el estado que guarda la población del laurel silvestre. Las tendencias de aprovechamiento de la especie la han llevado a estar en peligro de extinción, ya que no existe una organización entre los recolectores, ni esquemas de aprovechamiento legales que permitan su regeneración. La falta de coordinación institucional ha contribuido a que los usuarios del recurso no tengan procedimientos adecuados de gestión, debido a la aplicación ineficiente de las leyes e instrumentos jurídicos para el manejo de los recursos naturales

TRADITIONAL USE OF A PROTECTED SPECIES (Litsea glaucescens Kunth) IN THE “SIERRA DEL LAUREL

Whenever a natural area is proposed to be a protected area, the information that supports the proposal is usually based on the physical environmental, rather than on the social environment and on the management of natural resource. This study was conducted from May 2004 to June 2005 whit the objective to analyze the process of exploitation of Mexican Bay Leaf (Litsea glaucescens Kunth) in rural communities of Sierra del Laurel, a priority area in the state of Aguascalientes, México. Thirteen semi-structured interview were applied for local collectors and seven more were applied to inhabitants of the region to understand the trends in the use of resource. Three interviews were also applied to environmental officials from federal and local institutions to know about their position regarding the use of the Mexican Bay Leaf. A participative workshop with producers and collectors was conducted to document their opinion on the status of the population of this specie. Trend in utilization of the Mexican Bay Leaf have led it to be in danger of extinction: there is not organization nor legal utilization schemes among collectors that allow regeneration; the lack of institutional coordination, along whit inefficient implementation of laws and other legal instruments to regulate the management of natural resources, has contributed so that users have not adequate risk management procedures

Metallurgical Parameters Controlling the Eutectic Silicon Charateristics in Be-Treated Al-Si-Mg Alloys

The present work was carried out on Al-7%Si-0.4%Mg-X alloy (where X = Mg, Fe, Sr or Be), where the effect of solidification rate on the eutectic silicon characteristics was investigated. Two solidification rates corresponding to dendrite arm spacings (DAS) of 24 and 65 μm were employed. Samples with 24 μm DAS were solution heat-treated at 540 °C for 5 and 12 h prior to quenching in warm water at 65 °C. Eutectic Si particle charateristics were measured using an image analyzer. The results show that the addition of 0.05% Be leads to partial modification of the Si particles. Full modification was only obtained when Sr was added in an amount of 150–200 ppm, depending on the applied solidification rate. Increasing the amount of Mg to 0.8% in Sr-modified alloys leads to a reduction in the effectiveness of Sr as the main modifier. Similar observations were made when the Fe content was increased in Be-treated alloys due to the Be-Fe interaction. Over-modification results in the precipitation of hard Sr-rich particles, mainly Al4SrSi2, whereas overheating causes incipient melting of the Al-Cu eutectic and hence the surrounding matrix. Both factors lead to a deterioration in the alloy mechanical properties. Furthermore, the presence of long, acicular Si particles accelerates the occurrence of fracture and, as a result, yields poor ductility. In low iron (less than 0.1 wt%) Al-Si-Mg alloys, the mechanical properties in the as cast, as well as heat treated conditions, are mainly controlled by the eutectic Si charatersitics. Increasing the iron content and, hence, the volume fraction of Fe-based intermetallics leads to a complex fracture mode