Thesis (M.S.)--University of Wisconsin--Madison, 1984. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 151-161).

Thesis (M.S.)--University of Wisconsin--Madison, 1982. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 147-149).

Th.--Sci. phys.--Nancy--I.N.P.L., 1984.

Th. doct.-ing.--Sci des matér.--Nancy--I.N.P.L., 1984.

The development of aluminium alloys that can be processed by Rapid Solidification (RS) techniques for use in high temperature applications has recently been an area of intense study. One of the alloy systems of interest is Al-Cr-Zr-Mn. This work comprises a study of the microstructure and tensile properties of alloys of this system processed by melt spinning, high pressure gas atomisation (HPGA) and chill casting. The RS microstructures of Al-Cr and Al-Zr binary alloys were also compared with those of the quaternary alloys. The variety of microstructures observed in the powders of the quaternary alloys was consistent with the different cooling rates and nucleation temperatures experienced by droplets of different sizes, A cubic phase not previously reported was observed in the finer powder. The transition from a partitionless to a cellular microstructure occurred at estimated solidification front velocities similar to those predicted by morphological stability theory. The distribution of discrete Al[13]Cr[2] intermetallic particles within Al-Cr gas atomised powders of different sizes was found to be consistent with a probabilistic model of nucleants distributed in the volume of the alloy melt. Based on these results the original Al-5.2Cr-1.4Zr-1.3Mn (wt%) alloy was diluted to give an Al-3.3Cr-0.7Zr-0.7Mn (wt%) alloy so that the bulk of the powder (the sub-45mum size fraction) did not contain coarse intermetallic particles but exhibited a mainly cellular microstructure. A relationship has been determined between the thickness of wedge shaped chill castings and powder diameters for. similar microstructures. Prediction of alloy compositions designed to give a particular microstructure in a specified powder size can therefore be tested by a simple casting technique. The mechanical properties of the original and optimised quaternary alloy powders consolidated by Conform and extrusion have been determined and related to the as-consolidated and aged microstructures. The extruded powders of both alloys exhibited better properties than the Conformed powder. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The optimised alloy had a consistently better ductility. Neither of the alloys retained its strength after prolonged treatment at 400°C, but the results suggest that a service temperature of 300°C may be possible.

Orientador: Kamal Abdel Radi Ismail / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-16T04:30:19Z (GMT). No. of bitstreams: 1 Martins_KarlosRobertodaSilvaBraga_M.pdf: 2548852 bytes, checksum: 0bf50d46e50c30e28a01c42aeb127042 (MD5) Previous issue date: 2010 / Resumo: Este trabalho apresenta os resultados experimentais e numéricos de uma investigação sobre a formação de frost em torno de um cilindro vertical, dois cilindros verticais em série e três cilindros verticais em arranjo triangular na presença de ar úmido. A Formação de frost sobre superfícies frias em contato com ar úmido provoca sérios problemas em muitos equipamentos de refrigeração, prejudicando o seu desempenho térmico resultando em graves gargalos, gastos energéticos desnecessários, além da interrupção do processo de congelamento para eliminar o "frost" formado ocasionando grandes paradas de manutenção desses equipamentos. O principal objetivo desta investigação é estudar as influências dos parâmetros geométricos e operacionais sobre a camada de frost depositada, sua espessura e sua velocidade da frente de solidificação. Tendo em vista que os evaporadores são compostos de arranjos de tubos onde passa o fluido refrigerante, esta simulação e modelagem serão de grande valia para caracterizar o problema e investigar os diferentes efeitos desse fenômeno. Um código computacional foi adaptado para simular o processo de formação de frost em torno de um cilindro vertical. As medidas experimentais são utilizadas para validação do modelo e corroborar a possibilidade de sua utilização na previsão de desempenho desse tipo de arranjo de tubos. São apresentados e discutidos os resultados experimentais e numéricos dos efeitos das variações da vazão volumétrica de ar úmido, da vazão mássica e da temperatura do fluido secundário (álcool) sobre a taxa de deposição de frost, sua velocidade e espessura, mostrando boa concordância / Abstract: This paper presents experimental and numerical results of an investigation into the formation of frost about a vertical cylinder, two vertical cylinders in series and three vertical cylinders in triangular arrangement in the presence of humid air. The formation of frost on cold surfaces in contact with moist air causes serious problems in many refrigeration, impairing its thermal performance resulting in serious bottlenecks, unnecessary energy expenses, beyond the cessation of the freezing process to eliminate the "frost" formed leading major stops maintenance of such equipment. The main objective of this research is to study the influences of operational and geometrical parameters on the layer of frost deposited, its thickness and its rate of deposition. Considering that the evaporators are composed of arrays of tubes where it passes the refrigerant, this modeling and simulation will be of great value to characterize the problem and investigate the different effects of this phenomenon to evaluate our contribution to the formation of frost. A computer code was adapted to simulate the process of formation of frost around a vertical cylinder. Samples are used to validate the model and corroborate the possibility of their use in predicting the performance of such an arrangement of pipe. The experimental and numerical results of the effects of changes in volume flow of moist air, the mass flow and temperature of secondary fluid (alcohol) on the frost deposition rate, their rate of deposition and thickness showed good agreement and are displayed and discussed / Mestrado / Termica e Fluidos / Mestre em Engenharia Mecânica

The present work reports the effect of heat input, cooling rate, parent plate deformation and restraining conditions on the formation and morphology of delta ferrite in welds on AISI 318L and 321 steels. The experiments were carried out on commercially produced plates in the following conditions: (i) as received condition (ii) further deformed by cold rolling. (iii) further deformed by hot rolling. The effect of heat input and cooling rates were examined using bead-onplate Submerged Arc welds on the same parent plate material. The parent plate condition was assessed using bead-on-plate metal Inert Gas (MIG) and Subm rged Arc butt welds. The results suggest that 1. The weld metal solidification proceeds epitaxially from the existing unmelted base metal. The weld exhibited surface marking i.e. deformation bands or close packed plane, in the austenite matrix, but not passing through delta ferrite phase. 2. The solidification substructure, the ferrite content, and morphology are influenced by tile (i) thermal stress induced during welding, (ii) parent plate chemical composition, (iii) the parent plate microstructural and deformation characteristics i.e. strain energy. 3. Rapidly cooled welds have lower ferrite content than welds produced with slower cooling rates. The randomly distributed elongeed ferrite with some lath type ferrite morphology was predominantly attributed with the welds produced with high cooling rates. 4. The ferrite is the first solidified phase to form and austenite is formed from the liquid rather than by solid phase transformation of primary ferrite to austenite. 5. The ferrite formation is a diffusion controlled phase transformation, the degree of its dendrites development depends upon the welding parameters i.e. heat input, cooling rates. 8. Two types of ferrite morphology were observed: (a) elongated type dendrites identified as vermicular by previous investigators and (b) the cellular type morphology which has not been classified in previous investigations.

No description available.

Understanding the fundamental mechanisms of solidification and the relative significance of different parameters governing these mechanisms is of vital importance for controlling the evolution of microstructure during solidification, and consequently, for improving the efficacy of a casting process. Towards achieving this goal, the present work attempts to study the effect of convection and shrinkage on solidification and microstructure formation primarily through the development of computational models which are complemented with experimental investigations and analytical solutions. Convection strongly influences the solutal and thermal distribution adjacent to the solidification interface and affects the growth rate and morphology of dendrites. To investigate this, a numerical model based on the enthalpy method is developed for binary alloy dendrite growth in presence of convection. The model results are validated with corresponding predictions using level-set method and micro-solvability theory. Subsequently, the model is applied for studying the effect of convection on the growth morphology of single dendrites. Results show that the presence of flow significantly affects the thermo-solutal distribution and consequently the growth rate and morphology of dendrites. Parametric studies performed using the model predict that thermal and solutal Peclet number and melt undercooling strongly influence the tip velocity of dendrites. Additionally, an analytical model is developed to quantify the effect of convection on dendrite tip velocity through the definition of an equivalent undercooling. An expression for this equivalent undercooling is derived in terms of the flow Nusselt and Sherwood numbers and the analytical equivalent undercooling values are compared with corresponding predictions obtained using the numerical model. Subsequently, the interaction of multiple dendrites growing in close proximity is studied. It is observed that the presence of neighbouring dendrites strongly influences the thermo-solutal distribution in the domain leading to significant changes in growth pattern. The effect of seed density on the growth morphology is investigated and it is observed that a higher initial seeding density leads to more spherical dendritic structure. Comparison with results from chilled casting of Al-6.5% Cu alloy with and without grain refiners show qualitative similarity in both the cases. The next part of the thesis presents a eutectic solidification model developed using the general enthalpy-based framework for dendritic solidification. New parameters and rules are defined and suitable modifications are made to incorporate the physics of eutectic solidification and account for the additional complexities arising due to the presence of multiple solid phases. The model simulates the presence of buoyancy driven convection and its interaction with the solidification process. i The model predictions are found to be in good agreement with the Jackson-Hunt theory. At first, the model is applied to simulate regular eutectic growth in a purely diffusive environment and it is observed that the model predicts the variation in interface profile with change in lamella width similar to those observed in experimental studies on eutectic solidification. Subsequently, a few case studies are performed to demonstrate the ability of the model in handling complex scenarios of eutectic growth such as width selection, lamella division and presence of solutal buoyancy. It is observed that solutal buoyancy gives rise to flow cells ahead of the eutectic interface facilitating the transfer of solute between the two phases. Apart from forced and natural convection, another important factor affecting solidification is the presence of shrinkage. Currently, solidification shrinkage is mostly modelled using empirical relations and criteria functions. In the present work, a phenomenological model for shrinkage driven convection is developed by incorporating the mechanism of solidification shrinkage in an existing framework of enthalpy based macro-scale solidification model. The effect of shrinkage flow on the free surface deformation is accounted for by using the volume-of-fluid method. The results predicted by the model are found to be in excellent agreement with analytical solutions for one-dimensional solidification with unequal phase densities. A set of controlled experiments are designed and executed for validating the numerical model. The experiments involve in-situ X-ray imaging of casting of pure aluminium in a rectangular cavity. The numerical predictions for solidification rate, free surface movement and temperature profiles are compared with corresponding experimental results obtained from the in-situ X-ray images and thermocouple data. Subsequent case studies, performed using the model, show significant influence of applied heat flux and mould geometry on the formation of shrinkage cavities. The shrinkage flow model provides the foundation for development of a generalized model to accurately predict the formation and morphology of internal porosity. The validated macro-scale shrinkage model is extended to the microscopic scale to study the influence of shrinkage flow on the growth rate of dendrites. Results demonstrate that shrinkage driven convection towards the dendrite strongly influences the solutal and thermal distribution adjacent to the solidification interface and consequently decreases the growth rate of the dendrite. Additionally, an analytical model is developed to quantify the effect of shrinkage driven convection through the definition of an equivalent undercooling for shrinkage flow. The present models provide significant physical insight into various mechanisms governing the process of solidification. Moreover, due to their similar framework, the individual models have the potential to be an effective foundation for the development of a generalized multi-scale solidification model incorporating the presence of important phenomena such as shrinkage and convection.

Using molecular dynamics (MD) and boundary element method (BEM), different aspects of solidification in the aluminum silicon system are studied. The angular embedding atom model (AEAM) was implemented on LAMMPS, and the necessary potentials are developed. Firstly, a modified version of the Stillinger-Weber (SW) interatomic potential for pure Si is proposed. The advantage of this potential is that, in contrast to the original SW form, the modified version allows one to grow diamond cubic crystal structures from the melt at high temperatures. Additionally, an Al-Si binary potential of the AEAM type is able to accurately predict the experimental enthalpy of mixing. It is also able to predict an Al-Si phase diagram with a eutectic concentration for the liquid that agrees with experiment within 4 at% and a eutectic temperature that differs from experiment by only 13 K. Considering the importance of step mobility and step free energy on the solidification growth rate, chapters 3 and 4 are devoted to calculation of these concepts using MD simulations. In chapter 3 the step mobility, which is the proportionality constant between the velocity and driving force, was determined for the alloy with melt composition of Al-90%Si as a function of temperature and composition. It was found that mobility decreases fairly rapidly with the addition of Al solute. Also, from the variation with temperature, it appears the mobility is proportional to the interdiffusion coefficient in the liquid. It is observed that for the Al-60%Si alloy diffusion-controlled growth is the dominant scenario, even for a few degrees of undercooling. In chapter 4 equilibrium molecular dynamics (MD) simulations and the capillaryfluctuations method (CFM) are employed to calculate crystal-melt step free energies at three different melt compositions. Anisotropy of steps are investigated by setting up the systems with different crystal orientations of steps on the high-symmetry interface plane, (111) in this case. A complete isotropy of step free energy is observed for Al-60%Si and Al-90%Si alloying systems, while CFM failed in determining step free energy in Al-30%Si due to lack of step roughness. In chapter 5 the BEM is utilized to numerically compute the concentration profile in a fluid phase in contact with an infinite array of equally spaced surface steps. In addition, under the assumption that step motion is controlled by diffusion through the fluid phase, the growth rate is computed and the effect of step spacing, supersaturation and boundary layer width is studied. BEM calculations were also used to study the phenomenon of step bunching during crystal growth and it is found that, in the absence of elastic strain energy, a sufficiently large perturbation in the position of a step from its regular spacing will lead to a step bunching instability. / Thesis / Doctor of Philosophy (PhD)