Papers by Author: Carlos González-Rivera

Abstract: In this work a fundamental Eulerian mathematical model is developed to simulate fluid flow and mixing phenomena in aluminum ladles equipped with an impeller for dehydrogenization treatment. The effect of rotating speed and type of impeller, depth of immersion, and gas flow rate, on the mixing behavior and vortex formation is analyzed with this model. The model simulates operation with and without gas injection and it is developed in the commercial PHOENICS 3.4 CFD code in order to solve all conservation equations governing the process, i.e., continuity, 3D turbulent Navier-Stokes and k-ε turbulence model for a two-phase fluid flow problem using the Inter Phase Slip Algorithm. In order to realistically represent the process, the shape of the furnace and the impellers are modeled by employing Body Fitted Coordinates. It is concluded that the mixing behavior is highly dependent on the rotation speed and impeller type. Mixing time is improved when: the impeller is located at a depth of 0.229 m into the aluminum bath, and by using high rotation speeds, ladles with a high ratio of diameter to height, and impellers with notches.

Abstract: The purpose of this work is to explore the effect of the presence of two different primary phases on the microstructure and solidification kinetics of Pb-Sn alloys. The experimental results have been compared with predictions obtained from the Newton Thermal Analysis of cooling curves generated by a conventional heat transfer-solidification kinetics model. Three Pb-Sn alloys have been considered in this work in order to explore the solidification characteristics of the eutectic in hypoeutectic, hypereutectic and eutectic compositions. Experimental results indicate that the Pb-Sn eutectic morphology and the solidification rate depend on the nature of the pre-existent primary phase during eutectic solidification. From the observed discrepancies between experimental and simulated results it is concluded that further improvements are needed to simulate the solidification kinetics of eutectic microconstituents in the presence of pre-existing primary phases.

Abstract: The purpose of this work is to analyze the capability of Newton Thermal Analysis (NTA) to detect differences of the solidification kinetics between eutectic gray and ductile irons obtained from the same base alloy. The NTA experimental output has been analyzed with a simple micromacro modeling approach. The outcome of this work suggests that NTA has a good potential as a qualitative tool to characterize the solidification kinetics of alloys.

Abstract: This work explores the feasibility of using the Newton Thermal Analysis method (NTA) as an alternative technique for quantitative determination of the relative volume fractions of the microconstituents formed during solidification of three hypereutectic Pb-Sn alloys. The results suggest that NTA has a good potential as a quantitative microstructure analysis tool. However, it is required to further improve the method in order to match the microstructure predictions with the experimental measurements obtained by quantitative metallography.

Abstract: A Ni57Fe18Al25 alloy is produced by vacuum induction melting under an argon atmosphere and gravity cast into a copper chill mould. Ingots of 2 x 10 x 50 mm are re-heated at 1100 °C for 24 hours and hot rolled until a 25 % reduction is reached. Immediately, hot rolled plates are heat treated at 1100 °C for 1 hour and water cooled to room temperature. Microstructures of as cast, hot rolled and hot rolled plus heat treated specimens are characterized using scanning and transmission electron microscopy and x-ray diffraction. Specimens in the as-cast and hot rolled conditions show the presence of equiaxial and partially elongated β-grains, respectively, with the presence of a thin γ-phase decorating the grain boundaries. Hot rolled plus heat treated specimens have partially recrystallized β-grains with γ-phase and (Ni,Fe)3Al at the grain boundaries. The hot rolled specimens have a room temperature ductility of approximately 5 % in tensile testing. This enhancement in ductility is attributed to the modification of the β-grains by coexistence with a γ- phase along the grain boundaries.

Abstract: The microstructural and mechanical properties of an ultra-clean low carbon steel sheet with 0.035% Cr have been evaluated. The isothermal recrystallization kinetics at 800 oC is analyzed by using the Kolomogorov-Johnson-Mehl-Avrami (KJMA) equation. The obtained results indicate that Cr retards the recrystallization kinetics owing to the presence of fine particles of chromium carbides. The annealing process produces a fully recrystallized microstructure. The main texture components in the specimens are: {111}<112>, {112}<110> and {111}<110>. These are related with the Lankford value and the elongation. The obtained results suggest that the formability of this steel is excellent.