Papers by Keyword: Directional Solidification

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Authors: Guillaume Reinhart, Henri Nguyen-Thi, Brice Sarpi, Aboul Aziz Bogno, Bernard Billia
Abstract: Temperature Gradient Zone Melting (TGZM) occurs when a liquidsolid zone is submitted to a temperature gradient and leads to the migration of liquid droplets or channels through the solid, up the temperature gradient. TGZM has a major influence on the preparation of the initial solid-liquid interface during the stabilization phase following the directional melting of an alloy and is at the origin of the diffusion of solute towards the top part of the mushy zone. TGZM is also causing the migration up the temperature gradient of dendrite secondary arms during directional solidification, which can have a significant impact on the micro-segregation pattern of the final microstructure. In this communication we report on a directional solidification experiment carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France) on Al4.0 wt.% Cu alloy to study the dynamics induced by the TGZM phenomenon on an equiaxed grain that nucleated in front of a columnar structure. Based on in situ experimental observations obtained by synchrotron X-ray radiography, the dissolution of the bottom part of the equiaxed grain is characterized and measurements are compared with predictions of the TGZM theory in diffusive regime.
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Authors: Paul L. Schaffer, Ragnvald H. Mathiesen, Lars Arnberg
Abstract: Hypermonotectic alloys are distinguished by a temperature region for which the homogeneous melt decomposes into two liquid phases. In Al-based hypermonotectics, the minority phase is much higher in density than the matrix melt phase and consequently macro-segregation due to sedimentation is an inherent problem when casting these alloys. However, under the correct solidification conditions, it may be feasible to counteract sedimentation by thermocapillary forces that arise due to the thermosolutal dependence of the surface tension between the two liquid phases. The current investigation involved in-situ X-ray video microscopy studies during directional solidification of Al-Bi samples of various compositions employing a Bridgman furnace. It was found that large undercoolings were required to initiate the L  L1 + L2 reaction and L2 nucleation occurred heterogeneously on the monotectic front. L2 droplets were then set in collective size-dependent motion by hydrodynamic forces coupled to external fields (i.e. temperature gradient and gravity) and internal fluctuations. The resulting flow fields are superimposed onto the short range coagulation mechanisms, such as diffusion coupling, and are all crucial in determining the final size distribution and dispersion of the Bi-rich phase in the cast material.
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Authors: Bin Zhou, Ludo Froyen
Abstract: A special type of divorced eutectic growth mode (symbiotic growth) in a ternary Al-Mn-Si alloy, triggered by addition of titanium boride (TiB2) has been studied under both ground and microgravity conditions. During directional solidification, α (AlMnSi) particles nucleate ahead of the planar solidification front and are pushed and later engulfed by the interface forming a banded particle layer structure. The presence of fine titanium boride particles (clusters) in front of the growing α (AlMnSi) particles makes the interaction between the intermetallic α (AlMnSi) particles and solidification front much more complex than most proposed models for particle/interface interactions. Microgravity experiments can eliminate the gravity related effects and thus provide an opportunity to better understand the formation mechanism of symbiotic growth. In this study, hypoeutectic Al-1Mn-3Si alloys with addition of 0.33 wt% TiB2 were directionally solidified in ESA Solidification and Quenching Furnace (SQF) on board of the International Space Station (ISS). The ground experiment was conducted in a replica of this furnace prior to the microgravity experiments. Non-destructive X-ray tomography and its 3D reconstruction software was used to characterize the particles and their distribution. Comparison between ground and microgravity experiment results is presented. The particle pushing and engulfment of symbiotic growth is discussed based on a particle pushing and engulfment model.
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Authors: Alexandre Furtado Ferreira, José Adilson de Castro, Ivaldo Leão Ferreira
Abstract: The microstructure evolution during the directional solidification of Al-Cu alloy is simulated using a phase field model. The transformation from liquid to solid phase is a non-equilibrium process with three regions (liquid, solid and interface) involved. Phase field model is defined for each of the three regions. The evolution of each phase is calculated by a set of phase field equations, whereas the solute in those regions is calculated by a concentration equation. In this work, the phase field model which is generally valid for most kinds of transitions between phases, it is applied to the directional solidification problem. Numerical results for the morphological evolution of columnar dendrite in Al-Cu alloy are in agreement with experimental observations found in the literature. The growth velocity of the dendrite tip and the concentration profile in the solid, interface and liquid region were calculated.
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Authors: Sonja Steinbach, Johannes Dagner, Marc Hainke, Jochen Friedrich, Lorenz Ratke
Abstract: A quantitative understanding of the effect of fluid flow on the microstructure of cast alloys is still lacking. The application of time dependent magnetic fields during solidification offers the possibility to create defined flow conditions in solidification processing. The effect of rotating magnetic fields (RMF) on the microstructure formation in cast Al-alloys (Al-7wt.%Si, Al-7wt.%Si- 0.6wt.Mg) is studied experimentally and numerically. The forced fluid flow conditions result in pronounced macrosegregation effects and affect microstructural parameters. With increasing fluid flow the primary dendrite spacing decreases whereas the secondary dendrite arm spacing increases. The experimental analysis is supported by a rigorous application of numerical modeling with the software package CrysVUn.
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Authors: Lan Li, Lin Sheng Li, Chang Jun Qiu
Abstract: In order to meet the need of high-strength and high-electrical conductivity copper alloys in industry. A method of making high-strength and high-electrical conductivity copper alloys is discussed in this paper. This method uses the technology of heated mold continuous casting to make Cu-Cr alloy. Because it utilizes the high electrical conductivity of copper matrix and high strength of the chromium phase, the in-situ composite Cu-Cr alloy with directional solidification structure is got. The in-situ composite Cu-Cr alloy has good properties and will be widely used in industry.
804
Authors: Jian Min Zeng, You Bin Wang, Hai Feng Cheng
Abstract: Hydrogen is a harmful element in aluminum. How to accurately measure hydrogen content in molten aluminum alloy has been significant for improving the metallurgical qualities of aluminum products. Currently, Reduced Pressure Test (RPT) based on the famous Sievert’ law is the most commonly used method to measure the hydrogen in aluminum. However, due to the changes in solidification morphology of alloys under different solidification conditions, hydrogen will be closed more or less in the sample by the developed dendrites, which results in the dispersion of test results. According to the principle of solute redistribution, an improved reduced pressure technology under directional solidification was proposed in this paper. The solidification interface was pushed on along the single direction by controlling the temperature gradient of solid-liquid interface. Hydrogen is released in front of the solid-liquid interface through the solute redistribution. The result shows that the hydrogen content increases and the mean variance decreases with the increasing of the temperature gradients. The method was proved to be of higher accuracy.
1205
Authors: Heng Zhi Fu, Jun Shen, Shuang Ming Li, Jin Shan Li, Lin Liu
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Authors: Camila Yuri Negrão Konno, Angela J. Vasconcelos, André Santos Barros, Adrina P. Silva, Otávio Fernandes Lima da Rocha, Amauri Garcia, José Eduardo Spinelli
Abstract: Monotectic alloys are of great industrial importance because of their favorable tribological behavior. Many studies in order to better understand the morphologies obtained by monotectic reaction have been developed. To better understand the morphologies obtained by monotectic Al-1.2wt%Pb alloy reaction, especially in relation to induced convective flow, this paper aims to compare the microstructural evolution of the alloy obteind by directional solidification in transient heat-flow conditions in upward, downwand and horizontal solidification devices. It was observed a morphology transition from particles to fibers on upward and downward cases and a morphology of only particles on the horizontal case. The classical relationship used for eutectic growth, λ2v = C, which was considered applicable to monotectic reactions, didn’t seemed to be valid in the interphase spacing evolution for the downward device, however power functions (λ = C.va) were found in all cases.
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