Papers by Keyword: Dendrite

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Authors: Doru Michael Ştefănescu
Abstract: This paper is a review of the marvelous development of mathematical and computer models that describe the fundamentals of microstructure evolution during the solidification of cast alloys, from the 1966 seminal paper by Oldfield, the first to attempt computational modeling of microstructure evolution during solidification, to the current prediction of mechanical properties. The latest analytical models for irregular eutectics such as cast iron, as well as numerical models with microstructure output, to include cellular automaton, will be discussed. Phase field models will not be discussed because of their inapplicability to casting solidification at the present time.
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Authors: D. Ruvalcaba, Dmitry G. Eskin, Laurens Katgerman
Abstract: In the present investigation, serial sectioning and 3D reconstructions are made on samples quenched at selected temperatures during unconstrained solidification in order to observe the evolution in morphology of coarse dendrites in 3D. The 3D microstructure reconstruction during the solidification of an Al−7 wt.% Cu alloy allowed the identification of a complex coarse morphology of dendrites. High-ordered branches present different morphologies at different temperatures and locations in the microstructure due to coarsening and coalescence. 3D visualization of complex dendritic structures is discussed in the present investigation.
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Authors: Liang Yu, Liu Shun Wu, Liao Sha Li, Yuan Chi Dong
Abstract: Dendrite structure in solidification process has been studied by many researchers for it’s widely existence. In present work, a cellular automata model was proposed according to the basic physical chemistry concepts, which was helpful for a better understanding of the dendrite crystal growth and its physical chemistry mechanism. Two kinds of structures were considered in the model: hexagonal and rectangle. The status of every site was set as 0 and 1 which represent non-solidified and solidified state. Temperature field was simulated using finite difference method on the same mesh. The states of sites were changed according to the overcooling condition only. The computer simulation results showed that dendrite structure could be obtained under overcooling condition and temperature field calculation only, the structure of the dendrite was decided by the geometry of the model. The simulation resulted similar pattern as that obtained by experimental observation. The present model suggested that there exist a very simple basic for the typical complex phenomena, dendrite structure.
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Authors: Xin Lin, Lei Wei, Meng Wang, Wei Dong Huang
Abstract: A modified cellular automaton model for describing the dendritic solidification of pure substance was developed. Instead of using the high mesh-induced anisotropy capture rules, such as Von Neumann’s and Moore’s method, a new capture rule---random zigzag method was developed, which greatly reduced the mesh-induced anisotropy in crystallographic orientation. The calculation method for the solid/liquid interface curvature was also improved. The effect of interfacial energy anisotropy on the dendritic growth behavior was analyzed.
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Authors: Bo Wei Shan, Xin Lin, Lei Wei, Wei Dong Huang
Abstract: A modified cellular automaton model was proposed to simulate the dendrite growth of alloy. Different from previous models, this model used neither an analytical equation(such as KGT model) nor an interface solute gradient equation to solve the velocity of solid-liquid interface, but used the interface solute and energy conservation and thermodynamic equilibrium condition to describe the solid/liquid interface growth kinetics process. In present model, once the temperature field and solute field were solved by finite different method in the entire domain, the material thermodynamic properties can be substituted into four algebraic equations to easily determine the variation of solid fraction, interface temperature and solute concentration, instead of calculating interface moving velocity. As a result, the complexity of the calculation can be largely reduced. The simulated dendrite growth was in a good agreement with the Lipton–Glicksman–Kurz (LGK) model for free dendritic growth in undercooled melts.
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Authors: Kenji Miwa, Ming Jun Li, Takuya Tamura
Abstract: We have developed the refinement process of the microstructure of metallic materials by imposition of electromagnetic vibration force during solidification. This process is effective for both wrought magnesium alloys and cast ones. By simultaneous imposition of a static magnetic field of 10 Tesla under an alternative electric current of 60A, the average grain sizes of the AZ31B wrought alloy and the AZ91D cast alloy were obtained about 50 micron in both alloys. The grain size was affected by electric current frequency and had the minimum value at the special electric current frequncy of 500 to 2000 Hz and 900 Hz for wrought alloy and cast alloy, respectively. From experimental results, we suggested the mechanism of refinement of microstructure during solidification by imposition of electromagnetic vibration force. The cavitation phenomenon in liquid phase during electromagnetic vibration was effective to break down th esolid phase. And also the difference of electric conductivity between the solid phase and the liquid one brought vigorous vibration of the solid phase. Then the solid phase was suppressed its growth.
1453
Authors: Udo Brückner, Alexander Epishin, Thomas Link, Pedro D. Portella
363
Authors: Hironori Morishita, Hisao Esaka, Kei Shinozuka
Abstract: As-solidified structure of an ingot is composed of the chill, columnar and equiaxed zones. The whole solidified structure is strongly affected by the chill crystals. Some initial solidification grains have been observed on the ingot surface and thought to be traces of the nucleation point. The aim of this study is, therefore, to develop the experiment technique to make one ‘grain’ and to crystallographically investigate the initial solidification grain using EBSD analysis. In order to start solidification at a very specified position, a small metallic protrusion was installed on an insulating plate. Al-6 wt%Si alloy was melted at 800 °C and was poured on the metallic protrusion. In this study, the amount of protrusion was varied to investigate the growth mechanism of the initial solidification grain. The longitudinal cross section of the specimen was observed by an optical microscope, a scanning electron microscope. The starting position of solidification was the area that was on the metallic protrusion. In this initial solidification grain, it was difficult to observe the dendritic structure. The shape of this grain was about hemispherical. The grain area seemed to increase with increasing the amount of protrusion. The results of EBSD analysis showed that almost all initial solidification grains were composed by several crystals. The reason of this is that the nucleation frequency may increase with the amount of protrusion. The dendrite grew radially from the initial solidification grain continuously. The crystallographic structure was also continuous on the boundary of the initial solidification grain.
1328
Authors: Chao Wang, Zhen Rui Li, Yong Ji Niu, Shi Feng Shi, Ning An
Abstract: The causes and the influence factors of the defects of center porosity and secondary pipe of the casting superalloy ingot are briefly introduced, and the study on the principle and technical characters of grain refinement technology with dynamic methods carried out. The mechanism, working parameter and the test result of the typical application methods including the technologies of vibration of ultrasonic, the vibration of mould, the stir of mould and oscillation in the riser were investigated in detail. By comparing the advantages and disadvantages of different methods and also according to the practical production situation of our company, the most appropriate one for improving the defects of center porosity and secondary pipe is proposed.
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