Materials Science Forum Vols. 575-578

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.

Abstract: A stochastic mathematic model contained the effects of dendrite morphology, solidification shrinkage and dissolved gases was formed to simulate microporosity formation and growth. Microporosities appear in the interspaces of primary dendrites as well as secondary dendrites from microscopic view of A356 aluminum alloy experimental ingot with a metal mold. In the past literatures it took the volumetric fraction of microporosities as a function of the local density. In the present work a single pore size and distribution were predicted concerning the combination of shrinkage and dissolved gases and dendritic spacing. The dendritic spacing is a main parameter to decide the pore pattern. For shrinkage and dissolved gases causes, the favorable one is determined by dendritic spacing, also the local cool rate and tip growth rate. The dense degree of the experimental ingots in different casting conditions was discussed. The variations of dens degree from the measured values in different casting conditions are similar to that of porosity volume fraction from the predicted results.

Abstract: Double-stream-pouring continuous casting (DSPCC) is a novel technique to prepare gradient composites. In this paper, a two-dimensional steady mathematical model coupled flow, heat and solution transport in the solidification zone of DSPCC has been established based on the standard k-ε turbulent model. The variation of thermo-physical properties of the alloys against the temperature is also considered. The effects of casting speed on the temperature field, velocity field and composition of the researched composite are analyzed based on the proposed mathematical model. An experimental 2024/3003 aluminum composite was prepared by using a laboratory-made DSPCC facility. Selected results of experiment are in a good agreement with that of numerical simulation.

Abstract: Based on the solidification features of ductile iron and affecting factors for ductile iron shrinkage defect, the model of the ductile iron solidification is built and put forward a new defect predictive method EIECAM (Enclosed-Isolated area Expansion and Contraction Accumulation Method) model to predict defect. in DECAM, the liquid shrinkage, solidified shrinkage and graphitizing expansion during solidification are computed dynamically in the enclosed-isolated area , and the effect of graphite expansion on the wall movement is also accounted. Based on this method end cover of QT500 ductile iron casting is simulated and made the defect predictive, study its solidification process and the defect generation position, and make the experimental identification on the defect. It is resulted that the method can be able to predict the casting defect authentically.

Abstract: A cellular automaton (CA) based model for two-dimensional simulation of the grain morphology of high pressure die casting magnesium alloy was developed. The heterogeneous nucleation, the solute redistribution both in liquid and solid, the interface curvature and the growth anisotropy during solidification were also considered in the model. By fitting the curve of grain density distribution, parameters of continuous nucleation equation based on Gaussian distribution were calculated. The microstructure simulation of step-shape die castings of AM50 magnesium alloy was studied. The metallographic microstructure of the castings at the surface and center of three steps with different initial die temperature was investigated. The simulation results were compared with the experimental results and they were in good agreement on average grain size.

Abstract: A study of thermal field in induction melting with cold crucible and dual induction frequencies is presented in this paper. Numerical simulation was done by finite element based software COMSOL with use of multi-physical modeling. A method of superposing of magnetic vector potential is proposed to solve the complexity of dual induction frequencies. Different conditions of high and low frequencies were given in the calculation to find the effects on distribution of thermal fields. The computed temperature distribution was compared with the macro etched section of solidified ingots and the results showed that the orientation of dendritic grain structures follow to the similar direction of heat flow. This indicates that proper set of induction coils could enable more uniform thermal field distribution and attaining controlled solidification morphology in the ingots.

Abstract: This paper gives an advanced control to a strip caster with tilting-ladle-type automatic pouring system. The strip caster has been used for producing ferrous and nonferrous sheets. In the process, in order to obtain the high-quality products, it is required that the molten metal in the tundish keep the high liquid level. Therefore, the control system to liquid level in the tundish is proposed in this present paper. In this approach, the mathematical model from the control input into the motor for tilting ladle to the liquid level in the tundish is derived by using hydrodynamics. Then, for reaching quickly the molten metal to the high liquid level and keeping stably the liquid level, the control input into the motor is designed systematically by using the mathematical model. The proposed control system has advantage that it can be constructed simply. And, any sensors for measuring the liquid level are not required in the proposed system.

Abstract: For the purpose of the prediction of casting structures, heterogeneous nucleation rate in the undercooled melt of solififying Al-Si alloys were evaluated by comparing experimentally observed macrostructures of solidified ingots with numerically simulated ones. Molten alloys were unidirectionally solidified in an adiabatic mold from a steel chill block located at the bottom of the mold. In the experiment, columnar to equiaxed transition (CET) was observed. A numerical simulation for grain structure formation of the sample ingots was carried out using a cellular automaton (CA) method, and heterogeneous nucleation rate in the solidifying alloys were evaluated by producing the similar structures to experimental ones. An attempt was made to predict the grain structure of conventionally cast ingots using the evaluated heterogeneous nucleation rate. However, the simulation could not predict the structure of ingot with low superheat due to crystal multiplication near the mold wall. The crystal multiplication mechanism, so-called "Big Bang mechanism", was introduced into the simulation and the simulation could predict the grain macrostructure composed of columnar and equiaxed crystals that were similar to experimentally observed one.

Abstract: The hot deformation behaviors of a ferritic spheroidal cast iron (FSCI) have been investigated by compression testing on a Gleeble 3500 machine of the DSI-YSU Joint Laboratory. The temperature rang was from 1073K to 1273K and strain rate from 10-3 to 1 s-1. The total true stain was 0.7. The result shows that the flow curves obtained are typical of dynamic recrystallization processes. The plots of either the natural logarithms of the corresponding temperature or the natural logarithms of strain rate against the hyperbolic of flow stresses satisfy straight line relationships over the experimental data, indicating that the hot compression of the FSCI is thermally activated. The material constants, including activation energy 0H as 240.8 kJ/mol, stress-level coefficient α as 1.352×10-8 Pa-1, stress exponential n as 3.9937, structural factor A as 5.64×108 s-1, are derived .

Abstract: This paper investigated the forming defects causation and related resolving measures by combining numerical method with experimental technique. A practical case of one auto-body panel stamped parts with forming defects was studied in detail. A new approach determining the causation of forming defects and finding out resolving ways was proposed. Firstly, uses numerical method to analyze the characteristics of the whole forming process by dividing the forming process into virtual steps, so as to obtain the forming feature such as stress & strain distribution during the stamping process. Secondly, uses experimental grid method to measure the real plastic strain distribution of the defective area thus to analyze the forming rule of this area. By synthesizing both methods and carrying out extensive analysis, it is possible to make sure the cause of the defects and put out solving scheme further. The study shows that numerical combined with experimental method is an effective way in analyzing and resolving forming defects for auto-body parts.