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Modelling of the primary structure formation in the thin wall hypo- and hypereutectic ductile iron casting using CA method

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Nodular graphite cast iron, also known as ductile iron (DI), has major applications in critical engineering parts due to its mechanical properties and castablility. The mechanical and physical properties of this material depend on the shape and number of the graphite grains and microstructure of the metallic matrix. Solidification of DI was a subject of many computer modeling programs described in literature [1÷5], in which the stationary conditions of carbon diffusion in austenite is pre-assumed. Recently, a tendency for production of thin-walled castings has been observed [6÷8]. In this technology, the process of the fast solidification is very far from equilibrium and steady-state conditions. The purpose of the present work is a two-dimension model development for simulation of the DI structure formation during the solidification in the condition of non steady-state temperature and diffusion fields in the thin-wall casting. process Model The CA-FD (CA stands for Cellular Automata, and FD stands for Finite Difference) is one of the known methods of the simulation of microstructure formation during the solidification [9, 10]. In the CA microstructure modelling the outer grain shape is the result of the simulation which is not superimposed beforehand. The model development for a one-phase microstructure evolution is a subject of numerous research [11÷20]. Model of the eutectic solidification of DI in the uniform temperature field and superimposed cooling rate is known [21]. Presented model is based on the CA-FD technique and will predict solidification of DI in the non-uniform temperature field during the cooling of the thin-wall casting. Model takes into account the continuous nucleation of austenite and graphite grains from liquid controlled by undercooling, separate non-equilibrium growth of graphite nodules and austenite dendrites at the first solidification stage, and the following cooperative growth of graphite-[...]

Averaged Voronoi polyhedron in the peritectic transformation modelling

  Peritectic solidification of the alloys is in the centre of attention of researchers. This is a mechanism of the structure formation for many technical alloys. Peritectic solidification is believed to be the major cause of crack formation during the solidification of many steels [1]. During the peritectic solidification primary solid phase vanishes simultaneously along with liquid phase and new secondary (peritectic) phase grows as a solidification product. In the carbon steel γ-phase (austenite) appears, replacing δ-phase (ferrite) and liquid. Two separate mechanisms of peritectic solidification are known as: - peritectic reaction: when dissolution of primary phase, directly in liquid phase, is possible independently from the secondary one [2], - peritectic transformation: when primary phase is completely separated from liquid by the layer of secondary phase [3]. An analytical model of the kinetics of peritectic transformation, based on the linearized concentration gradient of the solute, has been presented by Das et al. [4]. The results shown in this paper exhibit a good agreement with the experimental results, but the difference between rates obtained by the computer modelling and relevant experimental data increase at the later stages of transformation. According to authors of [4] one of the reasons of this disagreement is the deviation from the idealized geometry. The idealized geometry of the elementary peritectic cell is usually used in the known numerical models. Furthermore, the idealized initial concentration profile of the solute in the preperitectic phase is assumed. Spherically symmetrical cell of radius (3/4n)1/3 was applied in [5], where n is the number of peritectic cells per unit volume. In the Ref. [6] only a plane spatial segment of the melt pool was used as the simulation domain. According to the analysis of diffusion transformations presented in [7], a single common mathematical model allows fo[...]

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