Dmytro Svyetlichnyy

Modeling of Microstructure Evolution in Process with Severe Plastic Deformation by Cellular Automata

Prediction of microstructure evolution and properties of ultrafine-grained materials is one of the most significant, current problems in materials science. Recently, an interest to apply the cellular automata (CA) to the simulation of different phenomena in materials has been rising constantly. The main asset of the CA is the ability for accurate modeling of the microstructure. Deformation in micro-scale shows anisotropy, which is related with the different crystallographic orientation of the grains in the polycrystalline material. To improve the accuracy of modeling, CA and FEM must be combined with crystal plasticity theory. In present model, deformation in macro-scale is transferred to meso-scale, where a representative element contains several, score or hundreds grains, and then is applied in micro-scale to each grain. Strain and strain rate are decomposed into the crystallographic directions. For each crystallographic direction, development of dislocation and subgrain boundaries are considered. In each grain development of dislocation structure is distinctive because their orientation is unique. Creation of low-angle boundaries and their development into high-angle boundaries are simulated by the cellular automata on the base of calculations using finite element method and crystal plasticity theory. Some algorithms implemented into CA are described in the paper, as well as simulation results.

A three-dimensional frontal cellular automaton model for simulation of microstructure evolution—initial microstructure module

This paper presents a three-dimensional frontal cellular automaton (FCA)-based model for modelling of microstructure evolution during technological processes. It is a hierarchical system. The first level is the FCAs, the second level contains modules of microstructural phenomena and the third level is presented by the models of technological processes. The module of the initial microstructure (IM) is one of the components of the second level. The IM allows one to obtain a digital material representation of given parameters, which can be used by other modules for further simulation. The parameters that must be assured by the IM module are the following: shape of the grains and distributions of the grain size, crystallographic orientation and boundary disorientation angles.To obtain the required parameters, the FCAs are first used as a tool for the creation of the basic microstructure characterized by the shape of the grains. The grain size distribution is obtained by the method, which changes nucleation and grain growth conditions. After the creation of the microstructure, crystallographic parameters are established. Distribution of the crystallographic orientation and boundary disorientation angles can be obtained independently or as associated parameters. Some examples of microstructures obtained by the IM module are presented in this paper.

Modeling Of Microstructure Evolution Of BCC Metals Subjected To Severe Plastic Deformation

Prediction of microstructure evolution and properties of ultrafine‐grained materials is one of the most significant, current problems in materials science. Several advanced methods of analysis can be applied for this issue: vertex models, phase field models, Monte Carlo Potts, finite element method (FEM) discrete element method (DEM) and finally cellular automata (CA). The main asset of the CA is ability for a close correlation of the microstructure with the mechanical properties in micro‐ and meso‐scale simulation. Joining CA with the DEM undoubtedly improves accuracy of modeling of coupled phenomena during the innovative forming processes in both micro‐ and macro‐scale. Deformation in micro‐scale shows anisotropy, which connected with that the polycrystalline material contains grains with different crystallographic orientation, and grain deformation is depended from configuration of directions of main stresses and axis of grain. Then, CA and DEM must be joint solutions of crystal plasticity theory. In t...

Evolution of Microstructure during the Shape Rolling Modeled by Cellular Automata

The numerical modeling of microstructure evolution in the forming processes is used different schemes and different tools. One of the methods is cellular automata (CA), and recently an interest in cellular automata for modeling of forming processes is increased constantly. Cellular automata are considered as one of the most optimal tool for microstructure modeling because of the computational effort and the obtained results. The paper presents the use of frontal cellular automata (FCA) for modeling microstructure evolution during the rolling process. The use of frontal cellular automata in combination with other methods is related to the possibility of obtaining more accurate and reliable results and has undeniable advantages. Results of the modeling of the process by finite element method (FEM) are input data for further simulation by FCA. Some examples of microstructure as results of FCA simulation are presented in the paper.

Modification of Flow Stress Model Based on Internal Variables

In the paper a flow stress model based on the dislocation theory in consideration of the recrystallization is shortly presented. The model contains two parts: a classic model of dislocation evolution and recrystallization model. The latter considers different kinds of recrystallization as the same process rooted in nucleation and grain growth. The results of the model parameters identification and the simulation are presented in this paper. Then disadvantages of the model are considered and new proposal for improvement the model are presented. Results of preliminary simulation are presented as well

Frontal cellular automata simulations of microstructure evolution during shape rolling

Abstract The use of numerical modelling allows us to shorten the time for development of new technologies and to optimise the existing ones in view of final material properties. The paper presents the modelling of shape rolling of 5 mm round bars in diamond and oval grooves. Modelling scheme consists of three stages. The first stage is a development of rolling schedule, which is based on finite-element method simulations. The schedule design begins from the last pass. The paper presents the schedule developed for the last six passes. Then, in the second stage, finite-element method simulations were fulfilled in the proper rolling sequence. The aim of this stage is obtaining the thermomechanical parameters of the process. These parameters are transferred to frontal cellular automata model that takes into account the deformation and simulates microstructure evolution. Some results of finite-element method and frontal cellular automata modelling are presented in this paper.

Multiscale Model of Shape Rolling Taking into Account the Microstructure Evolution - Schedule Design by Finite Element Method

The material properties are strongly depended on the microstructure. Recently, for modeling and prediction of microstructure evolution during the forming processes a cellular automata method is used. Combination of several methods in multiscale model allows to extend the possibilities of each method and obtain more reliable results, which are close to the real conditions. The objective of this study is development of multiscale model of microstructure evolution during the shape rolling process and use it for simulation of rolling of 5 mm round bars. Model uses for calculations the finite element (FEM) and cellular automata (CA) methods. Modeling consists of three stages: design of the shape rolling schedule with the definition of shape and sizes of grooves (FEM simulation of each pass, starting from the last pass), FEM modeling of shape rolling in the proper sequence of the passes, modeling of microstructure evolution by frontal cellular automata (FCA). Stages (especially the last two) can be repeated several times to optimize the technology in view of final microstructure. The paper presents the first stage of modeling, which includes design and selection of grooves scheme with used the finite element method. The last six passes were modeled. The rolling scheme obtained from the modeling in the next stage is simulated by FEM to obtain thermomechanical parameters of the process. Then, temperature, strain and strain rate distributions in bar cross-sections, rolling time and inter-pass time will be used as input data for modeling by FCA.

Modeling of Recrystallization with Recovery by Frontal Cellular Automata

The objective of the paper is modeling of material softening after the deformation. The main problem of almost every model with digital material representation is consideration of recrystallization as the only mechanism of material softening. Static recovery is introduced into the model based on frontal cellular automata. An influence of static recovery on softening process is twofold. Static recovery effects on a decrease of dislocation density directly and on growing rate of recrystallized grain indirectly. Because of static recovery the recrystallization slows down and the time of recrystallization is extended. Simulation consists of two stages. During the deformation, distortion of the cells, evolution of dislocation density, nucleation and grain growth are considered, while after the deformation, the processes of softening are considered only. Comparison of simulation results with experimental data are presented as well.

Application of Cellular Automata and Lattice Boltzmann Methods for modelling of Additive Layer Manufacturing

The holistic numerical model based on cellular automata (CA) and lattice Boltzmann method (LBM) are being developed as part of an integrated modelling approach applied to study the interaction of different physical mechanisms in laser-assisted additive layer manufacturing (ALM) of orthopaedic implants. Several physical events occurring in sequence or simultaneously are considered in the holistic model. They include a powder bed deposition, laser energy absorption and heating of the powder bed by the moving laser beam, leading to powder melting or sintering, fluid flow in the melted pool and flow through partly or not melted material, and solidification. The purpose of this study is to develop a structure of the holistic numerical model based on CA and LBM applicable for studying the interaction of the different physical mechanisms in ALM of orthopaedic implants. The model supposed to be compatible with the earlier developed CA-based model for the generation of the powder bed.,The mentioned physical events are accompanied by heat transfer in solid and liquid phases including interface heat transfer at the boundaries. The sintering/melting model is being developed using LBM as an independent numerical method for hydrodynamic simulations originated from lattice gas cellular automata. It is going to be coupled with the CA-based model of powder bed generation.,The entire laser-assisted ALM process has been analysed and divided on several stages considering the relevant physical phenomena. The entire holistic model consisting of four interrelated submodels has currently been developed to a different extent. The submodels include the CA-based model of powder bed generation, the LBM-CA-based model of heat exchange and transfer, the thermal solid-liquid interface model and the mechanical solid-liquid interface model for continuous liquid flow.,The results obtained can be used to explain the interaction of the different physical mechanisms in ALM, which is an intensively developing field of advanced manufacturing of metal, non-metal and composite structural parts, for instance, in bio-engineering. The proposed holistic model is considered to be a part of the integrated modelling approach being developed as a numerical tool for investigation of the co-operative relationships between multiphysical phenomena occurring in sequence or simultaneously during heating of the powder bed by the moving high energy heat source, leading to selective powder sintering or melting, fluid flow in the melted pool and through partly (or not) melted material, as well as solidification. The model is compatible with the earlier developed CA-based model for the generation of the powder bed, allowing for decrease in the numerical noise.,The present results are original and new for the study of the complex relationships between multiphysical phenomena occurring during ALM process based on selective laser sintering or melting, including fluid flow and heat transfer, identified as crucial for obtaining the desirable properties.

Flow Stress Model Based on Internal Variables

In the paper a flow stress model based on internal variables is shortly presented. The multiplicative model contains three parts. In the model, the normalized dislocation density ρm was considered, as a strain function only, independently to the strain rate and the temperature. Influence of varying processing conditions (the strain rate and the temperature) is introduced as a factor. The first one is a model of so called master curve. It is an internal variable model based on dislocation density and its output value strongly depends on strain and very weakly on temperature and strain rate. The second factor introduces varying deformation conditions. Changes of flow stress do not occur instantly with the change of deformation conditions, but it requires some strain for transition. The third part considers influence of recrystallization. The results of the model parameters identification and verification in varying deformation conditions are presented in this paper.