Modelling microstructure evolution during equal channel angular pressing of magnesium alloys using cellular automata finite element method

Equal channel angular pressing (ECAP) is one of the most popular methods of obtaining ultrafine grained (UFG) metals. However, only relatively short billets can be processed by ECAP due to force limitation. A solution to this problem could be recently developed incremental variant of the process, so called I-ECAP. Since I-ECAP can deal with continuous billets, it can be widely used in industrial practice. Recently, many researchers have put an effort to obtain UFG magnesium alloys which, due to their low density, are very promising materials for weight and energy saving applications. It was reported that microstructure refinement during ECAP is controlled by dynamic recrystallization and the final mean grain size is dependent mainly on processing temperature. In this work, cellular automata finite element (CAFE) method was used to investigate microstructure evolution during four passes of ECAP and its incremental variant I-ECAP. The cellular automata space dynamics is determined by transition rules, whose parameters are strain, strain rate and temperature obtained from FE simulation. An internal state variable model describes total dislocation density evolution and transfers this information to the CA space. The developed CAFE model calculates the mean grain size and generates a digital microstructure prediction after processing, which could be useful to estimate mechanical properties of the produced UFG metal. Fitting and verification of the model was done using the experimental results obtained from I-ECAP of an AZ31B magnesium alloy and the data derived from literature. The CAFE simulation results were verified for the temperature range 200-250 °C and strain rate 0.01-0.5 s-1; good agreement with experimental data was achieved

Model of Curvature of Crankshaft Blank during the Heat Treatment after Forging

AbstractThe manufacturing process of large crankshafts is affected by undesirable bending of the shaft blank at the heating and heat treatment stages, as well as at the forging step. In line elimination of the shaft blank bending is a time consuming and expensive operation. Identification of reasons of the shaft bending and identification of possibilities of avoiding this bending is an important theoretical and practical problem. In previous works, the Authors developed the finite element model and applied this model to optimization of the forging process and to predict microstructure evolution. The model was extended to predict bending of shaft and applied in the present work to simulate the influence of various parameters of forging on shaft bending.The work is devoted to further extension of the model by including deformation of the crankshaft during heat treatment after forging. The model estimates the contribution of the elastic-plastic deformation, including thermal expansion and dilatometric effect due to transformations, on the deformation and bending of the shaft. This process generates also residual stresses. The aim of the paper was to simulate phenomena occurring during heat treatment. A program based on the finite element method was developed to solve the three-dimensional thermal and mechanical problems. Shaft material model was developed and the elastic-plastic characteristics were implemented in the FE code. Heat exchange with the cooling medium and the dependence of thermal properties on temperature and heat of phase transformations were accounted for in a solution of the thermal problem. Dilatometric tests were performed to supply data for identification of the phase transformation model during cooling. The results of calculations of bending of shaft during the process of heat treatment, as well as the distribution of residual stresses and strains are presented in the paper. The calculations were performed for several modes of the heat treatment. Parameters affecting bending of the shaft were identified

Stress-Strain Analysis in TiN Nanocoating Deposited on Polymer with respect to Au Nanointerlayer

The multiscale analysis in the authors’ finite element code confirmed possibility of fracture, because of not sufficiently high level of compressive residual stress in the TiN deposited by physical deposition method and varied mechanical properties of the thin film and substrate. The residual stress cannot be identified by X-ray technique for amorphous polymer and layer with domains of crystalline TiN. It is assumed that the buffer biocompatible thin film of Au in the TiN/Bionate II material system will alter the evolution of residual stress and, therefore, will allow to determine the residual stress in profilometry studies, and helps to improve toughness of the connection between TiN and Bionate II. The introduction of Au nanocoating in the material system results in bending of the sample and a compressive residual stress in the TiN coating. Results of finite element simulation show improvement of connection between the polymer and TiN, and an increase of compressive residual stress in the coating by introduction of Au nanointerlayer results in reduction of stress and strain in the substrate (close to the boundary between substrate and coating)

Stress-Strain Analysis in TiN Nanocoating Deposited on Polymer with respect to Au Nanointerlayer

The multiscale analysis in the authors’ finite element code confirmed possibility of fracture, because of not sufficiently high level of compressive residual stress in the TiN deposited by physical deposition method and varied mechanical properties of the thin film and substrate. The residual stress cannot be identified by X-ray technique for amorphous polymer and layer with domains of crystalline TiN. It is assumed that the buffer biocompatible thin film of Au in the TiN/Bionate II material system will alter the evolution of residual stress and, therefore, will allow to determine the residual stress in profilometry studies, and helps to improve toughness of the connection between TiN and Bionate II. The introduction of Au nanocoating in the material system results in bending of the sample and a compressive residual stress in the TiN coating. Results of finite element simulation show improvement of connection between the polymer and TiN, and an increase of compressive residual stress in the coating by introduction of Au nanointerlayer results in reduction of stress and strain in the substrate (close to the boundary between substrate and coating)

Model of Residual Stresses in Hot-rolled Sheets with Taking into Account Relaxation Process and Phase Transformation

AbstractThe problem of calculations the residual stresses in hot-rolled sheets is considered in the paper. Residual stresses become of practical importance when the laser cutting of sheets is applied. The main factors influencing the residual stresses are the non uniform distribution of elastic-plastic deformations in the volume and unloading of the sheet material during cooling, phase transformation occurring during cooling and relaxation of the stresses. The goal of this paper is development of a model of residual stresses in hot-rolled sheets based on the elastic-plastic material model, taking into account the above factors. In this work the individual models for cooling of hot rolled sheets in the laminar cooling line and in the coil were developed. Elastic-plastic properties of the material were determined experimentally using tests on GLEEBLE 3800. Model of the thermal deformation during cooling was obtained on the basis of the dilatometric test. Thermal model was based on the solution for two dimensional cross-section of the sheet and the roll longitudinal section. Experimental verification of the thermal model was performed in industrial conditions

Structural Evolution of Thin Lamellar Cementite during Cold Drawing of Eutectoid Steels

Eutectoid steel wires are heat treated at isothermal temperatures to generate not only pearlitic microstructures but also bainite and martensite. These wires are subsequently drawn to increasing strain levels. The microstructural evolution is observed using scanning electron microscopy SEM and transmission electron microscopy TEM. Measurements of the lamellar spacing at different drawing stages reveal a thinning and a branching of the lamellae. In addition to this, the type of heat treatment influences the appearance of the lamellae and the formation of fine carbides. A numerical model was employed to compute the deformation conditions and the interlamellar spacing. The transformation and dissolution of cementite, branching of cementite and diffusion along dislocations were analysed with respect to various models from the scientific literature