Coupling the thermal and mechanical fields to metallurgical evolutions within a finite element description of a forming process

Finite element formulations are commonly used to predict stress, strain and temperature fields in metal forming. As these models have now gained robustness, an increasing attention is placed on the metallurgical evolutions associated with the thermal and mechanical fields. A finite element can be associated to a microstructure, and the microstructure description allows the modeling of a constitutive behavior, using conventional homogenization theories. These theories can be further refined with the help of finite element calculations describing the material structure at multiple scales. The paper concentrates on numerical strategies than can be developed to couple finite element formulations to metallurgical models of two kinds: those predicting crystallographic textures and mechanical anisotropy, and those dealing with phase changes controlled by diffusion. Multiscale finite element models describing key features of metallic structures are also discussed, within a digital material framework. © 2005 Elsevier B.V. All rights reserved

A review of dynamic recrystallization phenomena in metallic materials

The dynamic recrystallization (DRX) phenomena occurring in different thermo-mechanical processing (TMP) conditions for various metallic materials are reviewed. Several types of DRX are described: discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX). The terminologies used in this field are summarized, together with the key factors influencing the DRX processes including stacking fault energy, initial grain size, TMP conditions and second-phase particles. Both standard and advanced experimental techniques used to characterize DRX processes are examined. The focus is placed on the mechanisms of these three types of DRX, and the related numerical models

Microstructure and flow stress evolution during hot deformation of 304L austenitic stainless steel in variable thermomechanical conditions

Most of industrial hot deformation processes are performed in variable conditions where the strain rate and/or deformation temperature are not constant. In this work, hot compression tests in both constant and varying strain rate conditions were performed on 304L austenitic stainless steel using a Gleeble 3800 machine. The variations in microstructure and flow stress during and after the transient deformation stage are carefully analysed. It is clearly shown that, following the abrupt increase of strain rate, both the flow stress and substructural changes are subjected to a transient period over strains of ~0.2, before reaching states similar to those developed through constant strain rate conditions at the new strain rate. When the strain rate was rapidly decreased, the flow stress transient stage extended over a lower strain interval than the substructure transient period. It is shown that local misorientation distributions are good indicators of the deformation microstructure of low SFE materials as they capture small variations in deformation structures which cannot be analysed from stress-strain curves alone

Level set framework for the finite-element modelling of recrystallization and grain growth in polycrystalline materials

The paper describes a level set framework for the finite-element modelling of recrystallization in polycrystalline materials, including the nucleation, primary recrystallization and grain growth stages. A set of convection- reinitialization-diffusion equations is implemented within a finite-element formulation to describe boundary motion, according to a kinetic law relating the velocity of the boundary to a thermodynamic driving force. Different theoretical test cases illustrate the potential of the method for simulations of recrystallization and grain growth following large plastic deformation and/or considering complex microstructures. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

3D finite element model of semi-solid permeability in an equiaxed granular structure

A multi-domain finite element formulation has been applied to determine the semi-solid permeability of an equiaxed granular structure. The granular model was constructed from a Voronoï tessellation algorithm. The liquid-solid interface is represented implicitly by a level set function, and an anisotropic meshing technique is employed for an accurate description of the interface geometry with reasonable computation resources. The liquid phase is considered as an incompressible Newtonian fluid, and permeability is computed based on Darcy's law. A good agreement is found with available literature data and with the Kozeny-Carman prediction. The proposed method, associating the finite element method with a level set framework, is shown to be an effective technique for examining microstructure effects on the semi-solid permeability. This is illustrated by considering the influence of the Gibbs-Thomson effect. © 2010 Elsevier B.V. All rights reserved

Fast in-situ annealing stage coupled with EBSD: A suitable tool to observe quick recrystallization mechanisms

A heating stage has been developed to perform in-situ annealing in a SEM equipped with an EBSD system in order to study recrystallization mechanisms. High temperature treatments could then be performed inside the SEM, up to 1180°C and with high heating and cooling rates (∼ 100°C s - 1). Samples were cooled down to room temperature to perform EBSD orientation mapping in between successive short-duration heat treatments. Microstructure evolution snapshots obtained this way allow gaining an insight into recrystallization mechanisms. The interest of such experiments is shown for two examples: static recrystallization of cold deformed pure tantalum and post-dynamic evolution of hot-deformed Zircaloy4. © 2012 Elsevier Inc. All rights reserved

Parameter identification method for a polycrystalline viscoplastic selfconsistent model based on analytical derivatives of the direct model equations

An inverse method for automatic identification of the parameters involved in a polycrystalline viscoplastic selfconsistent (VPSC) model is presented. The parameters of the constitutive viscoplastic law at the single-crystal level, i.e. the critical resolved shear stresses (CRSS) of slip and twinning and the micro-hardening coefficients, can be identified using experimental data at the polycrystal level, i.e. stress-strain curves and deformation-induced textures. The minimization problem is solved by means of a Gauss-Newton scheme and the sensitivity matrix is evaluated by analytical differentiation of the direct model equations. As a particular case, the optimization procedure for the Taylor full constraints (FC) formulation is also presented. The convergence and stability of the identification scheme are analysed using several validation tests for different deformation paths imposed to a polycrystal of hexagonal structure. As an example of application of this inverse method, the relative CRSS of the active deformation systems of a Zircaloy-4 sheet are identified, based on several textures measured for different reductions and rolling directions