Véronique Favier

Micromechanical modeling of the elastic-viscoplastic behavior of polycrystalline steels

Abstract A self-consistent model developed to describe the elastic–viscoplastic behavior of heterogeneous materials is applied to low carbon steels to simulate tensile tests at various strain rates in the low temperature range. The choice of crystalline laws implemented in the model is discussed through the viscoplastic flow rule and several strain-hardening laws. Comparisons between three work-hardening models show that the account of dislocation annihilation improves the results on simulations at large strains. The evolution of the Lankford coefficients and texture development are also successfully simulated. Some microstructural aspects of deformation such as the stored energy and the evolution of the flow rates are discussed. By including the dislocation density on each slip system as internal variable, intragranular heterogeneities are underscored.

Residual and Internal Stress States in Duplex Steel with TWIP Effect

A new variety of duplex steels with superior mechanical properties has been studied. They exhibit a very interesting combination of strength (tensile strength of 680 MPa) and ductility values (more than 45% total elongation) due to the competition between different plasticity mechanisms. These steels contain two phases: austenite and ferrite and are characterized by low stacking fault energy at room temperature. In this work, four duplex steels with different chemical composition and phase volume fraction are studied. Residual and internal stresses in each phase were determined using the classical X-ray diffraction sin²ψ method. In the as-received state, both longitudinal and transverse residual stresses are in compression (until -350 MPa) for the ferrite and in tension (until +410 MPa) for the austenite. However, residual stresses in the austenitic phase decrease when its volume fraction increases. Moreover, internal stress distribution in one alloy was determined by X-ray diffraction during an in situ tensile test. The austenitic phase stress along the loading direction is higher than the macroscopic applied one, which is higher than the ferritic stress state, verifying a mixture rule and consistent with the initial residual stresses. For an applied macroscopic strain of about 1%, the austenite phase is subjected to a stress of about 600 MPa whereas the stress in the ferritic phase is about 300 MPa. It was also observed that as macroscopic strain increases, stress difference between the austenite and the ferrite decreases.

Analysis of semi-solid response under rapid compression tests using multi-scale modelling and experiments

Simulating semi-solid metal forming requires modelling semi-solid behaviour. However, such modelling is difficult because semi-solid behavior is thixotropic and depends on the liquid-solid spatial distribution within the material. In order to better understand and model relationships between microstructure and behavior, a model based on micromechanical approaches and homogenisation techniques is presented. This model is an extension of a previous model established in a pure viscoplastic framework to account for elasticity. Indeed, experimental load-displacement signals reveal the presence of an elastic-type response in the earlier stages of deformation when semi-solids are loaded under rapid compression. This elastic feature of the behaviour is attributed to the response of the porous solid skeleton saturated by incompressible liquid. A good quantitative agreement is found between the elastic-viscoplastic predicted response and the experimental data. More precisely, the strong initial rising part of the load-displacement curve, the peak load and the subsequent fall in load are well captured. The effect of solid fraction on mechanical response is in qualitative agreement with experiments.

Thixoforming of Steel: Experiments on Thermal Effects

This paper presents experimental results on steel thixoforming. The influence of thermal exchanges with tools and environment on the semi-solid response is analysed.

Does Shear Thickening Occur in Semisolid Metals

In the various forms of semisolid processing such as thixoforming and thixoforging, the entry into the die occurs in a fraction of a second so it is the transient rheological behavior which governs the initial stages of flow. In experiments in the literature, this rheological behavior is probed through applying rapid transitions in shear rate under isothermal conditions. There is contradictory evidence as to whether the behavior during these transitions is shear thinning or shear thickening, although it is clear that once in the die the material is thinning. Here the data in the literature are reanalyzed to obtain a rationalization of the contradictions which has not previously been available. It is argued that if a suspension is initially in a disagglomerated state (i.e., one which is initially sheared), the instantaneous behavior with a jump-up in shear rate is shear thickening (even if the long-term steady-state behavior is shear thinning) provided the fraction solid is greater than about 0.36 and the final shear rate at the end of the jump is greater than about 100 s−1. If the jump-up in shear rate is made from rest then yield masks the shear thickening.

Finite element simulations of thixoforging by using a micro‐macro constitutive equation and comparison with experiments

This paper presents different finite element simulations of thixoforging realised on Forge2© software. Compression and axial extrusion tests are particularly studied. The constitutive equation used for these simulations is based on a micro‐macro approach accounting for the semi‐solid microstructural evolution. The constitutive equation parameters are identified thanks to successive comparisons of the simulated load‐displacement responses with experimental ones for compression tests on steel semi‐solid at different temperatures. The relevance of the modelling is then analysed through extrusion tests. It is shown that the constitutive equation developed by the authors provides simulated results closer to the experimental ones than the classical Norton‐Hoff law. It demonstrates the capacity of prediction of such a modelling. Moreover, these simulations give a lot of information concerning the role of parameters such as solid fraction, initial solid skeleton degree of agglomeration, thermal exchanges, or stra...

Calorimetric Studies and Self-Heating Measurements for a Dual-Phase Steel Under Ultrasonic Fatigue Loading

The objective of the present research is to study the self-heating behavior of a dual-phase (DP) steel under ultrasonic fatigue loading and to investigate the effect of frequency on intrinsic heat dissipation of the material. The steel studied in this work is DP600 commercial DP steel. Fatigue tests were conducted using an ultrasonic fatigue machine at a testing frequency of 20 kHz with flat specimens. An infrared camera was used to measure the mean temperature evolution during the tests. A specific form of heat diffusion equation was adopted in this work to calculate the heat dissipation per cycle from temperature measurements. The variation of this dissipation versus stress amplitude in cyclic loading was also studied.

Analysis by Micromechanical Modeling on Material Flow under Rapid Compression in the Semi-Solid State

Different types of semi-solid processing are used to produce a variety of components. In this context, the use of FE simulations to obtain the filling of the dies and to optimize the semi-solid processing is clearly of a great interest. To carry it out properly in an isothermal case, the semi-solid flow into the die and friction phenomena have to be correctly described. In addition, comparisons between experiments and simulations are needed to assess the reliability of the modeling and to improve the understanding of the processing. In situ visualization of the semi-solid flow during processing is complex since the dies are closed and opaque. One of the main recent work with transparent glass sided dies to film die filling is that by Atkinson and Ward (2006). The purpose of this work is to compare numerical simulations to these experiments. Numerical simulations were performed with the solid mechanics-based software FORGE©. A micromechanical model accounting for the liquid and solid behaviour and their spatial distribution within the material (Favier et al, 2009) was used. The model parameters were identified using rapid compression tests on the A357 aluminium alloy (Favier and Atkinson, 2011). The slurry temperature corresponds to 0.5 solid fraction. Comparisons were focused on the flow behaviour. The impact of the presence of an obstacle and of the shape of the obstacle was investigated. The numerical simulations reproduced quite well the flow behaviour for the case with and without central obstacle. However, the change in flow due to an increase of the ram speed from 250 mm/s to 1000 mm/s is not captured.

Thixoforming of Steel - Influence of thermal parameters

This paper presents experimental results on steel thixoforming. The influence of thermal effects on the semi-solid response is analysed through the influence of thermal exchanges with tools and environment. Several rheological experiments such as compression, extrusion or radial filling test were developed to understand the semi-solid steel behaviour and determine the parameters that have a major influence on thixoforming. Actually, in our experiments, the temperature of the slug and consequently the solid fraction were found first order parameters while; the morphology of the solid phase plays a minor role.

Experimental Investigation on 7075 Aluminium Response during Thixoextrusion

The deformation behavior of semi-solid aluminium alloy is strongly dependent on the microstructure. This paper illustrates several experimental research works concerning thixoextrusion of 7075 aluminium alloy which was carried out at “Arts et Métiers ParisTech” of Metz. The microstructure obtained by two reheating systems and flow behavior during thixoextrusion tests were investigated. To minimize the heat losses, a sample obtained from a direct extruded bar is inserted in a die for reheating in semisolid state and for thixoextrusion. Reheating the steel die and the aluminium billet placed into the die at the same time using an induction furnace provides rapidly a very homogeneous microstructure suitable for thixoforming. During the experimental re-heating process, the temperature was directly controlled using thermocouples for temperature measurements in the sample and also in the die. The experimental results on extrusion load and microstructure evolution of the component are reported.