Prediction of tantalum microstructure evolution during thermomechanical treatments using FEM calculations with a dislocation density based constitutive law

In this paper, the behaviour of the tantalum subjected to a complex range of thermomechanical solicitations is studied and modelled. The objective of this work is the prediction of microstructure evolution of a tantalum part that is cold flow-formed, then stress-relieved and softened by heat treatments. Flow forming is a cold chipless process that elongates and thins the wall of a tubular part by applying a free rotating roller on the wall of the rotating part. (See Figure 1) This process leads to large strain up to 5 and one material point may be deformed at a fluctuating strain rate. To model this process, it is thus recommended to use a constitutive law that takes into account the history of the material. Microstructure evolution is controlled through the whole process by monitoring the state variable, that is, the dislocation density, within a FEM code. The 3D FEM software FORGE2007® is used to model the entire process. The code is enhanced with physical constitutive laws relative to the tantalum and derived from works of Klepaczko and Buy et al. [1,2,3]. The formalism of Klepaczko and Buy et al. is useful since it is based on laws regulating physical mechanisms of dislocations multiplication, annihilation and kinetics of glide depending on strain rate and thermal activation. Static recovery is modelled subsequently with an incremental approach where the softening is related to the dislocation density evolution. A set of thermomechanical experiments and treatments are done to identify the constitutive law and associated microstructure evolution. Compression, dynamic and static torsion tests covering a wide range of strain rates are used to fit the mechanical constitutive law and the dislocation density evolution law both inspired from Klepaczko and Buy et al. works. The same samples are then annealed with variable temperatures and times. Microstructure of annealed samples is characterized by means of micro-hardness. They give information to evaluate the dislocation density evolution. The data is used to model static recovery.0

3D FEM simulation of the flow forming process using lagrangian and ALE methods

The process of flow forming is numerically modeled using finite element codes based on the Forge2005® software. Two numerical approaches are considered. The first one uses an updated Lagrangian formulation. The problem is solved with help of a self-contact management algorithm. The second approach consists in using an ALE formulation that permits to optimize meshing with an adaptive method based on the Zienkiewicz-Zhu error estimation. The ALE method is well adapted to incremental forming processes such as flow forming and allows dealing with difficulties generated by the contact between the work piece and tools. Both formulations are coupled with complex tool kinematics. The Lagrangian formulation gives realistic results. The ALE formulation is promising with regard to computational time, and simulations on simple configurations show fairly good agreements with Lagrangian results. © 2007 American Institute of Physics

On the sluggish recrystallization of a cold-rolled Al–Mn–Fe–Si alloy

Annealing of supersaturated AA3xxx alloys at low temperatures usually results in sluggish recrystallization kinetics. This is due to the joint effect of the following factors: low nucleation rate, reduced grain boundary mobility, and large amount of fine precipitates (dispersoids). In this paper, samples of Al–Mn–Fe–Si alloy were appropriately homogenized in different conditions to produce different microchemistries before deformation, i.e. solutes and second-phase particles. The sluggish recrystallization behaviour of these cold-rolled Al–Mn–Fe–Si samples annealed in three different conditions was then investigated, the first condition being recrystallization without precipitation, followed by recrystallization and precipitation occurring concurrently, and finally precipitation occurring before recrystallization. The results clearly show that in all these conditions, an incubation time is involved, which decreases with increasing annealing temperature and cold deformation, as well as with decreasing solute amount. Qualitative analysis of the microstructure evolution after a sudden increase of annealing temperature suggests that the effective retarding force from solute and/or particles decreases at higher temperatures. When recrystallization occurs concurrently with precipitation, the growth of successful nuclei can still be suppressed by concurrent precipitation

Identification of cyclic and anisotropic behaviour of ODS steels tubes

In this paper, an elastic plastic behaviour model based on internal state variables is investigated. The final aim is to describe the mechanical stress-strain cyclic response of oxide dispersion strengthened (ODS) steels during the cold pilgering process. In this tube forming operation, a material element undergoes a series of small incremental deformations, alternatively under tensile and compressive stresses. The cyclic model considers isotropic hardening and kinematic hardening which can be related to the typical continuous cyclic softening of the ODS steels. Moreover, the identification process of the cyclic model parameters involves experimental data from only one sample. ODS steels tubes usually reveal an anisotropic strength in the radial, ortho-radial and longitudinal directions due to a crystallographic and strongly elongated grain morphology in the rolling direction. Identification of 3 Hill's parameters is done using compression tests of cylindrical specimens cut in three different directions (longitudinal, radial and ortho-radial) combined with an inverse analysis. © 2011 Published by Elsevier Ltd

Competition between intragranular and intergranular deformation mechanisms in ODS ferritic steels during hot deformation at high strain rate

Oxide Dispersed Strengthened (ODS) ferritic stainless steels present well-known fine grains microstructures where dislocation movement is hindered by a dense precipitation of nano-oxides particles. Previous research, on the thermomechanical behavior at high temperature and strain rates, was focused on torsion tests (Karch in J Nucl Mater 459:53–61, 2014). Considering texture evolution and grain shape as indicators of the intragranular dislocation glide activity, it was shown that, for high temperature and strain rate, intragranular deformation was in competition with intergranular accommodation. The latter phenomenon was related to early damaging at grain boundaries. The occurrence of a transition phenomenon from an intragranular to an intergranular deformation mechanism, with increasing temperature, was recently confirmed by neutron diffraction spectroscopy (Stoica in Nature Commun 5:5178, 2014). In the present paper, hot extrusion (HE) tests are performed, avoiding damage due to the high stress triaxiality, and allowing further investigation of intragranular and intergranular plasticity at large strains. Three ferritic steels exhibiting various precipitation size anddensitywere hot extruded.Microstructure evolution at different stages of deformation is investigated using the Electron Back-Scattered Diffraction (EBSD) technique. After extrusion at 1373 K (1100°C), the microstructure of ODS steels consists of a mixture of small round shape grains and larger elongated grains containing low-angle grain boundaries. Texture measurements show the appearance of the a-fiber (\110[//extrusion direction) and an increase in its intensity during the extrusion process in the larger grains. The fragmentation of the large elongated grains by Continuous Dynamic Recrystallization (CDRX) partially occurs in ODS materials depending on precipitation reinforcement. For smaller grains, plastic deformation has no effect on crystallographic orientation and grain shape, indicating a grain boundary accommodation phenomenon as the major deformation mechanism. Precipitation density not only impacts the intragranular dislocation glide activity, but also reduces CDRX kinetics in coarse grains

Using cross stamping to test Zinc sheets formability

Sheet metal formability has been studied for a half century. The sheet formability is mostly described by the Forming Limit Diagram (FLD). A prediction of this FLD is a source of interest for industrial companies. Indeed knowing the FLD of a material allows optimization of the production processes which leads to money saving. Nevertheless, the formability tests (tensile, bulge and Nakazima tests) which give the experimental FLD do not really represent the process that the sheet will undergo in industrial conditions. The paper therefore focuses on a cross stamping test. The material of interest is a Zinc sheet. FLD prediction is reported for a wide variety of metals [1] but literature about Zinc is nearly non existent. The studied Zinc sheets exhibit a highly anisotropic mechanical behaviour due to the hcp lattice structure and the inherited rolling texture. This anisotropic behaviour results in an anisotropic formability. The Zinc sheet FLD is influenced by the orientation of the rolling direction during the process. Experimental cross stamping of this material allows describing the studied material behaviour in a large range of mechanical solicitations from tensile to biaxial tension. The experimental results are compared with the finite element simulation and permit to understand where and why failures appear, which leads to a better understanding of Zinc anisotropic formability. © (2012) Trans Tech Publications

Theoretical and experimental evaluation of the formability of anisotropic zinc sheets

Formability of metal sheet has been widely studied for the past 40 years. This study leads to the well known Forming Limit Diagram (FLD) proposed by Keeler and Backhofen [1]. Such a diagram needs typical drawing and stretching experiments to be achieved. Lots of different metals have been considered as steel, aluminium, titanium or magnesium alloys [2]. Despite of the large amount of papers about sheet metal forming, few deal with Zinc sheets. The material has an anisotropic mechanical response due to its hexagonal crystallographic lattice and its microstructural texture. In the presented work, Nakazima and tensile tests have been performed for different mechanical orientations (0°, 45° and 90° angle to the rolling direction) in order to characterise this typical response. A high anisotropic behaviour has been noticed for the hardening and for the critical strains. The FLD is therefore a function of the orientation. Moreover thickness sensitivity is observed and leads to some criticisms about the plane stress assumption usually used in the FLD predictive models [3, 4]. The Modified Maximum Force Criterion (MMFC) is evaluated, and discussed. Then, this model is compared to a damage model used in [5] within an FEM formulation. © (2011) Trans Tech Publications