Marc Seefeldt

Texture Evolution during Cold Rolling of Low and High Carbon Steel. Measurement and Simulation

This paper presents simulations of the texture development during cold rolling of fully pearlitic steel. In order to investigate the importance of including microstructural aspects into texture simulations the calculations were carried out with the FC Taylor Pearlite model which takes the lamellar microstructure into account and with the FC Taylor and LAMEL model which are dedicated for one phase materials. The results are compared with those for ULC steel. For both materials the LAMEL model results are in good agreement with experimental data, while for the pearlitic steel, the FC Taylor Pearlite model improves the predictions only slightly with respect to the FC Taylor model.

Mesoscopic EBSD Analysis and Mesomechanical Behavior of Ridging or Roping in AA6XXX Alloys

Aluminium alloys for car body manufacturing often show a specific type of band-shaped surface roughening upon stretching, called “ridging” or “roping”. Experimental research as well as modelling attempts have indicated that the evolving surface roughness profiles cannot be understood based on banding of individual surface texture components, like cube or Goss, only. Therefore, it is proposed to study banding on the “mesoscopic” level of texture banding rather than on the “microscopic” one of orientation banding. In mechanical terms, such patterning in the texture would lead to patterning in, for instance, the Lankford coefficient (r-value), so that the mechanical response can be calculated on an intermediate length scale. The present contribution presents a method for calculating r-value patterning from Electron Backscatter diffraction (EBSD) orientation maps. In a first test case of a strongly ridging AA6xxx sheet, indeed patterning in the r-value is found which corresponds to reported patterning of the surface roughness.

A New Analytical Approach for the Velocity Field in Rolling Processes and Its Application in Through-Thickness Texture Prediction

A new analytical model is presented that expresses kinematically admissible velocity fields in rolling processes. Opposed to conventional streamline approaches, the current model does not force the material to flow along the prescribed lines, but introduces a new coordinate that is constant over these lines, to prescribe a fixed component of the velocity in the rolling direction as a function of that coordinate and the coordinate along the rolling direction. The interaction between the rolls and the surface is incorporated in the model via two scalar parameters which depend on the friction conditions between the roll and the sheet, and the properties of rolled material. The scalar parameters can be tuned with experimental observations of deformation flow across the thickness. The modelled material flow does not reveal significant deviation from the one calculated by streamlines. The obtained analytical expressions for the velocity gradient tensor components combined with polycrystal plasticity models enables the prediction of the through-thickness texture evolution for various friction conditions.

Neutron and X-ray diffraction analysis of residual stresses in cold-rolled pearlitic steel sheet

This paper reports about a combined neutron and X-ray diffraction study on the residual stresses in the ferrite matrix of cold-rolled fully pearlitic steel sheet. Neutron diffraction revealed compressive residual phase microstresses of about – 500 MPa in rolling direction. However, even in normal direction there are significant tensile residual microstresses, indicating that the morphology of the lamellar microstructure cannot be properly described as a “sandwich structure”. Neutron diffraction was also used during an in-situ tensile test to estimate the microstress level in the cementite phase. The combination of neutron and X-ray diffraction allows to separate, near the surface, the residual phase microstresses from the macrostresses. The latter are also important in rolling direction and imply some risk of undesirable shape changes after forming operations.

On the correlation of meso-scale local texture and roping profile in AA6xxx sheet alloys

Roping as a heterogeneous plastic deformation is generally attributed to the occurrence of the meso-scale clustering of grains with similar orientations. Large-scale electron backscattered diffraction (EBSD) orientation maps are customarily used to correlate the orientation topography with the roping profile. The most common way of investigating this phenomenon is to extract the predominant texture components and then to correlate them with the roping profile, since grains belong to different texture components lead to different plastic responses. Instead of using a microscopic representative volume element in the length scale of the grain size, the present work proposes a moving window mechanical model to use a representative volume element of the meso-scale, corresponding to a grain cluster, to simulate roping. For a tensile test in the transverse direction, a quantitative prediction of surface roping profile can be obtained. For an artificial EBSD orientation map, the proposed model can yield both roping wavelength and amplitude.

Online Use of Physically Based Plasticity Models for Steady State Cold Rolling Processes

A procedure has been developed to incorporate computationally costly physically based crystal plasticity models to calculate texture and anisotropy for steady state forming processes online. When using these models, at every point in the deformed zone, an average and a nonlinear solution procedure for stresses and/or strains in all these grains is required. The online calculation cost is avoided by offline creating a database with texture and anisotropy data for all possible deformation modes of the process. The case studied is a cold rolling process, but can easily be extended to any type of forming process, when the deformation field is known in advance. Textures and anisotropy data are predicted using a viscoplastic self-consistent model, but the method is suitable for any kind of crystal plasticity model. Single crystal plastic parameters, such as the critical resolved shear stress, the single crystal hardening parameters, and the strain-rate sensitivity, have been calibrated based on mechanical tests by means of a direct search simplex algorithm. The online calculated deformation history is compared to the histories stored in the database and the best match is selected. The deformation history is divided in two zones, the one before the neutral point where forward shearing occurs and the one after the neutral point where backward shearing occurs. One online deformation generation and selection procedure requires 0.005xa0s of CPU time for a database with a division in deformation gradients fine enough to accurately cover all deformations. The method allows calculating yield surfaces at any point in space based on microstructural effects modeled by crystal plasticity, without incremental material updating and necessity to define a kinematic and isotropic hardening, which makes the method suitable for fast models to calculate rolling forces and torques online.

Modeling of distortions after carburization and quenching processes of large gears

A new finite element model is developed to predict the deformations, stresses, phase compositions and carbon concentration gradients that arise as a consequence of the physical processes involved in a carburization and quenching process of a large steel gear. Firstly, the diffusion of carbon at elevated temperatures in the austenitic range is studied in a diffusion model. Secondly, the calculated carbon concentration distribution is used as an input for a model that couples the thermal, metallographic and mechanical effects during the quenching process and calculates the evolution of the temperature, phase composition and deformation history at any point in the gear. Two effects typical for oil quenching of large gears are incorporated in the model. The first is the influence of the gears own weight while hanging on chains before, during and after entering the quench bath. The second is the three-dimensional effect that it takes time between the moment the gear enters the oil quenching bath and the moment when the gear is fully immersed. The non-uniform temperature distribution over the gears axis causes a non-homogeneous plastic deformation. A diffusion-thermo-metallo-mechanical model that takes these effects into account is compared with a model that does not. The results show that these effects should be incorporated.

Experimental Study on the Influence of Grain Boundaries on the Subdivision Behaviour of Al-3%Mg Polycrystals during Cold Deformation Using Electron Backscatter Diffraction

The substructure of a single grain in an electron backscatter diffraction (EBSD) data map is studied, focusing on the influence of the grain boundary configuration on the misorientation to the average grain orientation of data points close to the grain boundary. For most grain boundary segments a certain degree of linking between the misorientations to the average orientation of the grain exists and large deviations from the average orientation of the grain are observed close to the triple junctions of the boundary segments. Changes of the misorientation over one boundary segment are analysed and possible explanations for these variations are discussed. It is suggested that the variations of the misorientation over the boundary segment can be attributed to the requirements of stress equilibrium and strain compatibility. Also the tendency of the grain boundary to lower its surface energy might have a significant influence on the misorientation profile and therefore on the subdivision behaviour of the grains.

Orientation Fragmentation in Copper, Nickel and Aluminum: A Comparative Study of the Nucleation Process

An attempt to model the nucleation of fragment boundaries during cold plastic deformation of f.c.c. metals is presented. The paper focuses on intrinsic nucleation in the grain interior due to elementary processes on the dislocation level. Since orientation fragmentation seems to be linked to slip banding and the underlying mechanisms should be the same, the model is based on the elementary process of double cross-slip. Simulations were carried out for Cu, Ni and Al. Fragment boundary spacings and misorientations could be predicted in reasonable agreement with experiment for Cu. For Ni, comparable results were obtained, when a stacking fault energy at the lower end of the range of literature data was chosen. The resulting rate equation for the generation of partial disclinations as carriers of orientation fragmentation can be implemented into an earlier model for the coupled substructure and texture development during cold plastic deformation.