Papers by Author: Marc Seefeldt

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: The paper focuses on the multi-level character of existing or currently developed models for polycrystal deformation. A general multilevel frame is presented, which can be applied to models for the simulation of plastic anisotropy to be implemented in FE codes for the simulation of metal forming processes, or to models for the simulation of deformation textures. A short overview is presented of two-level models ranging from the full-constraints Taylor model to the crystalplasticity finite element models, including the description of a few recent and efficient models (GIA and ALAMEL). Validation efforts based on experimental cold rolling textures obtained for steel and aluminium alloys are discussed. Finally a recent three-level model which also takes the microscopic level (dislocation substructure) is discussed.