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Metal plasticity

Modeling and simulation of anisotropic hardening during metal forming process with application to springback



Work hardening of materials plays an important role in metal forming processes. One example is springback in sheet metal components after forming. Springback is a serious problem in the manufacturing of sheet metal components. In numerical simulations of deformation processes, especially in the area of sheet metal forming, purely phenomenological models are insufficient to account for the effect of strain path changes on the forming process. As a consequence new material models have to be developed, which for example consider the anisotropy development during the forming process as well as the initial anisotropy of the material. Parallel to the typical anisotropic material model "kinematic hardening" which results in the materials shift of the centre of the elastic region in the direction of the plastic flow. Furthermore a new type of anisotropic model which accounts for arbitrary directional hardening, taking the microstructure of the material into consideration is developed. To determine the material parameters and validate the model, suitable testing methods are to be developed for a variety of deformation circumstances.



Micromechanically motivated phenomenological modeling of induced flow anisotropy and its application to sheet forming processes with complex strain path changes



Sheet metal forming involves large strains and severe strain path changes. In many metals large plastic strains lead to the development of persistent dislocation structures resulting in strong flow anisotropy. Iin the case of a strain path change it manifests trough very different stress-strain responses, e.g., a drop of the yield stress (Bauschinger effect) after a load reversal and an increase of the yield stress after an orthogonal strain path change (cross hardening). The Bauschinger effect can be modeled with the help of kinematic hardening. However, the usage of the back stress leads automatically to a drop of the yield stress after an orthogonal strain path change. Consequently, the concept of the combined isotropic-kinematic hardening used in the conventional plasticity has to be extended in order to better simulate processes with complex strain path changes. In this work a macroscopic material model is presented whose structure is motivated by polycrystalline modeling that takes into account the evolution of polarized dislocation structures on the grain level and crystallographic texture. The model considers, besides the movement of the yield surface and its proportional expansion, the changes of the yield surface shape (distortional hardening) and is able to better describe the induced anisotropic behaviour with regard to complicated loading histories. The model is applied to sheet forming processes with complex strain path changes.



A thermomechanical material model for the behaviour of aluminium alloys during extrusion



Extrusion is a manufacturing process where a material is pressed through a die. It is used to produce long formed objects of constant cross sections from different materials such as aluminium, copper, stainless steel and various types of plastic. Mostly rods, cables and pipes are made from copper and steel, whereas complex profiles are produced using aluminium. This work focuses on a direct extrusion process with aluminium alloys of the series 6000 (Al-Mg-Si) and 7000 (Al-Zn-Mg).



Modeling and Simulation of Chip Formation in High Speed Cutting



High speed cutting is a process of high interest in modern production engineering. Besides the advantage of high productivity, a decrease of cutting forces can be observed as a result of the increase of cutting speed. This phenomenon is connected to the occurrence of localized adiabatic shear bands, as these are the main mechanism of deformation and chip formation. In order to take advantage of the potential of the high speed cutting process, the modeling and simulation of the working mechanisms in a finite element frame is essential. The fact of high deformation speed and the mechanism of localized shear bands forces the implementation of a temperature and rate dependent material model. The key problem in the context of FEM simulation of the high speed cutting process is the strong mesh dependence on the developing of localized shear bands, as orientation and characteristic length of the mesh force direction respectively expansion of localization.



Modeling of electromagnetic sheet metal forming processes



Electromagnetic forming (EMF) is a dynamic, high strain-rate forming method in which strain-rates of are achieved. In this process, deformation of the work piece is driven by the interaction of an induced current in the work piece with a magnetic field generated by a coil adjacent to the work piece. EMF is just one of a number of high deformation-rate forming methods which offer certain advantages over other forming methods such as increase in formability for certain kinds of materials, reduction in wrinkling, reduced tooling costs, and many others.