Lisa Ehle

Surface integrity in electrochemical machining processes: An analysis on material modifications occurring during electrochemical machining

In contrast to most other manufacturing technologies, in electrochemical machining processes only slight changes in material characteristics in the rim zone of workpieces are stated in the literature. Due to the physical active principle, no thermo-mechanically induced phase changes or the evolution of a so-called white layer were ever observed. Aside of this fact, a not inconsiderable number of smaller modifications in the rim zone were found in the past. The most common effects occurring during electrochemical machining are the generation of a passive layer on the surface by changing the local chemical composition of the material, the selective dissolution of one metallic phase, or the occurrence of flow marks. Consequently, the last two effects also change the surface roughness as the marks and dissolved phases represent ditches in the surface. Therefore, in this article, material modifications occurring during electrochemical machining are presented. Their influence on the surface integrity is exemplarily analyzed for the heat-treatable steel 42CrMo4. In addition, first steps for a correlation of material loadings that promote these changes, the so-called process signature, are made. Based on this, the influence of different machining parameters can be compared to set up rim zone properties purposefully.

Material Loadings during Electrochemical Machining (ECM) - A First Step for Process Signatures

Properties of workpieces, like residual stress in the rim zone, cannot be predicted for manufacturing technologies reproducible in advance. This lack of predictability shall be solved by a new approach, called Process Signatures. These Process Signatures will combine the material loadings forced by the manufacturing process with the change of state variables, e. g. the variation of residual stress in the surface layer. As the Process Signatures shall achieve comparability for different processes with same physical working principle, it is necessary to describe the transition from material loadings to the change of material properties in a uniform way. Consequently an energy based approach is chosen that considers these transitions by the dissipation of the several kinds of energy brought into the manufacturing process and especially in the respective working area.A first step for the development of such Process Signatures is the identification of all process specific material loadings. This paper presents several material loadings generated during the electrochemical sinking process. In a further step the contribution of the individual material loadings to the material removal process are estimated. Finally first approaches for the combination of the main material loadings and the change of material properties are presented.

Efficient Multiscale FE-FFT-Based Modeling and Simulation of Macroscopic Deformation Processes with Non-linear Heterogeneous Microstructures

The purpose of this work is the prediction of micromechanical fields and the overall material behavior of heterogeneous materials using an efficient and robust two-scale FE-FFT-based computational approach. The macroscopic boundary value problem is solved using the finite element (FE) method. The constitutively dependent quantities such as the stress tensor are determined by the solution of the local boundary value problem. The latter is represented by a periodic unit cell attached to each macroscopic integration point. The local algorithmic formulation is based on fast Fourier transforms (FFT), fixed-point and Newton-Krylov subspace methods (e.g. conjugate gradients). The handshake between both scales is defined through the Hill-Mandel condition. In order to ensure accurate results for the local fields as well as feasible overall computation times, an efficient solution strategy for two-scale full-field simulations is employed. As an example, the local and effective mechanical behavior of ferrit-perlit annealed elasto-viscoplastic 42CrMo4 steel is studied for three-point-bending tests. For simplicity, attention is restricted to the geometrically linear case and quasi-static processes.