Additive manufacturing processes fulfill the actual market demands with regard to a high individuality and complexity of products. Hence, these processes are used nowadays in different branches (e. g. aerospace, automotive, medical industry). Further-more, a high process stability and reproducibility is requested by the user for an eco-nomic application of this technology. Up to now, these targets are reached by numer-ous test rigs on the manufacturing system which causes high resource consumption. For increasing the efficiency of metal-based additive manufacturing (AM), the em-ployees of the iwb application center Augsburg in corporation with the CADFEM GmbH and four further partners develops a simulation-based process chain (founded by the Bavarian Research Foundation). Before the production process is started, an analysis of the structural part behavior as well as a process optimization should be per-formed using the finite element analysis (FEA). Due to the complexity of the thermal-ly activated process, it is necessary to select the appropriate FE-modeling strategy for enhancing the target figures calculation efficiency and accuracy. Hence, in this work a strategy will be presented, which can map different levels of detail for the preprocess-ing definitions. These local and global descriptions can be realized by using suitable contact definitions (contact elements) to link different element meshes. Additionally, the user can select different layers of the part geometry, which should be analyzed in detail within the simulation. Also layer-specific distortions and residual stresses can be calculated while saving calculation time. Furthermore, with this approach the process history and therefore the whole part geometry can be considered in the structural cal-culations. A validation of the transient temperature field and the mechanical part properties is presented by the comparison with measured values.
The reliable prediction of local formability, for example, the performance in hole expansion test (HET), requires suitable material parameters. One proposed parameter for AHSS is true fracture strain (TFS), that is, “local ductility”. In this study TFS is measured by tensile tests and by hole expanding tests with Nakajima setup. The main outcome is that a general correlation of local ductility and hole expansion ratio from HET, as claimed for cold rolled steel grades in recent publications, cannot be confirmed, particularly when considering hot rolled steel grades. Thus, an additional parameter is introduced named “crack resistivity” which is defined as the maximum local true fracture strain at the edges of samples pierced by shear cutting and tested in the Nakajima setup. It is shown that crack resistivity correlates with the hole expansion ratio whereas a correlation with local ductility does not exist. Hence, when regarding steels of different process routes (i.e., hot and cold rolled steels grades), the prediction of local formability (in a general sense) requires measures of local ductility and crack resistivity. Consequently, for any general classification scheme, local ductility and crack resistivity must be considered besides global ductility (i.e., uniform elongation).
In order to be able to make targeted use of the advantages of AHSS grades in forming processes it is necessary to determine local material properties. Hence, in addition to edge crack sensitivity measures, such as hole expansion ratio according to ISO 16630 (HERISO), it is currently intensively discussed whether measures, that assess the local ductility of the material, should also be specified in material specifications. One representative is given by the parameter true fracture strain (TFS) that is based on the measurement of the fracture area of a cracked flat tensile specimen. By means of 16 different cold- and hot-rolled steel batches, the relation between the parameters TFS and HERISO was investigated. The outcome is that these parameters do not correlate to a satisfying degree when considering all materials and consequently are regarded as representatives of different material properties. Thus, the issue of the reliability of the measures is considered separately. For the regarded steel grades, the overall scattering of TFS, i.e. neglecting the sample orientation, is found to be 13 – 44 % and the scattering of HERISO is within 5 – 35 %. The high scattering of TFS is mainly caused by the anisotropy of the materials. In case of parallel measurements of the same batch the relative standard deviations of TFS and HERISO are on the same level for cold-rolled strips, whereas in case of hot-rolled strips the relative standard deviation of HERISO is significantly higher. By investigating the influence of the tensile specimen shape for a bainitic hot-rolled strip numerically, it could be shown that the width-to-thickness ratio, that takes the hole fracture area into account, influences the TFS value directly.
Additive Fertigungsverfahren beruhen auf dem Grundgedanken des Schicht-oder elementweisen Aufbaus.Die Fertigung der Geometrien erfolgt aus formlosen Materialien (Flüssigkeiten, Pulver) oder formneutralen Materialien (Band, Draht, Papier, Folie) mittels chemischer und/oder physikalischer Prozesse über eine CAD/CAM-Kopplung direkt aus den digital erzeugten CAD-Datenmodellen (vgl. V D I3405).
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