P 1247 - Development of incremental microforming for the production of functional samples of metallic bipolar plates

P 1248 - Qualification of the instrumented indentation test for the determination of characteristic values for high-strength steels with welds The use of high-strength steels in automotive bodywork has increased significantly in recent years. These include dual-phase and complex-phase steels, which combine the advantages of different microstructural constituents, as well as TRIP (TRansformation Induced Plasticity) and manganese-boron steels, which combine very good forming properties with high strength due to martensite formation during forming. TWIP (Twinning Induced Plasticity) steels achieve similar effects through forced twinning. The reasons for using these steels lie in the potential of these materials to reduce weight and costs while at the same time increasing passenger safety. Due to the suitability of these steels for welding in principle, the classic joining processes in car body construction such as low-cost and effective resistance spot welding, gas metal arc welding (GMAW) or laser welding are used. However, challenges sometimes arise, for example due to structural changes in the joints that lead to unwanted hardening or softening. In this project, a process is being developed with which the local material properties of materials typically used in automotive engineering and their joints can be determined. Relevant characteristic values are primarily the stress-strain behaviour of the various zones of a welded joint; relevant zones in turn are, in addition to the base material, the heat-affected zone and the weld metal. For this purpose, the method of instrumented indentation testing is being further developed for use with high-strength steels. Firstly, tensile tests are carried out on simple base material geometries. Subsequently, the optical strain field measurement is carried out on heavily waisted, welded tensile test specimens. The purpose of the waisted shape is to be able to generate the HAZ over the entire cross-section of the specimen using WPS and also to induce strains in the relevant areas during the test. The evaluation method used in the project, which is based on non-linear regression models in the form of artificial neural networks, enables the strength behaviour of the material to be predicted on the basis of the measured force indentation displacement data. Publication: 2020 Authors: Prof. Dr.-Ing. M. Rethmeier, E. Javaheri M.Sc., Dr.-Ing. M. Biegler

P 1249 - Increasing the wear resistance of forging tools by using an intelligent hot-work tool steel in combination with a material-specific nitriding treatment Forging tools made of an intelligent hot-work tool steel developed by alloying have good wear resistance in thermomechanically highly stressed tool areas. This is due to an alloy-specific mechanism of cyclic surface layer hardening, which ensures continuous protection of these wear-prone areas. However, due to the varying stresses in the different tool areas, the increased wear resistance is locally limited. The aim of the current research project was to increase the wear resistance of forging tools by using this intelligent hot-work tool steel in combination with a material-specific nitriding treatment. In order to realise global wear protection, forging tools made of an intelligent hot-work tool steel were subjected to a surface layer treatment by nitriding, so that an increase in wear resistance was ensured in areas subject to low thermomechanical stress. In addition to characterising the transformation and material behaviour, the main focus of the research project was on developing a nitriding treatment adapted to the intelligent hot-work tool steel and determining the performance of the surface layer-treated forging tools in model tests and industrial practice. Cyclic re-hardening of the tool surface layer was demonstrated in series forging tests after removal of the nitriding zone. Areas subject to insufficient thermomechanical stress were protected from wear by the nitriding zone. In an industrial environment, the nitrided tools made from the modified hot-work tool steel predominantly achieved tool life comparable to the reference condition. Crack formation in the tool contour was considered to be the main cause of failure. The results obtained contribute to the efficient utilisation of the mechanism of cyclic surface hardening in combination with a nitriding zone. Small and medium-sized steel production and processing companies gain a new knowledge base for the development of innovative materials and tools adapted to forming technology processes, gain the opportunity to place new products on the market and benefit from the results in the form of an effective wear protection measure to increase the tool life of hot forging tools. Publication: 2021 Authors: Prof. Dr.-Ing. B.-A. Behrens, Prof. Dr.-Ing. H. J. Maier

P 1243 - Method development for long-term prediction of bonded joints under combined temperature and media exposure The prediction of the ageing behaviour of adhesives is one of the major unresolved issues in adhesive bonding technology. Understanding the long-term behaviour under hygrothermo-mechanical stresses is an essential step towards confidence in adhesive bonding technology and thus to its increased use. However, most approaches to describing the effects of the state of the art only provide a rough estimate of the service life. The water absorption of the adhesive is analysed by means of gravimetric tests. This results in the diffusion rate and the saturation mass as a function of temperature and relative humidity. The water sorption of the adhesive layer is modelled using the FICK diffusion model. The influences of humidity and temperature on the saturation state are taken into account. The dependence of the diffusion coefficient on temperature is modelled using an ARRHENIUS-type approach. The influence of combined hygro-thermo-mechanical (htm) stress on the short and long-term behaviour of the bonded joints is investigated using perforated head tension and thick tensile shear specimens. Firstly, the quasi-static strength of the bonded joint is tested under different constant htm loads. The influence of moisture and temperature on the creep rupture strength is then systematically analysed using creep tests under different climatic conditions. The adhesive layer is modelled with an adhesive layer replacement model, which consists of a rheological model with damage. The rheological model describes the linear viscoelastic behaviour and the damage model describes the failure due to htm stress. The influences of temperature and moisture on the viscoelasticity are recorded with the time-temperature-water concentration shift. Multiaxiality during failure is taken into account by defining an equivalent stress. The concept developed for the long-term prediction of structural bonded joints is applied to the sheet metal tensile shear specimen as part of a case study. The FE simulation is carried out with LS-DYNA, utilising the analogy of the heat conduction and diffusion problem. The case study shows how the service life is predicted on the basis of the transient FE simulation. The influences of the viscoelastic properties on the failure behaviour and the resulting fracture times are shown. Publication: 2021 Authors: Prof. Dr.-Ing. G. Meschut, Prof. Dr.-Ing. A. Matzenmiller

P 1242 - Local concept for the design of elastic bonded joints As part of this research project, a method for direct high-resolution deformation measurement in bonded joints was developed to support the classification of existing design methods for bonded joints and to open up the possibility of new ones. At the Institute of Joining and Welding Technology at the TU Braunschweig (ifs), the new method based on in-situ computer tomography was used and further developed in such a way that measurements can be carried out on various bonded joints under different mechanical stresses. At the same time, the Laboratory for Materials and Joining Technology at Paderborn University (LWF) carried out the modelling and verification of the measurement method. Tensile tests with gradually increasing loads were carried out in coordination between the participating research institutes on adhesive substance samples and inhomogeneously stressed sample geometries in the form of simply overlapped tensile shear and head tensile samples. In-situ computed tomography measurements and measurements using grey value correlation for selected load levels were carried out after relaxation had ceased. From the detected movements of the particles, three-dimensional displacement fields corresponding to the respective load level could be calculated using a suitable evaluation algorithm. A comparison of the displacements measured on the surface using grey value correlation and the three-dimensional displacement fields calculated from in-situ computer tomography measurements served to verify the generated FE models. As a suitable simulation model, the MARLOW model provided the best matches for the numerical modelling. The parameterisation was carried out using the stress and longitudinal strain values determined from tensile tests as well as the transverse contraction coefficient or, alternatively, the transverse strain curve. The parameter sets and simulation models determined and validated by the research institutes for analysing bonded joints can be used directly by SMEs, such as manufacturers of software for calculating bonded joints. In general, confidence in adhesive bonding technology is strengthened by the in-depth understanding of adhesive behaviour under certain influences and improved design methods through simulation, which strengthens the companies active in this field of technology. Publication: 2021 Authors: Prof. Dr.-Ing. K. Dilger, Prof. Dr.-Ing. G. Meschut