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PublicationHybrid Wire- and Powder-based Laser Metal Deposition for Modification of Local Properties of H11 Dies( 2019-05-13)
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PublicationNumerical-experimental investigation of load paths in DP800 dual phase steel during Nakajima test( 2018)Fuel efficiency requirements demand lightweight construction of vehicle body parts. The usage of advanced high strength steels permits a reduction of sheet thickness while still maintaining the overall strength required for crash safety. However, damage, internal defects (voids, inclusions, micro fractures), microstructural defects (varying grain size distribution, precipitates on grain boundaries, anisotropy) and surface defects (micro fractures, grooves) act as a concentration point for stress and consequently as an initiation point for failure both during deep drawing and in service. Considering damage evolution in the design of car body deep drawing processes allows for a further reduction in material usage and therefore body weight. Preliminary research has shown that a modification of load paths in forming processes can help mitigate the effects of damage on the material. This paper investigates the load paths in Nakajima tests of a DP800 dual phase steel to research damage in deep drawing processes. Investigation is done via a finite element model using experimentally validated material data for a DP800 dual phase steel. Numerical simulation allows for the investigation of load paths with respect to stress states, strain rates and temperature evolution, which cannot be easily observed in physical experiments. Stress triaxiality and the Lode parameter are used to describe the stress states. Their evolution during the Nakajima tests serves as an indicator for damage evolution. The large variety of sheet metal forming specific load paths in Nakajima tests allows a comprehensive evaluation of damage for deep drawing. The results of the numerical simulation conducted in this project and further physical experiments will later be used to calibrate a damage model for simulation of deep drawing processes.
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PublicationDamage Evolution in Nakajima Tests of DP800 Dual Phase Steel( 2018)In order to extend the understanding of damage evolution in sheet metal forming, standardized Nakajima tests are carried out on a DP800 dual phase steel. Sample geometries for characteristic stress-strain states are drawn in incrementing stages and their damage states are analyzed using light and scanning electron microscopy as well as micro hardness measurements. Numerically analyzed load paths are correlated with the respective damage states to allow prediction of damage evolution in deep drawing processes. The influence of anisotropy is investigated by testing samples cut at various angles to the rolling direction of the sheet material. The result of the conducted research is the understanding of interactions between load paths and damage evolution in sheet forming. These results will later be used to optimize load paths in a deep drawing process, taking into account Lode parameter and stress triaxiality to produce damage controlled parts.
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PublicationVernetzte, adaptive Produktion( 2017)
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PublicationCorrelation between the wear behavior of coated cemented carbide tools and coatings properties assessed by nano-indentations, nanoand micro-impact tests( 2016)The application of appropriate PVD coatings on cemented carbide tools plays an important role for achieving an enhanced milling performance. In the conducted investigations, two different PVD coatings with an overall thickness of about 3.5 mm were deposited on cemented carbide tools possessing a fine, or a normal grain size structure. The coated tools were applied in milling hardened steel and Ti-Al alloy. Nano-indentations supported by appropriate FEM-algorithms for the results evaluation were carried out for determining the stress-strain curves of the employed coatings and substrates. The films brittleness was assessed by conducting nano-impact tests. The fatigue behavior of the applied coatings under dynamic loads was evaluated via impact tests at various temperatures and impac t force durations similar to those developed during milling. Based on the determined coating properties, the attained tool wear results were sufficiently explained.
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PublicationImpact of clamping technology on horizontal and vertical process chain performance( 2014)Clamping technology plays a major role in optimization of holistic process chains, determines auxiliary process times, enables process performance and affects workpiece quality. In this study we evaluate three different referencing strategies in horizontal process chains and discuss the effect of fixtures on workpiece dynamics. In order to achieve an optimal process chain performance an optimization of clamping solutions is crucial.
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PublicationCoating systems for precision glass molding tools( 2008)For the production of optical components with complex or micro geometries, the most suitable technology to replace grinding and polishing is precision glass molding. In this single-step replicative process, heated glass blanks are molded into complex optical components using ultra-precise forming tools. In practice the molds have a short lifetime, which can be attributed to the great thermal and mechanical stresses and to the chemical interactions between the hot glass and the mold surface. In order to increase the lifetime, protective coatings must be applied on the molds. As a result of the unique stress profile, the standard Ti, Al or Cr-based coatings widely used for cutting tools are inappropriate for this application. Instead, Pt-based coatings are preferred.
