Dr.

Günther, Daniel

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  • Publication
    On the mechanism of binder migration in furan binder jetting of sand molds and cores
    ( 2024-03-05)
    Hartmann, Christoph
    ;
    Silberhorn, Jonas
    ;
    Erhard, Patricia
    ;
    Günther, Daniel
    Binder Jetting is a layer-based additive manufacturing process in which a printhead deposits droplets onto a preprepared layer of particles. Upon droplet impact, the binder begins to migrate and infiltrate adjacent areas away from its originally intended location. The purpose of this study is to investigate the mechanism of furan binder migration. Observing in-situ binder spreading is challenging, especially for the furan binder used in this work. The strong discoloration of the surrounding sand makes it difficult to distinguish between the printed pattern and the surrounding loose sand. For this reason, a fluorescent dye is added to the binder. A wavelength-matched laser in the binder jetting machine provides the excitation energy, and in-situ observation of binder migration becomes feasible. The results show an unexpected behavior where the actual microscopic fluid redistribution of the binder does not match the observed macroscopic measurements of other reports. It becomes clear that the migration mechanism of furan binder in sand binder jetting is strongly influenced by gaseous mass transport. To support this theory, a phenomenon called boundary zone is studied by micrographic and computed tomographic analysis. This outer shell region surrounds samples with higher binder contents and extends over a thickness of approximately 400 µm. The binder content here is significantly higher than that of the core of the specimen and its target value, demonstrating that liquid concentration equalization is not feasible. A plausible explanation is evaporation and condensation of binder, resulting in localized binder accumulation in areas of high catalyst to binder ratios. Since binder evaporation is an overlooked issue in furan binder jetting, additional experiments are performed to demonstrate the extent of evaporation. For this, particle layers are deposited on a scale, the printhead deposits binder, and the resulting mass loss is recorded. With a better understanding of binder migration, new strategies can be developed to reduce geometric deviation, improve geometric precision, and possibly allow for higher layer thicknesses in furan sand binder jetting.
  • Publication
    Production of inorganic hollow cores using sacrificial ice cores
    ( 2024)
    Locke, Christopher
    ;
    Polzin, Hartmut
    ;
    Bissels, Joshua
    ;
    Günther, Daniel
    The production of hollow cores using 3D sand printing is state-of-the-art. Hollow cores are advantageous in decoring, gas permeability and material consumption. The methodology for producing hollow cores, as in 3D sand printing, is not transferable to core shooting. However, using sacrificial ice cores (SaIC) as a tool to produce hollow cores in a core shooting process enable complex hollow structures, as in 3D sand printing. Recently there has been an effort towards more environmentally friendly production. In this regard, water glass binders are in the focus of the metal casting industry and research institutions. This work presents a new method for producing water glass-bonded hollow cores using SaIC in a core shooting process. The manufacturing principle is detailed using a bending test bar and a near-series prototype core. The bending strength of the hollow bending test bars reaches up to 300 N/cm2. Due to the hollow structure, the decoring behaviour is significantly improved compared to solid cores.
  • Publication
    Hybrid Joining of Cast Aluminum and Sheet Steel Through Compound Sand Casting and Induction Heating To Enable Thin-Walled Lightweight Structures
    ( 2024)
    Locke, Christopher
    ;
    Guggemos, Martin
    ;
    Maier, Lorenz
    ;
    Hartmann, Christoph
    ;
    Volk, Wolfram
    ;
    Günther, Daniel
    Combining different joining processes to form a hybrid process offers new manufacturing possibilities. Adding induction heating to compound sand casting with additively manufactured lost sand moulds to preheat a metallic solid insert increases the degree of the metallic bond between sheet metal and casting metal. In this study, the manufacturability of thin-walled sheet steel/cast aluminum structures with reduced cast wall thickness in sand casting is characterized for the first time. Enabling lower wall thicknesses of sheet metal/cast metal structures in sand casting shifts the current limits and offers more significant lightweight construction potential. Shear tensile, compression shear, and pullout tests characterize the mechanical properties of the joints. Light microscopic imaging of metallographic samples quantifies the compound zone intermetallic (IMC) thickness. The shear tensile test specimens fail at wall thicknesses below 10 mm in the cast material, so metallurgical bond strength characterization does not occur. Therefore, the compression shear test is used to evaluate the metallurgical bond. Sound metallic bonding with smaller cast wall thicknesses of 8, 6 and 4 mm is achieved. Pullout specimens with 3 mm cast wall thickness further investigate the force-transmitting mechanisms of metallic bond, force-fit and form-locking. It is shown that metallic bonding is the predominant mechanism for force transmission when the compound sand casting process is enhanced by induction heating.
  • Publication
    Predicting and Evaluating Decoring Behavior of Inorganically Bound Sand Cores, Using XGBoost and Artificial Neural Networks
    ( 2023-07-06)
    Dobmeier, Fabian
    ;
    Li, Rui
    ;
    Ettemeyer, Florian
    ;
    Mariadass, Melvin
    ;
    Lechner, Philipp
    ;
    Volk, Wolfram
    ;
    Günther, Daniel
    Complex casting parts rely on sand cores that are both high-strength and can be easily decored after casting. Previous works have shown the need to understand the influences on the decoring behavior of inorganically bound sand cores. This work uses black box and explainable machine learning methods to determine the significant influences on the decoring behavior of inorganically bound sand cores based on experimental data. The methods comprise artificial neural networks (ANN), extreme gradient boosting (XGBoost), and SHapley Additive exPlanations (SHAP). The work formulates five hypotheses, for which the available data were split and preprocessed accordingly. The hypotheses were evaluated by comparing the model scores of the various subdatasets and the overall model performance. One sand-binder system was chosen as a validation system, which was not included in the training. Robust models were successfully trained to predict the decoring behavior for the given sand-binder systems of the test system but only partially for the validation system. Conclusions on which parameters are the main influences on the model behavior were drawn and compared to phenomenological-heuristical models of previous works.
  • Publication
    3D Printed Sand Tools for Thermoforming Applications of Carbon Fiber Reinforced Composites - A Perspective
    ( 2021)
    Günther, Daniel
    ;
    Erhard, Patricia
    ;
    Schwab, Simon
    ;
    Taha, Iman
    Tooling, especially for prototyping or small series, may prove to be very costly. Further, prototyping of fiber reinforced thermoplastic shell structures may rely on time-consuming manual efforts. This perspective paper discusses the idea of fabricating tools at reduced time and cost compared to conventional machining-based methods. The targeted tools are manufactured out of sand using the Binder Jetting process. These molds should fulfill the demands regarding flexural and compressive behavior while allowing for vacuum thermoforming of fiber reinforced thermoplastic sheets. The paper discusses the requirements and the challenges and presents a perspective study addressing this innovative idea. The authors present the idea for discussion in the additive manufacturing and FRP producing communities.
  • Publication
    Sealing of glass with titanium by glass pressing at the softening point
    ( 2019)
    Winkler, Sebastian
    ;
    Edelmann, Jan
    ;
    Günther, Daniel
    ;
    Wieland, Sebastian
    ;
    Selbmann, Franz
    ;
    Baum, Mario
    ;
    Schubert, Andreas
    Hermetic and mechanically strong glass-to-metal seals are required for many applications in technological fields such as aerospace engineering or medical engineering. While traditional glass-to-metal bonding technologies require melting of the glass, modern technologies such as anodic bonding use glass in its solid state. In this publication, a novel glass-to-metal bonding method with process temperatures around the softening point of the glass material is investigated. A glass window (silica based crown glass B270) in a titanium (grade 5) housing is manufactured by applying compressive force to the glass in a controlled low pressure argon atmosphere. Adherence of the glass-to-metal interface is determined with a universal testing machine. Hermeticity is measured directly with either pressure gain test or helium leak test. Experiments were performed in a full factorial design with 3 different process temperatures, 3 different process forces and 3 different methods for preparing the titanium surface. The results indicate that the bonding method is capable of producing hermetic seals with leak rates below 10−8 mbar l/s. Roughening of the metal surface generally improves both hermeticity and interface strength. Bonding strength can be further improved by increasing either processes temperature or, especially for rough surfaces, process force. For improving hermeticity either processes temperature or, especially for smooth surfaces, process force must be increased. The results indicate that successful bonding of glass and titanium with the new bonding method is influenced by the effects of mechanical interlocking and chemical reactions at the material interface.