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
    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
    Advanced Procedures for Series Production with 3D-Printed Core Packages
    ( 2023)
    Erhard, Patricia
    ;
    Hartmann, Christoph
    ;
    Li, Rui
    ;
    Volk, Wolfram
    ;
    Günther, Daniel
    The application of additive-manufactured cores and molds is of great interest for complex cast components. Nevertheless, several challenges still exist in utilizing binder jetting in the multi-step additive manufacturing process for foundry applications to its fullest extent. This contribution shows methods that facilitate the use of 3D-printed sand molds and cores in casting series applications. The binder jetting process itself is assessed from an overall process chain perspective to highlight the benefits of its application in series production. The challenges associated with automating mold cleaning for highly complex casting contours are depicted. In particular, employing the method of cleanable mold partitioning is shown to enhance the automation level of the overall process. Mold design tailored to 3D printing is demonstrated to contribute to overall cost and time savings in enhanced core packages. Topology-optimized, lightweight part designs involving complex freeform surfaces may require mold partitioning associated with laborious burr removal processes. A new approach in answer to the shortage of skilled workers in the harsh and hazardous foundry environment is shown. Implementing motion tracking technology is demonstrated to enable economical automated burr removal for minor quantities or high variant diversity in the future foundry. All the methods shown are of great importance for introducing printed core packages into series production.
  • Publication
    Removal of Stair-Step Effects in Binder Jetting Additive Manufacturing Using Grayscale and Dithering-Based Droplet Distribution
    ( 2022)
    Hartmann, Christoph
    ;
    Bosch, Lucas Valentin van den
    ;
    Spiegel, Johannes
    ;
    Rumschöttel, Dominik
    ;
    Günther, Daniel
    Binder jetting is a layer-based additive manufacturing process for three-dimensional parts in which a print head selectively deposits binder onto a thin layer of powder. After the deposition of the binder, a new layer of powder is applied. This process repeats to create three-dimensional parts. The binder jetting principle can be adapted to many different materials. Its advantages are the high productivity and the high degree of freedom of design without the need for support structures. In this work, the combination of binder jetting and casting is utilized to fabricate metal parts. However, the achieved properties of binder jetting parts limit the potential of this technology, specifically regarding surface quality. The most apparent surface phenomenon is the so-called stair-step effect. It is considered an inherent feature of the process and only treatable by post-processing. This paper presents a method to remove the stair-step effect entirely in a binder jetting process. The result is achieved by controlling the binder saturation of the individual voxel volumes by either over or underfilling them. The saturation is controlled by droplet size variation as well as dithering, creating a controlled migration of the binder between powder particles. This work applies the approach to silica sand particle material with an organic binder for casting molds and cores. The results prove the effectiveness of this approach and outline a field of research not identified previously.