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.
  • Patent
    Verfahren und Vorrichtung zum Formen einer aushärtbaren Formmasse
    ( 2023-11-22)
    Volk, Wolfram
    ;
    Günther, Daniel
    ;
    Klan, Steffen
    ;
    Erhard, Patricia
    ;
    Hartmann, Christoph
    ;
    Lechner, Philipp
    ;
    Steinlehner, Florian
    ;
    Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
    ;
    TU München
    Die Erfindung betriff eine Vorrichtung (10) und ein Verfahren zum Formen einer aushärtbaren Formmasse (202), mit:- Aufnehmen einer aushärtbaren Formmasse (202), die sich in einem flüssigen Zustand befindet, in einer Kavität (14), die von einem ersten Formteil (100) und wenigstens einem zweiten Formteil (101) eines Formwerkzeugs (12) begrenzt wird;- Erzeugen wenigstens einer ersten Relativbewegung zwischen dem ersten und dem zweiten Formteil (100, 101), so dass sich die Kavität (14) verkleinert;- Aushärtenlassen der Formmasse (202).
  • 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
    Influence of the Resin System and Sand Type on the Infiltration of 3D-Printed Sand Tools
    ( 2023)
    Erhard, Patricia
    ;
    Taha, Iman
    ;
    Günther, Daniel
    Binder jetting is a highly productive additive manufacturing (AM) method for porous parts. Due to its cost-effectiveness, it is used for large components and quantities ranging from prototyping to series production. Post-processing steps like sintering or infiltration are common in several applications to achieve high density and strength. This work investigates how 3D-printed sand molds can be infiltrated with epoxy resins without vacuum assistance to produce high-strength molds for thermoforming applications. Specimens 3D-printed from different sand types are infiltrated with resins of different viscosity and analyzed for infiltration velocity and depth. The infiltration velocities corresponded well with the correlation described in Washburn’s equation: The resins’ viscosities and the saturation level were decisive. Amongst the investigated sand types commonly used in foundries, sand type GS19 was found most suitable for infiltration. However, the sand type proved to be a less relevant influencing factor than the resins’ viscosities and quantities applied. Infiltration of topology-optimized 3D-printed sand tools up to a wall thickness of 20 mm for thermoforming applications was found to be feasible.
  • Publication
    Compound Casting of Aluminum with Sheet Steel in 3D Sand Casting Using an Inductive Heating System
    ( 2023)
    Locke, Christopher
    ;
    Guggemos, Martin
    ;
    Gruber, Maximilian
    ;
    Maier, Lorenz
    ;
    Mayr, Lukas
    ;
    Weiß, Tony
    ;
    Volk, Wolfram
    ;
    Günther, Daniel
    Compound casting is a process in which a single component is made from two metallic materials, such as aluminum and steel. Solid-liquid bimetallic compounds can be produced by suitable process control. This technology can reduce the number of joining processes, and the specific properties of the respective metal component can be used for specifically designed product properties, for example, where lightweight and high strength are needed. This paper presents an experimental methodology for producing a purely material-bonded bimetallic joint from cast aluminum and zinc-coated sheet steel in 3D sand casting using an inductive heating system. The process-related temperature characterisation in the compound zone is described using a heating test rig and temperature measurements. It shows that inductive preheating can only produce a material bond between the aluminum casting and the coated steel sheet. Shear tensile tests showed strengths between 15 MPa and 22 MPa. Laser surface pre-treatment using laser ablation cutting on the coated steel sheet was carried out to investigate the benefit of possible microform-locking. The results show a strength-reducing influence on the tensile shear tests. Micrographs showed the formation of Al4.5FeSi and Al7Fe2Si, as well as the formation of other undefined intermetallic phases. The thickness of the compound zone is 10 µm.
  • 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
    Innovative Salzkerne für komplexe Geometrien
    ( 2022-08)
    Erhard, Patricia
    ;
    Günther, Daniel
    Das Fraunhofer IGCV und die K+S Aktiengesellschaft haben in einem gemeinsamen F&E-Projekt eine übergreifende nachhaltige Prozesskette für das Schießen von Salzkernen zur Darstellung innen liegender Kavitäten im Aluminiumguss entwickelt. Im Vergleich zu konventionellen Sandkernen nutzt das vorgestellte alternative Verfahren ein Salzgemisch zur Herstellung binderfreier Salzkerne, welches mit Wasser rückstandsfrei ausgespült werden kann. Die dabei entstehende wertstoffhaltige Salzlösung kann, so das erarbeitete Konzept, von der K+S Aktiengesellschaft recycelt werden. Die neue Technologie überzeugt durch hohe Geometriefreiheit und besonders durch die erleichterte Entkernbarkeit der Bauteile.
  • 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.