Now showing 1 - 10 of 11
  • Publication
    Characterization of Ti-6Al-4V Fabricated by Multilayer Laser Powder-Based Directed Energy Deposition
    ( 2022)
    Ávila Calderón, Luis Alexander
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    Graf, Benjamin
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    Rehmer, Birgit
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    Petrat, Torsten
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    Skrotzki, Birgit
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    Laser powder-based directed energy deposition (DED-L) is increasingly being used in additive manufacturing (AM). As AM technology, DED-L must consider specific challenges. It must achieve uniform volume growth over hundreds of layers and avoid heat buildup of the deposited material. Herein, Ti-6Al-4V is fabricated using an approach that addresses these challenges and is relevant in terms of transferability to DED-L applications in AM. The assessment of the obtained properties and the discussion of their relationship to the process conditions and resulting microstructure are presented. The quality of the manufacturing process is proven in terms of the reproducibility of properties between individual blanks and with respect to the building height. The characterization demonstrates that excellent mechanical properties are achieved at room temperature and at 400 C.
  • Publication
    Microstructure of Inconel 718 parts with constant mass energy input manufactured with direct energy deposition
    ( 2019)
    Petrat, Torsten
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    Graf, Benjamin
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    The laser-based direct energy deposition (DED) as a technology for additive manufacturing allows the production of near net shape components. Industrial applications require a stable process to ensure reproducible quality. Instabilities in the manufacturing process can lead to faulty components which do not meet the required properties. The DED process is adjusted by various parameters such as laser power, velocity, powder mass flow and spot diameter, which interact with each other. A frequently used comparative parameter in welding is the energy per unit length and is calculated from the laser power and the velocity in laser welding. The powder per unit length comparative parameter in the DED process has also be considered, because this filler material absorbs energy in addition to the base material. This paper deals with the influence of mass energy as a comparative parameter for determining the properties of additively manufactured parts. The same energy per unit length of 60 J/mm as well as the same powder per unit length of 7.2 mg/mm can be adjusted with different parameter sets. The energy per unit length and the powder per unit length determine the mass energy. The laser power is varied within the experiments between 400 W and 900 W. Energy per unit length and powder per unit length are kept constant by adjusting velocity and powder mass flow. Using the example of Inconel 718, experiments are carried out with the determined parameter sets. In a first step, individual tracks are produced and analyzed by means of micro section. The geometry of the tracks shows differences in height and width. In addition, the increasing laser power leads to a higher dilution of the base material. To determine the suitability of the parameters for additive manufacturing use, the individual tracks are used to build up parts with a square base area of 20×20 mm². An investigation by Archimedean principle shows a higher porosity with lower laser power. By further analysis of the micro sections, at low laser power, connection errors occur between the tracks. The results show that laser power, velocity and powder mass flow must be considered in particular, because a constant mass energy can lead to different geometric as well as microscopic properties.
  • Publication
    Heat treatment of SLM-LMD hybrid components
    ( 2019) ;
    Düchting, Jan
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    Petrat, Torsten
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    Graf, Benjamin
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    Additive manufacturing is no longer just used for the production of prototypes but already found its way into the industrial production. However, the fabrication of massive metallic parts with high geometrical complexity is still too time-consuming to be economically viable. The combination of the powder bed-based selective laser melting process (SLM), known for its geometrical freedom and accuracy, and the nozzle-based laser metal deposition process (LMD), known for its high build-up rates, has great potential to reduce the process duration. For the industrial application of the SLM-LMD hybrid process chain it is necessary to investigate the interaction of the processes and its effect on the material properties to guarantee part quality and prevent component failure. Therefore, hybrid components are manufactured and examined before and after the heat treatment regarding the microstructure and the hardness in the SLM-LMD transition zone. The experiments are conducted using the nickel-based alloy Inconel 718.
  • Publication
    3D laser metal deposition: Process steps for additive manufacturing
    ( 2018)
    Graf, Benjamin
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    Marko, Angelina
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    Petrat, Torsten
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    Laser metal deposition (LMD) is an established technology for two-dimensional surface coatings. It offers high deposition rates, high material flexibility, and the possibility to deposit material on existing components. Due to these features, LMD has been increasingly applied for additive manufacturing of 3D structures in recent years. Compared to previous coating applications, additive manufacturing of 3D structures leads to new challenges regarding LMD process knowledge. In this paper, the process steps for LMD as additive manufacturing technology are described. The experiments are conducted using titanium alloy Ti-6Al-4V and Inconel 718. Only the LMD nozzle is used to create a shielding gas atmosphere. This ensures the high geometric flexibility needed for additive manufacturing, although issues with the restricted size and quality of the shielding gas atmosphere arise. In the first step, the influence of process parameters on the geometric dimensions of single weld beads is analyzed based on design of experiments. In the second step, a 3D build-up strategy for cylindrical specimen with high dimensional accuracy is described. Process parameters, travel paths, and cooling periods between layers are adjusted. Tensile tests show that mechanical properties in the as-deposited condition are close to wrought material. As practical example, the fir-tree root profile of a turbine blade is manufactured. The feasibility of LMD as additive technology is evaluated based on this component.
  • Publication
    Porosity of LMD manufactured parts analyzed by Archmimedes method and CT
    ( 2018)
    Marko, Angelina
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    Raute, Julius
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    Linaschke, Dorit
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    Graf, Benjamin
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    Pores in additive manufactured metal parts occur due to different reasons and affect the part quality negatively. Few investigations on the origins of porosity are available, especially for Ni-based super alloys. This paper presents a new study to examine the influence of common processing parameters on the formation of pores in parts built by laser metal deposition using Inconel 718 powder. Further, a comparison between the computed tomography (CT) and the Archimedes method was made. The investigation shows that CT is able to identify different kinds of pores and to give further information about their distribution. The identification of some pores as well as their shape can be dependent on the parameter setting of the analysis tool. Due to limited measurement resolution, CT is not able to identify correctly pores with diameters smaller than 0.1 mm, which leads to a false decrease in overall porosity. The applied Archimedes method is unable to differentiate between gas porosity and other kinds of holes like internal cracks or lack of fusion, but it delivered a proper value for overall porosity. The method was able to provide suitable data for the statistical evaluation with design of experiments, which revealed significant parameters on the formation of pores in LMD.
  • Publication
    Finite element analysis of in-situ distortion and bulging for an arbitrarily curved additive manufacturing directed energy deposition geometry
    ( 2018) ;
    Marko, Angelina
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    Graf, Benjamin
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    With the recent rise in the demand for additive manufacturing (AM), the need for reliable simulation tools to support experimental efforts grows steadily. Computational welding mechanics approaches can simulate the AM processes but are generally not validated for AM-specific effects originating from multiple heating and cooling cycles. To increase confidence in the outcomes and to use numerical simulation reliably, the result quality needs to be validated against experiments for in-situ and post process cases. In this article, a validation is demonstrated for a structural thermomechanical simulation model on an arbitrarily curved Directed Energy Deposition (DED) part: at first, the validity of the heat input is ensured and subsequently, the model's predictive quality for in-situ deformation and the bulging behaviour is investigated. For the in-situ deformations, 3D-Digital Image Correlation measurements are conducted that quantify periodic expansion and shrinkage as they occur. The results show a strong dependency of the local stiffness of the surrounding geometry. The numerical simulation model is set up in accordance with the experiment and can reproduce the measured 3 dimensional in-situ displacements. Furthermore, the deformations due to removal from the substrate are quantified via 3D scanning, exhibiting considerable distortions due to stress relaxation. Finally, the prediction of the deformed shape is discussed in regards to bulging simulation: to improve the accuracy of the calculated final shape, a novel extension of the model relying on the modified stiffness of inactive upper layers is proposed and the experimentally observed bulging could be reproduced in the finite element model.
  • Publication
    3D laser metal deposition in an additive manufacturing process chain
    ( 2017)
    Graf, Benjamin
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    ;
    Laser metal deposition (LMD) is an established technology for two-dimensional surface coatings. It offers high deposition rates, high material flexibility and the possibility to deposit material on existing components. Due to these features, LMD has been increasingly applied for additive manufacturing of 3D structures in recent years. Compared to previous coating applications, additive manufacturing of 3D structures leads to new challenges regarding LMD process knowledge. In this paper, the process chain for LMD as additive manufacturing technology is described. The experiments are conducted using titanium alloy Ti-6Al-4V and Inconel 718. Only the LMD nozzle is used to create a shielding gas atmosphere. This ensures high geometric flexibility, although issues with the restricted size and quality of the shielding gas atmosphere arise. In the first step, the influence of process parameters on the geometric dimensions of single weld beads is analysed based on design of experiments and statistical evaluation. The results allow adjusting the weld bead dimensions for the specific component geometry. In the second step, features of a 3D build-up strategy for high dimensional accuracy are discussed. For this purpose, cylindrical specimens consisting of more than 200 layers are built. Welding of multiple layers on top of each other leads to heat accumulation. Consequently, the molten pool is increased and weld bead height and width are changed. Furthermore, cooling times are prolonged. The build-up strategy has to be adjusted to deal with these issues. Process parameters, travel paths and cooling breaks between layers are varied. Temperatures during the deposition process are measured with pyrometer and thermography. The specimens are analysed with metallurgic cross sections, x-ray and tensile test. Tensile tests show that mechanical properties in the as-deposited condition are close to wrought material. The results are used to design guidelines for a LMD build-up strategy for complex components. As reality test, parts of a gas turbine burner and a turbine blade are manufactured according to these build-up strategies. Build-up rate, net-shape and microstructure of these demonstrative components are evaluated. This paper is relevant for industrial or scientific users of LMD, who are interested in the feasibility of this technology for additive manufacturing.
  • Publication
    Statistische Analyse der Schweißraupengeometrie von Ti6Al4V beim Laserbeschichten
    ( 2017)
    Marko, Angelina
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    Graf, Benjamin
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    Die Bedeutung des Laser-Pulver-Auftragsschweiß-Prozesses hat in den letzten Jahren enorm zugenommen, da er vielfältig eingesetzt werden kann. Ein Beispiel hierfür ist die Reparatur von Turbinenschaufeln. Um eine hohe Qualität und Zuverlässigkeit hierbei zu gewährleisten, ist eine Anpassung der Schweißraupengeometrie an die jeweilige Reparaturaufgabe notwendig. Die Schweißraupengeometrie beeinflusst die metallurgische Verbindung und den Grad der Aufmischung sowie eventuelle Anbindungsfehler. Aus diesem Grund ist es wichtig, die Effekte der unterschiedlichen Parameter auf die Spurgeometrie zu kennen. Ein bewährtes Werkzeug hierfür ist die statistische Versuchsplanung (DoE). Hierbei kann der Nutzer jedoch zwischen einer Vielzahl von Versuchsplänen wählen. Es wird davon ausgegangen, dass bei größeren Versuchsräumen ein höherer Informationsgewinn erfolgt. Aus diesem Grund wird in dieser Arbeit ein vollfaktorieller Versuchsplan, der in fünf Stufen variiert wird, mit einem zentral zusammengesetzten Versuchsplan (CCD) verglichen. Außerdem werden Grenzen des Prozesses aufgezeigt und der Versuchsraum entsprechend durch Eingrenzungen angepasst. Die Ergbnisse zeigen, dass sowohl der vollfaktorielle Versuchsplan als auch der zentral zusammengesetzte Versuchsplan die wichtigsten Effekte aufzeigt. Lediglich die Effektstärke kann durch den CCD-Versuchsplan nicht eindeutig bestimmt werden. Für den industriellen Einsatz wird daher unter Berücksichtigung der Kosteneffizienz der Einsatz eines CCD-Versuchsplanes empfohlen.
  • Publication
    Application of D-optimum experimental designs in consideration of restrictions for laser metal deposition
    ( 2017)
    Marko, Angelina
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    Graf, Benjamin
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    The process of laser metal deposition can be applied in many ways. Mostly, it is relevant to coating, for repair welding and for additive manufacturing. To increase the effectiveness and the productiveness, a good process understanding is necessary. Statistical test planning is effectual and often used for this purpose. For financial and temporal reasons, a restriction of the test space is reasonable. In this case, it is recommended to use a D-optimal experimental design which is practically applied to extend existing test plans or if process limits are known. This paper investigates the applicability of a D-optimum experimental design for the laser metal deposition. The results are compared to the current results of a full factorial test plan. Known restrictions are used for the limitation of the test space. Ti6Al4 is utilized as substrate material and powder. Comparable results of the D-optimal experimental design and of the full factorial test plan can be demonstrated. However, 80 % of time can be saved by the experimental procedure. For this reason, the application of D-optimal experimental design for laser metal deposition is recommend.
  • Publication
    Laser metal deposition as repair technology for a gas turbine burner made of Inconel 718
    ( 2016)
    Petrat, Torsten
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    Graf, Benjamin
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    Maintenance, repair and overhaul of components are of increasing interest for parts of high complexity and expensive manufacturing costs. In this paper a production process for laser metal deposition is presented, and used to repair a gas turbine burner of Inconel 718. Different parameters for defined track geometries were determined to attain a near net shape deposition with consistent build-up rate for changing wall thicknesses over the manufacturing process. Spot diameter, powder feed rate, welding velocity and laser power were changed as main parameters for a different track size. An optimal overlap rate for a constant layer height was used to calculate the best track size for a fitting layer width similar to the part dimension. Deviations in width and height over the whole build-up process were detected and customized build-up strategies for the 3D sequences were designed. The results show the possibility of a near net shape repair by using different track geometries with laser metal deposition.