Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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Characterization of Ti-6Al-4V Fabricated by Multilayer Laser Powder-Based Directed Energy Deposition

2022 , Ávila Calderón, Luis Alexander , Graf, Benjamin , Rehmer, Birgit , Petrat, Torsten , Skrotzki, Birgit , Rethmeier, Michael

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.

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Microstructure of Inconel 718 parts with constant mass energy input manufactured with direct energy deposition

2019 , Petrat, Torsten , Brunner-Schwer, Christian , Graf, Benjamin , Rethmeier, Michael

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.

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Application of additive manufactured tungsten carbide-cobalt electrodes with interior flushing channels in S-EDM

2020 , Uhlmann, Eckart , Polte, Julian , Bolz, Robert , Yabroudi, Sami , Streckenbach, Jan , Bergmann, André

Application fields of electrical discharge machining (EDM) are limited due to given process conditions. Manufacturing of parts with high aspect ratios and the application of multi-axis machining are limited due to process instabilities caused by removed particles. A promising approach to improve EDM process conditions, especially in sinking EDM (S-EDM), is the utilization of flushing channels in the tool electrode. However, with increasing complexity of the tool electrode geometry and the local integration of these flushing channels, conventional tool electrode manufacturing by cutting is limited. In contrast to that, the machining process selective laser melting (SLM) does not have such limitations. The appropriate integration of flushing channels, even for complex electrode geometries, improves process conditions during EDM in a variety of applications. This leads to a higher material removal rate and reduced tool wear compared to machining without flushing. Additionally, the number of required tool electrodes can be reduced, as SLM enables an efficient integration and miniaturization of all features in a single electrode. Because of its wear resistance and stability, tungsten carbide is an ideal tool electrode material, which is commonly applied in drilling EDM. After identifying suitable process parameters for roughing EDM with additively manufactured tungsten carbide cobalt tool electrodes, different forms of flushing channels were analysed in order to establish a fast process with minimum tool electrode wear. The results concerning material removal rate and the relative tool wear could be improved by applying internal flushing, though the tool wear stayed at a worse level compared to conventional tool electrode materials.

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Finite element analysis of in-situ distortion and bulging for an arbitrarily curved additive manufacturing directed energy deposition geometry

2018 , Biegler, Max , Marko, Angelina , Graf, Benjamin , Rethmeier, Michael

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.

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Laser Welding of SLM-Manufactured Tubes Made of IN625 and IN718

2019 , Jokisch, Torsten , Marko, Angelina , Gook, Sergej , Üstündag, Ömer , Gumenyuk, Andrey , Rethmeier, Michael

The advantage of selective laser melting (SLM) is its high accuracy and geometrical flexibility. Because the maximum size of the components is limited by the process chamber, possibilities must be found to combine several parts manufactured by SLM. An application where this is necessary, is, for example, the components of gas turbines, such as burners or oil return pipes, and inserts, which can be joined by circumferential welds. However, only a few investigations to date have been carried out for the welding of components produced by SLM. The object of this paper is, therefore, to investigate the feasibility of laser beam welding for joining SLM tube connections made of nickel-based alloys. For this purpose, SLM-manufactured Inconel 625 and Inconel 718 tubes were welded with a Yb:YAG disk laser and subsequently examined for residual stresses and defects. The results showed that the welds had no significant influence on the residual stresses. A good weld quality could be achieved in the seam circumference. However, pores and pore nests were found in the final overlap area, which meant that no continuous good welding quality could be accomplished. Pore formation was presumably caused by capillary instabilities when the laser power was ramped out.

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Manufacturing of carbide tools by Selective Laser Melting

2018 , Uhlmann, Eckart , Bergmann, André , Bolz, Robert

Application fields of electrical discharge machining (EDM) are limited due to given process conditions. When producing structures of high aspect ratios or using multi-axis machining, removed particles assemble at the machining zone, leading to process instabilities and increasing tool wear. A promising approach to improve EDM process conditions is the utilization of flushing channels in the tool electrode. However, with increasing complexity of the electrode geometry and the local integration of the mentioned flushing channels, conventional electrode manufacturing reaches its limitations. By applying Selective Laser Melting (SLM), these limitations are eliminated. An additional benefit is the efficient use of material during SLM, where nearly no waste is produced, because remaining powder can be used for the next SLM-process. The appropriate integration of flushing channels, even for complicated electrode geometries, improves process conditions during EDM in a variety of applications, leading to a higher material removal rate VW and reduced tool wear TH compared to machining without flushing. Additionally, the number of required tool electrodes can be reduced, as SLM enables an efficient integration and miniaturization of all features in a single electrode, what results in a far more sustainable process chain. Of particular interest in the field of EDM is carbide. Because of its wear resistance and stability, it is an ideal electrode material, which is commonly applied in µEDM. Tungsten carbide-cobalt is representative for this group of materials, which is already used in tool manufacturing. Several tests show a general suitability of carbide tool electrodes made by SLM for EDM-processing. However, the SLM process parameters and the composition of the carbide-cobalt show significant impact to the EDM results. A lower proportion of cobalt leads to reduced material removal rates, whereas the level of tool wear remains at a similar level. In order to benefit from the advantages of additive manufactured carbide tool electrodes, this investigation aims at decreasing waste of material and the number of required tool electrodes.

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