Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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Effects on the distortion of Inconel 718 components along a hybrid laser-based additive manufacturing process chain using laser powder bed fusion and laser metal deposition

2021 , Uhlmann, E. , Düchting, J. , Petrat, T. , Krohmer, E. , Graf, B. , Rethmeier, M.

The combination of laser powder bed fusion (LPBF), 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 additive manufacturing times for large metallic parts. For the industrial application of the LPBF-LMD hybrid process chain, it is necessary to investigate the influence of the LMD process on the LPBF substrate. In addition, the build plate material also has a significant impact on the occurrence of distortion along the additive manufacturing process chain. In the literature, steel build plates are often used in laser-based additive manufacturing processes of Inconel 718, since a good metallurgical bonding can be assured whilst reducing costs in the production and restoration of the build plates. This paper examines the distortion caused by LMD material deposition and the influence of the build plate material along the hybrid additive manufacturing process chain. Twin cantilevers are manufactured by LPBF and an additional layer is subsequently deposited with LMD. The distortion is measured in the as-built condition as well as after heat treatment. The effect of different LMD hatch strategies on the distortion is determined. The experiments are conducted using the nickel-base alloy Inconel 718. The results show a significant influence of LMD path strategies on distortion, with shorter tool paths leading to less distortion. The remaining distortion after heat treatment is considerably dependent on the material of the build plate.

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Quantifying mechanical properties of automotive steels with deep learning based computer vision algorithms

2020 , Javaheri, E. , Kumala, V. , Javaheri, A. , Rawassizadeh, R. , Lubritz, J. , Graf, B. , Rethmeier, M.

This paper demonstrates that the instrumented indentation test (IIT), together with a trained artificial neural network (ANN), has the capability to characterize the mechanical properties of the local parts of a welded steel structure such as a weld nugget or heat affected zone. Aside from force-indentation depth curves generated from the IIT, the profile of the indented surface deformed after the indentation test also has a strong correlation with the materials' plastic behavior. The profile of the indented surface was used as the training dataset to design an ANN to determine the material parameters of the welded zones. The deformation of the indented surface in three dimensions shown in images were analyzed with the computer vision algorithms and the obtained data were employed to train the ANN for the characterization of the mechanical properties. Moreover, this method was applied to the images taken with a simple light microscope from the surface of a specimen. Therefore, it is possible to quantify the mechanical properties of the automotive steels with the four independent methods: (1) force-indentation depth curve; (2) profile of the indented surface; (3) analyzing of the 3D-measurement image; and (4) evaluation of the images taken by a simple light microscope. The results show that there is a very good agreement between the material parameters obtained from the trained ANN and the experimental uniaxial tensile test. The results present that the mechanical properties of an unknown steel can be determined by only analyzing the images taken from its surface after pushing a simple indenter into its surface.

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Ersatzteilfertigung und Reparatur mittels additiver Fertigungsverfahren

2013 , Uhlmann, E. , König-Urban, K. , Rethmeier, M. , Graf, B.

Zur künftigen effektiven und kostengünstigen Wartung sowie Instandsetzung der am höchsten belasteten Bauteile einer Gasturbine sind neue Fertigungsverfahren zu entwickeln und in die bestehenden Prozessketten zu integrieren. Das größte Potenzial hiefür bieten das selektive Laserschmelzen (SLM) und das Laser-Pulver-Auftragschweißen (LPA). Das SLM-Verfahren basiert auf fünf Versuchsreihen: (1) Variierung der Fokuslage xF von Voll-, Gitter- und Supportstrukturen. (2) Variation der Laserleistung PL und der Verfahrgeschwindigkeit vs. (3) Ermittlung geeigneter Werte für den Spurabstand Dys und die Streifenüberlappung. (4) Erhöhung der Prozessgeschwindigkeit für die Fertigung von Voll-, Gitter- und Supportstrukturen durch die gleichzeitige Steigerung von PL und vs bei einer konstanten Streckenenergie E. Nach Abschluss der vierten Versuchsreihe liegt ein optimierter Parametersatz vor. (5) Fertigung von Zugstäben zur Ermittlung der erreichbaren Festigkeiten. Zur Qualifizierung eines neuen Werkstoffes für das SLM-Verfahren können Oberflächenrauigkeiten im Bereich von 45 µm eingestellt werden. Bei geringeren Rauigkeiten ist eine Nachbearbeitung erforderlich. Verfahren wie das Vibrationsgleitschleifen oder das Druckfließläppen weisen ein großes Potenzial für die Nachbearbeitung auf. Das LPA-Verfahren wird zunehmend beim Verschleißschutz sowie als Reparaturverfahren von Turbomaschinenkomponenten angewendet. Während beim SLM-Verfahren das Bauteil schichtweise im Pulverbett entsteht, wird der Zusatzwerkstoff beim LPA-Verfahren über eine Düse zugeführt. Bei der Reparatur von beschädigten Oberflächen oder Rissen findet zunächst eine spanende Vorbereitung statt. Ein ausgefräster Riss wird anschließend schichtweise mittels LPA wieder gefüllt.

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Geometric distortion-compensation via transient numerical simulation for directed energy deposition additive manufacturing

2020 , Biegler, Max , Elsner, B.A.M. , Graf, B. , Rethmeier, Michael

Components distort during directed energy deposition (DED) additive manufacturing (AM) due to the repeated localised heating. Changing the geometry in such a way that distortion causes it to assume the desired shape - a technique called distortion-compensation - is a promising method to reach geometrically accurate parts. Transient numerical simulation can be used to generate the compensated geometries and severely reduce the amount of necessary experimental trials. This publication demonstrates the simulation-based generation of a distortion-compensated DED build for an industrial-scale component. A transient thermo-mechanical approach is extended for large parts and the accuracy is demonstrated against 3d-scans. The calculated distortions are inverted to derive the compensated geometry and the distortions after a single compensation iteration are reduced by over 65%.

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Mechanical Properties Characterization of Welded Automotive Steels

2020 , Javaheri, E. , Lubritz, J. , Graf, B. , Rethmeier, M.

Among the various welding technologies, resistance spot welding (RSW) and laser beam welding (LBW) play a significant role as joining methods for the automobile industry. The application of RSW and LBW for the automotive body alters the microstructure in the welded areas. It is necessary to identify the mechanical properties of the welded material to be able to make a reliable statement about the material behavior and the strength of welded components. This study develops a method by which to determine the mechanical properties for the weldment of RSW and LBW for two dual phase (DP) steels, DP600 and DP1000, which are commonly used for the automotive bodies. The mechanical properties of the resistance spot weldment were obtained by performing tensile tests on the notched tensile specimen to cause an elongation of the notched and welded area in order to investigate its properties. In order to determine the mechanical properties of the laser beam weldment, indentation tests were performed on the welded material to calculate its force-penetration depth-curve. Inverse numerical simulation was used to simulate the indentation tests to determine and verify the parameters of a nonlinear isotropic material model for the weldment of LBW. Furthermore, using this method, the parameters for the material model of RSW were verified. The material parameters and microstructure of the weldment of RSW and LBW are compared and discussed. The results show that the novel method introduced in this work is a valid approach to determine the mechanical properties of welded high-strength steel structures. In addition, it can be seen that LBW and RSW lead to a reduction in ductility and an increase in the amount of yield and tensile strength of both DP600 and DP1000.

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Distortion-based validation of the heat treatment simulation of Directed Energy Deposition additive manufactured parts

2020 , El-Sari, Bassel , Biegler, Max , Graf, B. , Rethmeier, Michael

Directed energy deposition additive manufactured parts have steep stress gradients and an anisotropic microstructure caused by the rapid thermo-cycles and the layer-upon-layer manufacturing, hence heat treatment can be used to reduce the residual stresses and to restore the microstructure. The numerical simulation is a suitable tool to determine the parameters of the heat treatment process and to reduce the necessary application efforts. The heat treatment simulation calculates the distortion and residual stresses during the process. Validation experiments are necessary to verify the simulation results. This paper presents a 3D coupled thermo-mechanical model of the heat treatment of additive components. A distortion-based validation is conducted to verify the simulation results, using a C-ring shaped specimen geometry. Therefore, the C-ring samples were 3D scanned using a structured light 3D scanner to compare the distortion of the samples with different post-processing histories.

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Mechanical properties characterization of resistance spot welded DP1000 steel under uniaxial tensile tests

2019 , Javaheri, E. , Pittner, A. , Graf, B. , Rethmeier, M.

Resistance spot welding (RSW) is widely used in the automotive industry as the main joining method. Generally, an automotive body contains around 2000 to 5000 spot welds. Therefore, it is of decisive importance to characterize the mechanical properties of these areas for the further optimization and improvement of an automotive body structure. The present paper aims to introduce a novel method to investigate the mechanical properties and microstructure of the resistance spot weldment of DP1000 sheet steel. In this method, the microstructure of RSW of two sheets was reproduced on one sheet and on a bigger area by changing of the welding parameters, e. g. welding current, welding time, electrode force and type. Then, tensile tests in combination with digital image correlation (DIC) measurement were performed on the notched tensile specimens to determine the mechanical properties of the weld metal. The notch must be made on the welded tensile specimen to force the fracture and elongation on the weld metal, enabling the characterization of its properties. Additionally, the parameters of a nonlinear isotropic material model can be obtained and verified by the simulation of the tensile specimens. The parameters obtained show that the strength of DP1000 steel and the velocity of dislocations for reaching the maximum value of strain hardening, are significantly increased after RSW. The effect of sample geometry and microstructural inhomogeneity of the welded joint on the constitutive property of the weld metal are presented and discussed.

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