Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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Acoustic emission-based process monitoring in the milling of carbon fibre-reinforced plastics

2022 , Uhlmann, E. , Holznagel, Tobias

Milling of fibre-reinforced plastics is a challenging task. The highly abrasive fibres lead to high tool wear and coating failures, which cause increasing process forces and temperatures. Machining with a worn tool, in turn, can result in unwanted workpiece damages such as delamination or fibre protrusion. Reliable monitoring of the process must therefore be able to detect damages to the milling tool and the workpiece alike. The presented process monitoring approach measures the acoustic emission generated by the milling tool cutting edge entering the workpiece with a sensor attached to the tool holder. Specific acoustic emission frequency spectra and waveforms are emitted in the cutting zone for different tool wear states. Coating failures as well as other acoustic emission events due to workpiece damages can be robustly detected and distinguished by feature extraction and signal processing as well. The developed setup, the monitoring parameterisation techniques and signal processing algorithms as well as experimental and monitoring results are presented and discussed in this paper.

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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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CVD-Diamantwerkzeuge mit SiC-Zwischenschicht

2019 , Uhlmann, E. , Barth, E. , Gäbler, J. , Höfer, M.

Der Cobalt-(Co)-Anteil in Hartmetallen diffundiert während des Diamantbeschichtungsprozesses in die Diamantschicht und mindert deren Haftfähigkeit. Siliciumcarbid-(SiC)-Zwischenschichten können als Diffusionsbarriere für Cobalt dienen und die konventionelle Ätzvorbehandlung der Substrate ersetzen. Im Rahmen einer Forschungsarbeit werden Beschichtungsprozesse mit SiC-Zwischenschicht entwickelt, diese Schichtsysteme auf verschiedene Substrate aufgebracht und durch Zerspanungsuntersuchungen bewertet.

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Modelling of brushing processes

2019 , Sommerfeld, C. , Uhlmann, E. , Hoyer, A.

Brushes consist of a body with fixed highly flexible filaments and can be used for deburring and surface finishing operations. During the brushing process, axial and tangential deflections of the highly flexible filaments lead to an adaptation to the shape of the workpiece and interaction between the filaments. The described complex contact behavior has been insufficiently investigated so far. For a better understanding of the contact between a brush and the workpiece surface, this paper presents a model based on physical principles. The model describes the dynamic behavior of a brush in contact with different workpiece geometries and consists of separate physical descriptions for the filaments of the brush, the workpiece surface and the occurring contacts. A description of a single filament is given by a multi-body system of rigid links. The rigid links are connected by joints which approximate the material behavior of the filaments. To approximate different geometries, the workpiece surface is specified by a polynomial. Contact can occur between the filaments and the workpiece surface as well as between the filaments. For the description of the occurring contacts, Hertz's theory of elastic contact and Coulomb's law of friction are used. The aforementioned physical descriptions are included in the Lagrange's equations to obtain a system of equations of motion that calculates the deflection of the filaments of the brush and the acting forces during the contact with the workpiece surface. A numerical solution to the system of equations of motion was calculated by using experimentally determined material and contact properties of the filament. A comparison of the calculated forces with experimentally determined values shows good correlations for different workpiece surfaces and process parameters. In this context, the developed model calculates the progression and the maximum value of the acting contact forces. The results show a shorter contact length of the filament lc for a circular surface compared to a plane surface, and a rise of the maximum normal force Fn with the depth of cut ae. Furthermore, consideration of the filament interactions leads to a more accurate approximation of the brush-workpiece contact. Based on the findings, the developed model can be used to calculate predictions for different brushing processes which reduce the number of time-consuming preliminary tests for the process design.

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Influence of superimposed low frequency oscillations on single-pass honing of long-chipping steel

2022 , Uhlmann, E. , Rozek, André

Single-pass honing is used as a finishing process to meet high demands regarding form and dimensional accuracy of drilled bores. The disadvantages of single-pass honing compared to the conventional long-stroke honing are high process forces and torques as well as an increased risk of chip space clogging of the abrasive stones. A significant reduction in process forces and torques can be achieved by superimposing the axial movement with oscillations. In this work the kinematic basics of different oscillation parameters and their effects on single-pass honing of long-chipping steel are analyzed. It can be concluded that by superimposing low frequency oscillations in single-pass honing, the process forces and torques as well as the specific energy consumption can be reduced significantly without a decline in surface quality and form accuracy.

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Verfügbarkeitssteigerung durch gezielte Datenanalyse

2020 , Uhlmann, E.

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Die-sinking EDM of a SiC-boride-composite

2019 , Uhlmann, E. , Polte, J. , Jahnke, C. , Wolf, C.-S. , Degenhardt, U.

Silicon carbide-based composites are highly demanded for industrial applications, like heat exchangers in corrosive environments. In consequence of the mechanical properties like high hardness H and brittleness, cutting processes are still challenging. An opportunity for processing difficult-to-cut materials with sufficient electrical conductivity s is electrical discharge machining (EDM). In order to develop suitable machining technologies, known parameters for common material combinations can serve as a starting point. Hence, standard technologies from the database of a commercial die-sinking EDM machine tool were applied for machining a silicon carbide-boride-composite. The material removal rate QW and the arithmetical mean deviation Ra for finishing and roughing operations are observed. The formation of cracks and the extend of the typical deposition layer formed at the surface of the workpiece is analysed through scanning electron microscopy. While the material removal rate QW is increasing with higher discharge energies W in this investigation, the arithmetical mean deviation Ra is not dependent on this property. Furthermore, spalling is identified as main material removal mechanism occurring in this process. Overall, the machinability of silicon carbide-boride composites with EDM, applying commercial available parameter technologies, is successfully demonstrated.

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Residual stress assessment during cutting tool lifetime of CVD-diamond coated indexable inserts

2022 , Uhlmann, E. , Hinzmann, Daniel

Insufficient coating adhesion limits reproducibility regarding tool lifetime as well as workpiece quality during the application of CVD-diamond coated cutting tools. Depending on the combination of tungsten carbide substrate material, coating thickness as well as coating morphology, individual residual stress conditions exist within CVD-diamond coated cutting tool specifications. The application of these tools is accompanied by coating delamination as primary cutting tool failure. The tool lifetime of the respective cutting tool composition depends on the corresponding residual stress condition until crack development within the CVD-diamond coating initiates tool failure. During external cylindrical turning of hypereutectic aluminium silicon alloy AlSi17Cu4Mg-T6 the residual stress condition of a CVD-diamond coated cutting tool is assessed along the cutting edge, the rake face as well as flank face throughout the respective tool lifetime. Consequently, the progression of the residual stress condition until cutting tool failure regarding coating delamination is observed. During the tool lifetime of the investigated CVD-diamond cutting tools, compressive residual stress ∆σR,c shifts to tensile residual stress ∆σR,t underneath the cutting edge corner. The approximated residual stress difference of ∆σR ≈ 5 GPa indicates stress peak relaxation processes, such as crack initiation, within the CVD-diamond coating.

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Manufacturing of graphite electrodes with high geometrical requirements

2019 , Uhlmann, E. , Kuche, Y. , Polte, J. , Polte, M.

Graphite is widely used for the die-sinking electrical discharge machining (EDM) process, especially for the roughing process. For the manufacturing of graphite electrodes the milling process is mainly used. The process enables fast processing times tP and high geometrical flexibility. In consequence of the cutting behaviour of graphite micro-components with geometrical features can be manufactured. In this contribution the manufacturing of micro-pins and bridges with aspect ratios of A = 1:50 were machined with diamond coated milling tools. By variation of the depth of cut ap and the width of cut ae it can be shown that the influence of the depth of cut ap is quite bigger than the influence of the width of cut ae. This results in consequence of the higher stability of the geometrical features by improved force distribution.

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Mikrostrukturierte CVD-Diamant-Werkzeuge

2019 , Bachmann, H. , Meier, P. , Uhlmann, E. , Schröter, D.

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