Dr.-Ing. Dipl.-Phys. Reiner Zielke

Lehrstuhl für Werkstofftechnologie
TU Dortmund University

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  • A numerical method to improve the representativeness of real microstructure cut-outs applied in finite element simulations
    Schneider, Y. and Wasserbäch, W. and Schmauder, S. and Zhou, Z. and Zielke, R. and Tillmann, W.
    Crystals 11 (2021)
    To improve the representativeness of a real microstructural cut-out for modeling purposes, a numerical method named as “boundary pixel color alteration (BPCA)” is presented to modify measured 2D microstructure cut-outs. Its physical background is related to the phase growth. For the application, the precondition is that the representativeness of the microstructure is already satisfied to a certain extent. This method resolves the problem that the phase composition of a small cut-out can have a large discrepancy to the real one. The main idea is to change the pixel color among neighboring pixels belonging to different phases. Our process simultaneously maintains most of the characteristics of the original morphology and is applicable for nearly all kinds of multi-phase or polycrystalline metallic alloys, as well. From our axisymmetric finite element (FE) simulations (ABAQUS ) applied with 2D real microstructures, it shows that the volume ratios of microstructural phases, as a function of the structure position to the symmetric axis, converge to phase area ratios in the 2D cut-out, even though the axisymmetric element volume is position dependent. A mathematical proof provides the reason for the aforementioned convergence. As examples to achieve real compositions and to numerically prove the aforementioned convergence, four different materials including multiphase polycrystals are implemented. An improvement of the predicted FE result is presented for the application of a modified microstructure (with a higher representativeness) compared to the original one. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/cryst11040382
  • Experimental investigations of micro-meso damage evolution for a co/wc-type tool material with application of digital image correlation and machine learning
    Schneider, Y. and Zielke, R. and Xu, C. and Tayyab, M. and Weber, U. and Schmauder, S. and Tillmann, W.
    Materials 14 (2021)
    Commercial Co/WC/diamond composites are hard metals and very useful as a kind of tool material, for which both ductile and quasi-brittle behaviors are possible. This work experimentally investigates their damage evolution dependence on microstructural features. The current study investigates a different type of Co/WC-type tool material which contains 90vol.% Co instead of the usual < 50vol.%. The studied composites showed quasi-brittle behavior. An in-house-designed testing machine realizes the in-situ micro-computed tomography (µCT) under loading. This advanced equipment can record local damage in 3D during the loading. The digital image correlation technique delivers local displacement/strain maps in 2D and 3D based on tomographic images. As shown by nanoindentation tests, matrix regions near diamond particles do not possess higher hardness values than other regions. Since local positions with high stress are often coincident with those with high strain, diamonds, which aim to achieve composites with high hardnesses, contribute to the strength less than the WC phase. Samples that illustrated quasi-brittle behavior possess about 100–130 MPa higher tensile strengths than those with ductile behavior. Voids and their connections (forming mini/small cracks) dominant the detected damages, which means void initiation, growth, and coalescence should be the damage mechanisms. The void appears in the form of debonding. Still, it is uncovered that debonding between Co-diamonds plays a major role in provoking fatal fractures for composites with quasi-brittle behavior. An optimized microstructure should avoid diamond clusters and their local volume concentrations. To improve the time efficiency and the object-identification accuracy in µCT image segmentation, machine learning (ML), U-Net in the convolutional neural network (deep learning), is applied. This method takes only about 40 min to segment more than 700 images, i.e., a great improvement of the time efficiency compared to the manual work and the accuracy maintained. The results mentioned above demonstrate knowledge about the strengthening and damage mechanisms for Co/WC/diamond composites with > 50vol.% Co. The material properties for such tool materials (> 50vol.% Co) is rarely published until now. Efforts made in the ML part contribute to the realization of autonomous processing procedures in big-data-driven science applied in materials science. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14133562
  • Investigation of the stress corrosion cracking behavior on T24 material under the operational conditions in the water wall
    Ullrich, C. and Tillmann, W. and Rademacher, H.-G. and Zielke, R. and Körner, P.
    International Journal of Pressure Vessels and Piping 190 (2021)
    The requirements for materials and their strength significantly increased with the new generation of coal fired power plants operating at steam temperatures of up to 620 °C. Therefore, new materials were introduced to fulfill the defined needs. During the commissioning process of the first plant many cracks occurred in welds of T24 material. The cracks showed clear characteristics of stress corrosion cracking (SCC). Not knowing the exact parameters that lead to cracking, experiments in high temperature water were carried out. Slow tensile tests in a controlled environment are extremely well suited to generate information about material's SCC sensitivity. In the present paper, the influence of the temperature, the oxygen concentration of water, the pre-treatment of the specimen and the heat treatment to the SCC are investigated. Furthermore critical limits for the cracking are defined where possible. © 2021
    view abstract10.1016/j.ijpvp.2021.104317
  • A numerical method for the generation of hierarchical Poisson Voronoi microstructures applied in micromechanical finite element simulations—part I: method
    Schneider, Y. and Weber, U. and Wasserbäch, W. and Zielke, R. and Schmauder, S. and Tillmann, W.
    Computational Mechanics 66 (2020)
    Poisson Voronoi (PV) tessellations as artificial microstructures are widely used in investigations of material deformation behaviors. However, a PV structure usually describes a relative homogeneous field. This work presents a simple numerical method for generating 2D/3D artificial microstructures based on hierarchical PV tessellations. If grains/particles of a phase cover a large size span, the concept of “artificial phases” can be used to create a more realistic size distribution. From case to case, detailed microstructural features cannot be directly achieved by commercial or free softwares, but they are necessary for a deep or thorough study of the material deformation behavior. PV tessellations created in our process can fulfill individual requirements from material designs. Another reason to use PV tessellations is due to the limited experimental data. Concerning the application of PV microstructures, four examples are given. The FE models and results will be presented in consecutive works, i.e. “part II: applications”. © 2020, The Author(s).
    view abstract10.1007/s00466-020-01869-3
  • Modeling the separation performance of depth filter considering tomographic data
    Hoppe, K. and Maricanov, M. and Schaldach, G. and Zielke, R. and Renschen, D. and Tillmann, W. and Thommes, M. and Pieloth, D.
    Environmental Progress and Sustainable Energy 39 (2020)
    Fibrous depth filters are frequently used for the purification of gas streams with low dust loadings, as well as processes where a high initial filtration efficiency is required (e.g., clean rooms for aseptic production). One tool suitable for supporting the development of optimized filter media is the use of numerical simulations. The drawback of this technique is the high computational resources required. In this work, a new and fast approach based on a one-dimensional model was applied. Structural characteristics (e.g., porosity distribution and fiber diameter) of two different filter media were successfully determined using a novel X-ray microscope. These characteristics were incorporated in the filtration model, and their influence on the calculations was evaluated. It was found that the porosity distribution does have an impact on local (microscopic) deposition rates, but only a minor influence on the macroscopic filtration efficiency (around 3%). Benefits of the model are the application of measured structural data and the low computational expense. Compared to experimental data (VDI 3926 / ISO 11057), the prediction of the filtration efficiency can be improved by incorporating the structural data in the model. © 2020 The Authors. Environmental Progress & Sustainable Energy published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers.
    view abstract10.1002/ep.13423
  • Void distribution in a brazed cemented carbide steel joint analyzed by X-ray microscopy
    Yared, W. and Chen, C.-Y. and Sievers, N. and Tillmann, W. and Zielke, R. and Schimpfermann, M.
    Measurement: Journal of the International Measurement Confederation 141 (2019)
    Brazing is a relatively fast process that offers sufficient strength in the joint of dissimilar materials. Cemented carbides are often brazed onto steel components in order to improve the wear resistance of engineering tools. In the case of brazing such materials in an ambient atmosphere, a flux is necessary to improve the wetting of the liquid filler alloy on the surfaces. In some cases, the flux cannot be sufficiently removed from the small joint, thus forming voids during solidification. This phenomenon can greatly affect the integrity of the joint. Such voids are not adequately detectable by visual inspection or common nondestructive testing methods, such as ultrasonic scanning, acoustic emission testing, or thermography. In this study, X-ray microscopy is shown to provide adequate visualization and a quantitative analysis of the dispersion of voids within brazed components of cold work steel, 115CrV3, and cemented carbide, K10 (ISO 513). One of the challenging tasks when analyzing the aforementioned brazed materials is achieving a sufficiently high resolution within the joint gap, since the sample materials have similar X-ray absorption coefficients. Such high resolution was successfully achieved in this study by means of multiple scanning and image reconstruction techniques, such as beam filtering, dataset levelling, and noise removal. The voids on the 115CrV3-side are found to expand radially towards the edges of the specimen up to a maximum volume of 1.18E + 07 µm3. The same radial pattern was detected on the side of the K10, where the voids contracted in volume towards the center of the specimen. However, the K10-side was found to exhibit relatively larger voids with a maximum volume of 7.70E + 07 µm3, that is approximately seven times larger than that detected on the 115CrV3-side. © 2019 Elsevier Ltd
    view abstract10.1016/j.measurement.2019.04.045
  • Analysis of the ductility dip cracking in the nickel-base alloy 617mod
    Eilers, A. and Nellesen, J. and Zielke, R. and Tillmann, W.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    While testing steam leading power plant components made of the nickel-base alloy A617mod at elevated temperatures (700 °C), ductility dip cracking (DDC) was observed in welding seams and their surroundings. In order to clarify the mechanism of crack formation, investigations were carried out on welded specimens made of A617mod. Interrupted tensile tests were performed on tensile specimens taken from the area of the welding seam. To simulate the conditions, the tensile tests were conducted at a temperature of 700 °C and with a low strain rate. Local strain fields at grain boundaries and inside single grains were determined at different deformation states by means of two-dimensional digital image correlation (DIC). Besides the strain fields, local hardnesses (nanoindentation), energy dispersive X-Ray spectroscopy (EDX), and electron backscatter diffraction (EBSD) measurements were performed. Besides information concerning the grain orientation, the EBSD measurement provides information on the coincidence site lattice (CSL) at grain boundaries as well as the Schmid factor of single grains. All results of the analysis methods mentioned above were correlated and compared to each other and related to the crack formation. Among other things, correlations between strain fields and Schmid factors were determined. The investigations show that the following influences affect the crack formation: orientation of the grain boundaries to the direction of the loading, the orientation of the grains to each other (CSL), and grain boundary sliding. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/181/1/012020
  • Examination of the porosity in Reactive Air Brazed joints by ultrasonic testing
    Tillmann, W. and Sievers, N. and Pfeiffer, J. and Wojarski, L. and Zielke, R. and Poenicke, A. and Schilm, J.
    Advanced Engineering Materials 16 (2014)
    Reactive air brazing (RAB) has been developed as a method to join ceramics and steel, using CuO as a reactive agent that interacts with the surface of the ceramic, enabling a wetting by the molten filler metal. A major benefit of this method is the fact that the joining process can be carried out in an ambient atmosphere, in contrast to active brazing processes, which need to be performed in a vacuum furnace. In the past, several investigations were conducted to improve the mechanical bonding properties for both methods. A sealed gas-tightness is also important for innovative applications such as solid oxide fuel cells. In this regard, the reduction of porosity, which is necessary in order to achieve reproducible joints with a long lifetime, presents a challenge. Therefore, it is necessary to conduct fundamental analyses of the driving forces and mechanisms of the formation of voids in the interfacial area to guarantee a reliable joint quality. In this study, the authors used an ultrasonic testing method in the realm of the immersion technique to evaluate the porosity in brazements produced with varying process parameters. A major goal was to assess the influence of the cooling stage on the pore formation. The advantage of this non-destructive method is the possibility to scan the entire joint area using just one scan. The investigations were flanked by SEM analyses on different cross-sections. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/adem.201400055
  • Quality prediction of twin wire Arc sprayed coatings using acoustic Emission Analysis
    Tillmann, W. and Abdulgader, M. and Wang, G. and Zielke, R.
    Journal of Thermal Spray Technology 22 (2013)
    In this work, acoustic emission analysis is utilized in the twin wire arc spraying (TWAS) process to study the influence of the adjustable process parameters on the simultaneously obtained acoustic signals at the nozzle and at the substrate. The amplitude of recorded signals at the substrate was in general much higher than those recorded at the nozzle. At the substrate side, the amplitude of emitted acoustic signals is dependent on feedstock materials and is higher when using solid wires. The acoustic signals were recorded at the spraying gun for different gas pressures without arc ignition (as dry runs) in order to reveal the effect of the arc on the emitted acoustic signals. A correlation between controllable parameters, the acoustic signals, and the obtained in-flight particle characteristics was observed. This work contributes to the online control of TWAS processes and is one of many proposed publications in the research field of the conducted acoustic emission analysis. © 2012 ASM International.
    view abstract10.1007/s11666-012-9861-z
  • Investigation of PLC band nucleation in AA5754
    Feng, X. and Fischer, G. and Zielke, R. and Svendsen, B. and Tillmann, W.
    Materials Science and Engineering A 539 (2012)
    The purpose of the present work is the experimental investigation of the nucleation of PLC deformation bands in the aluminium alloy AA5754. The PLC bands are investigated using both mechanical methods and infrared (IR) thermography. The latter employs a high-speed IR camera which captures local changes of radiated power resulting from mechanical dissipation and heating due to the nucleation of PLC bands. The resulting IR images are used to determine spatio-temporal power field variations via image subtraction. Furthermore, band trajectories obtained from the IR images are used to study possible correlations between the spatio-temporal evolution of stress and radiated power in the specimens and PLC band development. © 2012 Elsevier B.V.
    view abstract10.1016/j.msea.2012.01.082
  • acoustical measurements

  • non-destructive methods

  • x-ray spectroscopy

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