Dr.-Ing. Stefan Kleszczynski

Product Engineering
University of Duisburg-Essen

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  • Properties of gas-atomized Cu-Ti-based metallic glass powders for additive manufacturing
    Soares Barreto, E. and Frey, M. and Wegner, J. and Jose, A. and Neuber, N. and Busch, R. and Kleszczynski, S. and Mädler, L. and Uhlenwinkel, V.
    Materials and Design 215 (2022)
    Laser powder bed fusion (PBF-LB/M) of bulk metallic glasses permits large and complex components to solidify to an amorphous state, thus expanding the processing possibilities of this material class. Here, the Cu-Ti-Zr-Ni family, also known as Vitreloy 101, is systematically investigated for processing of the PBF-LB/M powder itself. Gas atomization was used to produce powder of Vit101 and derivates micro-alloyed with Si and Sn. The influence of atomization and alloy composition on glass formation, oxygen content, particle morphology, and flowability were investigated. Amorphous powder was successfully obtained using industrial-grade purity as feedstock for the atomization. The oxygen content within the powder was controlled by the surface-to-volume ratio, without significant influence of the different atomization parameters and the microalloying itself. The powder displayed high circularity with sufficient flowability after drying. Our results contribute to the investigation of Vitreloy 101 alloys as promising candidates for PBF-LB/M applications. © 2022 The Author(s)
    view abstract10.1016/j.matdes.2022.110519
  • Experimental and numerical thermal analysis of the laser powder bed fusion process using in situ temperature measurements of geometric primitives
    Schnell, N. and Schoeler, M. and Witt, G. and Kleszczynski, S.
    Materials and Design 209 (2021)
    Laser powder bed fusion (PBF-LB/M) is a potent technology for manufacturing demanding geometries using innovative materials. The complex thermal conditions during the process are nontrivial to describe and have a significant impact on final material properties. Therefore, these conditions are analyzed using an experimental setup based on thermocouples embedded into the substrate plate close to the substrate-part interface of the respective sample. The in situ data allows for an in-depth investigation of correlations between core process parameters (laser power, scan velocity, exposed area) and the temperature progression at the base of the part. The alternative view on the conditions during the process enables a novel analytical description of the thermal history. Additionally, a macroscopic FEM-model is presented. It is calibrated and validated through the empirical data of geometric primitives to emphasize the added value of the setup as a calibration tool for thermal simulations. © 2021 The Authors
    view abstract10.1016/j.matdes.2021.109946
  • Impact of single structural voids on fatigue properties of AISI 316L manufactured by laser powder bed fusion
    Kotzem, D. and Kleszczynski, S. and Stern, F. and Elspaß, A. and Tenkamp, J. and Witt, G. and Walther, F.
    International Journal of Fatigue 148 (2021)
    The laser powder bed fusion of metals (PBF-LB/M) process is already exploited in several industrial applications. The process itself allows to introduce artificial defects which can later be characterized by their influence on the resulting mechanical properties. In this study, the influence of isolated single structural defects (0.3 mm ≤ √area ≤ 1.5 mm) on the fatigue properties is discussed and the √area-parameter model is applied. The obtained results show that the investigated material is highly defect tolerant as artificial defects with √area = 0.3 mm are not crack initiating. Specimens with a defect of √area = 1.0–1.5 mm clearly show crack initiation and propagation starting from the defect, and a fatigue strength estimation tends to be more conservative. © 2021 Elsevier Ltd
    view abstract10.1016/j.ijfatigue.2021.106207
  • Influence assessment of artificial defects on the fatigue behavior of additively manufactured stainless steel 316LVM
    Stern, F. and Grabowski, J. and Elspaß, A. and Kotzem, D. and Kleszczynski, S. and Witt, G. and Walther, F.
    Procedia Structural Integrity 37 (2021)
    The laser powder bed fusion of metals (PBF-LB/M) is one of the most promising techniques to realize lightweight optimized parts and structures. Possible design elements are internal cooling channels or topology optimized geometries. However, not only does the process suffer of instabilities causing pores and lack-of-fusion defects but also a low surface quality. A further knowledge about these defects and their influence on the mechanical behavior are needed to use additively manufactured parts in structural relevant applications. In this work, the austenitic stainless steel 316LVM (X2CrNiMo18-15-3) has been processed by PBF-LB/M. In total, four different batches were manufactured with either no intended porosity or specific cubic defects ranging between 0.3 and 1.5 mm edge length. The fatigue behavior was evaluated at stress ratio R = -1 up to 1E7 cycles. The fracture surface was analyzed by scanning electron microscopy and the relationship between artificial defect size and fatigue strength was investigated by Kitagawa-Takahashi (KT) diagram and its modification by El Haddad's intrinsic crack length. The results show that the KT-diagram underestimates the fatigue strength of the investigated steel indicating a high defect tolerance and possible hardening mechanisms during cyclic loading such as possible nano-twinning. An influence of the entrapped process gas could also play a role. As long as this is unclear, the models can only be used conservatively as the full potential of the PBF-LB/M steel cannot be fully exploited. © 2022 The Authors.
    view abstract10.1016/j.prostr.2022.01.071
  • Influence of powder characteristics on the structural and the mechanical properties of additively manufactured Zr-based bulk metallic glass
    Wegner, J. and Frey, M. and Piechotta, M. and Neuber, N. and Adam, B. and Platt, S. and Ruschel, L. and Schnell, N. and Riegler, S.S. and Jiang, H.-R. and Witt, G. and Busch, R. and Kleszczynski, S.
    Materials and Design 209 (2021)
    Additive manufacturing of Zr-based bulk metallic glasses (BMGs) is subject to growing scientific and industrial attention. Laser-based powder bed fusion of metals (PBF-LB/M) becomes a key technology to overcome current restrictions of size and geometry in the manufacturing of BMGs. For industrial application, further knowledge about defect formation, such as porosity and crystallization, is mandatory to develop processing strategies and suitable quality assurance. In this context, the influence of the particle size distribution, oxygen contamination, and applied process parameters during the PBF-LB/M of the glass-forming alloy AMZ4 (in at.% Zr59.3Cu28.8Al10.4Nb1.5) on the structural and mechanical properties were evaluated. It was found that the addition of SiO2 flow aid to the feedstock is suitable to increase flowability without impeding fabrication of the amorphous material. Furthermore, the processing of partially crystalline powder particles into amorphous samples is demonstrated. It indicates that today's high effort producing amorphous powders and thus the production costs can be reduced. Flexural bending tests and high-energy synchrotron X-ray diffraction reveal that the powder feedstock's oxygen content is crucial for the amorphization, embrittlement, and flexural strength of PBF-LB/M processed Zr-based BMGs. © 2021
    view abstract10.1016/j.matdes.2021.109976
  • Investigation of the Roughness Influence on the Absorption Behavior of additively manufactured Metals by the Laser Speckle Photometry
    Dang, D. and Elspas, A. and Cikalova, U. and Kleszczynski, S. and Bendjus, B. and Witt, G.
    Proceedings of the International Spring Seminar on Electronics Technology 2021-May (2021)
    The Laser Powder Bed Fusion of Metals (LPBF-M) is one of the most important methods in the additive manufacturing. This process can be used to produce components with a high degree of complexity and design freedom as well as with material density. Unfortunately, hundreds of factors influence the quality of the processes and thus the material characteristics which limits the reproducibility and the economic viability. Therefore, quality control during process, as well as the testing of material properties afterward, is one of the key development fields. This paper presents the applicability of the Laser Speckle Photometry (LSP) for determination of the surface topology in additive manufacturing of metals. The LSP is a non-destructive testing method which examines optical interference patterns bases on the speckle phenomena for the defect detection on surfaces. For this purpose, samples with a specific pore structure in the near-surface zone which influences the surface characteristics were manufactured. With the LSP, the surfaces were measured to verify a correlation between the roughness and the LSP signal. Depended on the surface roughness different absorption behaviors of the fabricated specimens were determined during external laser excitation as a part of LSP measurement and simultaneously measured temperature. © 2021 IEEE.
    view abstract10.1109/ISSE51996.2021.9467607
  • Speeding up Additive Manufacturing by Means of Forming for Sheet Components with Core Structures
    Rosenthal, S. and Hahn, M. and Tekkaya, A.E. and Platt, S. and Kleszczynski, S. and Witt, G.
    International Journal of Precision Engineering and Manufacturing - Green Technology (2021)
    A new process combination route consisting of additive manufacturing (AM) with a subsequent forming operation is proposed. The process route has the opportunity to increase the efficiency of the AM process route up to 360%. Stainless steel 316L sheets with different core structures (similar to sandwich sheets) are produced by AM, characterized, and formed in a die bending operation. The bending characteristics of this novel semi-finished product can be accurately predicted in a numerical simulation. The new process route is discussed in detail and compared to conventional AM parts in terms of the production efficiency. © 2021, The Author(s).
    view abstract10.1007/s40684-021-00384-x
  • Thermoplastic forming of additively manufactured Zr-based bulk metallic glass: A processing route for surface finishing of complex structures
    Frey, M. and Wegner, J. and Neuber, N. and Reiplinger, B. and Bochtler, B. and Adam, B. and Ruschel, L. and Riegler, S.S. and Jiang, H.-R. and Kleszczynski, S. and Witt, G. and Busch, R.
    Materials and Design 198 (2021)
    Additive manufacturing of bulk metallic glasses (BMGs) through laser powder bed fusion (LPBF) has drawn growing interest in the last years, especially concerning industry-relevant alloys based on iron or zirconium. The process-inherent high cooling rates and localized melting pools allow to overcome geometrical restrictions given for the production of BMGs by classical casting routes. Yet, the achievable surface qualities are still limited, making an adequate post-processing necessary. In this work, we report on applying thermoplastic forming on LPBF-formed parts for the first time to decrease surface roughness and imprint finely structured surface patterns without the need for complex abrasive machining. This BMG-specific post-processing approach allows to functionalize surface areas on highly complex LPBF-formed specimens, which could be of interest especially for medical or jewelry applications. © 2020 The Authors
    view abstract10.1016/j.matdes.2020.109368
  • Diamond-impregnated 316L metal matrix composites fabricated by powder bed fusion with laser beam - Influences of the energy input on the microstructural properties
    Wegner, J. and Fehr, A. and Platt, S. and Kleszczynski, S. and Witt, G. and Tillmann, W.
    Diamond and Related Materials 109 (2020)
    Fabricating diamond metal matrix composites (DMMCs) by means of powder bed fusion of metals using a laser beam (PBF-LB/M) is a new approach to extensively expand the spectrum of geometrical freedom for diamond tools. However, it must be borne in mind that the temperature input has a significant influence on the diamond condition since graphitizations are likely to occur. Therefore, it was analyzed how varying volume energy densities and substrate heating affect the microstructure and the densification of a 316 L stainless steel matrix, which was impregnated with 5 vol.-% Ni-coated diamonds. With regard to the densification, it was shown that an elevated substrate temperature (473 K) allowed to apply reduced volume energy densities and reduce stress-induced cracking. Thus, a relative density of 99.5% could be achieved. Furthermore, decreasing the volume energy density avoided graphitizations of the diamonds. Cr and Fe contents of the matrix material dissolved at the Ni-coated diamond surface revealing a diamond-metal interaction. A longer heat flux generally supported these diffusion processes at the interfaces. Finally, it became obvious that increased laser powers resulted in a higher densification, while low scan speeds and laser powers are desirable to foster diffusion and to avoid graphitizations. © 2020 Elsevier B.V.
    view abstract10.1016/j.diamond.2020.108040
  • Influence of isolated structural defects on the static mechanical properties of PBF-LB/M components
    Kleszczynski, S. and Elspaß, A.
    Procedia CIRP 94 (2020)
    Since the microstructure of components made by Laser-based powder bed fusion of metals (PBF-LB/M) is directly affected by the melting and solidification, mechanical properties are very sensitive to deviating process conditions. If deviations occur, local defects may remain within the microstructure. The impact of isolated defects on the resulting mechanical properties is only poorly understood. Since PBF-LB/M processes are increasingly used in industrial applications, fundamental systematic studies on the effects of structural defects on the mechanical properties are necessary to ensure quality. Thus, single defined structural defects of different dimensions are reproducibly placed in PBF-LB/M samples and used for static-mechanical tests. The results of tensile testing are compared with structural mechanical simulations, which provide insights into the prevailing stress and strain distributions. The achieved results show that isolated internal defects have only a minor influence on static strength parameters. Particularly with larger defect sizes, the values determined for tensile strength and yield strength showed only minor impairment. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.036
  • Influence of process gas during powder bed fusion with laser beam of Zr-based bulk metallic glasses
    Wegner, J. and Frey, M. and Kleszczynski, S. and Busch, R. and Witt, G.
    Procedia CIRP 94 (2020)
    Zr-based bulk metallic glasses offer a unique combination of hardness, high strength, and high elastic limits. Yet, manufacturable size and complexity are limited due to the required cooling rates. Short laser-material interaction times together with layer-wise and selective energy input allows the laser powder bed fusion process to largely overcome those restrictions. Still, the complex process-material interactions inhere numerous uncertainties. In the present work, additively manufactured Zr-based bulk metallic glasses produced under three different process gases are investigated by calorimetry, x-ray diffraction, and bending tests. A strong dependence between the thermophysical properties, flexural strength, and the applied atmosphere is found. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.039
  • LPBF-M manufactured Zr-based bulk metallic glasses coated with magnetron sputtered ZrN films
    Tillmann, W. and Fehr, A. and Wegner, J. and Stangier, D. and Kleszczynski, S. and Witt, G.
    Surface and Coatings Technology 386 (2020)
    The fabrication of Zr-based bulk metallic glasses (BMGs) by means of laser powder bed fusion of metals (LPBF-M) is recently emerging. This production route allows to widely overcome current geometrical restrictions of casting routes while maintaining the amorphous character, which is decisive for the unique mechanical properties, for instance. However, the roughness of the LPBF-M fabricated BMGs is still a challenging property, impeding the application of near-net shaped thin films that modify BMG surfaces, e.g. with respect to wear resistance. Zr59.3Cu28.8Al10.4Nb1.5 (at.%) substrates were manufactured by means of LPBFM, applying various exposure strategies, including laser remelting of the last solidified layer to influence the surface topography. Furthermore, BMG substrates were post-treated by grinding and polishing. Thus, varying degrees of crystallinity as well as surface roughness states were generated to analyze the effect of these characteristics on the microstructural properties of additionally applied magnetron sputtered ZrN films. Substrates that were fabricated with higher energy densities during LPBF-M exhibited (101)-Zr as well as (013)- and (110)-CuZr2 phases, which were accompanied by a decreased surface roughness. It was shown that all films had a crystalline structure on amorphous and partly crystalline BMG surfaces. A decreased surface roughness of the BMG substrates could be directly correlated with a higher hardness and a better adhesion of the ZrN film. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.125463
  • Material Characterization of Additively Manufactured Metals by Laser Speckle Photometry
    Dang, D. and Elspas, A. and Cikalova, U. and Kleszczynski, S. and Bendjus, B. and Witt, G.
    Proceedings of the International Spring Seminar on Electronics Technology 2020-May (2020)
    In contrast to traditional time-consuming and costly manufacturing processes, additive manufacturing offers an effective production of prototypes. In addition to the numerous process developments of recent years, quality assurance as well as the testing of material properties afterward have now become one of the main requirements. Previous research approaches such as thermography or optical imaging have essential disadvantages, why there is a need for simple and effective solutions. For this purpose, the Laser Speckle Photometry (LSP) as a novel nondestructive approach for evaluating the material quality manufactured by Laser Powder Bed Fusion of Metals (LPBF-M) is presented. For the validation of this testing method exactly defined defective samples were reproducibly fabricated by specific adaptation in the building process. Voids were introduced into the structure, which are not visible on the surface. The simple LSP setup and the adjusted evaluation algorithm, based on the correlation function, are decisive for characterizing different material density states. This paper presents the potential of LSP for the use in LPBF-M processes stated with the first results of the validation. © 2020 IEEE.
    view abstract10.1109/ISSE49702.2020.9120869
  • Forming properties of additively manufactured monolithic Hastelloy X sheets
    Rosenthal, S. and Platt, S. and Hölker-Jäger, R. and Gies, S. and Kleszczynski, S. and Tekkaya, A.E. and Witt, G.
    Materials Science and Engineering A 753 (2019)
    Additive manufacturing (AM) of semi-finished sheets for a subsequent forming operation has not been investigated yet. The potentials in resource efficiency and effective use of build-chamber-volumes, by combining laser powder bed fusion of metals and forming technology are demonstrated. The overarching aim of this process chain are time savings of up to 50% and the benefit of material strengthening by work hardening. The scope of this paper is the understanding and characterization of the flow behavior of additively manufactured semi-finished parts for the use in a subsequent forming application of the nickel-based superalloy, Hastelloy X. Characterization methods used in sheet metal forming are applied to monolithic additively manufactured tensile, compression and in-plane torsion specimens. The resulting characterization and yield criteria can be used to predict the forming behavior of additively manufactured semi finished parts with integrated functions like cooling channels that are formed in its final geometry. A correction function is introduced to consider the surface roughness in the stress-strain diagrams. The material shows a high anisotropic yield stress with a nearly isotropic hardening behavior in the as-build condition. The heat treatment reveals a homogenization of the material accompanied with an isotropic initial yield stress but anisotropic yield behavior. To numerically model those effects, different yield surfaces based on the preceding material characterization are discussed. It turns out, that the additively manufactured Hastelloy X shows high potential in terms of formability combined with high tensile strengths. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.03.035
  • In situ measurement of part geometries in layer images from laser beam melting processes
    zur Jacobsmühlen, J. and Achterhold, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    Progress in Additive Manufacturing 4 (2019)
    Laser beam melting (LBM) enables production of three-dimensional parts from metallic powder with very high geometrical complexity and very good mechanical properties. In LBM, a thin layer of metallic powder is deposited onto the build platform and melted by a laser according to the desired part geometry. Until today, the potential of LBM for critical applications such as medical devices and aerospace has not been exploited due to the lack of build stability and quality management. We present an image analysis method, which segments part contours in high-resolution images of LBM-produced layers. Based on the reference contour from 2D slices of the 3D part model and edge-detection results, a graph model is built and segmented using Graph Cuts (min-cut max-flow algorithm). Our method is evaluated on 124 part contours from 5 build jobs with different part geometries. Iterative GrabCut segmentation on nonlinearly smoothed images achieves the best results with a median Jaccard distance of 0.035 (32 % improvement over the reference geometry masks) and a mean contour distance below 2.4 px (36.4 % improvement). © 2018, Springer Nature Switzerland AG.
    view abstract10.1007/s40964-018-0068-9
  • Investigation of LBM-processed bimodal powder mixtures of the nickel base alloy HX and WC–Co
    Schaak, C. and Kleszczynski, S. and Tillmann, W. and Witt, G.
    Progress in Additive Manufacturing 4 (2019)
    Laser beam melting (LBM) is an additive manufacturing technology (AM), which enables the production of individual, complex metal parts. The development of AM-specific materials is a necessary step to exploit the full technological potential and should be a useful contribution for the ongoing process of implementing, establishing, and realizing AM processes in industrial fields of application. Theoretically, the LBM process offers the opportunity to process any weldable metal powder feedstock. Since LBM parameters affect the solidification conditions directly, this process is predestinated for future material developments. In the present paper, first research results to process the conventional LBM powder (Hastelloy X; HX) that was blended with a fine WC–Co powder are presented. Due to its high temperature and corrosion resistance, HX is widely used in industrial AM applications. WC–Co was added to improve the wear and mechanical properties. In this study, the blended, bimodal powder feedstock was analyzed, and the influence of the key process parameters on the porosity, morphology, and grain structure was investigated. The aim was to obtain the first basic information concerning the behavior of such powder blends during LBM processing and the resulting material properties. In summary, processing powder blends is possible. Since an increased porosity was determined for a high WC–Co content, the process parameters need to be further improved. In addition, it was determined that the energy input during the powder melting process directly affects the distribution of WC–Co particles within the HX matrix. High-energy inputs lead to a degradation of the WC particles. Low-energy inputs foster a consistent embedding of WC–Co particles within the HX matrix. © 2018, Springer International Publishing AG, part of Springer Nature.
    view abstract10.1007/s40964-018-0058-y
  • Laser speckle photometry - Optical sensor systems for condition and process monitoring
    Chen, L. and Cikalova, U. and Bendjus, B. and Gommlich, A. and Sudip, S.R. and Schott, C. and Steingroewer, J. and Belting, M. and Kleszczynski, S.
    Materialpruefung/Materials Testing 61 (2019)
    Laser speckle photometry (LSP) is an innovative, non-destructive monitoring technique based on the detection and analysis of thermally or mechanically activated speckle dynamics in a non-stationary optical field. With the development of speckle theories, it has been found that speckle patterns carry information about surface characteristics. Therefore, LSP offers a great potential for the characterization of material properties and monitoring of manufacturing processes. In contrast to the speckle interferometry method, LSP is very simple and robust. The sample is illuminated by only one laser beam to generate a speckle pattern on the surface. The signals obtained are directly recorded by a CCD or CMOS camera. By appropriate optical solutions for the beam path, typically, resolutions of less than 10 μm are reached if larger areas are illuminated. LSP is definitely a contactless, quick and quality relevant material characterization and defect detection method, allowing process monitoring in many industrial fields. Examples from online biotechnological monitoring and laser based manufacturing demonstrate further potentials of the method for process monitoring and controlling. © Carl Hanser Verlag GmbH & Co. KG
    view abstract10.3139/120.111308
  • Mechanical properties of honeycomb structured zr-based bulk metallic glass specimens fabricated by laser powder bed fusion
    Wegner, J. and Frey, M. and Stiglmair, P. and Kleszczynski, S. and Witt, G. and Busch, R.
    South African Journal of Industrial Engineering 30 (2019)
    Laser powder bed fusion of bulk metallic glasses offers great potential to overcome the existing restrictions of the geometrical size and complexity of bulk metallic glasses in conventional manufacturing routes due to high cooling rates during laser powder bed fusion. Bulk metallic glasses exhibit extraordinary strength, paired with high elasticity. Yet insights into additive manufactured bulk metallic glasses, especially of complex structures, are limited. The present article investigates the mechanical behaviour of Zr-based bulk metallic glasses, fabricated into honeycomb structures through laser powder bed fusion, by performing three-point bending tests. The results reveal a significant increase in specific strength, quasi-plasticity, and high elastic elongation. These structures thus offer great potential for light-weight applications and compliant mechanisms. © 2019, South African Institute of Industrial Engineering. All rights reserved.
    view abstract10.7166/30-3-2265
  • Compound quality assessment in laser beam melting processes using layer images
    Zur Jacobsmühlen, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    I2MTC 2017 - 2017 IEEE International Instrumentation and Measurement Technology Conference, Proceedings (2017)
    Laser beam melting (LBM) is an additive manufacturing (AM) technology that allows the layer-based production of geometrically complex parts from metal powder. Non-destructive evaluation of part quality is a pre-requisite for widespread application of this promising technology and an ongoing research topic. In-process measurement of part layer compound quality will be beneficial during optimization of process parameters for best part quality. We inspect the part surface in high resolution layer images acquired after laser exposure. Based on a fast Fourier transform (FFT) we decompose surface images into laser scan lines (oriented component) and underlying powder structure (not oriented component). By computing the signal energy ratio we obtain a measure for the prominence of laser scan lines which is correlated with the compound quality. For 25 parts with varied laser power and scan velocity the signal energy ratio is compared to the computed energy input. A linear model achieves a root-mean-square error (RMSE) of 9.76Jmm-3. Additionally we automatically classify part regions as good/not good and compare our results to a manual selection based on light microscopy. © 2017 IEEE.
    view abstract10.1109/I2MTC.2017.7969870
  • Nanoparticle improved metal materials for additive manufacturing
    Sehrt, J.T. and Kleszczynski, S. and Notthoff, C.
    Progress in Additive Manufacturing 2 (2017)
    A variety of laser systems and powder materials is available for additive manufacturing processes such as powder bed fusion of metallic parts (laser beam melting). The required energy density for a sufficient melting of powder materials strongly depends on the optical properties of the used powder (e.g., absorption, reflection and transmittance). During laser irradiation a moving melt pool is generated in the laser heat affected zone. Re-solidification of the molten particles results in interconnected welding lines similar to those of traditional welding processes. Here, the layer by layer approach combined with a selective laser exposure in cross-sectional areas of the parts enable the generation of 3D structures from the powder bed. The mechanical properties of such fabricated structures are usually comparable to the mechanical properties of the bulk material the powder particles are made of. In this paper, a proof of principle is demonstrated to receive improved mechanical or other properties of parts being manufactured by laser beam melting. The approach addresses laser beam melting of the commonly available powder materials tool steel (1.2709) and Hastelloy X (2.4665) which are additionally modified with nanoparticles (Al2O3) on their surfaces. Due to the shortage of these two available nanoparticle modified materials (about 100 g each) only relatively small test specimens are manufactured and, therefore, only limited typical characteristic values could be determined. However, the nanoparticle modified and laser beam molten 3D structures were systematically characterized by optical and scanning electron microscopy, energy-dispersive X-ray microanalysis, micro hardness indentation and etching analysis. It turns out that modification of the educt powder surfaces with nanoparticles prior to laser beam melting can improve e.g., mechanical properties of the generated 3D structures. © 2017, Springer International Publishing AG.
    view abstract10.1007/s40964-017-0028-9
  • Robust calibration marker detection in powder bed images from laser beam melting processes
    Zur Jacobsmuhlen, J. and Achterhold, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    Proceedings of the IEEE International Conference on Industrial Technology 2016-May (2016)
    Laser beam melting (LBM) systems produce parts by melting metal powder according to the sliced 3D geometry using a laser. After each layer, new powder is deposited and the process is repeated. Process monitoring via acquisition and analysis of layer images during the build job is a promising approach to thorough quality control for LBM. Image analysis requires orthographic images, which are usually not available as the camera cannot be placed directly above the build layer due to the position of the laser window. The resulting perspective distortions have to be corrected before analysis. To this end we compute a homography from four circular markers which are «drawn» into the powder bed by the machine's laser and detected in the acquired images. In this work we present a robust method for the automatic detection of calibration markers, which deals with the noise-like powder regions, disconnected lines, visible support structures and blurred image regions. Our homography estimation method minimizes the shape error between transformed circular reference marker shapes and detected elliptical markers yielding an image with correct aspect ratio and minimal distortions. Our method achieves a detection rate of 96.3 % and a spatial detection error of 2.0 pixels (median, 95 %-percentile: 5.17pixels) compared to a manually created ground truth. © 2016 IEEE.
    view abstract10.1109/ICIT.2016.7474873
  • Detection of elevated regions in surface images from laser beam melting processes
    Zur Jacobsmuhlen, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    IECON 2015 - 41st Annual Conference of the IEEE Industrial Electronics Society (2015)
    Laser Beam Melting (LBM) is a promising Additive Manufacturing technology that allows the layer-based production of complex metallic components suitable for industrial applications. Widespread application of LBM is hindered by a lack of quality management and process control. Elevated regions in produced layers pose a major risk to process stability as collisions between the powder coating mechanism and the part may occur, which cause damages to either one or even both. We train a classifier-based detector for elevated regions in laser exposure result images. For this purpose we acquire two high resolution layer images: one after laser exposure and another one after powder deposition for the next layer. Ground truth labels for critical regions are obtained from analysis of the latter, where elevated regions are not covered by powder. We compute dense descriptors (HOG, DAISY, LBP) on the surface image after laser exposure and compare their predictive power. The top five descriptor configurations are used to optimize parameters of Random Forest, Support Vector Machine and Stochastic Gradient Descent (SGD) classifiers. We validate the detectors with optimized parameters using cross-validation on 281 images from three build jobs. Using a DAISY descriptor with a SGD classifier we achieve a F1-score of 0.670. The presented method enables detection of elevated regions before powder coating is performed and can be extended to other surface inspection tasks in LBM layer images. Detection results can be used to assess LBM process parameters with respect to process stability during process design and for quality management in production. © 2015 IEEE.
    view abstract10.1109/IECON.2015.7392275
  • Robustness analysis of imaging system for inspection of laser beam melting systems
    Zur Jacobsmuhlen, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    19th IEEE International Conference on Emerging Technologies and Factory Automation, ETFA 2014 (2014)
    Laser Beam Melting (LBM) is an additive manufacturing process, which enables the layer-based production of complex parts from metal powder, i.e. '3D printing' with metal. In previous publications, we presented a high-resolution imaging system for inspection of LBM processes, which uses a high-resolution camera to acquire images of each powder layer and laser exposure result. The external camera position necessitates perspective correction, which is based on calibration markers which are 'drawn' onto the powder by the LBM system's laser in the first layer. As movements of the powder deposition mechanism cause vibrations, the orientation of the camera may be changed, which would invalidate the calibration results and lead to imprecise measurements or segmentations. To evaluate the effect of these disturbances, we placed calibrations markers in multiple layers and determined the position offset using template matching. We analyze the relative marker drift in three LBM processes and determine the spatial acquisition error. The maximum distance is 4.91 pixels (156.1 μm on the part), while most detected markers deviate by less than 1.5 pixels (46 μm). Compared to the pixel size of 20 μm to 32μm, these deviations are significant and require a repeated calibration in higher layers for valid high-resolution image-based measurements. © 2014 IEEE.
    view abstract10.1109/ETFA.2014.7005262
  • High resolution imaging for inspection of Laser Beam Melting systems
    Jacobsmuhlen, J.Z. and Kleszczynski, S. and Schneider, D. and Witt, G.
    Conference Record - IEEE Instrumentation and Measurement Technology Conference (2013)
    Laser Beam Melting (LBM) allows the fabrication of three-dimensional parts from metallic powder with almost unlimited geometrical complexity and very good mechanical properties. LBM works iteratively: a thin powder layer is deposited onto the build platform which is then melted by a laser according to the desired part geometry. Today, the potential of LBM in application areas such as aerospace or medicine has not yet been exploited due to the lack of process stability and quality management. For that reason, we present a high resolution imaging system for inspection of LBM systems which can be easily integrated into existing machines. A container file stores calibration images and all layer images of one build process (powder and melt result) with corresponding metadata (acquisition and process parameters) for documentation and further analysis. We evaluate the resolving power of our imaging system and show that it is able to inspect the process result on a microscopic scale. Sample images of a part built with varied process parameters are provided, which show that our system can detect topological flaws and is able to inspect the surface quality of built layers. The results can be used for flaw detection and parameter optimization, for example in material qualification. © 2013 IEEE.
    view abstract10.1109/I2MTC.2013.6555507
  • 3d printers

  • glass

  • laser beam melting

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