@phdthesis{BesongBesong2024, author = {Besong Besong, Lemopi Isidore}, title = {Development of novel hole-flanging processes}, doi = {10.26127/BTUOpen-6859}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-68597}, school = {BTU Cottbus - Senftenberg}, year = {2024}, abstract = {Flanges are essential elements in sheet metal parts, where they perform important functions such as increasing structural stiffness, serve as bearing seats and positioning aids. Currently, flanges are mainly formed by the conventional hole-flanging process, which uses dedicated dies and punches. The limitations of conventional hole-flanging include the high cost involved in die design and manufacture, poor accessibility to form flanges in some complicated dies, and low formability. High-speed tool rotation is introduced in hole-flanging processes to increase the process temperature and forming limit of hard-to-form materials. This is done using paddle-shaped tools and spherical punches in existing production lines. In addition, strategies are explored to enhance the geometrical accuracy in robot-based hole-flanging by single-point incremental forming (SPIF). This thesis investigates new hole-flanging process variants by using experiments and finite element (FE) analysis. Empirical material models are used to study the effects of the process parameters on flange shape, forming limits, process temperature, and forming mechanics. The process formability is highest in hole-flanging with punch rotation and reduces for paddle forming and hole-flanging by SPIF. The least formability is observed in conventional hole-flanging. High-speed tool rotation and tool feeds form flanges with high formability. Surface strain measurements of the flanges show that low-speed tool rotation combined with high tool feeds favor crack formation. Tensile tests revealed that the material's formability increases with temperature and strain rate. From FE analyses of the process variants, the deformation in hole-flanging was determined to be mainly because of membrane stretching and bending. Some shear was present in paddle forming and SPIF. Based on FE analyses and experiments, the temperature and strain rate are determined to be the main parameters that account for the difference in process formability. Hole-flanging by SPIF is conducted on a robot manipulator to enhance geometrical accuracy. New tool paths are explored to prevent flange conicity. A parametric study is conducted to determine the process's feasibility. The stiffness of a robot is compensated to achieve the target flange geometry.}, subject = {Bohrungsflanschen; Hole-flanging; Paddle forming; Robot incremental forming; Process development; Deformation analysis; Schaufelumformung; Inkrementelle Roboterumformung; Verformungsanalyse; Prozessentwicklung; Kragenziehen; Flansch; Prozessentwicklung ; Deformation; Blechumformen; Finite-Elemente-Methode}, language = {en} } @phdthesis{Berding2016, author = {Berding, Jens}, title = {Entwicklung eines wandlungsf{\"a}higen Pressensystems mit Servospindelantrieb}, publisher = {Winter-Industries GmbH}, address = {Berlin}, isbn = {978-3-86624-633-1}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-41227}, school = {BTU Cottbus - Senftenberg}, year = {2016}, abstract = {Unternehmen der Umformtechnik sehen sich vermehrt steigenden Anforderungen bei schwankenden Einflussfaktoren ausgesetzt. F{\"u}r eine wirtschaftliche Fertigung sind Pressen erforderlich, die sich den steigenden Anforderungen und schwankenden Einflussfaktoren anpassen und mit m{\"o}glichst hoher Produktivit{\"a}t, Qualit{\"a}t und Wirtschaftlichkeit eine breite Produktpalette fertigen k{\"o}nnen. Unter diesen Voraussetzungen bietet sich der Einsatz von kraftgebundenen Servospindelpressen an. Diese erm{\"o}glichen frei programmierbare St{\"o}ßelbewegungsprofile und stellen die Nennkraft {\"u}ber den gesamten St{\"o}ßelhub zur Verf{\"u}gung. Allerdings erf{\"u}llen die bekannten Servospindelpressen die Forderung nach wandlungsf{\"a}higen Pressen, welche an unterschiedliche Anforderungen angepasst werden, besonders die {\"A}nderung der Abfolge von Teilprozessen und der Austausch von Maschinenkomponenten, nur sehr eingeschr{\"a}nkt. Aus diesem Grund wird ein neuartiges wandelbares Pressensystem entwickelt. Dieses soll dem Anwender weitreichende Vorteile bringen: Die Nennkraft des Pressensystems wird auf mehrere, in Durchlaufrichtung der Teile frei positionierbare und miteinander koppelbare Pressenmodule mit eigenen Pressenantrieben und -st{\"o}ßeln aufgeteilt. Durch Servospindelantriebe werden prozessangepasste St{\"o}ßelbewegungsprofile f{\"u}r einzelne Teilprozesse erm{\"o}glicht. Dies erlaubt eine wandelbare Anordnung von Teilprozessen, wodurch die Prozesskette skaliert werden kann. Unterst{\"u}tzt wird die Wandlungsf{\"a}higkeit des Pressensystems, indem es durch den Anwender erweitert werden kann und sich weitere Prozesse wie Schweißen, L{\"o}ten oder Kleben integrieren lassen. Ziel der Arbeit ist die Entwicklung des beschriebenen wandlungsf{\"a}higen Pressensystems mit Servospindelantrieb f{\"u}r die Komplettbearbeitung von kleinen Bauteilen. Grundlage bildet die Untersuchung des Standes der Pressentechnik und der Methoden der Produktentwicklung. Der Entwicklungsprozess wird in die Phasen Anforderungsfindung, Konzeptfindung und Entwurfsfindung gegliedert. Zur {\"U}berpr{\"u}fung wird aus dem Entwurf ein Prototyp ausgearbeitet. Im Anschluss werden Maßnahmen zur Produktstrukturierung durchgef{\"u}hrt, aus denen die Weiterentwicklung des Entwurfes zu einer Typengruppe und einem Baukastensystem resultiert. Zum Abschluss wird das Pressensystem bewertet, sein Einsatzspektrum beschrieben und das Pressensystem innerhalb bestehender Pressen eingeordnet.}, subject = {Umformtechnik; Pressensystem; Wandlungsf{\"a}higkeit; Servopresse; Spindelpresse; Forming technology; Press system; Adaptability; Servo press; Spindle press; Presse ; Spindel ; Servomotor}, language = {de} } @phdthesis{Maqbool2021, author = {Maqbool, Fawad}, title = {Targeted generation and suppression of the deformation mechanism and residual stresses in incremental sheet forming to improve the geometric accuracy}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-55646}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {The geometric accuracy of an incrementally formed part is dependent on the underlying deformation mechanism and the residual stresses induced in the material during forming. This thesis presents new insights into the deformation mechanism and residual stresses of the Single Point Incremental Forming (SPIF) variant of the ISF process. In addition, strategies to control, suppress and adjust the deformation mechanism and residual stresses to increase the geometric accuracy and improve part properties are presented. The deformation mechanism of the ISF process is investigated. A novel methodology is developed to split the plastic energy dissipation during the SPIF process as a contributions of energies from dominant deformation modes. It is found that the dominant deformation mode can be bending, shear or membrane stretching, depending on the selected values of the process parameters. By controlling the contribution of each deformation mechanism, the outcome of the SPIF process can be designed for maximum geometric accuracy within the constraints of process time and formability. The build-up of the residual stresses in the SPIF process is investigated and a relation between the geometric accuracy and the residual stresses under changing process parameters is developed. The wall angle parameter has the highest influence on residual stresses. Moreover, the intensity and the magnitude of the residual stresses can be controlled by adjusting the process parameters. A direct approach based on post-forming Stress Relief Annealing (SRA) is developed to suppress residual stresses and increase geometric accuracy. In this regard, a modular tooling set-up is designed to perform the SRA under partial constraint. For maximum efficiency, optimal SRA parameters are determined. Two parts with practical significance are incrementally formed and subsequently stress relief annealed. A considerable increase in the geometric accuracy is observed with SRA in comparison to the parts without SRA. In the last part of this thesis, the very first approach is presented to generate a target distribution of high magnitude residual stresses in the ISF process for improving the mechanical properties. The desired residual stresses are generated in the conventionally formed disc springs in the surface treatment approach. In an integrated forming approach, the forming of the disc springs and the desired residual stresses are generated in a single step. Mechanical properties of the disc springs are better for both approaches in comparison to conventional disc springs. Hence, a targeted generation and distribution of the residual stresses to improve the mechanical properties is possible.}, subject = {Incremental sheet forming; Inkrementelle Blechumformung; Deformation mechanism; Residual stress; Geometric accuracy; Annealing; Verformungsmechanismus; Eigenspannung; Geometrische Genauigkeit; Gl{\"u}hen; Blechumformen; Eigenspannung; Deformation; Genauigkeit}, language = {en} } @phdthesis{Nguyen2022, author = {Nguyen, Qui Lam}, title = {Tool path planning for wire-arc additive manufacturing processes}, doi = {10.26127/BTUOpen-5982}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-59827}, school = {BTU Cottbus - Senftenberg}, year = {2022}, abstract = {Among variants of AM technology, wire-arc additive manufacturing (WAAM) process suitably produces bulky metal parts with a medium complexity. Although WAAM shows a great potential, this process has not been fully explored, and it is therefore a worthwhile subject to further investigate. This monograph engages with tool path planning for the WAAM process and existing problems in WAAM are subsequently addressed, as following: The existing overlapping models yield an uneven surface due to the inner beads overlapping with two neighbor beads while the outers have only a one-sided overlap. New mathematical models are established to obtain optimal distances between adjacent weld beads and to make surfaces more even. Lightweight structures have high strength as compared to their weight. Producing these structures by WAAM results in imperfections because of uneven weld beads - an inevitable phenomenon when starting a new track. A new tool path strategy to produce lightweight structures is developed using contour patterns, which transforms any arbitrary geometry into that of a continuous path. Unfortunately, voids are formed at junctions when using contour patterns. An adaptive correction using machine learning is then developed to overcome this defect. Large overhangs and inclined features are out of reach of WAAM when operating in the x-y plane in Cartesian system. This limits the ability of the WAAM process. To overcome that, each overhang feature should be welded with a distinct direction. This approach is realized within the robot based WAAM process. Establishing the kinematics of the FANUC robot arm and the positioner helps compute the new coordinates, the angular displacements of the positioner, and the orientation of the torch. The proposed strategy is examined through the construction of an overhang part. The capabilities of the multi-axis deposition are extended by constructing curved thin-walled structures. Irregular thickness layers are deposited by controlling the travel speed, resulting in a different deposition rate. The curved thin-walled structures can be properly fabricated by combining a multi-axis deposition with non-uniform thickness layers. Bonding quality as well as geometric accuracy are also to be investigated. To ensure geometric accuracy, a compensation strategy is developed. The performance of the proposed algorithm is validated on different geometries. This monograph will conclude with a summary of this work's main achievements and contributions as well as outlooks on future research.}, subject = {Wire-arc additive manufacturing; Lightweight structures; Curve thin-walled structures; Multi-bead overlapping models; Lichtbogenadditive Fertigung; Leichte Strukturen; Gekr{\"u}mmte d{\"u}nnwandige Strukturen; {\"U}berlappende Modelle; Rapid Prototyping ; D{\"u}nne Schale; Lichtbogen; {\"U}berlappung}, language = {en} } @phdthesis{Ulbricht2022, author = {Ulbricht, Alexander}, title = {Multi-scale correlation between defects and internal stresses in additively manufactured AISI316L structures}, doi = {10.26127/BTUOpen-6374}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-63746}, school = {BTU Cottbus - Senftenberg}, year = {2022}, abstract = {Additive Manufacturing of metals has become relevant for industrial applications. The near net-shape production of components produced by Laser Powder Bed Fusion (PBF-LB/M) enables new possibilities in component design combined with a reduction of the amount of needed material. Omitting the extra material, that was part of conventionally produced components due to machining constraints, results in components which in consequence lack the inherent additional safety margins provided by the higher material consumption of conventionally produced components. Therefore, to use PBF-LB/M metals in safety critical applications an in-depth understanding of porosity and internal stresses in parts made by PBF-LB/M is needed. Only non-destructive testing methods—such as computed tomography and residual stress analysis using neutrons—enable the assessment of porosity and stresses in the whole part. In this thesis I tackled creep and tensile static deformed specimens to fill research gaps in this field in terms of analysing PBF-LB/M stainless steel AISI 316L: from structural properties to in-situ behaviour. The initial void population of AISI 316L is studied using X-ray and synchrotron micro computed tomography. Specimens produced with different process parameters were analysed to quantify the influence of process parameters on the initial void population. The possibility to close voids using the laser illumination of subsequent layers is discussed by a quantitative study of the ability of the laser to melt different multiples of the applied layer thickness. The formation of internal stress is inherent to components produced by the PBFLB/ M process. These stresses remain in the components after production as residual stresses. In this thesis a study is presented which aims to unravel the mechanisms that define the spatial distribution of the residual stresses, and their magnitude. In the end, the population of internal voids during mechanical testing is studied by X-ray micro computed tomography. The evolution of damage accumulation in creep specimens is studied at different stages of the creep test. Results are compared to a creep tested conventionally made specimen and to a PBF-LB/M specimen from a tensile test. An interconnection between the PBF-LB/M microstructure and the pattern of damage is revealed.}, subject = {Additive manufacturing; Computed tomography; Neutron diffraction; Residual stress; 316L; Additive Fertigung; Computertomographie; Neutronenbeugung; Edelstahl; Rapid Prototyping (Fertigung); Selektives Laserschmelzen; Zerst{\"o}rungsfreie Werkstoffpr{\"u}fung; Computertomografie; Neutronenbeugung}, language = {en} } @phdthesis{Imran2021, author = {Imran, Muhammad}, title = {Investigating and modeling the effect of metal forming-related measures for damage-controlled hot forming}, doi = {10.26127/BTUOpen-5710}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-57105}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {Within the process chains of the semi-finished production, hot forming is used to eliminate pores and voids from the casting process under compressive stresses and to adjust the microstructure for further processing. In the case of caliber rolling process, tensile stresses occur at certain roll gap ratios which promote void formation (damage) on non-metallic inclusion. Decohesion of matrix and inclusion in void formation is determined by local flow stress. Since the process is carried out above recrystallization temperature, damage cannot only be controlled through stress state (stress triaxiality and Lode parameter) but also the softening processes like dynamic recovery (DRV) and recrystallization (DRX) which can relieve local stresses. Such processes can be used to delay or prevent the damage initiation and growth and hence the mechanical properties of components can be improved. In spite of this, the influence of DRX on damage development has not been quantified so far. This study focuses on the understanding the interaction between softening processes and damage during hot forming and to derive a model for coupled microstructure and damage evolution. For precise damage modeling, in-depth damage characterization of the specimens deformed during hot tensile tests under different stress states and deformation conditions (temperature and strain rate) is conducted. The stress states are varied by designing the specimens with different notch radii. To model the influence of local stresses on damage, continuum-based damage modeling approach is adapted where a system of matrix-inclusion-interface is considered. The influence of different parameters such as DRX, stress state, temperature, strain rate, total strain, inclusion size and type (soft/hard) is analyzed. Based on the damage quantification results, an extension to Gurson-Tvergaard-Needleman (GTN) damage model is devised where a new nucleation criterion couples the stress state and DRX to the void formation at inclusions. The extended GTN model is utilized to predict the internal damage during multi-step hot caliber rolling process and validated with experimentally observed damage at different locations of stress state and recrystallized volume fraction. Based on the model, a damage-controlled strategy is proposed by finding the solution of optimal control problem (OCP) to obtain an optimal variable deformation speed that can induce low damage as compared to constant speed while keeping the process time constant. To implement the damage-controlled strategy, an isothermal forming simulator is designed and manufactured that can replicate the real load paths during hot forming. It is suggested that damage can be controlled if material is allowed to recrystallize at low deformation speed without inducing damage and then, speed up the deformation after onset of recrystallization.}, subject = {Damage modeling; Dynamic recrystallization; Damage; Hot forming; Hot caliber rolling; Sch{\"a}digungskontrolle; Sch{\"a}digungsmodellierung; Dynamische Rekristallisation; Warmumformung; Spannungszustand; Warmumformen; Werkstoffsch{\"a}digung; Schadensanalyse}, language = {en} } @phdthesis{Babel2025, author = {Babel, Christoph Josef}, title = {Virtual evaluation of car body panel surfaces}, doi = {10.26127/BTUOpen-7196}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-71960}, school = {BTU Cottbus - Senftenberg}, year = {2025}, abstract = {The progression of the automotive industry has introduced a slew of complex designs to meet the personalized style expressions of consumers. However, these intricate designs in premium automobiles present significant manufacturing challenges, especially in detecting and prioritizing the varying severities of cosmetic surface defects. Traditionally, defect identification is performed at the end of the development phase, which is a costly and time-consuming process. This thesis seeks to enhance early defect detection efficiency in the concept phase using artificial intelligence (AI). Through examination of unpainted parts, auditors' perceptual practices, and deep draw simulations, the research aims to establish a virtual prediction method to accurately classify surface defects. The findings of this thesis prove that the geometry of unpainted parts sufficiently predicts potential defects post-painting, refuting the need for physical painted part analysis. The identification process adopted by auditors, grounded in analyzing light distortions on surfaces, is replicated within a neural network. Furthermore, by incorporating physiological optical aspects, the thesis improves the virtual representation of defects, creating a visible and analyzable dataset for AI algorithms. The accuracy of the neural networks trained on such datasets is substantiated by the successful translation of auditors' classification methods into a machine learning environment. In particular, a machine learning approach using Random Forest (RF) algorithms excelled in prefiltering areas of interest based on curvature and strain values. A convolutional recurrent neural network (CRNN) is developed to classify the severity of defects, with the introduction of an annotation application to label data by experts. The CRNN demonstrated a remarkable accuracy of 86 \% in classifying defect severity based on simulation data. Moreover, the study assessed the frame-by-frame localization of defects, where the PatchCore anomaly detection algorithm proves optimal, achieving an F1 Score of 0.86. Overall, the research successfully shows the transferability of expert auditors' perception into a neural network architecture, highlighting the feasibility of implementing AI in early phases of automotive design to predict and classify defects, which has the potential to significantly reduce development costs and improve manufacturing efficiency.}, subject = {Computer vision; Neural network; Deep draw simulation; Rendering; K{\"u}nstliche Intelligenz; Maschinelles Lernen; Umformsimulation; Bildgenerierung; Kraftfahrzeugindustrie; Produktentwicklung; Tiefziehen; Fehlererkennung; Simulation; K{\"u}nstliche Intelligenz; Maschinelles Lernen}, language = {en} }