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Gefördert durch den Publikationsfonds der Universität Kassel

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Electron Beam Melting (EBM) is a powder-bed additive manufacturing technology enabling the production of complex metallic parts with generally good mechanical properties. However, the performance of powder-bed based additively manufactured materials is governed by multiple factors that are difficult to control. Alloys that solidify in cubic crystal structures are usually affected by strong anisotropy due to the formation of columnar grains of preferred orientation. Moreover, processing induced defects and porosity detrimentally influence static and cyclic mechanical properties. The current study presents results on processing of a metastable austenitic CrMnNi steel by EBM. Due to multiple phase transformations induced by intrinsic heat-treatment in the layer-wise EBM process the material develops a fine-grained microstructure almost without a preferred crystallographic grain orientation. The deformation-induced phase transformation yields high damage tolerance and, thus, excellent mechanical properties less sensitive to process-induced inhomogeneities. Various scan strategies were applied to evaluate the width of an appropriate process window in terms of microstructure evolution, porosity and change of chemical composition.

Die vorliegende Arbeit beschäftigt sich mit der chemischen Aktivierung des Einwegeffekts in einem Formgedächtnispolymer. Ein kommerziell erhältliches TPU wurde hinsichtlich seiner Formgedächtnisperformance analysiert. Die Auswirkungen kleiner Moleküle (Aceton, Wasser und Ethanol) auf die funktionellen und strukturellen Eigenschaften des Block-Copolymers wurden untersucht. Zu dem etablierten Spritzgussprozess wurde Probenmaterial auch durch Schmelzschichtung additiv gefertigt, um die Aktorperformance durch Optimierung der Probengeometrie zu erhöhen und diese mit Referenzproben zu vergleichen. Mittels 3D-Technik konnten die Auswirkungen der Oberflächen- zu Volumenverhältnisse hinsichtlich der Eindiffusion zum Triggern des Formgedächtniseffekts charakterisiert werden. Es zeigte sich, dass die Aktivierung insgesamt deutlich schneller erfolgt und eine Ausrichtung der Makromoleküle (Druckstrategie) eine höhere Festigkeit in Richtung der ausgerichteten Polymerketten hervorruft.

Im Rahmen dieser Arbeit wurden mikrostrukturelle, chemische und thermo-mechanische Einflussfaktoren auf die mechanischen Eigenschaften von einkristallinen Co-Basis-Superlegierungen untersucht. Dafür wurden systematische Untersuchungen durchgeführt und die zu charakterisierenden Einflussfaktoren sukzessive variiert, sodass ein Einflussfaktor möglichst isoliert von anderen betrachtet werden konnte. Bei der Untersuchung einer Legierungsserie, bei der nur der γ‘ Ausscheidungsvolumenanteil, aber nicht die Phasenzusammensetzung variiert, wurde herausgefunden, dass der Härtungsbeitrag mit zunehmendem Ausscheidungsanteil steigt. Die Druck- und Druckkriechfestigkeit nimmt mit steigendem γ‘ Gehalt zu und erreicht sogar Festigkeitsbeiträge, die mit Ni-Basis-Superlegierungen vergleichbar sind. Dabei nimmt die Stabilität der zweiphasigen Mikrostruktur allerdings ab. An zwei multinären Legierungen wurde herausgefunden, dass die Größe der γ‘ Ausscheidungen einen signifikanten Einfluss auf die Kriechfestigkeit hat, deren Form und die Anordnung aber eher eine untergeordnete Rolle für das Verformungsverhalten spielen. Bei einer genaueren Charakterisierung des Kriechverhaltens mit doppeltem Dehnratenminimum wurde festgestellt, dass die gerichtete Vergröberung einen großen Einfluss auf die Ausprägung und Lage der Minima hat, aber auch die Verformungsmechanismen und Segregation von Elementen an Planardefekte nicht außer Acht gelassen werden dürfen., The scope of this work was to investigate microstructural, chemical and thermomechanical influences on the mechanical properties of single crystalline Co–based superalloys. All investigations were carried out systematically by varying one feature gradually to characterize one specific influencing factor isolated from others. The investigation of an alloy series with varying precipitate fraction but similar phase compositions revealed that the strengthening contribution of the γ' phase increases with increasing volume fraction. The compressive and compressive creep strength is improved in a way that it is comparable to conventional Ni–based superalloys, however, the stability of the two–phase microstructure is diminished. Two multinary Co–base superalloys showed that the initial γ' size has a significant influence on the creep properties, whereas the shape and arrangement of the precipitates do not play a major role. It was confirmed that the directional coarsening of the γ' phase significantly influences the double minimum creep behavior, however, also the importance of the deformation mechanisms and segregation to planar defects was shown. Therefore, the appearance of the minima could clearly be correlated to the formation of different planar defect configurations. Additionally, a preliminary study on the tension–compression asymmetry during creep was conducted over a large temperature range. It was found that the asymmetry is pronounced differently at every temperature, however, differences between tensile and compressive creep appear at all conditions. Furthermore, the influence of the base elements Co and Ni on the deformation mechanisms was investigated. An alloy series with gradually varying base element content exhibited significantly better creep properties in the Ni–rich alloys. After evaluating and discussing different strengthening contributions, the changing deformation mechanisms are mainly responsible for these observations. The characterization of deformed samples using a transmission electron microscope revealed a variety of occurring mechanisms according to the Co/Ni content, ranging from superlattice stacking faults on the Ni–rich side to extended antiphase boundaries in the Co–rich alloys. These results helped to achieve a better understanding of the plastic deformation behavior of Co-base superalloys and to develop new design criteria for future alloy development., FAU Studien Materialwissenschaft und Werkstofftechnik; 25

In the present study the structural and functional properties of Fe–Mn–Al–Ni shape memory alloy sheets produced on an industrial process route focusing on hot rolling were investigated. The as-processed condition is characterized by a high fraction of the non-transforming γ-phase, which ensures good workability, but is associated with poor superelasticity. The alloy shows good structural properties with a yield strength of about 600 MPa, which is well above the usual transformation stress related to the martensitic phase transformation for the investigated alloy composition. After solution annealing, a microstructure showing no preferred orientation being characterized by distinctly larger grains is present. The results obtained reveal that the previous thermo-mechanical processing had no impact on the subsequent texture, however, provided a sufficient amount of driving force for abnormal grain growth. Imposed by a cyclic heat treatment, oligocrystalline structures with grain sizes above 10 mm can be achieved in the industrially processed material, which show superelastic properties similar to material processed in small batches in the laboratory.

Out-of-plane fiber waviness, also referred to as wrinkling, is considered one of the most significant effects that occur in composite materials. It significantly affects mechanical properties, such as stiffness, strength and fatigue and, therefore, dramatically reduces the load carrying capacity of the material. Fiber waviness is inherent to various manufacturing processes of fiber-reinforced composite parts. They cannot be completely avoided and thus have to be tolerated and considered as an integral part of the structure. Because of this influenceable but in many cases unavoidable nature of fiber waviness, it might be more appropriate to consider fiber waviness as effects or features rather than defects. Hence, it is important to understand the impact of different process parameters on the formation of fiber waviness in order to reduce or, in the best case, completely avoid them as early as possible in the product and process development phases. Mostly depending on the chosen geometry of the part and the specific manufacturing process used, different types of fiber waviness result. Fiber-reinforced composite materials allow for a significant mass reduction due to the comparably low density (c.f. 4-5 times less than steel) and, in addition, fibers can be aligned in accordance with the load paths. This possibility of alignment allows the fibers to be placed at the exact position where they are needed to provide the component with the required stiffness and strength. However, this can lead to a load path-optimized composite structure, which is not necessarily easy to produce and free of defects. The placement of the fibers or semi-finished textile products is still often carried out by hand-lay-up, especially in the aviation industry. This allows a diverse draping of the unidirectional (UD) layers, woven textiles or non-crimped fabrics (NCF) onto the production tool. However, manufacturing effects such as fiber waviness, porosity, delamination and distortion cannot be completely avoided. The increased demand for composite components and their production process stability for the aviation and automotive industries requires a transition to at least partially automated manufacturing processes. Those systems come with a higher deposition rate and ensure reproducible quality, but also imply production effects, e.g. fiber waviness. This necessitates a sophisticated understanding of those implicit effects on the mechanical properties of the manufactured structure. The decision as to whether these unwanted irregularities are considered as manufacturing features (effects), or as defects, depends on the size, number and location in the component. Those allowance limits depend on the strength and stiffness reserve at the location of the feature, as well as on functional requirements, e.g. water tightness. The assessment of manufacturing effects further depends on the industry. In the aviation industry, the allowance limits for defects are very restricted, while in the automotive industry the need for short cycle times leads to a trade-off between robust processes and tolerated manufacturing imperfections. To this point, there is still no generally accepted approach to quantitatively support accept/reject/repair-decisions and make a consistent assessment of wavy layers in composites. If the effect is termed to be a defect, typically a deviation from design must be requested in the aviation industry and an individual decision must be made on "use as is", repair or reject entirely. In some cases, experiments on representative test samples are performed at the subcomponent-level on a statistical basis. However, this is both time consuming and cost intensive. It is necessary to strive for a fiber-oriented and in particular a manufacturing-oriented design and construction of composite components. Towards this goal, design and production engineers aim to expand the permissible margin of safety by assessing the effect on stiffness and strength of those production effects, i.e. fiber waviness, porosity, delamination etc. Additionally, they aim to reduce or, in the best case, avoid them on the process side, increasingly with the help of finite element based process simulations. In this thesis, numerous mechanisms of wrinkling were analyzed, leading to several recommendations to prevent wrinkle formation not only during composite processing, but also at an earlier design stage, where generally several influencing factors are defined. Based on that, an overview of typically occurring wave shapes is presented and a classification scheme based on ten characteristic features is suggested for categorization purposes. The assessment of out-of-plane fiber waviness in composite materials is strongly dependent on the accuracy of detection and quantification of the wave parameters such as amplitude, wavelength and position in the laminate. In the aviation industry, ultrasonic testing (UT) is the preferred method for the evaluation of composite materials. The evaluation of the ultrasound signal from different manufacturing effects is difficult and it often cannot be clearly determined whether there are actually wavy regions in the laminate or not. In this thesis, different non-destructive testing (NDT) methods, such as infrared thermography (IRT), digital shearography, eddy current testing (ET) and X-ray computed tomography (CT) have been used to assess their potential for the detection and characterization of embedded out-of-plane fiber waviness in composite materials. These methods were applied on test plates with artificially embedded waviness with varying amplitudes, wavelengths and positions in the laminate and evaluated with respect to their ability of detecting the wrinkle morphology. The experimental non-destructive procedures of infrared thermography and digital shearography were simulated using the Finite Element Method (FEM) to gain a deeper understanding on the influence of fiber waviness on the measured results. To understand the complex failure behaviour of composite materials containing out-of-plane fiber waviness under compressive and tensile loading, numerous experimental tests have been carried out. Digital image correlation (DIC), passive thermography (IRT) and acoustic emission (AE) test methods have been used to investigate damage initiation and propagation on specimen level. In addition to that, an extensive material characterization on planar specimens was also performed. Composite materials exposed to harsh environmental conditions, i.e. hot-wet, show considerably reduced mechanical properties, governed by a degrading matrix. To investigate the effect of fiber waviness on the mechanical properties at both room temperature and after 12 months hot-wet conditioning at 70°C and 85% relative humidity, mechanical tests (compressive and tensile loading) were conducted. The basic strategies for the assessment of fiber waviness are briefly described. In engineering practice several approaches are used, i.e. empirical, generic, and semi-empirical. These include experimentally obtained knockdown factors, simplified simulations or extensive testing on subcomponent level, both experimentally and numerically. A developed micromechanical model is implemented in a MATLAB GUI to determine the effective elastic properties as well as the resulting complex stress state of uniform and graded fiber waviness. The well-established Puck failure criterion was implemented and applied on the calculated stresses to predict local ply failure and determine the strength of wavy plies. The mechanical behavior of out-of-plane fiber waviness is investigated for both unidirectional and quasi-isotropic laminates by numerically simulating damage initiation and propagation. A nonlinear material model was implemented in ABAQUS/Explicit as a material user-subroutine, which is able to capture the material behavior including shear nonlinearities, failure initiation and propagation in unidirectional laminates reasonably accurate.

Salzkavernen gelten als vielversprechende Möglichkeit Wasserstoff unter Tage zu speichern. Da aktuell keine Salzkaverne zur kommerziellen Wasserstoffspeicherung in Deutschland existiert, wurden Forschungsvorhaben initiiert, um offene Fragen in diesem Bereich zu klären. Die vorliegende Arbeit beschäftigt sich mit der Bestimmung der Dichtheit eines ausgewählten technischen Bohrungsbarriereelements: der letzten zementierten Rohrtour. Laborative Permeabilitätsmessungen stellen eine Möglichkeit dar, den Nachweis der Dichtheit zu erbringen. Zur Messung der Permeabilität werden zwei Versuchsanlagen gebaut, die auf einem instationären Messprinzip basieren. Mit Hilfe dieser Anlagen wird die Durchlässigkeit von Einzelmaterialproben und Verbundproben bestehend aus Steinsalz, Anhydrit, Zementstein und Futterrohr bestimmt und bewertet. Zur Auswertung der Versuche wird eine Software programmiert, die die eindimensionale Strömungsgleichung mit Hilfe der Finiten-Volumen-Methode numerisch löst. Die Arbeit beschreibt die weltweiten Erfahrungen im Bereich untertägiger Wasserstoffspeicherung. Weiterhin wird der Stand der Technik von Permeabilitätsmessungen dargestellt und die Eigenschaften des verwendeten Messverfahrens beschrieben. Mit Hilfe der Auswertung von Dichtheitstest kann gezeigt werden, dass die Anlagen zum Nachweis niedrigster Permeabilitäten geeignet sind. Das grundliegende mathematische Modell und dessen numerische Approximation wird hergeleitet. Die numerischen Fehler und der Modellfehler werden mit Hilfe einer Genauigkeitsanalyse bestimmt. Über die Analyse der Messunsicherheiten der Eingangsparameter erfolgt eine Abschätzung der Messunsicherheit der berechneten Permeabilität. Die Ergebnisse der Permeabilitätsmessungen zeigen, dass der untersuchte Zementstein dichte Verbunde mit Steinsalz und Futterrohr gegenüber Wasserstoff ausbilden kann. Als wichtigste Einflussgröße auf die Permeabilität wird der Effektivdruck identifiziert. Ein Einfluss der Messgase (Wasserstoff und Methan) auf das Strömungsverhalten kann, mit Ausnahme des Klinkenberg-Effekts, nicht nachgewiesen werden. Erfahrungen aus dem Bereich der Erdgasspeicherung sind prinzipiell auf die Wasserstoffspeicherung übertragbar. Die Ergebnisse der Untersuchungen sind in die Erstellung eines Leitfadens zur Errichtung von Wasserstoffkavernen für Genehmigungsbehörden und zukünftige Investoren eingeflossen.

Glass fibers (GF) and regenerated cellulose fibers (RCF) are possible partners in the hybrid reinforcement of thermoplastics because of their different properties. Due to the weak bonding properties of polypropylene, coupling agents are used and the fiber volume content is set high to achieve high reinforcing effects. A lower fiber content of GF can raise the toughness properties of a reinforced polypropylene which is investigated in this study with different ratios of GF and RCF. The composites are tested in tensile tests, flexural tests and also in notched Charpy impact tests. The results can be used to compare whether a substitution of GF with RCF or the addition of more GF leads to higher mechanical properties. The tensile and Charpy impact results are compared with the Rule of Hybrid Mixtures (RoHM) to show the deviation to the prediction. Better results in terms of stiffness and strength are seen with a higher total fiber volume, while hybrid reinforced specimens show lower toughness values compared to the RCF reinforced reference specimens. Adding 5 vol% GF to 16 vol% RCF results in an increase in tensile strength by 26%, but also a significant decrease in elongation at break by 65%.

Electron beam powder bed fusion (E-PBF) is a well-known additive manufacturing process. Components are realized based on layer-by-layer melting of metal powder. Due to the high degree of design freedom, additive manufacturing came into focus of tooling industry, especially for tools with sophisticated internal cooling channels. The present work focuses on the relationships between processing, microstructure evolution, chemical composition and mechanical properties of a high alloyed tool steel AISI H13 (1.2344, X40CrMoV5-1) processed by E-PBF. The specimens are free of cracks, however, lack of fusion defects are found upon use of non-optimized parameters finally affecting the mechanical properties detrimentally. Specimens built based on suitable parameters show a relatively fine grained bainitic/martensitic microstructure, finally resulting in a high ultimate strength and an even slightly higher failure strain compared to conventionally processed and heat treated AISI H13.

The fiber type, orientation of the fiber, fiber-matrix adhesion, and the fiber length are very important for the performance of a short fiber reinforced plastic. Hybrid reinforced polybutylene terephthalate and reference compounds were tested using tensile, Charpy impact, and three-point bending mechanical tests. The interaction of regenerated cellulose fiber and glass fiber was investigated using a polybutylene terephthalate matrix at a fiber volume content of 10%. The ratios of each fiber type was varied. The compounds were pultruded with an extrusion die to have an even fiber length of 3 mm after granulating. In a second step, the specimens were injection molded for mechanical testing. The results were compared to the rule of hybrid mixtures (RoHM) prediction. It was shown that the results of the hybrid reinforced compound were close to the RoHM prediction. The Charpy impact tests show a high positive hybrid effect. The fiber length shows an interaction that is dependent on the ratio of each fiber type.

Laser distance sensors are a widespread, fast and contactless approach for distance and surface topography measurements. Main characteristics of those sensors are given by resolution, measurement speed and sensor geometry. With decreasing sensor size, the alignment of the optical components in sensor setup becomes more challenging. The depth response of optical profilers is analyzed to obtain characteristic parameters and, thus, to value the alignment and the transfer behavior of those sensors. We present a novel miniaturized sensor setup comprising of confocal and interferometric confocal signals within one sensor in order to compare both principles simply by obscuring the reference arm by an absorber. Further, we introduce a theoretical signal modeling in order to analyze influences such as spatial coherence, Gaussian beam characteristics and tilted reflectors on depth response signals. In addition to this, we show that the coherent superposition significantly reduces the axial resolution due to the confocal effect in interferometric signals compared to simple confocal signals in measurement and simulation results. Finally, an appropriate fit function is presented, in order to figure out characteristic sensor parameters from the obtained depth response signal. In this context, a good agreement to simulated and measured signals is achieved.

The present work focuses on the effect of thermo-mechanical processing on the mechanical properties and microstructural evolution of AA6082 and AA7075 aluminum alloys using a novel forming process, i. e. integrating solution heat treatment, hot forming and tool quenching. Different tool temperatures ranging from 24 °C to 350 °C were applied to investigate their influence on mechanical strength and ductility. By using various tool temperatures, this study aims to provide insights needed for tailoring the mechanical properties of two different high-strength aluminum alloys. Further it is shown, how the different resulting cooling rates affect the final property distribution. Upon processing, uniaxial tensile tests were carried out at room temperature to characterize the mechanical properties of the investigated conditions. Microstructural investigation was further conducted by using scanning electron microscopy to reveal the prevalent deformation and strengthening mechanisms. Results obtained by mechanical testing reveal that reduction in tool temperature finally results in the realization of parts with higher strength upon aging. Tool temperatures above 200 °C deteriorate the strength of both alloys, however, improve ductility. Most importantly, the thermo-mechanical process used in the present work has only minor impact on the grain size of both alloys. However, the process appears to have a strong influence on the final morphology and size of precipitates. Elevated tool temperatures and, thus, lower cooling rates, make both alloys prone to the formation of coarse precipitates eventually deteriorating strength.

With regard to the sustainability and biological origin of plastic components, regenerated cellulose fiber (RCF)-reinforced polymers are expected to replace other composites in the future. For use under severe conditions, for example, as a housing in the engine compartment, the resistance of the composites and the impact on the fiber and fiber-matrix adhesion must be investigated. Composites of bio-polyamide with a reinforcement of 20 wt.% RCF were compounded using a twin-screw extruder. The test specimens were manufactured with an injection molding machine and aged under conditions of high humidity at 90% r. H, a high temperature of 70 °C, and water storage using a water temperature of 23 °C for 504 h. Mechanical tests, single-fiber tensile tests (SFTT), single-fibre pull-out tests (SFPT), and optical characterization revealed significant changes in the properties of the composites. The results of the SFPT show that accelerated aging had a significant effect on the bio-polymer and an even stronger effect on the fiber, as the single-fiber tensile strength decreased by 27.5%. Supplementary notched impact strength tests revealed a correlation of the impact strength and the accelerated aging of the RCF-reinforced composites. In addition, it could be verified that the tensile strength also decreased at about 37% due to the aging effect on the RCF and a lowered fiber-matrix adhesion. The largest aging impact was on the Young’s modulus with a decrease of 45% due to the accelerated aging. In summary, the results show that the strengthening effect with 20 wt.% RCF was highly decreased subsequent to the accelerated aging due to hydrolysis and debonding because of the shrinkage and swelling of the matrix and fiber. These scientific findings are essential, as it is important to ensure that this bio-based material used in the automotive sector can withstand these stresses without severe degradation. This study provides information about the aging behavior of RCF-reinforced bio-based polyamide, which provides fundamental insights for future research.

In der vorliegenden Arbeit wurde eine quinäre, einphasige \(\sigma\)-Phase mit der Zusammensetzung \(Cr_{46,0}Mn_{15,2}Fe_{16,3}Co_{17,0}Ni_{5,5}\) (in At.-%) hergestellt, ihre Phasenstabilität sowie ihre elastischen und mechanischen Eigenschaften charakterisiert und mit denen einer binären \(Cr_{48}Fe_{52}\) (in At.- %) verglichen. Die Ergebnisse zeigen, dass die quinäre \(\sigma\)-Phase oberhalb von 1170 °C in eine kubisch-raumzentrierte (krz)-Phase umwandelt. Im Gegensatz dazu, wandelt die binäre \(\sigma\)-Phase bereits ab ~800 °C in krz um. Interessanterweise zeigen die quinäre und binäre \(\sigma\)-Phasen ähnliche Elastizitätsmodule, Härte und Bruchzähigkeiten auf, d.h. sie sind beide steif, hart und spröde. Im Gegensatz dazu ist die quinäre krz-Legierung doppelt so hart wie die binäre, was darauf hindeutet, dass die Mischkristallverfestigung in ungeordneten Mischkristallen effektiver als in intermetallischen Verbindungen ist.

Scientific reports 12(1), 21862 (2022). doi:10.1038/s41598-022-26052-x, The complex thermal history imposed by the laser-based powder bed fusion of metals (PBF-LB/M) process is known to promote the evolution of unique microstructures. In the present study, metastable CrMnNi steels with different nickel contents and, thus, different phase stabilities are manufactured by PBF-LB/M. Results clearly reveal that an adequate choice of materials will allow to tailor mechanical properties as well as residual stress states in the as-built material to eventually redundantize any thermal post-treatment. The chemical differences lead to different phase constitutions in as-built conditions and, thus, affect microstructure evolution and elementary deformation mechanisms upon deformation, i.e., twinning and martensitic transformation. Such alloys designed for additive manufacturing (AM) highlight the possibility to tackle well-known challenges in AM such as limited damage tolerance, porosity and detrimental residual stress states without conducting any post treatments, e.g., stress relieve and hot isostatic pressing. From the perspective of robust design of AM components, indeed it seems to be a very effective approach to adapt the material to the process characteristics of AM., Published by Macmillan Publishers Limited, part of Springer Nature, [London]

Due to their high specific strength and temperature resistance, γ-titanium aluminides (γ-TiAl) have a growing importance for automotive and aerospace applications. However, conventional processing is very challenging due to the inherent brittleness of the material. Therefore, new manufacturing techniques and methods have to be established. Additive manufacturing techniques such as electron powder bed fusion (PBF-EB/M) are favored, since they enable near net shape manufacturing of highly complex geometries. The high preheating temperatures, which typically occur during PBF-EB/M, can significantly improve the processability of TiAl and facilitate the fabrication of complex parts. In this study, a previously optimized material condition of the β-solidifying TNM alloy TNM-B1 (Ti-43.5Al-4Nb-1Mo-0.1B) was manufactured by PBF-EB/M. The resulting microstructure, defect distribution and morphology, and mechanical properties were characterized by means of characterization methods, e.g., CT, SEM, light microscopy, hardness measurements, and tensile tests. A special focus was on the mechanical high-temperature behavior. The pronounced sensitivity of the material to defects and internal notches, e.g., due to lack of fusion defects (misconnections) which were found in the as-built condition, was identified as a main cause for premature failure below the yield point due to the low ductility. This failure was analyzed and potential improvements were identified., Progress in additive manufacturing;7(3)

Die Arbeit befasst sich mit der Ausscheidungshärtung und deren Einfluss auf mechanische und elektrische Eigenschaften von Kupfer-Titan-Legierungen mit Legierungsgehalten unter 1 Ma.-% Titan. Ein besonderer Fokus lag dabei auf der Optimierung der erzielbaren elektrischen Leitfähigkeit bei gleichzeitig hoher Festigkeit. Die zentralen Inhalte der Untersuchungen waren: • Der Einfluss des Titangehalts, der Temperatur und Dauer der Wärmebehandlung auf die aus der Ausscheidungshärtung resultierenden mechanischen und elektrischen Eigenschaftsänderungen wurde anhand von Legierungen mit 0,2 Ma.-% bis 1 Ma.-% Titan untersucht. • Der Einfluss einer erhöhten Versetzungsdichte auf die Ausscheidungshärtung und die daraus resultierenden Eigenschaften wurde anhand von unterschiedlich stark kaltumgeformten und ausscheidungsgehärteten Proben ermittelt. • Die Untersuchung des Optimierungspotentials der elektrischen Leitfähigkeit durch den Einsatz von geringen Mengen von Aluminium, Nickel, Silizium, Zink und Zinn. • Die Bestimmung des Einflusses einer Kombination von vorgelagerter Kaltumformung und Einsatz eines dritten Legierungselements auf die aus der Ausscheidungshärtung resultierenden mechanischen und elektrischen Eigenschaften. In der vorliegenden Arbeit wurde erstmals nach dem derzeit bekannten Stand der Technik und der Literatur umfassend die Ausscheidungshärtung von Kupfer-Titan-Legierungen mit weniger als 1 Ma.-% Titan untersucht. Bisherige Untersuchungen beschäftigten sich mit dem Bereich von 1,5 Ma.-% bis 6 Ma.-% Titan und in der industriellen Anwendung kommt bisher nur eine Legierung mit 3 Ma.-% Titan zum Einsatz. Aufgrund der guten, durch Ausscheidungshärtung erreichbaren, mechanischen Eigenschaften werden Kupfer-Titan-Legierungen in binärer Form oder als Mehrstofflegierungen mit Titan als Hauptlegierungselement als Substitutionswerkstoffe für Kupfer-Beryllium-Legierungen angesehen. So kommen diese vermehrt bei Steckverbindern im Automobilbau oder als Werkstoff für den Batteriekontakt oder die Kamera in Smartphones zum Einsatz. Jedoch weisen Kupfer-Titan-Legierungen eine deutlich geringere elektrischen Leitfähigkeit als Kupfer-Beryllium-Legierungen auf. Der Ansatz für die im Rahmen dieser Arbeit durchgeführten Untersuchungen ist, dass durch den Einsatz geringerer Titan-Gehalte in Kombination mit geeigneter Wärmebehandlung und Optimierung durch vorgelagerte Kaltumformung und den Einsatz weiterer Legierungselemente eine möglichst hohe elektrische Leitfähigkeit bei Erhalt guter mechanischer Eigenschaften erreichbar wird. Die experimentellen Untersuchungen von Kupfer-Titan-Legierungen mit bis zu 1 Ma.-% Titan und deren Wärmebehandlungen zur Erzielung verschiedener Auslagerungszustände zeigen, dass vor allem Legierungen mit 0,8 Ma.-% bis 1 Ma.-% Titan ein großes Potential hinsichtlich einer Ausscheidungshärtung aufweisen. Bei diesen lassen sich durch die Ausscheidungshärtung gute mechanische Eigenschaften und eine gute elektrische Leitfähigkeit einstellen. Eine deutliche Steigerung dieser Werkstoffkennwerte ist vor allem durch eine vorgelagerte Kaltumformung zu erzielen. Die Verwendung geringer Anteile an Silizium und Zink führten zu einer Beschleunigung der Ausscheidungshärtung bei einem ähnlichen Eigenschaftsprofil. Durch den Einsatz einer vorgelagerten Kaltumformung bei den Legierungen mit geringen Zusätzen von dritten Elementen zeigte sich noch eine erheblich schnellere Aushärtung bei Erzielung guter Ergebnisse für die Härte, Zugfestigkeit und elektrische Leitfähigkeit. Vor allem die schnelle Aushärtung liefert eine gute Grundlage für eine wirtschaftliche und energieeffiziente Herstellung dieser Legierungen in einer industriellen Anwendung.

Ziel der vorliegenden Arbeit war die Beschreibung der mechanischen Kennwerte im Zusammenhang mit verschiedenen Schichten von Multilayer-Zirkoniumdioxidkeramiken für die frästechnische Herstellung. Untersucht wurden vier Schichten einer jeden geprüften Keramik. Die Dichte, die biaxiale Biegefestigkeit und die Vickers-Härte wurden getestet. Eine Hälfte der Proben (n=30) wurde einer künstlichen Alterung im Thermocycler (10.000 Zyklen, 5°/55°) unterzogen, während die andere Hälfte (n=30) ohne den Einfluss künstlicher Alterung geprüft wurde. Zudem wurde an jedem Material eine EDX-Analyse durchgeführt. Ermittelt wurden die mechanischen Kennwerte der Multilayer-Zirkoniumdioxidkermamiken IPS E.max ZirCAD Prime (Ivoclar Vivadent AG; Schaan, Liechtenstein), Optimill Multilayer 3D (Dentona AG; Dortmund, Deutschland) und Ceramill Zolid fx multilayer (Amann Girrbach GmbH; Koblach, Österreich). Die Dichtewerte veränderten sich bei allen drei Materialien leicht über die Schichten hinweg. Bei den IPS-Proben zeigte sich ein Zusammenhang zwischen der biaxiale Biegefestigkeit und der Schichtung. Für die Härte gab es keine eindeutigen Rückschlüsse auf einen Zusammenhang mit der Transluzenz. Die Dentona- und die Ceramill-Proben zeigten keine an- oder absteigende Tendenz der biaxialen Biegefestigkeit über die Schichten hinweg. Insgesamt zeigten sich signifikante Unterschiede zwischen den mechanischen Kennwerten der unterschiedlichen Multilayer-Zirkoniumdioxidkeramiken (p, The aim of the present work was to describe the mechanical characteristics associated with different layers of multilayer zirconia ceramics for milling production. Four layers of each tested ceramic were investigated. The density, biaxial flexural strength and Vickers hardness were tested. Half of the samples (n=30) were subjected to artificial aging in a thermocycler (10,000 cycles, 5°/55°), while the other half (n=30) was tested without the influence of artificial aging. In addition, EDX analysis was performed on each material. The mechanical properties of the multilayer zirconia ceramics IPS E.max ZirCAD Prime (Ivoclar Vivadent AG; Schaan, Liechtenstein), Optimill Multilayer 3D (Dentona AG; Dortmund, Germany) and Ceramill Zolid fx multilayer (Amann Girrbach GmbH; Koblach, Austria) were determined. The density values changed slightly across the layers for all three materials. For the IPS specimens, there was a correlation between biaxial flexural strength and layering. For hardness, there was no clear inference of a relationship with translucency. The Dentona and Ceramill specimens did not indicate an increasing or decreasing trend in biaxial flexural strength across the layers. Overall, there were significant differences between the mechanical properties of the different multilayer zirconia ceramics (p

An essential step in the manufacturing or development of a product, or in inspecting the durability of an existing structure or machinery, is to have information about the strength of its consisting materials. Such an information, which can be summarized in the form of a stress-strain diagram, can be obtained by performing a conventional tensile test on the specimens made of the target material. However, when the fabrication of a product requires different manufacturing processes such as welding, the final structure is made of different materials that each has its own unique behavior, e.g., the unique stress-strain curve. In many cases, it is not possible to prepare a homogeneous tensile specimen from such a product, e.g. a welded joint contains three different zones such as base metal, weld seam and heat affected zone. Therefore, it is proposed to reproduce the microstructure of the interest zone in a large area by using a thermomechanical simulator to finally provide a standard tensile specimen or prepare the micro tensile specimens from the target area that require significant effort or the infrastructures which are not available in many small companies or research institutes. Consequently, an alternative approach is needed to determine the mechanical properties of a structure locally with straightforward implementation for the end user. The current research work aims to further develop the instrumented indentation technique (test) to establish a correlation between the indentation test information and the material parameters of an investigated sample. In the first step, the force-indentation depth curves obtained as the output of the instrumented indentation machine are connected with the mechanical properties of the indented samples using a trained artificial neural network. Subsequently, the methodology has been developed by training the artificial neural network based on the data from the surface of an indented sample, which is summarized in the form of the indented surface profile. The latter approach is not only more precise than the first one, it is additionally independent of the instrumented indentation machine which makes it applicable in many small companies. Moreover, there is a strong agreement between the output of the two trained artificial neural networks and the experimental results, indicating the robustness of the employed methodology. Additionally, at the end of the current report and in further work, the introduced methodology was advanced and a concept was presented to perform material characterization with artificial neural networks which are trained with the images taken from the indented surface of a specimen using a high-resolution 3D measurement system and a light microscope. Although, these trained artificial neural networks show acceptable performance and further ease material characterization for the end user, the accuracy of the predicted material parameters can be increased enormously by enlargement of the training datasets., Ein entscheidender Schritt bei der Herstellung oder Entwicklung eines Produkts oder bei der Inspektion der Haltbarkeit einer bestehenden Struktur oder Maschine ist es, detaillierte Kenntnisse über die Festigkeit der verwendeten Materialien zu haben. Eine solche Information, die in Form eines Spannungs-Dehnungs-Diagramms zusammengefasst werden kann, kann durch eine klassische Zugprüfung an den Proben aus dem Zielmaterial bestimmt werden. Erfordert die Herstellung eines Produkts jedoch verschiedene Fertigungsprozesse wie z. B. Schweißen, besteht die fertige Struktur aus verschiedenen Materialien, die jeweils ein einzigartiges Verhalten aufweisen, z. B. die einzigartige Spannungs-Dehnungskurve. In vielen Fällen ist es nicht möglich, eine homogene Zugprobe aus einem solchen Produkt herzustellen, z.B. enthält eine Schweißverbindung drei verschiedene Zonen wie Grundwerkstoff, Schweißnaht und Wärmeeinflusszone. Eine Möglichkeit besteht darin, die zu untersuchende Zone in einem großen Bereich mit Hilfe eines thermomechanischen Simulators zu reproduzieren, um somit eine normierte Zugprobe aus dem Zielbereich zu fertigen. Diese Methode bedeutet einen erheblichen Aufwand und erfordert Infrastrukturen, die in vielen kleinen Unternehmen oder Forschungsinstituten nicht verfügbar sind. Folglich wird ein alternativer Ansatz mit einfacher Implementierung für den Endanwender benötigt, um die mechanischen Eigenschaften einer Struktur lokal zu bestimmen. Die aktuelle Forschungsarbeit zielt darauf ab, die instrumentierte Eindringprüfung weiterzuentwickeln, um eine Korrelation zwischen den Informationen der Eindringprüfung und den Materialparametern einer untersuchten Probe herzustellen. Im ersten Schritt werden die als Output der instrumentierten Eindringmaschine erhaltenen Kraft-Eindringtiefen-Kurven mit Hilfe eines trainierten künstlichen neuronalen Netzes mit den mechanischen Eigenschaften der eingedrückten Proben verbunden. Anschließend wurde eine Methodik entwickelt, mit der das künstliche neuronale Netz auf Basis der Daten der Oberfläche einer eingedrückten Probe, die in Form des eingedrückten Oberflächenprofils zusammengefasst werden, trainiert werden kann. Der letztere Ansatz ist nicht nur präziser als der Erste, er ist zusätzlich unabhängig von der instrumentierten Eindringmaschine, was ihn in vielen kleinen Unternehmen anwendbar macht. Außerdem gibt es eine starke Übereinstimmung zwischen der Ausgabe der beiden trainierten künstlichen neuronalen Netze und den experimentellen Ergebnissen, was auf die Stabilität der verwendeten Methodik hinweist. Zusätzlich wurde am Ende des aktuellen Berichts und in „Further Work“ die vorgestellte Methodik weiterentwickelt und ein Konzept vorgestellt, bei dem die Materialcharakterisierung mit künstlichen neuronalen Netzen durchgeführt wird, welche mit den Bildern, aufgenommen mit einem hochauflösenden 3D-Messsystem und einem Lichtmikroskop, der eingedrückten Oberfläche einer Probe trainiert werden. Obwohl diese trainierten künstlichen neuronalen Netze eine akzeptable Performance zeigen und die Materialcharakterisierung für den Endanwender weiter erleichtern, kann die Genauigkeit der vorhergesagten Materialparameter durch Vergrößerung der Trainingsdatensätze enorm gesteigert werden.

In the present study, compositionally-graded structures of AISI 316L and CoCrMo alloy are manufactured by powder-based laser-beam directed energy deposition (DED-LB). Through a process-integrated adjustment of powder flow, in situ alloying of the two materials becomes feasible. Thus, a sharp and a smooth transition with a mixture of both alloys can be realized. In order to investigate the phase formation during in situ alloying, a simulation approach considering equilibrium calculations is employed. The findings reveal that a precise compositional as well as functional gradation of the two alloys is possible. Thereby, the chemical composition can be directly correlated with the specimen hardness. Moreover, phases, which are identified by equilibrium calculations, can also be observed experimentally using scanning electron microscopy (SEM) and energy-dispersive X-ray-spectroscopy (EDS). Electron backscatter diffraction (EBSD) reveals epitaxial grain growth across the sharp transition region with a pronounced -texture, while the smooth transition acts as nucleus for the growth of new grains with -orientation. In light of envisaged applications in the biomedical sector, the present investigation demonstrates the high potential of an AISI 316L/CoCrMo alloy material combination.

Background: Many metals exhibit a stress overshoot, the so-called cross-hardening when subjected to a specific strain-path change. Existing tests for sheet metals are limited to an equivalent prestrain of 0.2 and show varying levels of cross-hardening for identical grades. Objective: The aim is to determine cross-hardening at large strains, relevant for forming processes. Mild steel grades (DC04, DC06, DX56) and high strength steel grades (BS600, DP600, ZE800) are investigated to quantify the level of cross-hardening between different grades and reveal which grades exhibit cross-hardening at all. Method: A novel test setup for large prestrain using hydraulic bulge test and torsion of curved sheets is developed to achieve an orthogonal strain-path change, i.e. the strain rate tensors for two subsequent loadings are orthogonal. The influence of strain rate differences between the tests and clamping of curved sheets on the determined cross-hardening are evaluated. The results are compared to experiments in literature. Results: Cross-hardening for sheet metal at prestrains up to 0.6 true plastic strain are obtained for the first time. For DX56 grade the maximum cross-hardening for all prestrains have a constant level of approximately 6%, while the maximum cross-hardening for DC04 and DC06 grades increases, with levels between 7 and 11%. The high strength grades BS600 and ZE800 do not show cross-hardening behavior, while, differencing from previous publications, cross-hardening is observed for dual phase steel DP600. Conclusion: Depending on the microstructure of the steel grade the cross-hardening increases with large prestrain or remains constant., Experimental mechanics;Bd 62. 2022, H. 3, S. 441-458

Micromagnetic materials characterization is a nondestructive means of predicting mechanical properties and stress of steel and iron products. The method is based on the circumstance that both mechanical and magnetic behaviour relate to microstructure over similar interaction mechanisms, which leads to characteristic correlations between mechanical and magnetic properties of ferromagnetic materials. The prediction of mechanical properties or stress from micromagnetic parameters represents an inverse problem commonly addressed by regression and classification approaches. Challenges for the industrial application of micromagnetic methods lie in the development of robust sensors, definition of significant features, and implementation of powerful machine learning algorithms for a reliable quantitative target value prediction by processing of the micromagnetic features. This contribution briefly explains the background of micromagnetics, describes the typical challenges experienced in practice and provides insight into latest progress in the application of machine learning to micromagnetic data.

In der vorliegenden Arbeit wird hauptsächlich mittels Transmissionselektronenmikroskopie die Struktur von asymmetrischen Korngrenzen in gezielt hergestellten Kupfer-Bikristallen charakterisiert. Silbersegregation entlang der Korngrenze führte zu einem strukturellen Übergang in Form einer konzentrationsabhängigen Facettierungsreaktion. Durch die Aufnahme von Silberatomen und das Überschreiten einer kritischen Silberkonzentration an der Korngrenze werden symmetrische \(\Sigma\)5 (210) Segmente gebildet. Dabei variiert die Länge und Häufigkeit dieser Segmente mit der Silberkonzentration. Das zweite, asymmetrische Segment der facettierten Korngrenze ist konstant in Länge und damit konzentrationsunabhängig. Zuletzt wurde untersucht wie sich die Strukturänderung auf das Verhalten bei plastischer Verformung auswirkt. Es wurde gezeigt, dass der chemische Stimulus die Strukturänderung bewirkt, welche dann eine Änderung des Verformungsmusters durch das Einfügen von Facettenknotenpunkten bewirkt.

Gefördert im Rahmen des Projekts DEAL

In der vorliegenden Arbeit wurden Verarbeitungsrouten etabliert, um einphasig kfz CrCoNi und CrFeNi MEAs mit definierten Mikrostrukturen herzustellen. Beide polykristalline Legierungen wiesen zwei Arten von Zwillingen auf, die sich während der Rekristallisation (RX-Zwillinge), oder Verformung bei niedrigen Temperaturen (Verformungszwillinge) bildeten. Diese tragen durch unterschiedliche elementare Prozesse zu den mechanischen Eigenschaften bei. Um die Rolle der RX-Zwillinge auf die Streckgrenze zu untersuchen, wurden Hall Petch Plots als Funktion der Kristallitgröße (Berücksichtigung von RX Zwillings und Korngrenzen) mit denen als Funktion der Korngröße verglichen. Für CrFeNi wurden unterbrochene Zugversuche und Durchstrahlungselektronenmikroskopie kombiniert. Während in den frühen Stadien der Plastizität das planare Gleiten von Versetzungen dominierte, reduzierten mit fortschreitender Verformung Verformungszwillinge die mittlere Korngröße dynamisch (dynamischer Hall Petch-Effekt)., In the present work, processing routes were established to produce single phase FCC CrCoNi and CrFeNi MEAs with well defined microstructures. Both polycrystalline alloys exhibited two types of twins forming either during recrystallization (annealing twins) or deformation at low temperatures (deformation twins), which both contribute to the mechanical properties through different elementary processes. To investigate the role of annealing twins on yield strength, Hall-Petch plots as a function of crystallite size (accounting for both annealing twins and grain boundaries) were compared with the more traditional plots as a function of grain size. For CrFeNi, interrupted tensile tests in combination with transmission electron microscopy were performed. While, in the early stages of plasticity, planar glide of dislocations dominates, deformation twinning acts as an additional mechanism as deformation progresses, which dynamically reduces the mean grain size (dynamic Hall Petch effect).

Renewable and environmentally responsive materials are an energy- and resource-efficient approach in terms of civil engineering applications, e.g. as so-called smart building skins. To evaluate the influence of different environmental stimuli, like humidity or solar radiation, on the long-term actuation behavior and mechanical robustness of these materials, it is necessary to precisely characterize the magnitude and range of stimuli that trigger reactions and the resulting kinetics of a material, respectively, with suitable testing equipment and techniques. The overall aim is to correlate actuation potential and mechanical properties with process- or application-oriented parameters in terms of demand-oriented stimuli-responsive element production. In this study, the impact of solar radiation as environmental trigger on the cellulose-based humidity-sensing material Cottonid, which is a promising candidate for adaptive and autonomously moving elements, was investigated. For simulating solar radiation in the lab, specimens were exposed to short-wavelength blue light as well as a standardized artificial solar irradiation (CIE Solar ID65) in long-term aging experiments. Photodegradation behavior was analyzed by Fourier-transform infrared as well as electron paramagnetic resonance spectroscopy measurements to assess changes in Cottonid���s chemical composition. Subsequently, changes in micromechanical properties on the respective specimens���surface were investigated with roughness measurements and ultra-micro-hardness tests to characterize variations in stiffness distribution in comparison to the initial condition. Also, thermal effects during long-term aging were considered and contrasted to pure radiative effects. In addition, to investigate the influence of process-related parameters on Cottonid���s humidity-driven deformation behavior, actuation tests were performed in an alternating climate chamber using a customized specimen holder, instrumented with digital image correlation (DIC). DIC was used for precise actuation strain measurements to comparatively evaluate different influences on the material���s sorption behavior. The infrared absorbance spectra of different aging states of irradiated Cottonid indicate oxidative stress on the surface compared to unaged samples. These findings differ under pure thermal loads. EPR spectra could corroborate these findings as radicals were detected, which were attributed to oxidation processes. Instrumented actuation experiments revealed the influence of processing-related parameters on the sorption behavior of the tested and structurally optimized Cottonid variant. Experimental data supports the definition of an optimal process window for stimuli-responsive element production. Based on these results, tailor-made functional materials shall be generated in the future where stimuli-responsiveness can be adjusted through the manufacturing process., Functional composite materials;Vol. 2. 2021, Art.No. 9

Das Infrarotschweißen ist ein industriell etabliertes Verfahren zur Herstellung von Bauteilen in unterschiedlichsten Anwendungsbereichen. Die Prozesseinrichtung ist jedoch mit einem hohen Kosten- und Zeitaufwand verbunden, da komplexe Strahler-Werkstoff-Wechselwirkungen das Aufschmelzverhalten des Kunststoffbauteils bestimmen. In vielen industriellen Infrarotschweißprozessen ist daher ein Rauchen der infraroterwärmten Bauteilbereiche zu beobachten. Eine Erforschung des Zusammenhangs zwischen Rauchbildung, Kunststofftemperatur, thermisch-oxidativer Belastung des Kunststoffs und den resultierenden mechanischen Schweißnahteigenschaften steht bislang aus. Weiterhin sind in Infrarotschweißprozessen in der industriellen Fertigung oftmals hohe Umstellzeiten und schwankende Fügeteiltemperaturen festzustellen. In der vorliegenden Arbeit werden erstmals die mechanischen Eigenschaften von Infrarotschweißverbindungen mit der Rauchbildung und thermisch-oxidativen Kunststoffbelastung korreliert und zwei Ansätze zur schonenden Erwärmung untersucht. Die Ergebnisse weisen nach, dass eine thermisch-oxidative Kunststoffschädigung zu einer Verschlechterung der mechanischen Schweißnahteigenschaften führt und bei der Auslegung industrieller Prozesse in Betracht gezogen werden muss. Das Schweißen in Argonatmosphäre und mit aktiver Strahlerleistungsregelung verhindern die Kunststoffzersetzung und führen in der Regel zu besseren mechanischen Schweißnahteigenschaften. Weiterhin wird deutlich, dass die Fügeteiltemperatur einen vernachlässigbaren Einfluss auf die Schweißnahteigenschaften hat, wohingegen Umstellzeiten ≥ 5 s eine erhebliche Reduzierung ebendieser zur Folge haben. Eine weitere Herausforderung ist das Infrarotschweißen (endlos-)faserverstärkter Kunststoffe. Die derzeit übliche Stumpfanordnung der Fügeteile führt zu einer Faserumlenkung in der Fügenaht und hat zur Folge, dass die Faserverstärkung nicht über die Fügeebene hinweg genutzt werden kann. Im Rahmen der Arbeit wird aufgrund dessen das überlappende Infrarotschweißen von Organoblechen untersucht. Um Bauteile aus Organoblechen mit erhöhter Komplexität, Größe und Steifigkeit herstellen zu können, werden zudem zwei industriell nutzbare Verfahrensvarianten auf Basis des Infrarotschweißens entwickelt. Unter Nutzung dieser, können Organoblechhohlkörper mit Überlappverbindungen gefertigt werden. Die Ergebnisse zeigen, dass sowohl in Plattenprobekörpern als auch in Hohlkörpern eine Nutzung der Faserverstärkung über die Fügeebene hinweg möglich ist.:1 Einleitung und Zielsetzung 2 Grundlagen und Stand der Technik 3 Experimentelles 4 Analyseverfahren 5 Ergebnisse zum Stumpfschweißen 6 Ergebnisse zum Überlappschweißen 7 Bewertung der Ergebnisse 8 Zusammenfassung und Ausblick The infrared welding is a well-established process in the industrial production of parts in various applications. However, the complex emitter-material interaction, which influences the meltdown behaviour of the plastic parts, results in a high effort for the process setup. A smoking of the plastic parts is to observe in numerous industrial infrared welding processes. The correlations between the smoking of the plastic, its temperature and thermal-oxidative degradation as well as the mechanical properties of the resulting welds are unidentified yet. Furthermore, in industrial infrared welding processes often high changeover times and varying joining part temperatures are existent. Therefore, within the present work the connections between the mechanical joint properties of infrared welds and the thermal-oxidative degradation of plastics are elaborated for the first time and two approaches for the gentle infrared heating are investigated. The findings prove that the thermal-oxidative degradation of the plastic substantially decreases the mechanical weld properties and needs to be taken into account when setting up the industrial infrared welding process. The welding in argon atmosphere and the use of an active infrared emitter power control, which ensures the heating of the plastic below the degradation temperature, lead to better mechanical weld properties. In addition, the outcome of this work shows that the influence of the temperature of the joining part is negligible regarding the mechanical joint properties, whereas changeover times greater than or equal to 5 s lead to a dramatical decrease in the mechanical properties. Another challenge is the infrared welding of fibre reinforced plastics. The butt welding of fibre reinforced thermoplastics is common practice and prevents the use of fibres in the joint plane due to the fibre deflection in this area. As a result, the overlapping welding of organo sheets is investigated as well. In order to produce large and complex parts with high stiffness made of organo sheets, two process variants on the basis of the infrared welding technology are developed, which can be used at the industrial scale to manufacture hollow bodies. The overlapping welds of specimens and in hollow bodes made of organo sheets, enable the fibre utilisation across the joint plane.:1 Einleitung und Zielsetzung 2 Grundlagen und Stand der Technik 3 Experimentelles 4 Analyseverfahren 5 Ergebnisse zum Stumpfschweißen 6 Ergebnisse zum Überlappschweißen 7 Bewertung der Ergebnisse 8 Zusammenfassung und Ausblick

Gefördert im Rahmen des Projekts DEAL; Hessen State Ministry of Higher Education, Germany; Universiti Sains Malaysia

Das ultraschallunterstützte Rührreibschweißen (USE-FSW) als innovatives Hybrid-Pressschweißverfahren zeichnet sich durch eine Reihe von Vorteilen aus, welche es für die Kombination artfremder metallischer Werkstoffe mit deutlich unterschiedlichem Schmelzpunkt ermöglichen, qualitativ hochwertigere Verbunde zu realisieren. Die vorliegende Arbeit thematisiert experimentelle Untersuchungen der Auswirkungen des zusätzlich eingekoppelten Leistungsultraschalls auf das Prozessverhalten und die Bindungsmechanismen sowie daraus resultierender mechanischer Verbundeigenschaften beim ultraschallunterstützten Rührreibschweißen von Aluminium/Stahl-Verbunden. Im Anschluss an die Ermittlung geeigneter Parameter für das konventionelle Rührreibschweißen erfolgen grundlegende Betrachtungen des Einflusses des Leistungsultraschalls auf das Schwingungsverhalten, das thermische Verhalten und das insitu- Prozesskraftverhalten, aus denen bestmögliche Ultraschallparameter abgeleitet werden. Nachfolgende detaillierte, vergleichende Untersuchungen des konventionellen und des ultraschallunterstützten Rührreibschweißprozesses belegen unter anderem eine Reduktion der Dicke spröder, aluminiumreicher intermetallischer Phasen am Verbund-Interface des USE-FSW-Verbundes, was in einer Erhöhung der Zugfestigkeit und der Duktilität resultiert. The ultrasound enhanced friction stir welding (USE-FSW) as an innovative hybrid solid state joining process is characterized by a number of advantages that enable the realization of higher quality joints for the combination of dissimilar, metallic material combinations with strongly differing melting points. The present work addresses the impact of the additional power ultra- sound transmission on the process behavior and the bonding mechanisms as well as resulting mechanical joint properties for the ultrasound enhanced friction stir welding of aluminum/steel joints via experimental investigations. Subsequent to the determination of suitable parameters for the conventional friction stir welding basic considerations of the power ultrasound influence on the oscillation behavior, the thermal behavior and the in-situ process force behavior take place for deriving a best possible set of ultrasound parameters. Moreover, the conventional and the ultrasound enhanced friction stir welding process are investigated comparatively in detail, proving a reduction in thickness for brittle, aluminum-rich intermetallic phases at the USE-FSW joint interface among other things, resulting in an improved tensile strength and ductility.

In the investigation at hand, sheets of ferritic and pearlitic grey cast iron with spheroidal graphite are welded using a fiber-laser. The influence of varying laser power and welding speed on the macroscopic characteristics of the weld seam and crack formation are analyzed and discussed. Substantial crack formation with crack densities of up to 2.93/cm are found. Furthermore, hardness measurements are conducted to evaluate the formation of brittle phases within the weld metal and heat-affected zone. Peak hardness values of up to 860 HV0.3 and 975 HV0.3 are identified in EN-GJS-400-15 and EN-GJS-700-2, respectively. Subsequently, a proposition on the dependence of crack formation on weld seam width is given. Furthermore, the influence of nickel-containing filler material on the microstructure, crack formation and hardness is studied. It can be derived from the results that laser-beam welding of grey cast iron with spheroidal graphite requires further research in order to achieve crack-free weld seams and hinder the formation of undesirable, brittle phases.

Zugleich: Dissertation, Universität Kassel, 2020

Gefördert im Rahmen des Projekts DEAL

In den letzten Jahren hat das Recycling von technischen Thermoplasten aus industriellen Anwendungen, wie z. B. aus der Automobilbranche, stetig zugenommen. Die Technologien zum Recycling sind dabei Stand der Technik, problematisch hingegen ist die Charakterisierung der Eigenschaften der Recyclate, die oft einer erneuten Anwendung im technischen Bereich entgegensteht. Aufgrund des unbekannten Belastungsbildes des Recyclates sind Eigenschafts- und Lebensdauervorhersagen schwierig, die aber dringend benötigt werden, um die Recyclate auf dem Markt als Werkstoffanbieten zu können. Oftmals fehlt ein Datenblatt für diese Werkstoffe, was wiederum die Voraussetzung für den Einsatz und die Qualitätssicherung bei den Bauteilen ist. Die Arbeit setzt an dieser Stelle mit Polyamid-6-Recyclaten an, deren Eigenschaftsänderungen infolge von Alterungsprozessen, auch im Vergleich zu Frischmaterial, zu untersuchen sind. Die Zielstellung besteht in der Erstellung eines Datenblattes und der Erfassung der Werkstoffkennwerte. Zur Erfüllung der Zielstellung tragen die Konzipierung von Untersuchungsmethoden und Ausführung von Versuchen zur gezielten Alterung sowie die Bestimmung der resultierenden Eigenschaften der PA-Recyclingwerkstoffe bei, ergänzt durch tribologische Untersuchungen an Recyclaten. Das Ergebnis soll in eine Abschätzung von Nutzungsmöglichkeiten und Anwendungsgrenzen einfließen.:Ziel und Motivation Eigenschaften von PA 6 und PA 6-GF-15-Recyclaten Tribologische Untersuchungen Mehrfachverarbeitung Langzeitverhalten Fazit Danksagung

Zugleich: Dissertation, Universität Kassel, 2020

Nb3Sn Rutherford cables are used in CERN’s superconducting 11 T dipole and MQXF quadrupole magnets, which are proposed for the instantaneous luminosity (rate of particle collisions) upgrade of the Large Hadron Collider (LHC) by a factor of five to a High Luminosity-Large Hadron Collider (HL-LHC). Nb3Sn-based conductors are the key technology for the envisioned Future Circular Collider (FCC) with an operating magnetic dipole field of 16 T. The baseline superconductor of the LHC dipole magnets is Nb–Ti, whereas an operation above 10 T is not possible due to the current carrying performance limitations of this superconductor at higher magnetic fields. Therefore, a superconducting material such as Nb3Sn has to be used with proven performance capabilities of 10 T and above. The conductor choice towards Nb3Sn-based cables affects the magnet manufacturing process, as it requires a heat treatment up to 650°C, an epoxy resin impregnation and introduces mechanical diffculties as the superconducting filaments are brittle and strain sensitive. A mechanical over loading of the filaments lead to irreversible conductor damage. The designs of 11 and 16 T magnets are supposed to push the conductor towards its mechanical and electrical performance limitations. The magnetic field induced forces on the current carrying conductor are balanced by mechanical pre-loading of the magnet. Thereby the highest controlled mechanical pre-load for the 11 T dipole magnet is set at ambient temperature. The mechanical stress limits of Nb3Sn-based cables have been investigated at cryogenic temperatures. The material strength and stiffness of the cable insulation system, formed by glass-fibre-reinforced resin, is increased at low temperatures. The ultimate stress values, determined at cryogenic temperature, are therefore not conservative. The ultimate stress limitation of the insulated conductor is assumed to be lower at ambient temperature. The cable limitations at ambient temperature need to be known for the ongoing magnet manufacturing process and also for future design approaches. Furthermore, the compressive stress–strain behaviour of a coil conductor block at ambient temperature is the key material characteristic, in order to recalculate the stress level in the coil during the assembly process. Existing approaches using an indirect strain measurement method provide uncertainties in the low-strain regime, which is the essential strain range for a material compound consisting of major fractions composed of heat-annealed copper and epoxy resin. Compressive stress–strain data of an impregnated conductor block are required, based on a direct strain measurement system, as available data have been collected on samples based on a different strand type and insulation system. The elaborated direct strain measurements can be correlated to strain gauge data, measured directly on a coil. The stress distribution in a Nb3Sn Rutherford cable need to be understood and validated to understand strain-induced degradation effects in the insulated conductor. This knowledge can also help to optimise the stress distribution envisioned magnet designs. The stress–strain state in the copper and Nb3Sn phase of a loaded conductor block has to be determined experimentally. This dissertation describes a test protocol and first elaborated results on the investigated stress limitations of a Nb3Sn Rutherford cable under homogeneous load applied in transversal direction. The compressive stress–strain behaviour of impregnated Nb3Sn Rutherford cable stacks was investigated experimentally. This includes a detailed report on the sample manufacturing process, measurements performed and validation of results through a comparison with the elaborated data of cable stacks extracted from a coil. The presented results from neutron diffraction measurements of loaded cable stacks allow the determination of the stress–strain level of the copper and Nb3Sn phase in the impregnated conductor. The relevant measured results have been recalculated with numerical calculations based on the Finite Element Method (FEM).:1. Introduction 1 1.1. The LHC and the HL-LHC project 1.2. The FCC study 1.3. Superconducting materials for accelerator magnets 1.4. Multi-filamentary wires and Rutherford cables 1.5. Coil manufacturing process 1.6. Magnet coil assembly 1.7. Objectives of this thesis 2. Theory: fundamental principles 17 2.1. Analytical calculation: sector coil dipole 2.2. Mechanical behaviour of composite materials 2.3. Failure criteria and strength hypotheses for materials 2.4. Compressive tests 2.5. Fundamental principles of Neutron scattering 2.5.1. Test apparatus and measurement method 2.5.2. Lattice plane and Miller indices 2.5.3. Bragg diffraction and interference 2.5.4. Diffraction-based strain calculation 2.5.5. Diffraction-based stress calculation 2.6. Fundamental principles of FEM 3. Homogeneous transversal compression of Nb3Sn Rutherford cables 3.1. Superconducting cable test stations 3.2. The FRESCA test facility and specific sample holder 3.3. The sample description 3.4. Experimental procedure 3.5. Review of existing contact pressure measurement system 3.6. Compressive test station 3.7. Validation of the pressure-sensitive films 3.8. Press punch 3.9. Improvement of the contact stress distribution 3.9.1. First test: cable pressed between the bare tools 3.9.2. Second test: tool shimmed with a soft Sn96Ag4 3.9.3. Third test: tool shimmed with a soft Sn60Pb40 3.9.4. Fourth test: tool shimmed with a soft indium 3.9.5. Fifth test: tool shimmed with a polyimide film 3.10. Test results 3.11. Conclusion 4. Material characterisation by a compression test 4.1. Test set-ups for compressive tests and validation 4.2. Sample preparation 4.3. Compressive stress–strain measurement 4.4. Ten-stack sample stiffness estimation-based composite theories 4.5. Dye penetration test on loaded and unloaded samples 4.6. Conclusion 5. Neutron diffraction measurements 80 5.1. Test set-up for neutron diffraction measurement 5.2. The samples 5.3. Experiment: lattice stress–strain measurements 5.4. Conclusion 6. Simulation and modelling of Nb3Sn cables 6.1. The models 6.2. The 2D simulation results 6.3. The 3D simulation results 6.4. Conclusion 7. Comprehensive summary 7.1. Summary 7.2. Critical review 7.3. Next steps Appendix 113 A. Calculation of the magnetic field components in a sector coil without iron B. Approaches for the determination of diffraction elastic constants C. Manufacturing drawings D. FEM calculation results of the 2D model E. FEM calculation results of the 3D model F. Source Codes Bibliography

Many researchers hold the view of silicon nanowires as a potential constituent for future nano-sensors and devices, owing to their electrical, mechanical and optical properties and inherent small diameters. Within the scope of potential silicon nanowire applications, high aspect ratio bottom-up grown silicon nanowires can be applied in scanning atomic force microscopy (AFM). In general, to fabricate silicon nanowire-based devices, nanowires are either synthesized directly in regions of interest or transferred to a device substrate. Fabricating a single silicon nanowire-based device using bottom-up grown silicon nanowires is still challenging and elaborate, as it is currently realized by applying advanced and sophisticated techniques such as focused-ion-beam deposition for confining the growth area. Also, current silicon nanowire integration techniques require still further development towards large-scale production and industrialization, as the main criteria including reproducibility, cost-effectiveness and control over the nanowire position and orientation were not met. The main goal of the thesis is a realization of a low-cost and facile approach for silicon nanowire integration based on both, growth and assembly strategies. Unlike the reported techniques and methods for fabrication of nanowire scanning probes, advanced and sophisticated approaches shall be avoided. In this work, silicon nanowires are applied in atomic force microscopy (AFM) probes as a scanning tip, by integrating silicon nanowires into fabricated micro-sized Si3N4-cantilevers. In order to realize a single silicon nanowire via the bottom-up growth, a new strategy is proposed by applying the contact printing method and regrowing silicon nanowires from the gold particles at the tip of the contact printed nanowires. In principle, by contact printing silicon nanowires into a region of interest (here on the tip region of cantilevers), the gold particles at the tip of the nanowires can be employed again as catalyst to yield silicon nanowire scanning tips. Beside the fabrication, characterization of the fabricated probes is also one of the thesis goals. The characterizations are targeted by testing the fabricated probes in AFM and comparing it to a pyramidal silicon tip. Furthermore, the mechanical behaviors of silicon nanowire tips on micro-cantilevers, including the silicon nanowire softness and oscillation behavior, are studied.

The use of micro fibres in Ultra-High-Performance Concrete (UHPC) as reinforcement increases tensile strength and especially improves the post-cracking behaviour. Without using fibres, the dense structure of the concrete matrix results in a brittle failure upon loading. To counteract this behaviour by fibre reinforcement, an optimal bond between fibre and cementitious matrix is essential. For the composite properties not only the initial surfaces of the materials are important, but also the bonding characteristics at the interfacial transition zone (ITZ), which changes upon the joining of both materials. These changes are mainly induced by the bond of cementitious phases on the fibre. In the present work, three fibre types were used: steel fibres with brass coating, stainless-steel fibres as well as nickel-titanium shape memory alloys (SMA). SMA fibres have the ability of &ldquo, remembering&rdquo, an imprinted shape (referred to as shape memory effect), triggered by thermal activation or stress, principally providing for superior performance of the fibre-reinforced UHPC. However, previous studies have shown that NiTi-fibres have a much lower bond strength to the concrete matrix than steel fibres, eventually leading to a deterioration of the mechanical properties of the composite. Accordingly, the bond between both materials has to be improved. A possible strategy is to roughen the fibre surfaces to varying degrees by laser treatment. As a result, it can be shown that laser treated fibres are characterised by improved bonding behaviour. In order to determine the bond strength of straight, smooth fibres of different metal alloy compositions, the present study characterized multiple fibres in series with a Compact-Tension-Shear (CTS) device. For critical evaluation, results obtained by these tests are compared with the results of conventional testing procedures, i.e., bending tests employing concrete prisms with fibre reinforcements. The bond behaviour is compared with the results of the flexural strength of prisms (4 &times, 4 &times, 16 cm3) with fibre reinforcements.