FG Füge- und Schweißtechnik
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- schweißen (26)
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- welding (12)
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- Laserstrahlschweißen (5)
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In the current paper, the correlation between the physical size of additively built wire arc specimens and their structure and properties is studied. For the purpose of this work, two oval shaped specimens of different lengths were manufactured under the same technological conditions. The specimens have a length of 200 mm and 400 mm and will be referred to as L200 and L400. The microstructure of the samples was studied using X-ray diffraction analysis (XRD), optical microscopy, and scanning electron microscopy (SEM). The microhardness, yield strength (YS), and ultimate tensile strength (UTS) were determined and their correlation with the technological conditions of specimen build-up was clarified. The results of the carried out experiments indicated that the crystallographic structure of both specimens is similar. The scanning electron microscopy images show a higher concentration of irregularly shaped micro-pores formed near the edge of the αAl grains in the structure of the L400 specimen compared to the L200 one. An increase in the size of the αAl solid solution grains in the case of the L200 specimen towards its top section was noticed using optical microscopy. A slightly lower magnitude change was noticed concerning the L400 specimen. The increase in the size of the aluminum crystals was determined to be the increasing interpass temperature. Due to the much smaller thermal dissipation capacity of the smaller specimen, the interpass temperature of the same increased faster compared to the larger specimen. All of the above-mentioned factors led to a decrease in the microhardness of the specimens at higher stages of build-up. Since the specimens were deposited using similar layer deposition conditions, the resultant YS and UTS data are also highly comparable.
The mass production of metallic components requires high agility in the working process conditioned by the necessity of building details of different shapes and sizes. Changing the size of the components theoretically influences the thermal dissipation capability of the same, which could lead to a change in their structure and mechanical properties. This is particularly important when aluminum alloys are concerned. For this reason, two Al5356 single-track specimens were built using the same technological conditions of layer deposition by varying only their geometrical size. In all cases, the specimens were wire and arc additively manufactured (WAAM) using a process based on gas metal arc welding (GMAW) in the cold arc pulse mode (CAP). The structure of both specimens was studied and defects along their surfaces were detected in the form of micro-pores and micro-cracks. A high concentration of undissolved Mg particles was also detected, along with some standalone Si particles. Uniformity in the build-up process was achieved, which led to the formation of nearly identical structures in the specimens. Subsequently, the resultant mechanical properties were also highly comparable. This indicates that the geometry-related variation in thermal conditions has an insignificant influence on the component’s structure and properties.
The effect of heat treatment at various temperatures (650, 850, 1050, and 1100°C) and dwell times (10 min and 1 h) on the metallurgical and microstructural evolution as well as on the related tensile properties of stainless steel 316L processed by selective laser melting (SLM) has been systematically evaluated. The metallurgical and microstructural features such as defects, stability of the columnar–cellular structure and substructure, second phase particles, and phase transformation imparted by SLM and heat treatment have been discussed. It has been shown that the processing conditions specific to SLM significantly alter the kinetics of phase evolution compared to standard welding techniques which affects the accuracy of the prediction. The influence of these characteristics on tensile properties and hardness was elucidated. It was disclosed that with increasing heat treatment temperature there was a gradual increase in elongation but a decrease in strength related to the dislocation density and the development of the microstructure.
In this publication, cold gas spraying (CGS) is investigated as an enabler for aluminum-steel joints. Using a powder-based coating process to adhere a steel layer to an aluminum substrate allows a steel component to be welded to the deposited layer by resistance spot welding. This method permits the metallurgical connection between similar materials to be separated, while mechanical bonding ensures the connection at the dissimilar aluminum-to-inlayer interface. A modification of the porous CGS layer, as well as the creation of the remelted zone in the aluminum, can be observed during the resistance spot welding process. Electron backscatter diffraction (EBSD) analyses show that the severely prestressed particles in the CGS coating recrystallize, which coincides with a decrease in defect density and hardness in the heat-affected zone. Microscopy of the aluminum substrate shows the creation of metallurgical pores as well as the expansion of pores attributed to the casting process. The rise in remelted aluminum hardness and decrease in the heat-affected zone of the CGS layer indicate the formation of a metallurgical notch.
Im vorgestellten Projekt wurden werkstoff- und konstuktionsgerechte Knotenformen aus Aluminiumschaumsandwich (engl. Aluminium Foam Sandwich, AFS) gestaltet und mit unterschiedlichen Schweißverfahren wie dem Metall-Inertgasschweißen und dem Rührreibschweißen gefügt und geprüft. Im Rahmen der Arbeiten stand dabei die schweißtechnische Verarbeitung von AFS mit anderen Produkten, wie den Strangpressprofilen oder dem Aluminiumvollmaterial, und deren Beanspruchbarkeit im Vordergrund. In umfangreichen Untersuchungen wurden hierzu sowohl die Verbindugsknoten selbst als auch der Sandwichverbund und die Einzelkomponenten des Sandwich, Schaum und Decklagen, anhand der statischen Festigkeit und Schwingfestigkeit bewertet. Zusätzlich wurden innere Schädigungsmechanismen betrachtet. Mit experimentellen und numerischen Nachweismethoden konnte die anwendungssichere Knotengestaltung, -anbindung und -berechnung der unterschiedlichen Verbindungstypen umgesetzt werden. Die Ergebnisse wurden im Rahmen der Nachweisführung gegenübergestellt und hinsichtlich eines praxisgerechten Einsatzes nach FKM-Richtlinie bzw. Eurocode9 bewertet. Mit den ermittelten Ergebnissen wurde die bisherige Gestaltungs- und Fügerichtlinie um neue Verbindungstypen erweitert.
Dieser Beitrag stellt eine Entwicklung, Anpassung und Untersuchung der neuartigen Pinschweißtechnik zur Verbindung thermoplastischer Faserkunststoffverbunde mit metallischen Fügepartnern dar. Die untersuchte Fügetechnik bietet, im Vergleich zu anderen Verfahren, neben einer einseitigen Zugänglichkeit, ein hohes Leichtbaupotenzial. An Multimaterial-Einzelpinverbindungen wurden die CMT-Pinschweißbarkeit charakterisiert und unterschiedliche Fügestrategien erprobt und ausgewertet. Als Bewertungskriterien wurden das Schweißgut sowie der Faser- und Matrixerhalt in Abhängigkeit von den Schweißparametern untersucht. Die mechanische Beanspruchbarkeit der mit dem entwickelten Verfahren erstellten Verbindungen wurde in Scherzugversuchen ermittelt. An Mehrpinverbindungen wurde anschließend der Einfluss der Pinanordnung untersucht und die Auslegung der Fügezone analysiert. Der Fügeprozess wurde an Funktionsmustern und diese wiederum in 3-Punkt-Biegeversuchen validiert sowie mit dem Kleben verglichen.
Modern material-compatible joining methods for fiber-reinforced plastics require the heating of the materials. In order to predict the respective complex temperature fields and curves, the Fraunhofer IAP and the BTU
Cottbus-Senftenberg have developed numerical methods, which are able to simulate different radiation sources and process sequences as well.
Investigations on the thermal conditions during laser beam welding of high-strength steel 100Cr6
(2023)
This study examines the thermal conditions during laser beam welding of 100Cr6 high-strength steel using a TruDisk5000 disc laser with a continuous adjustable power range of 100–5000 W. Two parameter sets, characterized by laser power and welding speeds, were analyzed by thermal-metallurgical FE simulations to determine their impact on the thermal conditions during welding. The results show a significant shift in heat coupling, with conduction transitioning to deep penetration welding. As a result of the high welding speeds and reduced energy input, extremely high heating rates up to 2∙104 K s−1 (set A) respectively 4∙105 K s−1 (set B) occur. Both welds thus concern a range of temperature state values for which conventional Time-Temperature-Austenitization (TTA) diagrams are currently not defined, requiring calibration of the material models through general assumptions. Also, the change in energy input and welding speed causes significantly steep temperature gradients with a slope of approximately 5∙103 K mm−1 and strong drops in the temperature rates, particularly in the heat affected zone. The temperature cycles also show very different cooling rates for the respective parameter sets, although in both cases they are well below a cooling time t8/5 of 1 s, so that the phase transformation always leads to the formation of martensite. Since the investigated parameters are known to cause a loss of technological strength and conditionally result in cold cracks, these results will be used for further detailed experimental and numerical investigation of microstructure, hydrogen distribution, and stress-strain development at different restraint conditions.
In recent years, lightweight construction and the demand for resource and energy efficiency have increasingly supported the use of high-strength steels. Laser beam welding (LBW) of these materials is used in industrial mass production to efficiently manufacture high-precise components and parts with the highest quality requirements. Avoiding welding-related defects such as weld distortion and cold cracking is critical. Conventional applications currently meet this requirement to a limited extent due to very restricted process tolerances and the use of non-critical materials, which limits the potential of the joining process. Based on FE welding process simulations, concepts have been developed to reduce distortion and cracking through active control of the LBW process. The underlying models consider the weld induced temperature field, microstructure transformations, and residual stresses to calculate distortion. In addition, the local hydrogen concentration is calculated, and the results of the welding process simulation are evaluated using a cold cracking tool that includes material-specific cracking criteria. The ability to simulate distortion and cold cracking behavior opens up the possibility of parameter variation. From the data collected, concepts of active force introduction with dynamic workpiece clamping have been derived that lead to distortion and cold cracking reduction and promote the weldability of high-strength materials.
Neuartige, werkstoffgerechte Fügeverfahren für Faserkunststoffverbunde setzen die Erwärmung der Materialien voraus. Um die damit verbundenen komplexen Temperaturfelder und -verläufe vorherzusagen, haben das Fraunhofer IAP und die BTU Cottbus-Senftenberg numerische Verfahren entwickelt. Mit diesen können auch verschiedene Strahlungsquellen und Prozessabläufe simuliert werden.
Das Widerstandspunktschweißen ist ein weit verbreitetes Schweißverfahren, insbesondere bei der Karosserieherstellung in der Automobilbranche. Im Hinblick auf den Leichtbau werden zunehmend Aluminiumlegierungen verwendet. Das Hauptproblem beim Widerstandspunktschweißen von Aluminiumlegierungen ist der schnelle Elektrodenverschleiß. Dadurch wird die Qualität und Effizienz des Schweißprozesses beeinträchtigt. Die Elektrodenstandmenge ist erreicht, wenn die Schweißverbindung eines der Qualitätskriterien nicht erfüllt. Um die Elektrodenstandmenge zu ermitteln, werden in der Industrie im Rahmen der Schweißnahtqualifizierung zeit- und ressourcenaufwändige Methoden, u. a. zerstörende Prüfmethoden, verwendet. In dieser Arbeit wird eine Methode zur Vorhersage des kritischen Elektrodenverschleißes beim Aluminiumschweißen durch Messung des dynamischen Widerstandes vorgeschlagen. In umfangreichen Versuchsreihen an einer automobiltypischen Al-Legierung der 5000er Gruppe wurden dynamische Widerstandsmessungen durchgeführt. Aus den Widerstandsmesswerten konnte ein Indikator ermittelt werden, welcher auf das Auftreten von Poren in Schweißverbindung und auf einen bald auftretenden kritischen Elektrodenverschleiß hinweist.
Представлены концепция и тестирование новой модификации изобретенного ранее ультразвукового преобразователя для толщинометрии в высокотемпературных иммерсионных средах. Суть модификации заключается в удалении нижнего электрода пьезоэлемента и использовании электропроводности окружающей жидкости для передачи электрического заряда на контактирующую с жидкостью поверхность пьезоэлемента. Благодаря отсутствию заземляющего электрода разработанная технология обеспечивает заметное повышение точности прибора, а также значительное улучшение отношения сигнал/шум. Кроме того, предложенная модификация позволяет снизить нижнюю границу измеряемой толщины. Это обеспечивается за счет укорочения посылаемого ультразвукового импульса благодаря минимизации рассеяния излучаемого сигнала, обусловленного в исходном варианте нижним электродом. Таким образом, несмотря на некоторые отдельные сложности, которые могут возникнуть при применении модифицированного преобразователя, представленная концепция обеспечивает заметное улучшение качества измерения.
This work presents the concept and testing of a new modification of a previously invented high-temperature resistant ultrasonic transducer for immersive thickness measurement. The modification lies in the removal of a front electrode and using the electrical conductivity of the surrounding liquid to transfer the electrical charge onto the liquid-contacted surface of a piezo actuator. By not using a front electrode the developed technology provides a considerable increase of the transducer sensitivity, as well as a significant improvement of the signal-to-noise ratio. Furthermore, it reduces the minimum measurable thickness by shortening the impulse due to minimization of the scattering of emitted signal caused by front electrode. Therefore, despite some isolated complications, which could arise in special cases of application of modified transducer, the presented concept provides an appreciable improvement of the measuring quality.
Among different joining methods, the electron beam welding is recently applied for manufacturing of turbine components from temperature-resistant nickel-based conventionally cast Alloy 247 LC CC. However, the high tendency to hot cracking, in particular the formation of solidification cracks, remains a major challenge. Experiments indicate a significant reduction in hot cracks if the welding is performed outside the common welding parameter range. To understand these observations, a study of local thermo-mechanical conditions during electron beam welding of Alloy 247 samples was carried out using numerical simulations. The results were subsequently compared with reference test welds. For this purpose, a finite element model for coupled transient thermal and mechanical analysis was created and used. The work presents a comparative analysis of the evolution of strain components in brittle temperature range during cooling, considering the distribution and orientation of the cracks. Various relations between local strain kinetics and crack appearance, with notable influence of the plastic strain vector, were observed. Finally, the aspects of assessment of hot crack susceptibility with aid of thermo-mechanical welding simulation are discussed.
Microhardness variations across the friction stir welded (FSW) and impulse friction stir welded (IFSW) AA2024–T351 joints have been elucidated by the transformations of the S–Al2CuMg phase with a special focus on a distinguished hardness peak within the heat-affected zone (HAZ) of the impulse welds. The increase in hardness within the stir zone (SZ) originated from the partial re-precipitation of the initial Guinier-Preston-Bagaryatsky zones (GPB) and metastable S needles, previously dissolved.) Formation and growth of stable S precipitates via coalescence accounted for the softening through the thermo-mechanically affected zone (TMAZ). The peak strengthening within the HAZ of the IFSW joints was mainly caused by the dense needle-shaped S particles, which can be explained by a mutual influence of the process specific temperature and strain cycles. Dislocations and subgrain boundaries introduced to the material due to plastic deformation facilitated the nucleation of strengthening S precipitates in the HAZ. It demonstrates that the impact of deformation should be considered by the characterization of the precipitation development in the HAZ.
Das pulverbasierte 3D-Metalldrucken ermöglicht die Fertigung von hochkomplexen Integralbauteilen, die als Trägerstruktur für Mikroreaktoren benutzt werden. Nach einer Beschichtung der Struktur mit einem Katalysatormaterial, können diese Mikroreaktoren höchst effizient für die CO2-Methanisierung eingesetzt werden. Durch die additive Fertigung mit dem selektiven Laserstrahlschmelzen (SLM) wurden adaptierte dreidimensionale periodische Gitterstrukturen mit sehr geringen Wandstärken hergestellt. Diese weisen ein sehr günstiges Verhältnis von Oberfläche zu Volumen (TPMS - Triply periodic minimal surface) auf. Des Weiteren zeigen die TPMS-Strukturen vorteilhafte Strömungseigenschaften, die sowohl für die Beschichtung mit dem Katalysator-Material als auch die Durchströmung mit den Reaktionsmedien im Betrieb essentiell sind. So ist es beispielsweise möglich, durch die Variation der Dimension einer TPMS-Struktur die Oberfläche pro Volumeneinheit, den hydraulischen Durchmesser des Reaktors und somit auch den Transportprozess des Fluids deutlich zu verbessern. Diese Faktoren ermöglichen eine effektivere Gas-Katalysator-Reaktion. Im Vergleich zu Mikroreaktorstrukturen aus den traditionellen Herstellungsverfahren liefern die additiv gefertigten Strukturen sowohl eine höhere CO2-Umwandlungsrate als auch eine CH4-Selektivität innerhalb des diffusionskontrollierten Bereichs.
Im vorgestellten Projekt wurden werkstoff- und konstruktionsgerechte Knotenformen aus Aluminiumschaumsandwich gestaltet und mit unterschiedlichen Schweißverfahren, wie dem Metall-Inertgas-Schweißen und dem Rührreibschweißen, gefügt und geprüft. In umfangreichen Untersuchungen wurden die Knoten, der Sandwichverbund und die Einzelkomponenten des Sandwich, Schaum und Decklagen, anhand der statischen Festigkeit und Schwingfestigkeit bewertet. Zusätzlich wurden innere Schädigungsmechanismen betrachtet. Mit experimentellen und numerischen Nachweismethoden konnte die anwendungssichere Knotengestaltung, -anbindung und -berechnung der unterschiedlichen Verbindungstypen umgesetzt werden. Die Ergebnisse wurden im Rahmen der Nachweisführung gegenübergestellt und hinsichtlich eines praxisgerechten Einsatzes nach FKM-Richtlinie bzw. Eurocode9 bewertet. Mit den ermittelten Ergebnissen wurde die bisherige Gestaltungs- und Fügerichtlinie um neue Verbindungstypen erweitert.
3D-printed structured catalysts for CO2 methanation reaction: Advancing of gyroid-based geometries
(2022)
This work investigates the CO2 methanation rate of structured catalysts by tuning the geometry of 3D-printed metal Fluid Guiding Elements (FGEs) structures based on periodically variable pseudo-gyroid geometries. The enhanced performance showed by the structured catalytic systems is mostly associated with the capability of the
FGEs substrate geometries for efficient heat usages. Thus, variations on the channels diameter resulted in ca. 25% greater CO2 conversions values at intermediate temperature ranges. The highest void fraction evidenced in the best performing catalyst (3D-1) favored the radial heat transfer and resulted in significantly enhanced catalytic activity, achieving close to equilibrium (75%) conversions at 400 ◦C and 120 mL/min. For the 3D-1 catalyst, a mathematical model based on an experimental design was developed thus enabling the estimation of its behavior as a function of temperature, spatial velocity, hydrogen to carbon dioxide (H2/CO2) ratio, and inlet CO2 concentration.
Its optimal operating conditions were established under 3 different scenarios: 1) no restrictions, 2) minimum H2:CO2 ratios, and 3) minimum temperatures and H2/CO2 ratio. For instance, for the lattest scenario, the best CO2 methanation conditions require operating at 431 ◦C, 200 mL/min, H2/CO2 = 3 M ratio, and inlet CO2 concentration = 10 %.
Due to the recent developments of hardware components and the hereby resulting ability to increase process parameters, the application area of the cold gas spray technology is expanding quickly. The present
research focuses on the influence of working gas pressure and working gas temperature on the adhesive strength of inner diameter coatings, which were produced with two different alloy steel powder variants. Gas pressure and gas temperature were varied in four different parameter sets. At first, the powder variants were examined for morphology and particle size distribution. Secondly, the influence of
four different process parameters on the achievable particle velocity was measured. In addition, the arithmetical mean height (Sa) of the coating was measured in order to determine the effect of the four parameter sets on the achievable surface roughness. Furthermore, the impact of the process parameters on the steel particles’ penetration
depth into the aluminum substrate was examined. Finally, adhesion strength measurements of the inner diameter coatings were carried out. The results reveal that with rising process parameters, the particle velocity increases, and the achievable surface roughness is lowered. It was also shown that the penetration depth of the particles into
the substrate increases with increasing particle velocity. In addition, this study demonstrated a dependence of the process parameters on the adhesion strength for inner diameter coatings.
The beam inclination leads to a change in the laser spot size on the material surface. The higher the inclination, the larger the irradiated area and the lower the laser intensity. Moreover, if the material surface is outside of the beam focal plane, the intensity distribution profile becomes asymmetric. In this study, a heat source model, which calculates the intensity distribution on the workpiece surface as a function of beam parameters (beam waist, divergence half-angle) and process parameters (laser power, incidence angle, and distance to focal plane) was developed. The applicability of the heat source model was demonstrated by simulating 4 different laser hardening regimes. Once the heat efficiency coefficient had been calibrated the developed finite-element model allowed computation of temperatures while hardening with perpendicular laser beams as well as with inclined beams. The open-source software FEniCSx was used for the finite element computations. The mathematical formulation, required for performing temperature simulations with FEniCSx was briefly introduced.
Due to recent developments, cold gas spraying technology can now be used to create inner diameter coatings for cylinder inner diameters > 70 mm. The present investigations focus on the process optimization and the specific properties of cold gas spray inner diameter coatings created with three different alloy steel powder variants. The cold gas spray coating properties were compared with the corresponding properties of coatings created with twin wire arc technology. The particle velocities and deposition efficiencies were measured with the aim of optimizing the process parameters. The most suitable process parameters were used to analyze the microstructure of the deposited coating in terms of porosity and interface quality. Furthermore, the hardness and adhesion strength properties of the coatings were measured. In addition, the different liners were honed, and the achievable surface roughness of each was determined. Finally, wear resistance was evaluated using ball-on-disk testing. The results reveal that with the maximum process parameters, the cold gas spray coating properties are comparable to the twin wire arc coating properties. Further investigations are necessary to determine whether cold gas spraying is a feasible alternative to the current series production process for cylinder surface coatings.
Durch den strukturellen Einsatz von Faser-Kunststoff-Verbunden (FKV) lassen sich hochwertige gewichtsoptimierte Karosserien in Mischbauweise umsetzen. Die untersuchte CMT-Pin-Schweißtechnik zur Verbindung von thermoplastischen Organoblechen mit Stählen bietet, im Vergleich zu anderen Verfahren, neben einer einseitigen Zugänglichkeit, ein hohes Leichtbaupotenzial. Das Vorhaben wurde auf eine werkstoff-, prozess- und konstruktionsgerechte Umsetzung des Verfahrens ausgerichtet.
Auf Basis experimenteller und numerischer Untersuchungen wurde eine Methode zum Vorwärmen von Organoblechen mittels Infrarotstrahlung entwickelt. Die Eignung der CMT-Pin-Technologie wurde sowohl für das Fügen von karbon- als auch glasfaserverstärktem PA6 betrachtet. Als Bewertungskriterien wurden das Schweißgut sowie der Faser- und Matrixerhalt in Abhängigkeit der Schweißparameter untersucht. Das entwickelte Verfahren wurde mit herkömmlichen Fügemethoden bzgl. der in Kopf- und Scherzugversuchen ermittelten Festigkeiten verglichen. Zur Bestimmung der Beständigkeit der Verbindungen gegen unterschiedliche Witterungsbedingungen wurden Salzsprühnebel- und Wechselkorrosionstests durchgeführt. Mit dem Fügen von Hutprofilen wurde die Anwendung der entwickelte CMT-Pin-Technik am Tunnel sowie den Längs- und Querträgern der Bodenstruktur eines Kraftfahrzeugs demonstriert.
For the purpose of this research, single track details were manufactured in the shape of thin walls with a length of 100 mm and a height of 80 mm. Two welding speeds were chosen for this experiment–13.3 mm/s and 20.0 mm/s corresponding to the following heat inputs: 120 J/mm and 80 J/mm. The gas metal arc welding (GMAW) method was used for the build-up of the specimens in the cold arc pulse mode. The structure of the specimens was studied using X-ray diffraction (XRD) analysis carried out with CuKα radiation with a wavelength of 1.5406 Ǻ, optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). Furthermore, the Vickers hardness of the samples was determined using a ZwickRoell DuraScan 10/20 G5 unit at a force of 1 N. A preferred crystallographic orientation towards the (200) plane was observed in all cases, however a vastly textured structure was observed with inclusions of peaks in the (111), (220), and (311) crystallographic planes. The full width at half maximum (FWHM) of samples taken from different stages of build-up was calculated indicating an increase of the dislocation density at the more advanced stages of specimen growth. Despite that an increase of the hardness was observed towards the top of both specimens. This is attributed to the change in the structure of the αAl + Si formations from an irregular one at the bottom of the specimens, towards a fibrous one at the top. The results are discussed in regard to the optimization of the build-up process during wire arc additive manufacturing (WAAM).
Impulse Friction Stir Welding (IFSW) was utilized to join 6082–T6 alloy plates at various impulse frequencies. A distinctive feature of IFSW is the generation of mechanical impulses that enhances the forging action of the tool, and thereby, alters the weld microstructure. The microstructural evolution in the Stir Zone (SZ) with special focus on the strengthening precipitation behavior, and overall mechanical properties of the IFSW joints have been investigated. It was demonstrated that the strengthening β″ precipitates reprecipitated in the SZ of the IFSW joints during natural aging. In contrast, no precipitates were found in the SZ of the Friction Stir Welding (FSW) weld. Partial reversion of β″ after IFSW is supposed to occur due to more developed subgrain network and higher dislocation density introduced by impulses that accelerated precipitation kinetics. Dynamic recrystallisation was facilitated by impulses resulting in a fine, homogeneous structure. There was no significant difference between the microhardness in the SZ, tensile and yield strength of the FSW and IFSW joints. However, the application of impulses demonstrated the smoothing of the hardness reduction in the transition region at the advancing side. The shift of the fracture location from the Heat-Affected Zone (HAZ) by FSW to the SZ as well as higher elongation of the joints by IFSW of lower frequencies could be related to the grain refinement and the change of the grain orientation.
Während in einem konventionellen Ultraschallprüfkopf ein Kunststoffdämpfungskörper mit Kleber oder Öl an einen Piezoschwinger angekoppelt ist, werden als Hochtemperaturlösung feinporöse sintermetallische Dämpfungsmaterialien mit einer Flüssigglasankopplung eingesetzt. Um dabei das Sintermetall vor dem korrosiven Angriff der Glasschmelze zu schützten, wird am Dämpfungskörper gegenwärtig Gold- oder Platinfolie zeit- und kostenaufwendig appliziert.
In der aktuellen Arbeit wurde eine Methode zum Korrosionsschutz der sintermetallischen Oberfläche aus rostfreiem Stahl mittels Laser-Pulverauftragschweißen entwickelt. Im Laufe einer Schweißparameterstudie auf dem Substratmaterial aus massivem rostfreiem Stahl wurde die Eignung unterschiedlicher Auftragsmaterialien auf Nickel- und Kobaltbasis untersucht. Dabei wurde zunächst der Einfluss verschiedener Schweißparameter wie Laserleistung, Vorschubgeschwindigkeit und Pulvermenge auf die Auftragsqualität (Nahtform, Vermischungsgrad, Porosität) metallografisch bewertet. Anschließend wurden die aufgetragenen Schichten hinsichtlich ihrer Korrosionsbeständigkeit getestet. Der Kontakt mit einer flüssigen Glasmischung bei 500 °C hat bei keiner der beschichteten Proben sichtbare Korrosionswirkung gezeigt. Als Resultat der Schweißparameterstudie haben sich besonders die nickelbasierten
Pulver als gut geeignete Materialien für den Prozess gezeigt. Die identifizierten Schweißparameter wurden im nächsten Schritt erfolgreich an das sintermetallische Substrat angepasst.
Crystallography and Mechanical Properties of Wire Arc Additively Manufactured Al5356 Components
(2021)
In this study, we present results of the evolution of the crystallographic texture and imperfections, as well as the corresponding mechanical properties of wire and arc additively manufactured (WAAM) Al5356 specimens, studied by X-ray diffraction (XRD) and tensile testing, respectively. The results show that the phase composition is in the form of a face-centred cubic (fcc) crystal structure, corresponding to the base material, as well as some traces of oxide phase Al2O3. The WAAM specimen growing is accompanied by a slight change in the preferred crystallographic orientation, from (111) to (200) and a decrease in the imperfections concentration. Also, it was found that the ultimate tensile strength (UTS), yield strength (YS), and elongation slightly decrease from the initial to the more advanced stages of growth.
Analysis of temperature field during electron beam welding of hot crack sensitive nickel base alloy
(2021)
The use of temperature-resistant nickel-based superalloys in the field of industrial gas turbines is increasing due to constantly rising operating temperatures. Among different joining methods the electron beam welding is commonly used for manufacturing of turbine components because it provides a reduced heat input. However, the highly localized beam impact causes high temperature gradients and leads to the formation of hot cracks. Welding of such material with a high brittle temperature range is challenging and needs an increasing effort of process adaption. Experiments indicate a significant improvement of hot crack susceptibility when welding outside the common welding parameter range. To understand these observations a study of the temperature field in the specimens using numerical simulations was carried out. For this purpose, a validated simulation model was used. Special attention was paid to the domain in close vicinity of the weld pool. The results illustrate the velocity of solidification isotherm as well as the temperature rates when crossing the melting point. Finally, the relations between thermal fields, crack types and distributions in the different specimens are discussed.
Structured materials can be progressive alternatives to commonly used
flat sheets because of their higher bending stiffness and stability compared to flat sheet metals, made of the same alloy. The application of sheet metals requires accurate information regarding their strength and deformation behavior. Such data are not commonly available and have to be measured by specific test setups and implementation
of tests. The aim of this work is to obtain new knowledge about deformation mechanisms of structured sheet metals. Structured sheet metals (SSM) made of conventional deep-drawing steel DC04 were investigated by means of tensile tests. The influence of the structure type arrangement on the deformation behavior was analyzed. The evolution of local strains was analyzed by means of strain gauge
measurements as well as 3D-displacement measurements with an ARAMIS highresolution camera system. Local orientation changes in different structural elements were measured using the electron backscatter diffraction technique.
Mixing of steel and aluminum within the weld pool during keyhole laser welding results in a complex dissimilar microstructure, which in turn, initiates a shift in weld metal mechanical properties. In this study, a numerical model for computation of distortions in laser-welded dissimilar overlap joints (austenitic stainless steel 304 – 6082-T6 aluminum alloy), which considers properties of the mixed steel-aluminum weld metal was developed. The required yield strength, Young's modulus, and strain hardening exponent of the weld metal were experimentally determined using the indentation technique coupled with energy-dispersive X-ray spectroscopy. The designed material model calculates the weld elastic-plastic properties as a function of the aluminum concentration. The softening of the alloys in the heat-affected zone was determined by physical simulations and considered as a function of maximum temperature. Computed and measured distortions showed good agreement for various welding regimes with an average deviation of 18.4%. The sensitivity analyses indicated that the application of the developed weld material model significantly improves the accuracy of the thermomechanical simulations.
Die lichtbogenbasierte additive Fertigung (WAAM) ist ein vielversprechendes Werkzeug, um die Ersatzteilbereitstellung von schwer verfügbaren oder aufwendig herstellbaren Einzelkomponenten in kürzester Zeit zu gewährleisten. Die Straffung innerbetrieblicher Abläufe sowie die Minimierung von Herstellungs- und Lagerkosten sind eine direkte Folge. Vor dem Einsatz im Betrieb muss sichergestellt werden, dass die Bauteile fehlerfrei additiv herstellbar sind und die mechanischen Eigenschaften den Anforderungen konventionell gefertigter Werkstücke entsprechen. Ein solcher Nachweis konnte am Beispiel eines additiv hergestellten Zwischenstücks für die Bergbauausrüstung mit einer abschließenden Charakterisierung der Werkstoffeigenschaften erfolgreich geführt werden. Das Zwischenstück mit einem Gewicht von ca. 31 kg zur Verbindung von geometrisch unterschiedlichen Profilen wurde ausgehend von einer Grundplatte aus S355-J2+N beidseitig additiv mit einem Massivdraht G3Si1 aufgebaut. Die Ermittlung geeigneter Parameter, Schweißfolgen und Zwischenlagentemperaturen erfolgte experimentell mit begleitenden metallografischen Untersuchungen. Nach erfolgreicher Fertigung wurden die mechanischen Eigenschaften des Zwischenstücks bestimmt sowie die Festigkeit und Zähigkeit mit den geforderten Werkstoffkennwerten des bisher eingesetzten konventionellen Konstruktionsmaterials verglichen.
This work presents a comparative study towards the development of efficient micro-reactors based on 3D-printed structures. Thus, the study evaluates the influence of the metal substrate geometry on the performance of structured catalysts for the CO2 methanation reaction. For this purpose, 0.5%Ru–15%Ni/MgAl2O4 catalyst is wash coated over two different micro-monolithic metal substrates: a conventional parallel channel honeycomb structure and a novel 3D-printed structure with a complex gyroid geometry. The effect of the metal substrate geometry is analyzed for several CO2 sources including ideal flue gas atmospheres, the presence of residual CH4 and CO in the flue gas, as well as simulated biogas sources. The advantages of the gyroid-3D complex geometries over the honeycomb structures are shown for all evaluated conditions, providing at the best-case scenario a 14% improvement of CO2 conversion. Moreover, this contribution shows that systematically tailoring geometrical features of structured catalysts becomes an effective strategy to achieve improved catalysts performances independent of the flue gas composition. By enhancing the transport processes and the gas-catalyst interactions, the employed gyroid 3D metal substrates enable boosted CO2 conversions and greater CH4 selectivity within diffusional controlled regimes.
This work investigates heat transfer mechanism of aluminum resistance spot welding process. The main target is to determine thermal contact conductance and heat transfer coefficients for natural convection and thermal radiation at ambient air and forced convection inside the water-cooled electrodes. For this purpose, the heat transfer of hot sheets in a welding gun for aluminum alloys AA5182 and AA6014 is analyzed experimentally and numerically. The transient temperature field is measured by several thermocouples in a simplified experimental setup. Subsequent thermal-mechanical coupled finite
element simulations of the experiments were used to calibrate the heat transfer coefficients. The heat transfer coefficient for natural convection and thermal radiation to ambient air is 13 W m2 K and the heat transfer coefficient for forced convection of electrode water-cooling is 25,000 W m2 K. The results indicate that the thermal contact conductance can be assumed ideal for welding process. Additionally, the finite element model is validated by the measured and calculated dissipated heat due to forced convection. Finally, a sensitivity analysis is performed to compare the influence of maximum and minimum heat transfer coefficients of forced convection (water-cooling) on transient temperature field and dissipated heat of sample
AA5182.
In this work, a numerical model and experiments are used to investigate heat transfer processes during resistance spot
welding process of aluminum. For this purpose, calibrated heat transfer conditions and thermal contact conductance are transferred from a previous work to a coupled thermal-electrical-mechanical finite element model. First, all domains of the numerical model are validated by an experimental study. The experimental setup includes the measurement of current, voltage drops, electrode force, electrode displacement, and temperatures while two sheets of aluminum alloy AA5182 are joined. Computational results show that most of the generated Joule heat (78%) is stored in the electrodes or transferred to cooling water until the end of weld time. Heat transfer by natural convection and thermal radiation is very small and can in general be neglected for complete process. Afterwards, the influence of electrode water-cooling on welding process is investigated numerically. The results indicate that the generation of Joule heat and thermal energy of the sheets during weld time is only slightly affected by electrode water-cooling. As a consequence, water-cooling conditions do not affect nugget formation. In contrast, electrode water-cooling highly influences cooling conditions during hold time.
For the reliability of many welded structures like pipelines or offshore wind parks, it is necessary to ensure high mechanical properties such as strength and toughness of the entire material. In order to improve quality of weld joints, flux cored rods and wires containing different nanoparticles (ZrO2, LaB6 and Al2O3) were manufactured and welded with the TIG and MAG welding processes. Influence of these nanoparticles on the microstructure and mechanical properties of the weld joints were investigated. The acicular ferrite and improved strength of the weld metals were observed by the addition of 0.2 wt.% of iron powder with ZrO2 nanoparticles to the filling of flux cored wires.
Cored wires are used in numerous welding processes with or without extern al gas or flux shielding. Submerged are welding (SAW) with eored wires, seamed or seamless, for joining mild and low alloyed steel grades is a technique that has demonstrated clear advantages during the three last decades. Ever since its invention, the SAW process has evolved with one main goal: to combine quality with produetivity. With low alloy wires, the benefits have already been clearly demonstrated and widely exploited. However, little has been written on SA Wand cladding with eored wires for eorrosion or he at resisting applieations. Its extension to high alloy eompositions brings eorresponding benefits and adds so me specific and unique features. This paper foeuses on consumable specificities and on the quality and productivity features of SAW with CRA (corrosion resistant alloy) cored wires. It deseribes the potential as weIl as the limitations ofthis technique. Cored wires are now used for submerged are welding of almost aIl stainless steels ranging from soft martensitic to super-duplex, and for aseries of nickel base compositions as weIl as for cladding cobalt base aIloys. Examples of industrial applications with austenitic, duplex, martensitic and heat resisting stainless steels are given to illustrate the potential ofthe eored wire solution.
The work presents an innovative approach for simulation of welding induced distortions, which allows taking into consideration the local effects of the welding process as well as the global interactions in the structure. The approach bases on an integration of analytical and numerical calculation’s procedures in a hybrid model, realized with the aid of the inherent strain concept. The analytical procedure calculates the shrinkage strain components and area of the plastic deformed zone, caused by welding. The calculated strains are then transferred to the global finite element model in order to predict the distortions and stresses in the structure after every welding stage, as well as their final state. The consideration of welding scenarios, weld sequence, clamping conditions and the assembling stages is done by taking into account the intermediate variation of the strain state during the manufacturing steps.
The article gives the theoretical basis of the analytical model. The consideration of the essential physical relations is put into discussion. The application of the model is illustrated on complex structures. Its reliability and practicability are demonstrated by means of a validation study.
Great attention is focused nowadays on laser welding of dissimilar steel-aluminum joints in overlap configuration in key-hole mode. It was found that elastic-plastic properties of the weld metal exhibit strong difference to those of the base alloys and can be defined as a function of aluminum content in the weld metal. A developed Finite-Element simulation model allows prediction of the aluminum content as a function of welding parameters and subsequently the elastic-plastic properties of the weld metal as a function of the determined content. The main goal of the present study is to show the impact of the weld metal properties on welding distortions and residual stresses. For that purpose, a sensitivity analysis of the thermomechanical model was performed, where the distortions and residual stresses were computed as a function of welding parameters and therefore as a function of corresponding weld metal properties. The analysis showed that the influence of the weld metal is essential, and its properties should be taken into consideration in the models for better prediction accuracy.
This study presents an experimental results of residual stress states in stainless steel plate samples with size 100 x 50 x 10 mm welded using a high-power fiber laser. The technological parameters of the welding process were: laser power Q = 15 kW; laser spot size = 0.65 mm; welding speed V = 3 m/min; 4 m/min; 5 m/min, using 30 L/min of protective Ar gas. The neutron diffraction method was used to determine the residual stresses in the bulk of the material. The neutron experiments were performed on the FSD diffractometer at the IBR-2 pulsed reactor in the Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research - Dubna, Russian Federation. The measured residual stress distributions exhibit maxima at weld seam centers. As expected, for all specimens the residual stress is falling down in regions distant from the weld zone. Maximal residual stress value of 492 MPa was observed for sample welded with speed of V = 5 m/min.
The objective of this study was to investigate the effect of the high welding speed on the mechanical properties and their relations to microstructural characteristics of butt friction stir welded joints with the use of 6082-T6 aluminum alloy. The aluminum sheets of 2.0 mm thick were friction stir welded at low (conventional FSW) and high welding speeds (HSFSW) of 200 and 2500 mm/min, respectively. The grain size in the nugget zone (NZ) was decreased; the width of the softened region was narrowed down as well as the lowest microhardness value located in the heat-affected zone (HAZ) was enhanced by HSFSW. The increasing welding speed resulted in the higher ultimate tensile strength and lower elongation, but it had a slight influence on the yield strength. The differences in mechanical properties were explained by analysis of microstructural changes and tensile fracture surfaces of the welded joints, supported by the results of the numerical simulation of the temperature distribution and material flow. The fracture of the conventional FSW joint occurred in the HAZ, the weakest weld region, while all HSFSW joints raptured in the NZ. This demonstrated that both structural characteristics and microhardness distribution influenced the actual fracture locations.
Leichtbau und das Streben nach Ressourcen- und Energieeffizienz fördern in den letzten Jahren vermehrt den Einsatz hochfester Stähle. Das Laserstrahlschweißen dieser Werkstoffe wird in der industriellen Großserienferti-gung zur effizienten Herstellung hochpräziser Bauteile und Komponenten mit höchsten Qualitätsanforderungen eingesetzt. Die Vermeidung schweißbedingter Fehler wie Schweißverzug und Kaltrissbildung sind dabei von ent-scheidender Bedeutung. Konventionelle Anwendungen erfüllen diesen Anspruch derzeit bedingt durch sehr enge Prozesstoleranzen und die Auswahl unkritischer Werkstoffe, was die Potentiale der Fügeverbindung begrenzt. Auf Basis von FE-Schweißprozesssimulationen konnten Konzepte entwickelt werden, welche eine Verzugs- und Riss-minderung durch aktive Steuerung des Laserschweißprozesses ermöglichen. Die zugrundeliegenden Modelle be-rücksichtigen zum einen das schweißbedingte Temperaturfeld, Gefügeumwandlungen und Eigenspannungen für die Berechnung des Verzugs. Zudem wird die lokale Wasserstoffkonzentration berechnet und die Ergebnisse der Schweißprozesssimulation mittels eines Kaltrisstools bewertet, welches werkstoffspezifische Risskriterien enthält. Die Fähigkeit das Verzugs- und Kaltrissverhalten abzubilden, eröffnet die Möglichkeit zur Parametervariation. Aus den erhobenen Daten wurden Konzepte der aktiven Krafteinleitung mit einer dynamischen Werkstückeinspannung abgeleitet, die zu Verzugs- und Kaltrissminimierung führen und die Schweißbarkeit hochfester Werkstoffe fördern.
Die Schweißposition beeinflusst erheblich die mechanischen Eigenschaften der Schweißnaht, insbesondere die Kerbschlagzähigkeit. Dies zeigt ein umfassender Vergleich der mechanischen Eigenschaften von Schweißgütern, die in der Steigposition (PF) und Wannenlage (PA) ausgeführt wurden. Schweißgüter von Kohlenstoff-Mangan-Stählen weisen in der PF-Position eine reduzierte Kerbschlagarbeit im Vergleich zur PA-Position auf. Dagegen ist bei austenitischen und Duplexstählen der Einfluss der Schweißposition auf die Kerbschlagarbeit genau entgegengesetzt. Der Vortrag fasst den Einfluss der Schweißpositionen auf die mechanischen Eigenschaften zusammen und liefert eine Erklärung für die gegensätzlichen Auswirkungen auf die Kerbschlagzähigkeit der Schweißgüter. Der Schwerpunkt liegt dabei auf hochlegierten Fülldrähten mit einer schnell erstarrenden Schlacke. Die Schlackeneigenschaften, Schweißgutmikrostruktur, Delta-Ferrit-Gehalte für Schweißgüter vom Typ Duplexstahl (2209) und von austenitischen Schweißgütern (308L) sowie die Auswirkungen der Schweißposition auf die Richtung des Schweißkornwachstums werden erläutert. Zum Vergleich werden Kohlenstoff-Mangan-Rutil-Fülldrähte mit schnell erstarrender Schlacke herangezogen.
The temperature influence on the microstructure evolution and microhardness of the agehardenable aluminium alloy 6082 T6 during friction stir welding was defined. In order to achieve this aim, the thermocycles calculated using the developed 3D Finite Element Model were physically simulated on the Gleeble-3800 in the points which located in the different zones of the weld. The microstructure in the chosen points after Gleeble testing was investigated as well as the microhardness was measured. The results were consequently compared with the relevant results obtained after friction stir welding. It was shown that the microstructure and microhardness profile are influenced not only by temperature but by deformation. The increase in hardness in different zones after FSW compared to Gleeble testing can be explained by the grain refinement in the nugget zone as well as the hardening precipitate distribution along the weld which can occur more rapidly due to the deformation influence.
Amethod for assessing the effect of the temperature of friction stir welding on the microstructure and mechanical properties of welded joints of aluminum alloy D16AT is suggested. The temperature cycles in different zones of a welded joint are determined by mathematical simulation. The computed temperature cycles are implemented in a Gleeble-3800 system. It is shown that the welding temperature affects little the evolution of the microstructure in different zones of the welded joints of alloy D16AT.
In the present work, laser beam build-up welding of AlSi12 alloy powder on AlSi1MgMn-alloy (EN AW-6082) substrate has been studied to determine the laser deposition strategy for fabrication of 3-D structure. First, the influence of laser power, scanning speed and powder feeding rate on the output parameters, such as clad geometry, dilution ratio, powder efficiency and porosity were analysed for deposition of a single track. Mathematical relationships were then established and the optimal parameters were identified by the desirability approach. Second, these optimal process parameters are furthermore adjusted to fabricate rectangular, cylindrical and complex combination volumes; different layer deposition strategies are used and evaluated. Finally, by using an adapted deposition strategy, one prototype consisted of different complexly shaped elements on one curved Al-alloy substrate without welding fusion defects was manufactured. The average tensile strength of deposited
samples with four different layer deposition directions (vertical 0°/90°; horizontal 0°/90°; horizontal 0° and horizontal 90°)
was measured, respectively, which are comparable strengths or higher than those of cast samples. The microstructure of the
laser-deposited material is investigated using optical microscopy and scanning electron microscopy as well as microcomputed
tomography. Microstructure and hardness of deposited multilayers exhibit inhomogeneous distributions, which vary with the deposit location
The work deals with the elastic-plastic thermo-mechanical deformation process after spot heating of large thin plates. It promotes a new theoretical model of the process. The main target is to clarify the kinetics of the plastic strain formation during thermal straightening and to explore the main interactions. For this purpose, a simplified mechanical problem is formulated and applied. As a result, definite thermo-mechanical relations between temperature, initial strain, and accumulated plastic strain are obtained. Finally, a verification study is presented and discussed. The results contribute to extending the knowledge about the process and to establishing a theoretical background for development of applied analytical or analytical-numerical approaches for practical use in the designing and planning phases in welding production. An application to a particular case is described in an accompanying paper (part 2).
An analytical model based on a theoretical model established in the previous work (part 1) is developed. The analytical model considers the particular case of an instantaneous high-density heat source acting on a thin pre-stressed plate. It calculates the plastic strain and the area of the zone of plastic deformation. A theoretical study is performed in order to quantify the major factors that influence the plastic strain formation. Furthermore, the work presents a calculation procedure for distortion analysis of lar ge and complex structures, intended to be used for solving industrial tasks. The calculation procedure consists of a combination of the analytical solution and an elastic finite element analysis with the aid of the inherent strain approach. Various aspects of its application are also discussed. The calculation procedure is finally explained using a simple example. Its reliability and practicability are demonstrated through verification and validation studies. The work is dedicated to the thermal straightening of thin-walled welded structures; however, it can also be related to other processes.
Welded joints of aluminum alloy 6082-T6 formed by the method of impulse friction stir welding are studied. The effect of the power and frequency of the pulses on the microstructure and mechanical properties of the welded joints is determined. Application of an additional pulse during the welding affects the surface quality and the shape of the weld, the distribution of the oxide layer and of particles of the hardening phase, and the grain size in the zone of dynamic recrystallization.
Direct bonding of copper and porous LaCrO₃ without an extra filler interlayer was successfully completed using local and fast Cu‐infiltration through laser cladding. This significantly reduced the susceptibility of the ceramic to cracking. A high‐speed camera investigation into the wetting and infiltration behavior of a Cu‐melt into LaCrO₃ with a porosity of ~63 vol% was performed. By adjusting the focal distance with a constant laser power of 300 W, the Cu‐melt was rapidly infiltrated into the ceramic preform in 10 seconds. This was completed under atmospheric air conditions, without added inert gas. The joining process developed can be used to fabricate ceramic/metal joints with targeted (micro‐) structure properties by adjusting the infiltrated melt and the infiltration depth, which would be suitable for many applications, such as multifunctional devices, solid oxide fuel cells or heating elements.
The properties of sandwiches with aluminum foam and the special features of their use in lightweight welded structures are determined. The weldability of such sandwiches in foamed and non-foamed conditions is analyzed under friction stir welding, laser beam welding and short arc welding in protective gas. The process and mechanical properties of the welds are studied. The results are generalized in a catalog that may be useful for materials scientists and designers.
The properties of the welded joints of 6082-T6 alloy produced by friction stir welding (FSW) and laser welding(LW) were studied. The maximum temperature and microhardness fields in the friction stir-welded joints were shown to be asymmetrical. The concentrated energy input during laser welding yields a narrow heat-affected zone (HAZ) with sharp changes in mechanical properties. The strength of the FSW joint constitutes 72% and that of the LWjoint—67% of the base metal strength. In case of LW, the minimum hardness of the joint corresponds to the metal of the welded joint, while in case of FSW, it corresponds to the heat-affected zone. At the same maximum heating temperature, the hardness of the heat-affected zone of the friction stir-welded joint is lower compared to laser welding due to substantially lower rates of heating and cooling.
The Influence of Geometrical and Welding Imperfections on the Strength of Stiffened Structures
(2018)
The strength calculation of stiffened plates and shells via finite element simulation has been part of the state of the art for a long time. Geometrical nonlinearities as well as the nonlinear material behavior are considered within the calculation. To simplify, both types of imperfections, geometrical and structural ones, are mostly combined in these strength calculations being considered as equivalent geometrical imperfections. Values for standard cases are included in EN 1993-1-5 in case of plated structures or 1-6 in case of shells. A second possibility is the separate definition of fabrication tolerances and residual stresses, as far as they are known. With increasing complexity of the structure, less information are however available on geometrical and particularly structural imperfections. Nevertheless, by means of a more realistic consideration of these values, the actual ultimate load can be determined significantly more exactly and the performance of high performance steel structures is increased.
In the investigated cases, deformations and stresses arise mainly due to welding. Therefore, the topic of this article is the application of an analytical numerical hybrid model for predicting imperfections induced by welds. This is presented on an example of a stiffened steel structure and a subsequent ultimate load calculation. At the beginning the analytical model, its physical basis as well as the physical interrelationships are introduced and explained. This is followed by the explanation of the coupling procedure between the analytical model and the numerical calculation for the calculation of weld-induced deformations and residual stresses. The significance of the hybrid model is proven by means of a few sample applications. For a realistic determination of the weld imperfections in the investigated stiffened structure, the coupled hybrid model is used afterwards. An ultimate load analysis gives information about the load carrying behavior under axial loading. The results are compared with the results of an ultimate load analysis from a literature example assuming different eigenvalues with different scaling. The results underline the potential additional utilization of load bearing capacity by this new approach.
Knowledge of the properties of dissimilar welds is of great significance for the development of multi-material lightweight structures. In this study, stainless steel (1.4301) and aluminum alloy (6082-T6) sheets were welded in overlap configuration in keyhole mode. The resulting weld metals were investigated with respect to their mechanical properties. Several samples were cut out of different locations along the welds and their cross-sections were subjected to indentation testing and energy dispersive X-ray (EDS) analysis. Young's modulus E, yield stress σy, and strain hardening exponent n, were determined by means of reverse analysis of the indentation load (P) – depth (h) curves, allowing construction of true stress – true strain relations. An essential increase in yield stress in comparison to the one of the base alloys was observed inside the weld metal. In contrary, Young's modulus and strain hardening exponent of the welds were almost identical to corresponding values of the base steel metal. Due to the sensitivity of yield stress to the aluminum content, slight variations of the welding parameters lead to significant changes in elastic-plastic behavior of the weld metal.
The microstructure and the distribution of chemical elements in laser-welded joints of Al – Mg – Si alloy 6005-T6 are studied. Segregations of chemical elements are detected over grain boundaries in the heat-affected zones of the welded joints. The joints fracture by the intergrain mechanism. A Gleeble 3800 device is used to determine the temperature dependences of the mechanical properties of the alloy with allowance for the special features of the welding cycle. Amethod for evaluating the sensitivity of welded joints of aluminum alloys to formation of liquation cracks with allowance for the local properties of the metal, the welding conditions, and the rigidity of the construction is suggested.
The formation of acicular ferrite in the structure of weld deposits when welding carbon steels is often associated with the introduction of nonmetallic additives into the electrode material. Fe–alumina composite powders with alumina particles less than 1 µm were used as a filler for cored welding wire and the influence of aluminum oxide additives up to 2,5 wt.% on the structure of the welded seam was studied. It was shown that the addition of aluminum oxide to the welding wire led to the initiation of the formation of intragranular acicular ferrite in the weld deposit and to a more even distribution of hardness through the zones of weld seam.
Numerical models for thermo-mechanical simulation of the welding
residual stresses in laser beam and multi-pass arc welded joints are developed. The models describe the resulting microstructure as a function of the peak temperature, austenization time, cooling time and take into account the microstructure formed after
single or multiple thermal cycles. The results of simulations are validated by neutron diffraction experiments performed on FSD diffractometer at the IBR-2 pulsed reactor in FLNP JINR (Dubna, Russia). The elaborated numerical models are in good agreement
with obtained neutron diffraction data, which enable to study the influence of different conditions and process parameters on the distribution of residual welding stresses.
In this study, a three-dimensional thermal conduction model based on the finite element analysis was established to predict a heat distribution during laser welding in overlap configuration of austenitic stainless steel 1.4301 and hardenable aluminum alloy 6016. A hybrid model based on two expanded curve-rotated volumetric heat sources is suggested to capture the complex weld geometry. Since thermal flow in the overlap area is a crucial part of the model, an analytic calculation of a thermal contact conductance between the plates and its subsequent implementation in the FE model through a superposition of two contact surfaces are described in details in this paper. Full thermal cycles, weld shapes, and thus mixing grades were successfully predicted by means of the developed model for different parameter combinations.
An influence of the arc welding and the laser welding on fatigue properties and destruction mechanisms of aluminum foam sandwich has been studied. It was found that specimens made by the arc welding had the best fatigue strength. Comparative results of four point bending fatigue tests of the non-welded specimens such as butt welded joints made by different welding methods are presented.
In this study, the residual stresses distribution induced by multi-pass arc welding of the steel S355J2+N are investigated experimentally and numerically. An extended approach is used for the simulations, which considers the change of the local microstructure properties due to multiple reheating. Experimental material data obtained from physical welding simulations with Gleeble® are used for the model calibration. The experimental stress study is performed using a neutron diffraction method on a fourier stress diffractometer. Numerical analysis of the welding stresses formation in the weldment is performed and compared to the experimental study. The results explain the influence of the welding thermal history on the resulting local thermo-mechanical properties in the heat-affected zone and, thus, on the residual stress distribution. The consideration of the local microstructure properties in the welding simulation leads to a significant increase in accuracy of the numerical results. The major influence factor on the residual stress formation is the change in the interpass microstructure yield strength. When a root pass with short cooling times is subjected to re-austenitisation in the fine-grained zone, the yield strength increases in this area and affects consequently the residual stress distribution. The influence of the reheating is detectable in the depth of the weldment, but it is less significant for the residual stress formation near the surface of the welded joint.
Welded joints of aluminum alloy 6082-T6 formed by the method of pulse friction stir welding are studied. The effect of the power and frequency of the pulses on the microstructure and mechanical properties of the welded joints is determined. Application of an additional pulse during the welding affects the surface quality and the shape of the weld, the distribution of the oxide layer and of particles of the hardening phase, and the grain size in the zone of dynamic recrystallization.
The series of impulse friction stir welding (IFSW) experiments for aluminum alloy 6082-T6 butt joints was performed to obtain micrographs, tensile and hardness test samples and to receive the validation data for a numerical model of the process. Impulse force and frequency were varied to determine the effect of individual process parameters on microstructure evolution and mechanical properties of the welded butt joints. A three-dimensional model was established to simulate temperature distribution, heat generation and contact state between the tool and the workpiece during IFSW. Based on the numerical simulation, the influence of the impulses on heat input, maximum weld temperatures and contact state was found. The combination of impulse force and frequency effected the grain refinement. The influence of impulses on the elongation and hardness of the welded joints during IFSW were determined.
Rollennahtelektrode
(2017)
Eine Rollennahtelektrode zur Verwendung beim Rollennahtschweißen umfasst eine segmentförmige Elektrodenfläche (2) zur Anpressung der zu verbindenden Bauteile und zur Einleitung des Schweißstroms und der Schweißspannung. Die Elektrodenfläche (2) ist derart dreieckförmig oder mäanderförmig oder zueinander beabstandet ausgebildet, dass in der ebenen Abwicklung der Elektrodenfläche (2) ein Zickzack-Muster oder ein Wellen-Muster oder ein Steppnaht-Muster oder ein Rechteck-Muster vorliegt.
In der vorgestellten Arbeit wird die Anwendbarkeit von Aluminiumschaumsandich in Schweißkonstruktionen vorgestellt.
Systematische Betrachtungen zur Schweißbarkeit mittels Laserstrahlschweißen, MSG - Kurzlichtbogen und
Rührreibschweißen sowohl im geschäumten als auch im ungeschäumten Zustand werden beschrieben. Dazu wurden verschiedene Knotenformen wie I-Stoß- bzw. T-Stoßverbindungen hergestellt und deren mechanischtechnologische Eigenschaften ermittelt. Neben der quasistatischen Prüfung, wurde jede Ausführungsform des Knotens
auf deren Schwingfestigkeit hin geprüft und die Anwendbarkeit bestehender Regelwerke wie die FKMRichtlinie oder der Eurocode 9 für die Nachweisführung betrachtet. Die Ergebnisse sind abschließend in einem Gestaltungskatalog als Hilfsmittel für den Konstrukteur zusammengefasst. Parallel dazu wurden die experimentellen Untersuchungen mittels FEM begleitet. Es wird die FE-Modellentwicklung für die Berechnung des Sandwichverbundes
an sich als auch von Fügeknoten dargestellt wie auch die Modellvalidierung mit einem Musterbauteil aus einer Hubarbeitsbühne.
The series of impulse friction stir welding (IFSW) experiments for aluminum alloy 6082-T6 butt joints was performed to obtain micrographs, tensile and hardness test samples and to receive the validation data for a numerical model of the process. Impulse force and frequency were varied to determine the effect of individual process parameters on microstructure evolution and mechanical properties of the welded butt joints. A three-dimensional model was established to simulate temperature distribution, heat generation and contact state between the tool and the workpiece during IFSW. Based on the numerical simulation, the influence of the impulses on heat input, maximum weld temperatures and contact state was found. The combination of impulse force and frequency effected the grain refinement. The influence of impulses on the elongation and hardness of the welded joints during IFSW were determined.
Dem Fügen mittels Schweißen kommt in vielen Industriezweigen eine zentrale Bedeutung zu. Durch die ungleichmäßige Erwärmung der Schweißverbindung kommt es jedoch zu unerwünschten Verformungen, die durch kostenintensive Nachbehandlungen beseitigt werden müssen. Ist der bleibende Schweißverzug bereits in einem frühen Stadium des Produktentwicklungszyklus bekannt, können geeignete Gegenmaßnahmen ergriffen oder Nachbehandlungen bereits in der Produktplanung festgelegt werden.
Das Kernziel dieser Arbeit ist die Entwicklung und Umsetzung eines gekoppelten analytisch numerischen Hybridmodells für die Schweißverzugsberechnung von Großstrukturen. Die Basis hierfür liefert das Schrumpfkraftmodell, welches um moderne Schweißverfahren und neue Werkstoffe erweitert wird. Sowohl experimentell als auch mittels numerischer Analysen wird die Instabilität dünnwandiger Strukturen untersucht und eine mögliche Erfassung im Schrumpfkraftmodell vorgeschlagen. Durch die analytisch numerische Kopplung stellt auch die Berechnung der Verzüge, hervorgerufen durch krummlinig wandernde W\"armequellen keine Hürde mehr dar und wird implementiert.
Um die Anwendung des Hybridmodells zu vereinfachen, wird ein Programm entwickelt, welches die umfangreiche mathematische Vorgehensweise des Schrumpfkraftmodells beinhaltet und aufgrund der Geometrie der Schweißverbindung, des Werkstoffs und der Schweißparameter die mechanischen Lasten berechnet. Die Verifizierung der Software, des Kopplungsalgorithmus sowie die Untersuchung der Stabilität und Sensibilität des Hybridmodells erfolgt an Stumpfstoßverbindungen.
Abschließend wird das Modell an unterschiedlich komplexen Schweißkonstruktionen angewandt und die berechneten Schweißverzüge mit experimentellen Messungen verglichen. Eingangs werden einfache I- und T-Stoßverbindungen betrachtet. Es folgt die Verzugsberechnung eines Demonstrators aus dem Schiffbau mit 8 Schweißnähten sowie einer gesamten Decksektion mit ca. 90 Nähten, wobei verschiedene Elemente auf eine Grundplatte von 20x16 Metern gefügt werden.
Calculation of thermal processes in welding thick plates with moving axisymmetric heat sources
(2016)
Thin sheet metals from deep drawing steel DC04 are very often used in the production of car body and case parts. Quality improvement of sheet
metal components by new constructive solutions (structuring) as well as
adapted joining technology is going on. Structured sheet metals differ from each other by their high bending stiffness. At the same time, they show certain anisotropy due to the structure. Therefore a typical testing
method of structured semi-finished parts (single sheet metals, sandwiches)is the bending test. The literature review revealed that in many
studies no special demands on tests of structured materials were made. This concerns particularly the structure arrangement, structure direction and structure location of the specimen relative to the mandrel position during bending tests, i. e., the direction of the fixed load relative to the structure. The aim of this study was to determine the influence of the test
specification on flexural behavior. In the present paper, honeycomb-structured
sheet metals were examined using 3-point bending tests. Bending stiffness and lightweight potential were calculated with respect to the location of load application and compared for different structure arrangements,directions and locations. The influence of the anisotropy on flexural
behavior of the honeycomb-patterned sheet metals was moderate.
Bleche mit regelmäßig angeordneten steifigkeitserhöhenden Strukturen werden zunehmend in Kon-struktionen zur Erhöhung der Steifigkeit dünner Bleche bzw. von kompletten Bauteilen verwendet. Damit diese Bauteile wirtschaftlich verarbeitet werden können, bietet das Rollennahtschweißen mit seinen hohen Prozessgeschwindigkeiten und ohne notwendigen Einsatz von Zusatzwerkstoffen bzw. Schweißhilfsstoffen eine kostengünstige Alternative zu anderen Verfahren. Beim Rollennahtschweißen strukturierter Bleche führen jedoch klassische Rollennahtelektroden zur teilweisen bzw. vollständigen Einebnung der Versteifungselemente, sodass deren positive Wirkung reduziert bzw. vollständig auf-gehoben werden kann.
Die geschweißten Bleche haben Wabenstrukturen mit unterschiedlichen Elementtiefen als regelmäßig angeordnete Versteifungselemente. Die Bleche finden vorrangig Anwendung im Fahrzeugbau und im Bauwesen. Aufgrund dieser Branchen und für zukünftige Anwendungen im Wärmetauscher- und Behäl-terbau wurden die Untersuchungen an Werkstoffqualitäten DC04 und X5CrNi 18-10 mit Blechdicken von 0,5 mm und 0,7 mm durchgeführt.
Die entwickelten neuartigen Elektroden berücksichtigen die Topologie der Bleche, um eine Einebnung der strukturierten Bleche zu vermeiden. Daher wurden Elektroden mit veränderten Geometrien an den Elektrodenarbeitsflächen gestaltet. Eine der Elektroden besitzt eine periodisch unterbrochene und die zweite eine laterale Profilierung mit in Schweißrichtung nichtlinearer Form der Elektrodenarbeitsfläche. Das Verhalten beim Schweißen wurde in Schweißversuchen bestimmt und Schweißbereichsdiagram-me bzw. Prozessfenster abgeleitet. Die Verbindungseigenschaften wurden durch Metallografie-, Fes-tigkeits-, Druck- und Dichtheitsuntersuchungen validiert. Als Vergleich und Referenz dienen Schweiß-untersuchungen mit konventionellen Rollennahtelektroden.
Die Untersuchungsergebnisse zeigen Unterschiede bei den Prozessparametern und der Ausprägung der Verbindung in Abhängigkeit von der geschweißten Struktur, dem ausgewählten Schweißpfad und des geschweißten Werkstoffes gegenüber den Referenzschweißungen. Eine anschließende Optimie-rung führte zu einer vergleichbar guten Schweißnahtqualität sowohl bei Erhalt der Versteifungselemen-te mit den neuen Elektroden als auch bei deren Einebnung mit konventionellen Elektroden in Referenz zu Schweißuntersuchungen am ebenen Blech.
Anschließend wurde ein Vergleich der Verbindungsfestigkeiten mit dem Widerstandspunktschweißen und dem Metall-Aktivgas-Schweißen durchgeführt.
Schweißeinrichtung (1) zum Schweißen von Werkstücken (100), umfassend eine Laserstrahlquelle (10) zur Emission eines Laserstrahls (11), eine Vorschubeinrichtung zur Realisierung einer Vorschubbewegung einer abzuschmelzenden Elektrode (70) aus Schweißzusatzwerkstoff (30) in einer Transportrichtung (21) in eine Abschmelzzone, und eine Stromkontakteinheit (60) zur Anlegung einer elektrischen Spannung an die abzuschmelzende Elektrode (70), wobei die Laserstrahlquelle (10), die Vorschubeinrichtung und die Stromkontakteinheit (60) derart relativ zueinander angeordnet und ausgebildet sind, dass die Längsachse des von der Laserstrahlquelle (10) aussendbaren Laserstrahls (11) im Wesentlichen parallel zur Transportrichtung (21) der Elektrode (70) verläuft, dadurch gekennzeichnet, dass sie eine Elektrodenformungseinrichtung (40) aufweist, mit der ein Querschnitt der Elektrode (70) erzeugbar ist, der den von der Laserstrahlquelle (10) aussendbaren Laserstrahl (11) zumindest in einem Winkelbereich von 180° umgibt.
Anwendungsnahe Simulation des thermischen Richtens : Forschungsvorhaben P 1008/IGF-Nr. 17970 BR
(2016)
Principles of welding
(2016)