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Eingeladener Vortrag
- nein (132)
Laser Powder Bed Fusion (L-PBF) allow the fabrication of lightweight near net shape AlSi10Mg components attractive to the aerospace, automotive, biomedical and military industries. During the build-up process, high cooling rates occur. Thus, L-PBF AlSi10Mg alloys exhibit a Si-nanostructure in the as-built condition, which leads to superior mechanical properties compared to conventional cast materials. At the same time, such high thermal gradients generally involve a deleterious residual stress (RS) state that needs to be assessed during the design process, before placing a component in service. To this purpose post-process heat treatments are commonly performed to relieve detrimental RS. In this contribution two low-temperature stress-relief heat treatments (SRHT) are studied and compared with the as-built state: a SRHT at 265°C for 1 hour and a SRHT at 300°C for 2 hours. At these temperatures microstructural changes occur. In the as-built state, Si atoms are supersaturated in the α-aluminium matrix, which is enveloped by a eutectic Si-network. At 265°C the Si precipitation from the matrix to the pre-existing network is triggered. Thereafter, above 295°C the fragmentation and spheroidization of the Si branches takes place, presumably by Al–Si interdiffusion. After 2 hours the original eutectic network is completely replaced by uniformly distributed blocky particles. The effect of the heat and the microstructure modification on the RS state and the fatigue properties is investigated. Energy dispersive x-ray and neutron diffraction are combined to investigate the near-surface and bulk RS state of a L-PBF AlSi10Mg material. Differences in the endurance limit are evaluated experimentally by high cycle fatigue (HCF) tests and cyclic R-curve determination.
n this work, three classes of fatigue models are reviewed according to the fatigue regimes commonly considered in the current components design. Particular attention is devoted to the so-called Class III fatigue models, covering the three fatigue regimes, namely, LCF, HCF and VHCF. The applicability and limitations of the pro-posed analytical sigmoidal solutions are discussed from the viewpoint of practical design. The compatible Weibull S-N model by Castillo and Canteli is revisited and improved by considering a new reference parameter GP = E⋅σM ⋅(dε/dσ)|M as the driving force alternative to the conventional stress range. In this way, the requirement, σM ≤ σu, according to the real experimental conditions, is fulfilled and the parametric limit number of cycles, N0, recovers its meaning. The probabilistic definition of the model on the HCF and VHCF regimes is maintained and extended to the LCF regime. The strain gradients may be calculated from the monotonic or cyclic stress–strain curve of the material although a direct derivation from the hysteresis loop is recommended. Some Class III fatigue models from the literature and another one improved by the authors are applied to the assessment of one experimental campaign under different stress ratios conditions and the results compared accordingly. Finally, the new probabilistic GP-N field is evaluated. The results confirm the practical confluence of the stress- and the strain-based approaches into a single and advantageous unified methodology.
Statement of problem
The design of the implant-abutment connection has been widely researched, but the impact of different crown-abutment geometries remains unclear.
Purpose
The purpose of this in vitro study was to evaluate the effect of different crown-abutment margin geometries on the mechanical behavior and fit of screw-retained implant-supported single-crown restorations by using mechanical static and fatigue tests and mastication simulation.
Material and methods
A total of 45 cobalt-chromium premolar-shaped metal frameworks were fabricated for single-unit implant-supported screw-retained restorations on stock abutments and internal hexagon Ø4.25×11-mm cylindrical implants. They were divided into 3 groups according to margin geometry: S, shoulder; C, chamfer; and F, feather-edge. Three static load until fracture and 24 dynamic load tests were performed by using the International Organization for Standardization 14801:2016 standard (ISO 14801:2016) (number of cycles limit: 5×106 cycles, frequency: 6 Hz). The ProFatigue software program was used to optimize the procedure (S, n=12 specimens; C, n=7 specimens; and F, n=5 specimens). Six additional specimens from each group were subjected to a mastication simulation (limit number of cycles: 1×106 cycles, cyclic loading from Pmin=30 N to Pmax=300 N, frequency: 6 Hz). Results from the fatigue tests were reported descriptively, and the Fisher exact test was used to analyze the difference in failure modes. Data from maximum misfit were evaluated by photogrammetry and statistically analyzed with the Anderson-Darling test and the Kruskal-Wallis and Dunn multiple comparison tests (α=.05).
Results
The fatigue limit was 456 N for group S, 512 N for group C, and 514 N for group F. The mean ±standard deviation misfit was 2.6 ±0.1 μm for group S, 3.8 ±1.1 μm for group C, and 3.6 ±0.8 μm for group F. Differences in misfit between groups S and C and between groups S and F were statistically significant (P<.05).
Conclusions
Crown-abutment connections with chamfer and feather-edge margins showed better mechanical behavior, while shoulder margin exhibited better fit. However, high levels of fit were achieved for all the evaluated geometries.
Design methodology of vessel produced by L PBF stainless steel using representative specimens
(2022)
This work presents the preliminary results of an ongoing project with a double objective: on the one hand, the characterisation of the mechanical properties against fatigue damage of an additively manufactured 316 stainless steel produced by laser powder bed-based (L-PBF) technology; on the other hand, the implementation of numerical simulation techniques able to predict the mechanical behaviour of the material in order to optimise and reduce the design costs of vessels used in the chemical sector. The current state of the work developed in this research framework allows showing the first batch of experimental results of crack propagation rate (FCGR) and high cycle fatigue (HCF) tests. The geometry of the vessels studied presents three clearly differentiated regions, either in terms of thickness (11-15 mm) or concerning the inclination of the walls to the direction of manufacturing (0º - 45º). The experimental campaign carried out so far allows identifying the differences in behaviour when comparing different extraction locations around the vessel. This is due to the variations in thermal cycles that the deposited material undergoes during the manufacturing process. Therefore, this causes variations in the microstructure which lead to changes in the response of the material. In this work, these differences are analysed qualitatively and quantitatively from the results of FCGR and HCF, thus allowing to locate the regions with the highest risk in terms of structural integrity against fatigue. This preliminary phase together with the numerical simulation of the additive manufacturing process are key to achieving a reliable description and modelling of the material. The latter will make it possible to address the priority aim of this project, involving the manufacture of independent samples whose properties are representative of the original material extracted from the reference vessels. It is, therefore, a comprehensive methodology for the design of additively manufactured components based on the localised fatigue mechanical properties of representative specimens.
La tecnología de fabricación aditiva (AM) continúan progresando y permitiendo alcanzar diseños cada vez más complejos y optimizados. La industria química es uno de los sectores donde componentes AM han adquirido un gran interés. La falta hasta la fecha de una directiva europea que regule la inspección, certificación y aceptación de equipos sometidos a presión hace necesario progresar en esta línea. El objetivo que se persigue en este trabajo es el de desarrollar una metodología de diseño sobre componentes fabricados aditivamente basada en la estimación de vida a fatiga de las zonas más susceptibles de sufrir dicho tipo de fallo. El estudio comprende diversas facetas de análisis, simulaciones numéricas, análisis de la microestructura del material y una extensa campaña experimental. La evaluación de la integridad estructural se realiza aplicando mecánica de fractura. La historia térmica a lo largo del proceso de fabricación determina la microestructura del componente en cada región y, por ende, influye en las propiedades mecánicas en cada una. Se presentan los resultados preliminares de un proyecto de investigación en curso dirigido a la caracterización de propiedades mecánicas en recipientes de presión producidos por fusión láser en lecho de polvo (L-PBF, por sus siglas en inglés) de acero inoxidable 316L. Se detallan los resultados preliminares en términos de velocidad de crecimiento de grietas por fatiga (FCGR), y se comparan los resultados de probetas extraídas de diferentes regiones de los depósitos.
It is already known that the laser beam welding (LBW) or hybrid laser-arc welding (HLAW) processes are sensitive to manufacturing tolerances such as gaps and misalignment of the edges, especially at welding of thick-walled steels due to its narrow beam diameter. Therefore, the joining parts preferably have to be milled. The study deals with the influence of the edge quality, the gap and the misalignment of edges on the weld seam quality of hybrid laser-arc welded 20-mm-thick structural steel plates which were prepared by laser and plasma cutting. Single-pass welds were conducted in butt joint configuration. An AC magnet was used as a contactless backing. It was positioned under the workpiece during the welding process to prevent sagging. The profile of the edges and the gap between the workpieces were measured before welding by a profile scanner or a digital camera, respectively. With a laser beam power of just 13.7 kW, the single-pass welds could
be performed. A gap bridgeability up to 1 mm at laser-cut and 2 mm at plasma-cut samples could be reached respectively.
Furthermore, a misalignment of the edges up to 2 mm could be welded in a single pass. The new findings may eliminate the need for cost and time-consuming preparation of the edges.
The robust determination of the threshold against fatigue crack propagation DKth is of paramount importance in fracture mechanics based fatigue assessment procedures. The standards ASTM E647 and ISO 12108 introduce operational definitions of DKth based on the crack propagation rate da/dN and suggest linear fits of logarithmic DK– da/dN test data to calculate DKth. Since these fits typically suffer from a poor representation of the actual curvature of the crack propagation curve, a method for evaluating DKth using a nonlinear function is proposed. It is shown that the proposed method reduces the artificial conservativeness induced by the evaluation method as well as the susceptibility to scatter in test data and the influence of test data density.
To assess the ability of cracks to grow, a robust determination of the threshold against fatigue crack propagation ∆Kth is of paramount importance. The standards ASTM E647 and ISO 12108 introduce operational definitions of ∆Kth based on the crack propagation rate da/dN. For evaluating ∆Kth, both suggest fitting a linear function to a defined subset of the logarithmic ∆K – da/dN test data, where ∆Kth follows by evaluating the linear function at da/dN = 10-7 mm/cycle and da/dN = 10-8 mm/cycle, respectively.
In general, this kind of fit suffers from a bad representation of the actual curvature of the crack propagation curve. Therefore, we propose a robust method for evaluating ∆Kth using a non-linear function that reduces the artificial conservativeness induced by the evaluation method as well as the susceptibility to scatter in test data and the influence of test data density. The method is calibrated against a large set of S690QL crack growth data obtained from a total of 48 specimens, and validated against a set of S355NL and S960QL data (3 specimens each), obtained as a part the IBESS (integral fracture mechanics determination of the fatigue strength of welds) project.
Die additive Fertigung (ugs. „3D-Druck“) ist bei der Herstellung von hochkomplexen metallischen Bauteilen, bionisch inspiriertem Leichtbau oder Prototypen nicht mehr wegzudenken.
Für die konventionelle Fertigung hat sich das bestehende System der Qualitätsinfrastruktur (QI) bewährt. Die additive Fertigung vergrößert den gestalterischen Spielraum von möglichen Bauteilgeometrien und Prozessfehlern jedoch erheblich. Hier gerät die QI an ihre Grenzen, sodass die Fertigung und Zulassung sicherheitsrelevanter Bauteile sehr zeit- und kostenintensive Versuche erfordern. Eine moderne digitale QI erlaubt eine effizientere Qualitätssicherung für additiv gefertigte Bauteile. Dies erfordert eine durchgängig digitale Abbildung des physischen Materialflusses.
The talk gives an introduction into gas metal arc welding and its relation to plasma science as well as current challenges in welding research concerning research data management and the application of the FAIR principles.
The WelDX project is introduced and the main goals are discussed and contrasted with the current features of the weldx API.
Different internal and public facing use cases focusing on research data management and their implementation using weldx are presented.
The interactive part of the presentation displays some advanced multi layer use cases and data analysis using the weldx API as well as the integration of materials properties into weldx.
This paper is focused on the approximation of absorbed Charpy impact energy-temperature curves (KV-T) and the management of sub-size test piece results in Charpy pendulum impact tests. Both subjects are frequently underestimated but may have a strong impact on the assessment of product quality and structural integrity. The status of related test standards, product standards, structural integrity assessment procedures and corresponding research is reviewed.
It is concluded that the state of regulation is partly insufficient and wrong. There is a wide gap between the state of knowledge in the field of materials testing and the requirements of product standards and structural integrity assessment procedures. The present paper aims at boosting and supporting the necessary discussion among the parties involved to raise the level of technical safety. Correspondingly, a large experimental database was created by Charpy impact testing of popular modern structural Steels with nominal yield strength from 235 MPa to 960 MPa. KV-T data were generated using test pieces with thicknesses between 2.5 mm and 10 mm. Different strategies for KV-T curve fitting, definition of transition temperatures, as well as upper and lower shelf energy levels were followed, and different scaling procedures were applied to the sub-size test piece results. The results show that proportional scaling of sub-size to full-size test piece results in terms of KV should be disallowed because it is a wrong approach.
Regulations still incorporating such procedures should be revised as soon as possible. Existing nonproportional scaling and conversion procedures were applied. The results obtained illustrate that neither an overall nor consistent conservatism nor an adequate conservatism level can be achieved. In summary, the standardizing bodies should enforce the discussion to revise relevant test standards considering more precise specifications on Charpy data fitting. Furthermore, proposals are made for an improved management of sub-size test piece results.
Duplex stainless steels (DSS) are frequently used, especially in applications requiring high strength combined with high corrosion resistance in aggressive media. Examples include power plant components and maritime structures. During welding of these steels, local variations in chemical composition can occur. This results in ferritization of the material and negatively affects the mechanical properties of the components. In this work, tungsten inert gas (TIG) welding experiments were performed with DSS. Chemical composition analysis was realized in situ by using Laser Induced Breakdown Spectroscopy (LIBS). The aim of the work is to quantitatively measure the chemical composition in the weld seam of various DSS and to identify possible influences of welding parameters on the microstructure of the material. The chemical concentrations of the main alloying elements Cr, Ni, Mn on the surface of the sample during the welding process and the cooling process were measured. Mn and Ni are austenite stabilizers and their content increases during welding by using certain high alloyed filler material. Spectra were recorded every 1.3 s at a spacing of approximately 2 mm. During the cooling process the location of the measurement was not changed. The LIBS method is proofed to be suitable for the quantitative representation of the chemical compositions during the welding process.
Laser powder-based directed energy deposition (DED-L) is a technology that offers the possibility for 3D material deposition over hundreds of layers and has thus the potential for application in additive manufacturing (AM). However, to achieve broad industrial application as AM technology, more data and knowledge about the fabricated materials regarding the achieved properties and their relationship to the manufacturing process and the resulting microstructure is still needed. In this work, we present data regarding the low-cycle fatigue (LCF) behavior of Ti-6Al-4V. The material was fabricated using an optimized DED-L process. It features a low defect population and excellent tensile properties. To assess its LCF behavior two conventionally manufactured variants of the same alloy featuring different microstructures were additionally tested. The strain-controlled LCF tests were carried out in fully reversed mode with 0.3 % to 1.0 % axial strain amplitude from room temperature up to 400°C. The LCF behavior and failure mechanisms are described. For characterization, optical microscopy (OM), scanning electron microscopy (SEM), and micro-computed tomography (µCT) were used. The low defect population allows for a better understanding of the intrinsic material’s properties and enables a fairer comparison against the conventional variants. The fatigue lifetimes of the DED-L material are nearly independent of the test temperature. At elevated test temperatures, they are similar or higher than the lifetimes of the conventional counterparts. At room temperature, they are only surpassed by the lifetimes of one of them. The principal failure mechanism involves multiple crack initiation sites.
Nichtrostende Duplexstähle (DSS) werden häufig verwendet, insbesondere bei Anwendungen, die eine hohe Festigkeit in Verbindung mit einer hohen Korrosionsbeständigkeit in aggressiven Medien erfordern. Beispiele hierfür sind Kraftwerkskomponenten und maritime Anwendungen. Beim Schweißen dieser Stähle kann es zu lokalen Schwankungen in der chemischen Zusammensetzung kommen. Dies führt zu einer Ferritisierung des Werkstoffs und beeinträchtigt die mechanischen Eigenschaften der Bauteile. In dieser Arbeit wurden Versuche zum Wolfram-Inertgas-Schweißen (WIG) mit DSS durchgeführt. Die Analyse der chemischen Zusammensetzung wurde in situ mit Hilfe der laserinduzierten Plasmaspektroskopie (LIBS) durchgeführt. Ziel der Arbeit ist es, die chemische Zusammensetzung in der Schweißnaht verschiedener DSS quantitativ zu messen und mögliche Einflüsse der Schweißparameter auf die Mikrostruktur des Werkstoffs zu ermitteln. Gemessen wurden die chemischen Konzentrationen der Hauptlegierungselemente Cr, Ni, Mn auf der Oberfläche der Probe während des Schweißprozesses und des Abkühlungsprozesses. Mn und Ni sind Austenitstabilisatoren, deren Gehalt beim Schweißen durch die Verwendung bestimmter hochlegierter Schweißzusatzwerkstoffe zunimmt. Die Spektren wurden alle 1,3 s in einem Abstand von etwa 2 mm aufgezeichnet. Während des Abkühlungsprozesses wurde der Ort der Messung nicht verändert. Die LIBS-Methode eignet sich nachweislich zur quantitativen Darstellung der chemischen Zusammensetzungen während des Schweißvorgangs.
This presentation focuses on the basic ideas and current status of the development of an arithmetical method to predict the failure rotational speed of turbine disks. The certification specification requires that a gas turbine aero-engine must hold 5 minutes at overspeed conditions without critical failure. Therefore, instead of experimental proof from spin-tests using test-disks similar to engine components, it is considered to use simple specimen with similar test conditions compared to real overspeed scenarios. These test conditions, or stress fields are determined using arithmetical method, e.g. finite element method, with consideration of fracture mechanics under quasi-static conditions with a given rotational speed.
Failure modes like hoop burst and rim peeling are considered during determination of stress fields. Various crack-tip parameters are used to explore the similarity of stress field between simple specimen and real overspeed scenarios. Additionally, probabilistic aspects and the implementation of a global stability criterion for overspeed analysis are also considered.
Aero-engine turbine disks are safety-relevant components which are operated under high thermal and mechanical stress conditions. The actual part qualification and certification procedures make use of spin-tests conducted on production-similar disks. While these tests provide, on the one hand, a reliable definition of the critical conditions for real components, on the other hand they represent a relevant cost item for engine manufacturers. The aim of this work is to present part of a fracture mechanics-based procedure under development which aims at replacing the tests on production-similar disks with lab tests on fracture mechanics specimens. In particular, the rim-peeling failure mode is considered as case study. A semi-circular surface crack is modelled at the most stressed region at the diaphragm of a turbine disk, with the crack plane perpendicular to the radial direction. The crack is therefore subjected to a biaxial stress state and grows under increasing rotational speed until it triggers the rim-peeling failure. The finite element simulation of the cracked disk considers the real thermal and mechanical loading conditions. In order to design a lab representative specimen, beside the crack driving force, expressed in terms of J-integral, also the constraint to plastic deformation e.g., stress triaxiality, at the crack-tip must be similar for the same crack in the specimen and in the disk. This has been achieved and as expected, both the highest J-integral and constraint factor are calculated at the same location along the crack front for both disk and specimen. The results of the structural integrity assessment in the form of a Failure Assessment Diagram (FAD) show good agreement between designed specimen and disk both in terms of expected failure mode and value of the critical speed. Probabilistic aspects are also considered in the calculations.
Ni alloys are cost intensive materials and generally classified as difficult-to-cut materials. However, machining of these materials is needed especially in case of alloy 36 (1.3912), which is commonly used in mould construction for the production of fibre-reinforced composites. With regard to repair, modification and manufacturing of such components, additive manufacturing offers significant economic advantages. Nevertheless, subsequent machining steps are needed to achieve the final component contour and defined surface conditions. Dependent on the material and machining process conditions, detrimental tensile residual stresses may be the result on the machined surface, having negative impact on the component performance and safety. Recent studies exhibited that hybrid machining processes, such as ultrasonic-assisted milling (USAM), are suitable for achieving favourable residual stress states at the surface of difficult-to-cut Ni-Cr alloys. In this investigation, wire arc additive manufactured (WAAM) specimens of alloy 36 were finish milled under different milling conditions. In addition to the machined surfaces condition and topology, the surface-near residual stresses were analysed using X-ray diffraction (XRD). Especially for low cutting speeds, significantly improved surface properties, roughness parameters and lower mechanical and microstructural degradations were found for the specimen machined with USAM compared to conventional milling. The improved surface integrity could furthermore be observed by a significant reduction of the tensile residual stresses in the surface boundary area.
Untersuchungsmethoden beim Fräsen additiv gefertigter schwer spanbarer Kobalt-Chrom-Legierungen
(2022)
Für den Anlagenbau werden hochbelastete Komponenten aus Co-Cr-Legierungen hergestellt. Dabei ist die Zerspanung so herausfordernd, dass additive Fertigungsschritte oft wirtschaftlicher sind. Diese bedingen aber vorwiegende heterogene, anisotrope Gefüge und Eigenschaften und bei nachfolgender Fräsbearbeitung unstetige Zerspankräfte und instabile Spanvorgänge. Hierbei können hybride Zerspanprozesse helfen, hohe Belastungen für Werkzeug und Werkstückoberfläche zu minimieren. Diesbezügliche Charakterisierungen erfordern speziell adaptierte Zerspanungsanalysen.
Anhand eines Anwendungsbeispiels, bei dem eine Modifikation der Ausgangslegierung mit 1 % Hf erfolgte, wird die Wechselwirkung zwischen Gefügemorphologie und Fräsprozess dargestellt, der konventionelle mit einem hybriden ultraschallunterstützten Fräsprozess verglichen und die Charakterisierung des Werkzeugverschleißes anhand zweier verschiedener Fräser betrachtet. Analysen auftretender Prozesskräfte mittels Piezo-Mehrkomponenten-Dynamometer, der Werkzeugoszillation mittels Laser-Vibrometer und zusätzliche In-situ-Temperaturmessungen der Werkzeugschneiden mittels Highspeed-Pyrometer bilden den Kern der neuen Zerspanungsprüftechnik.
Die vorgestellten Untersuchungsmethoden bilden die Vorgänge bei der Zerspanung additiv gefertigter Werkstoffe detailliert und reproduzierbar ab. Sowohl Legierungsmodifikation als auch hybride Prozessbedingungen lassen gezielte Einflüsse zur deutlichen Verbesserung der Zerspanungssituation zu.
Ni alloys are cost intensive materials and generally classified as difficult-to-cut materials. However, machining of these materials is needed especially in case of alloy 36 (1.3912), which is commonly used in mould construction for the production of fibre-reinforced composites. With regard to repair, modification and manufacturing of such components, additive manufacturing offers significant economic advantages. Nevertheless, subsequent machining steps are needed to achieve the final component contour and defined surface conditions. Dependent on the material and machining process conditions, detrimental tensile residual stresses may be the result on the machined surface, having negative impact on the component performance and safety. Recent studies exhibited that hybrid machining processes, such as ultrasonic-assisted milling (USAM), are suitable for achieving favourable residual stress states at the surface of difficult-to-cut Ni-Cr alloys. In this investigation, wire arc additive manufactured (WAAM) specimens of alloy 36 were finish milled under different milling conditions. In addition to the machined surfaces condition and topology, the surface-near residual stresses were analysed using X-ray diffraction (XRD). Especially for low cutting speeds, significantly improved surface properties, roughness parameters and lower mechanical and microstructural degradations were found for the specimen machined with USAM compared to conventional milling. The improved surface integrity could furthermore be observed by a significant reduction of the tensile residual stresses in the surface boundary area.