Development of friction stir processing of CNT - reinforced aluminum alloy composites

Owing their wide range of exceptional properties, for example thermal conductivity values of more than 3000W/(mK) and strength in the range of 100 GPa, Carbon Nanotubes (CNTs) have recently gained much attention as reinforcement for composite materials. Considering the practical application in metal matrix composites (MMCs), tranferring those favourable properties from nano to macro scale represents a main challenge. Methods of the powder metallurgy route show promising results so far, but also lead to long process times and damage of the tublar structure of the CNTs due to prolonged ball milling times. At this point, the application of Friction Stir Processing (FSP) for fabricating CNT-reinforced MMCs offers the possibility to reduce process times and realize the required reinforcement at the relevant location of the component. The process uses a specially designed tool with a pin to plasticize the base material by frictional heating and thus incorporate the reinforcing material by stirring it into the workpiece. Investigations, that have been carried out on this subject, generally consider the unifom dispersion of the CNTs within the matrix as the key challenge of this process. So far, the solution herefore is the application of multipass-FSP in order to distribute the CNTs uniformly by processing the weld up to five times with alternating welding direction. This method usually leads to damage or even destruction of the tubular structure of the CNTs. Regarding all investigations on this subject, it can be noticed, that only conventional tool profiles like cylindrical or threaded were used for the experiments, though other profiles like square or more complex ones are considered to exhibit an increased mixing effect. Therefore the objective of the thesis is to analyse the performance of four different tool geometries under varying parameters and their influence on the CNT dispersion and general MMC composite properties. Channels were cut into an Al 5086 plate, filled with CNT-powder and processed by the different tools. The results were evaluated by metallographic analysis, hardness and electrical resistance measurement and SEM analysis. Among the different geometries, the triangular profile produced defect-free welds over the whole parameter set and distributed the CNTs uniformly along a wide area close to the weld surface.Tesi

Influence of Pin Length and Electrochemical Platings on the Mechanical Strength and Macroscopic Defect Formation in Stationary Shoulder Friction Stir Welding of Aluminium to Copper

The fabrication of dissimilar joints for electrical applications raises challenges for conventional joining technologies. Within solid-state processes, friction stir welding (FSW) provides numerous advantages to realize different joint configurations. However, depending on the intermixing of the materials, defects like hooking and significant intermetallic compound formation around copper fragments are observed and lead to a decrease in joint properties. Therefore, stationary shoulder FSW was applied to produce 2 mm EN AW1050/CW024A lap joints with minimized intermixing at the interface. Compared to conventional FSW, the range of the friction-based heat input can be increased without risking excessive plastification under the tool shoulder. The influence of the pin length on the interfacial structure as well as the mechanical properties were investigated. A pin length of 2.2 mm and hence a plunging of approximately 0.2 mm into the lower copper sheet was found to obtain the highest failure load. A further increase caused the formation of hooking defects, which led to void formation at the interface and failure within the area of the thinned aluminium sheet. The results were also transferred to lap joints with a tin and silver interlayer of 10 µm and also showed good results in terms of bond strength and contact area

Hybrid Friction Eutectic Bonding (HFEB) – stoffschlüssiges Fügen von Aluminium und Kupfer unter Nutzung der eutektischen Reaktion

Ansätze des werkstofflichen Leichtbaus mit dem Ziel der Gewichts- und Ressourcenoptimierung in elektrischen Verbindungen haben das Fügen von Aluminium-Kupfer-Mischverbindungen zu einem stark verfolgten Forschungsschwerpunkt der letzten Jahre gemacht. Im Rahmen von Elektromobilität sowie allgemeiner Erhöhung von Komfort- und Sicherheitsstandards ergeben sich hierfür hohe Herausforderungen an die einzusetzenden Fügetechnologien. Der Zielzustand für elektrische Verbindungen besteht in großflächigen, duktilen Verbindungen mit geringem Kontaktwiderstand. Je nach Verfahren werden diese Ziele jedoch durch die Bildung spröder, intermetallischer Phasen oder erhöhte Grenzflächen- und Bauteilverformung nur zum Teil erreicht. In der vorliegenden Arbeit wurde gezielt die Bildung einer eutektischen Schmelze zwischen Aluminium und Kupfer genutzt, um den Lösungsraum zwischen Press- und Schmelzschweißverfahren zu nutzen. Während der Initialkontakt der Grenzflächen durch Fügedruck während des Rührreibpunktschweißens beschleunigt wird, findet die Vergrößerung und Benetzung der Fügefläche durch die Bildung einer eutektischen Schmelze statt. Diese bildet sich zwischen den Grundwerkstoffen, ohne dass diese in die flüssige Phase übergehen. In Abhängigkeit der Prozessführung kann der Pressschweißprozess hier erneut genutzt werden, um die Schmelze aus der Fügezone zu verdrängen. Somit kann das Erstarrungsverhalten gezielt beeinflusst werden, während sich weitere Vorteile durch den Abtransport von Einschlüssen und Verunreinigungen ergeben. Durch eine angepasste Prozessführung können vollflächige Anbindungen in kurzer Prozesszeit von weniger als 0,5 s erzielt werden. Die Bildung weiterer Sprödphasen außerhalb des eutektischen Systems wird durch die Limitierung der Spitzentemperatur und des Energieeintrags verhindert. Zusätzlich kann die Schmelzebildung als Reduzierung des Materialwiderstands während des kraftgeregelten Prozesses detektiert und diese für weitere Prozessansätze genutzt werden. Weitere Optimierungsansätze zeigen sich beim Übertrag auf weitere Pressschweißverfahren und Bauteilgeometrien.Considering the current approaches on energy efficiency and electro mobility, increasing requirements on electrical assemblies require a continuous development of available manufacturing technologies. Especially joining aluminum and copper for lightweight battery assemblies and connectors has become a focus topic for industrial research. The requirements on dissimilar joints for electrical applications include the reduction of the contact resistance as well as maximization of mechanical and thermal stability. However, there are major metallurgical obstacles for conventional joining technologies. High temperatures during joining of Al-Cu are known to facilitate the growth of brittle intermetallic compounds (IMCs), which show a negative effect on the joint properties even at a few microns layer width. Otherwise, solid state welding usually requires a significant deformation at the interface and hence shows limited applicability for sensitive components. Furthermore, the lack of melt formation causes oxide and surface remnants to remain within the joint and reduce the actual contact area. Therefore, the objective of this investigation was to trigger the eutectic reaction between aluminum and copper by a friction based heat input. This novel approach between fusion and solid state welding causes a local melt formation between the solid joining partners so that a large cohesive bond area can be formed without melting the base materials. At the same time, the IMC formation can be reduced by short process times and low peak temperatures. The applied joining force and the frictional heat input cause oxide breakup and solid state diffusion across the interface until the eutectic melt forms. The force can then also be used to displace the melt out of the joining area along with oxide fragments and further surface remnants. With an adjusted process setup, Ø 6–10 mm welds could be produced within less than 0,5 s. The formation of further IMCs outside of the eutectic composition was avoided or minimized by the low energy input and limited peak temperature. The melt formation can even be detected during the force controlled process and be used for optimizing the melt displacement. Further optimization approaches could be demonstrated by applying the concept onto resistance spot welding and connection of copper caps and aluminum stranded wire.Der zunehmende Einsatz von Aluminium-Kupfer-Mischverbindungen für Anwendungen im Bereich E-Mobility führt zu Herausforderungen an die einzusetzenden Fügetechnologien. In dieser Arbeit wurde daher gezielt die Bildung einer eutektischen Schmelze zwischen Aluminium und Kupfer während eines Pressschweißprozesses ausgelöst, um den Lösungsraum zwischen Press- und Schmelzschweißverfahren zu nutzen. Während der Initialkontakt durch den Fügedruck beschleunigt wird, findet die Vergrößerung der Fügefläche durch die Bildung einer eutektischen Schmelze zwischen den festen Grundwerkstoffen innerhalb von weniger als einer Sekunde statt. In Abhängigkeit der Prozessführung kann der Pressschweißprozess erneut genutzt werden, um die Schmelze aus der Fügezone zu verdrängen. Die Bildung weiterer spröder Phasen wird durch die Limitierung der Spitzentemperatur und des Energieeintrags minimiert. Durch eine geeignete Prozessführung zeigen sich Vorteile beim Übertrag auf weitere Verfahren und Bauteilgeometrien

Energy taxes and employment A do-it-yourself simulation model

Das Arbeitspapier beschaeftigt sich mit den wohlfahrts- und beschaeftigungspolitischen Effekten der Steuerreform der Gruenen. Im ersten Teil entwickeln die Autoren ein flexibles, inaktives Simulationsmodell, das unter http://brw.zew.de zugaenglich ist. Der Nutzer hat hier die Moeglichkeit, seine eigene gruene Steuerreform oder Emissionsquoten zu spezifizieren und die Wohlfahrts- und Beschaeftigungseffekte zu quantifizieren. Er kann zwischen verschiedenen Modellvarianten waehlen, z.B. zwischen einer geschlossenen oder gering geoeffneten Oekonomie mit oder ohne Arbeitslosigkeit. Im zweiten Teil beschreiben die Autoren die numerischen Modellspezifikationen und erlaeutern exemplarisch einige Simulationskalkulationen. Uebungen fuer Schulklassen sind unter o.g. web-site erhaeltlich. (ICAUebers)'This paper deals with the welfare and employment effects of green tax reforms. In the first part the authors develop a flexible, interactive simulation model which is accessible under http://brw.zew.de target=NewWindow> http://brw.zew.de . Users can specify their own green tax reforms or emission quotas and quantify welfare and employment effects. He or she can choose between different model variants, e.g. closed or small open economies with or without unemployment. In the second part the authors describe the numerical model specifications and explain some simulation calculations by means of examples. Some exercises for classroom use are available on the above mentioned web-site.' (author's abstract)German title: Energiesteuern und Beschaeftigung: ein do-it-yourself SimulationsmodellSIGLEAvailable from ftp://ftp.zew.de/pub/zew-docs/dp/dp0121.pdf / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

European soil seed bank communities across a climate and land-cover gradient

This is the data set used for the publication Buffering effects of soil seed banks on plant community composition in response to land use and climate, published in the journal Global Ecology and Biogeography. Aim. Climate and land use are key determinants of biodiversity, with past and ongoing changes posing serious threats to global ecosystems. Unlike most other organism groups, plant species can possess dormant life-history stages such as soil seed banks, which may help plant communities to resist or at least postpone the detrimental impact of global changes. This study investigates the potential for soil seed banks to achieve this. Location. Europe Time period. 1978 – 2014 Major taxa studied. Flowering plants Methods. Using a space-for-time/warming approach, we study plant species richness and composition in the herb layer and the soil seed bank in 2796 community plots from 54 datasets in managed grasslands, forests and intermediate, successional habitats across a climate gradient. Results. Soil seed banks held more species than the herb layer, being compositionally similar across habitats. Species richness was lower in forests and successional habitats compared to grasslands, with annual temperature range more important than mean annual temperature for determining richness. Climate and land use effects were generally less pronounced when plant community richness included seed bank species richness, while there was no clear effect of land use and climate on compositional similarity between the seed bank and the herb layer. Main conclusions. High seed bank diversity and compositional similarity between the herb layer and seed bank plant communities may provide a potentially important functional buffer against the impact of ongoing environmental changes on plant communities. This capacity could, however, be threatened by climate warming. Dormant life-history stages can therefore be important sources of diversity in changing environments, potentially underpinning already observed time-lags in plant community responses to global change. However, as soil seed banks themselves appear, albeit less, vulnerable to the same changes, their potential to buffer change can only be temporary, and major community shifts may still be expected.,This dataset is a collection of 41 published and 5 unpublished data sets, consisting of 2796 plots with corresponding seed bank and herb layer community data. Sampling effort varied across data sets, but involved sampling of the soil and subsequent germination trials in a greenhouse to determine seed bank composition. Herb layer communities were determined by the identification of plants in relevés. Please consult the readme file and published paper for further details.,Please contact database or individual data set authors for further information and collaboration when using the data set or any of its component parts. Please also note that some of these data sets have already been published alongside their orginal papers. Finally, please cite data and datasets according to community standards.

Many Labs 2: Investigating Variation in Replicability Across Samples and Settings

We conducted preregistered replications of 28 classic and contemporary published findings, with protocols that were peer reviewed in advance, to examine variation in effect magnitudes across samples and settings. Each protocol was administered to approximately half of 125 samples that comprised 15,305 participants from 36 countries and territories. Using the conventional criterion of statistical significance (p < .05), we found that 15 (54%) of the replications provided evidence of a statistically significant effect in the same direction as the original finding. With a strict significance criterion (p < .0001), 14 (50%) of the replications still provided such evidence, a reflection of the extremely high-powered design. Seven (25%) of the replications yielded effect sizes larger than the original ones, and 21 (75%) yielded effect sizes smaller than the original ones. The median comparable Cohen’s ds were 0.60 for the original findings and 0.15 for the replications. The effect sizes were small (< 0.20) in 16 of the replications (57%), and 9 effects (32%) were in the direction opposite the direction of the original effect. Across settings, the Q statistic indicated significant heterogeneity in 11 (39%) of the replication effects, and most of those were among the findings with the largest overall effect sizes; only 1 effect that was near zero in the aggregate showed significant heterogeneity according to this measure. Only 1 effect had a tau value greater than .20, an indication of moderate heterogeneity. Eight others had tau values near or slightly above .10, an indication of slight heterogeneity. Moderation tests indicated that very little heterogeneity was attributable to the order in which the tasks were performed or whether the tasks were administered in lab versus online. Exploratory comparisons revealed little heterogeneity between Western, educated, industrialized, rich, and democratic (WEIRD) cultures and less WEIRD cultures (i.e., cultures with relatively high and low WEIRDness scores, respectively). Cumulatively, variability in the observed effect sizes was attributable more to the effect being studied than to the sample or setting in which it was studied

Many Labs 2: Investigating Variation in Replicability Across Samples and Settings

We conducted preregistered replications of 28 classic and contemporary published findings, with protocols that were peer reviewed in advance, to examine variation in effect magnitudes across samples and settings. Each protocol was administered to approximately half of 125 samples that comprised 15,305 participants from 36 countries and territories. Using the conventional criterion of statistical significance (p < .05), we found that 15 (54%) of the replications provided evidence of a statistically significant effect in the same direction as the original finding. With a strict significance criterion (p < .0001), 14 (50%) of the replications still provided such evidence, a reflection of the extremely high-powered design. Seven (25%) of the replications yielded effect sizes larger than the original ones, and 21 (75%) yielded effect sizes smaller than the original ones. The median comparable Cohen’s ds were 0.60 for the original findings and 0.15 for the replications. The effect sizes were small (< 0.20) in 16 of the replications (57%), and 9 effects (32%) were in the direction opposite the direction of the original effect. Across settings, the Q statistic indicated significant heterogeneity in 11 (39%) of the replication effects, and most of those were among the findings with the largest overall effect sizes; only 1 effect that was near zero in the aggregate showed significant heterogeneity according to this measure. Only 1 effect had a tau value greater than .20, an indication of moderate heterogeneity. Eight others had tau values near or slightly above .10, an indication of slight heterogeneity. Moderation tests indicated that very little heterogeneity was attributable to the order in which the tasks were performed or whether the tasks were administered in lab versus online. Exploratory comparisons revealed little heterogeneity between Western, educated, industrialized, rich, and democratic (WEIRD) cultures and less WEIRD cultures (i.e., cultures with relatively high and low WEIRDness scores, respectively). Cumulatively, variability in the observed effect sizes was attributable more to the effect being studied than to the sample or setting in which it was studied

Additional file 33 of Implicating genes, pleiotropy, and sexual dimorphism at blood lipid loci through multi-ancestry meta-analysis

Additional file 33: Table S22. Mouse genes with lipid phenotypes (silver set)