1,724,258 research outputs found
Material constitutive behaviour and microstructure study on aluminium alloys for friction stir welding
Get PDFThe material constitutive behaviour and microstructure of Aluminium alloys 6082
and 7449 were studied with the Gleeble hot compression test. The novel aspect of
the work is that the testing was done at high strain-rates and at temperatures
within 5 K of the solidus. The results indicated that the strength was
maintained up to near solidus temperatures, with no dramatic strength reduction
being observed. There was however, a distinct change in the slope observed with
the 7449 results around 720 K. The experimental results were then fit to the
Zener-Holloman equation, which describes the relationship between the material
flow stress, temperature and strain-rates. The material microstructure of the
hot compression test samples was analysed, and the averaged grain size was
calculated to compare with friction stir weld nuggets. This will be used to
infer the processing conditions that exist in the dynamically recrystallized
weld nugget. Finally, a simple model was used to understand how processing
conditions affected the deformation behaviour
Model for predicting heat generation and temperature in friction stir welding from the material properties
Get PDFThis paper describes a simple numerical model for predicting the heat generation
in friction stir welding (FSW) from the material hot deformation and thermal
properties, the process parameters, and the tool and plate dimensions. The model
idealises the deformation zone as a two-dimensional axisymmetric problem, but
allowance is made for the effect of translation by averaging the three-
dimensional temperature distribution around the tool in the real weld. The model
successfully predicts the weld temperature field and has been applied with
minimal recalibration to aerospace aluminium alloys 2024, 7449 and 6013, which
span a wide range of strength. The conditions under the tool are presented as
novel maps of flow stress against temperature and strain rate, giving insight
into the relationship between material properties and optimum welding
conditions. This highlights the need in FSW for experimental high strain rate
tests close to the solidus temperature. The model is used to illustrate the
optimisation of process conditions such as rotation speed in a given alloy and
to demonstrate the sensitivity to key parameters such as contact radius under
the shoulder, and the choice of stick or slip conditions. The aim of the model
is to provide a predictive capability for FSW temperature fields directly from
the material properties and weld conditions, without recourse to complex
computational fluid dynamics (CFD) software. This will enable simpler
integration with models for prediction of, for example, the weld microstructure
and properties
Toughness-strength relations in the overaged 7449 Al-Based alloy
Get PDFThis article examines the relationship between plane strain fracture toughness, KIc, the tensile properties, and the microstructure of the overaged 7449 aluminum-plate alloy, and compares them to the 7150 alloy. The 7449 alloy has a higher content of ??/? precipitates; and, the 7150 alloy contains a greater amount of coarse intermetallic particles, as it contains an appreciable amount of coarse S phase (Al2CuMg), which is largely absent in the 7449 alloy. The toughness of the alloys shows an increase on overaging, and the 7449 alloy shows a reasonably linear toughness—yield strength relation on extended overaging. Several mechanisms of failure occur: coarse voiding at intermetallics and a combined intergranular/transgranular shear fracture mode, with the former becoming more important as overaging progresses. Drawbacks of existing models for toughness are discussed, and a new model for plane strain fracture toughness, based on the microstructurally dependent work-hardening factor, KA, introduced in Ashby's theory of work hardening, is developed. This model predicts a linear relation between KIc and K0.85A/?0.35ys, where ?ys is the yield strength, which is consistent with the experimental data
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Friction Stir Welding of Precipitation Strengthened Aluminum 7449 Alloys
Get PDFThe Al-Zn-Mg-Cu (7XXX series) alloys are amongst the strongest aluminum available. However, they are considered unweldable with conventional fusion techniques due to the negative effects that arise with conventional welding, including hydrogen porosity, hot cracking, and stress corrosion cracking. For this reason, friction stir welding has emerged as the preferred technique to weld 7XXX series alloys. Aluminum 7449 is one of the highest strength 7XXX series aluminum alloy. This is due to its higher zinc content, which leads to a higher volume fraction of eta' precipitates. It is typically used in a slight overaged condition since it exhibits better corrosion resistance. In this work, the welds of friction stir welded aluminum 7449 were studied extensively. Specific focus was placed in the heat affected zone (HAZ) and nugget. Thermocouples were used in the heat affected zone for three different depths to obtain thermal profiles as well as cooling/heating profiles. Vicker microhardness testing, transmission electron microscope (TEM), and differential scanning calorimeter (DSC) were used to characterize the welds. Two different tempers of the alloy were used, a low overaged temper and a high overaged temper. A thorough comparison of the two different tempers was done. It was found that highly overaged aluminum 7449 tempers show better properties for friction stir welding. A heat gradient along with a high conducting plate (Cu) used at the bottom of the run, resulted in welds with two separate microstructures in the nugget. Due to the microstructure at the bottom of the nugget, higher strength than the base metal is observed. Furthermore, the effects of natural aging and artificial aging were studied to understand re-precipitation. Large improvements in strength are observed after natural aging throughout the welds, including improvements in the HAZ
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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