Mapping and Modelling Single Scan Track Formation in Direct Metal Selective Laser Melting

When a laser beam scans the surface of a metallic powder bed, the resulting track may be continuous with a crescent shape cross-section, continuous with an elliptical section, discontinuously irregularly broken, discontinuously balled or only partially melted. This paper reports experimental observations of what CO₂laser powers and scan speeds lead to what type of track, for M2 and H13 tools steels and 314S stainless steel powder beds. It also presents measurements of bed physical properties relevant to predicting the behaviour and the results of modelling that agree with the experiments. The results are placed in the broader context of selective laser melting build strategies     

Further studies in selective laser melting of stainless and tool steel powders

Previous reported work on the selective laser melting of a H13 tool steel powder bed surface has shown that there is a scan speed range in which the layer mass increases/fluctuates with increasing speed. This paper expands the investigation towards M2 tool steel and 316L stainless steel powders to identify if they reveal similar behaviours. Wide ranges of scan spacings and scan speeds have been examined, at selected laser powers. Furthermore, the masses of the layers have been compared with those predicted from an existing finite element thermal model. It has been found that at a constant laser power, the variation of mass with scan speed is much less than might be expected from a constant assumed absorptivity into a powder bed.     

Visualization and Analysis‐Oriented Reconstruction of Material Interfaces

Reconstructing boundaries along material interfaces from volume fractions is a difficult problem, especially because the under‐resolved nature of the input data allows for many correct interpretations. Worse, algorithms widely accepted as appropriate for simulation are inappropriate for visualization. In this paper, we describe a new algorithm that is specifically intended for reconstructing material interfaces for visualization and analysis requirements. The algorithm performs well with respect to memory footprint and execution time, has desirable properties in various accuracy metrics, and also produces smooth surfaces with few artifacts, even when faced with more than two materials per cell.     

Assessing tropical cyclone impacts using object-based moderate spatial resolution image analysis: a case study in Bangladesh

The environmental and societal impacts of tropical cyclones could be reduced using a range of management initiatives. Remote sensing can be a cost effective, accurate, and potential tool for mapping the multiple impacts caused by tropical cyclones using high-to-moderate spatial resolution (5–30 m) satellite imagery to provide data on the following essential parameters – evacuation, relief, and management of natural resources. This study developed and evaluated an approach for assessing the impacts of tropical cyclones through object-based image analysis and moderate spatial resolution imagery. Pre- and post-cyclone maps of artificial and natural features are required for assessing the overall impacts in the landscape that could be acquired by mapping specific land cover types. We used the object-based approach to map land-cover types in pre- and post-cyclone Satellite Pour l’Observation de la Terre (SPOT) 5 image data and the post-classification comparison technique to identify changes in the particular features in the landscape. Cyclone Sidr (2007) was used to test the applicability of this approach in Sarankhola Upazila in Bangladesh. The object-based approach provided accurate results for classifying features from pre- and post-cyclone satellite images with an overall accuracy of 95.43% and 93.27%, respectively. Mapped changes identified the extent, type, and form of cyclone induced impacts. Our results indicate that 63.15% of the study area was significantly affected by cyclone Sidr. The majority of mapped damage was found in vegetation, cropped lands, settlements, and infrastructure. The damage results were verified through the high spatial resolution satellite imagery, reports and pictures that were taken after the cyclone. The methods developed may be used in future to assess the multiple impacts caused by tropical cyclones in Bangladesh and other similar environments for the purposes of tropical cyclone disaster management.     

The effect of gravity on R410A condensing flow in horizontal circular tubes

Heat transfer characteristics of R410A condensation in horizontal tubes with the inner diameter of 3.78?mm under normal and reduced gravity are investigated numerically. The results indicate that the heat transfer coefficients increase with increasing gravitational accelerations at a lower mass flux, whereas their differences under varying gravity are insignificant at a higher mass flux. The liquid film thickness decreases with increasing gravity at the top part of the tube, whereas the average liquid film thickness is nearly the same under different gravity accelerations at the same vapor quality and mass flux. The local heat transfer coefficients increase with increasing gravity at the top of the tube and decrease with increasing gravity at the bottom. The proportion of the thin liquid film region is important for the overall heat transfer coefficients for the condensing flow. A vortex with its core lying at the bottom of the tube is observed under normal gravity because of the combined effect of gravity and the mass sink at the liquid–vapor interface, whereas the stream traces point to the liquid–vapor interfaces under zero gravity. The mass transfer rate under zero gravity is much lower than that of normal gravity.     

Ductile shear failure damage modelling and predicting built-up edge in steel machining

This paper reports an improved ductile shear failure model for steels and its application, through finite element simulations, to predicting the conditions for built-up edge formation in steel machining. The model has two parts, a standard damage accumulation law and (the improved part) how damage affects the steel's flow stress after failure. The accumulation law includes a strain to failure with inverse exponential dependence on hydrostatic pressure and reducing in a blue-brittle temperature range. The flow stress after failure remains finite in compressive hydrostatic conditions, to create a friction resistance to shear across the failure surface. Predictions of built-up edge formation depend strongly on strain hardening behaviour. This affects the hydrostatic stress field in the chip formation region. Simulations show the general features of built-up edge formation (a finite cutting speed range with an upper limit determined by increased ductility with temperature and a lower limit determined, depending on conditions, by insufficient heating for blue-brittleness, lower chip/tool friction or a change to unsteady chip formation). The simulations are tested against previously published observations of built-up edge formation in orthogonal cutting of a Russian steel equivalent to AISI 5130. To extend the work to a wider range of steels requires more data to be gathered on individual steels’ damage accumulation law coefficients. Also, at this stage, the simulations only predict the conditions (cutting speed, uncut chip thickness) in which built-up edge forms. They are not able to follow the growth of the built-up edge to its final shape.     

MY RECOLLECTIONS OF G.2 A.6

Abstract

In this article, we consider an attack on the SIGABA cipher under the assumption that the largest practical keyspace is used. The attack highlights various strengths and weaknesses of SIGABA and provides insight into the inherent level of security provided by the cipher.     

Ultrasound-modulated twin-fluid atomization of a liquid jet

A resonant liquid capillary wave theory which extends Taylor's dispersion relation to include the sheltering effect of liquid surface inclination caused by air flow is presented. The resulting dispersion curves are compared to new experimental results of how drop-size and size distributions vary with surface tension and air velocity in both airblast and ultrasound-modulated twin-fluid atomization of liquids with a constant kinematic viscosity of 2 cSt. Good agreements between the theoretical predictions of relative growth rates of the capillary waves and the experimental results of drop-size and size distributions led to the conclusion that Taylor-mode breakup of capillary waves plays a very important role in twin-fluid (airblast) atomization of a liquid jet. Thus, the ultrasound-modulated twin-fluid atomization not only verifies the capillary wave mechanism but also provides a means for controlling the drop-size and size distributions in twin-fluid atomization, which has a variety of applications in fuel combustion, spray drying, and spray coating.