Effect of Discretization of Permeability Term and Mesh Size on Macro- and Meso-segregation Predictions

Macro- and meso-segregations correspond to heterogeneities of composition at the scale of a casting. They develop during the solidification. One of the parameters that has an essential effect on these segregations is the mush permeability which varies over a wide range of magnitude. We present simulation results for solidification of Sn-Pb alloy in a two-dimensional cavity. The role of discretization schemes and mesh size on the formation of channel segregates and macrosegregation is discussed

Modelling of Columnar-to-Equiaxed and Equiaxed-to- Columnar Transitions in Ingots Using a Multiphase Model

International audienceWe present a new method to handle a representative elementary volume (REV) with a mixture of columnar and equiaxed grains in ingot castings in the framework of an Eulerian volume averaged model. The multiscale model is based on a previously established fully equiaxed model. It consists of a three-phase (extra-granular liquid, intra-granular liquid and solid) grain-growth stage coupled with a two-phase (solid and liquid) macroscopic transport stage accounting for grain and nuclei movement. In this context, we take into account the formation of a columnar structure and its development using a simplified front-tracking method. Columnar solidification is coupled with the growth of equiaxed grains ahead of the columnar front. The particularity of the model is the treatment of concurrent growth of mixed columnar and equiaxed structures only in the volumes that contain the columnar front. Everywhere else, the structure is considered either fully columnar or fully equiaxed. This feature allows for reasonable computational times even in industrial size castings, while describing the solutal and mechanical blocking phenomena responsible for the Columnar-to-Equiaxed Transition. After a validation of the model, we discuss the numerical results for a 6.2-ton industrial steel ingot by comparison with experimental measurements. Final maps for macrosegregation and grain structures size and morphology are analysed. Furthermore, we quantify the impact of nuclei formation through fragmentation along the columnar front on the result. An attempt at predicting the occurrence of the Equiaxed-to-Columnar Transition in the later phases of the process is also made

Multi-scale finite element modelling of solidification structures by a splitting method taking into account the transport of equiaxed grains

International audienceIn solidification processes of large industrial castings and ingots, the transport of solid in the liquid has an important effect on the final grain structure and macrosegregation. Modeling is still challenging as complex interactions between heat and mass transfers at microscopic and macroscopic scales are highly coupled. This paper first presents a multi-scale numerical solidification model coupling nucleation, grain growth and solute diffusion at microscopic scales with heat and mass transfer, including transport of liquid and solid phases at macroscopic scales. The resolution consists of a splitting method, which considers the evolution and interaction of quantities during the process with a transport stage and a growth stage. This splitting reduces the nonlinear complexity of the set of considered equations and provides an efficient numerical implementation. It is inspired by the work of Založnik et al. [1,2], which used a finite volume method (FVM). The present work develops the solution based on the finite element method (FEM). Numerical results obtained with this model are presented and simulations without and with grain transport are compared to study the impact of solid-phase transport on the solidification process and on the formation of macrosegregation

First analysis of a numerical benchmark for 2D columnar solidification of binary alloys

International audienceDuring the solidification of metal alloys, chemical heterogeneities at the product scale (macrosegregation) develop. Numerical simulation tools are beginning to appear in the industry, however their predictive capabilities are still limited. We present a numerical benchmark exercise treating the performance of models in the prediction of macrosegregation. In a first stage we defined a "minimal" (i.e. maximally simplified) solidification model, describing the coupling of the solidification of a binary alloy and of the transport phenomena (heat, solute transport and fluid flow) that lead to macrosegregation in a fully columnar ingot with a fixed solid phase. This model is solved by four different numerical codes, employing different numerical methods (FVM and FEM) and various solution schemes. We compare the predictions of the evolution of macrosegregation in a small (10×6 cm) ingot of Sn-10wt%Pb alloys. Further, we present the sensitivities concerning the prediction of instabilities leading to banded channel mesosegregations

Micro-NRA and micro-3HIXE with He-3 microbeam on samples exposed in ASDEX Upgrade and Pilot-PSI machines

Micro nuclear reaction analysis (micro-NRA) exploiting the nuclear reaction D(3He,p)4He was used for post-mortem analyses of special marker samples, exposed to deuterium plasma inside ASDEX Upgrade (AUG) tokamak and to the deuterium plasma jet in the Pilot-PSI linear plasma gun. Lateral concentration profiles of deuterium and erosion/deposition profiles of the marker materials were obtained by a combination of micro-NRA and particle induced X-ray emission by 3He beam (3HIXE). In the case of AUG samples, where 25 nm thick W marker layers had been deposited on unpolished and polished graphite substrates, the effect of surface roughness on local erosion and deposition was also investigated. The lateral distribution of W concentration showed that erosion is much more distinct in the case of polished samples and the resulting surface shows a “leopard” skin pattern of W accumulated on carbon aggregates left on the surface from polishing. The Pilot-PSI samples indicated preferential accumulation of deuterium a few mm off from the centre of the region affected by the plasma beam. This is connected with the largest surface modifications while the thick deposited layers at the centre do not favour deuterium retention per se. The results were cross correlated with those obtained using laser-induced breakdown spectroscopy (LIBS). With its quantitative abilities, micro-NRA provided essential calibration data for in situ LIBS operation, as well as for complementary post mortem Secondary Ion Mass Spectroscopy (SIMS)Micro nuclear reaction analysis (micro-NRA) exploiting the nuclear reaction D(He-3,p)He-4 was used for post-mortem analyses of special marker samples, exposed to deuterium plasma inside ASDEX Upgrade (AUG) tokamak and to the deuterium plasma jet in the Pilot-PSI linear plasma gun. Lateral concentration profiles of deuterium and erosion/deposition profiles of the marker materials were obtained by a combination of micro-NRA and particle induced X-ray emission by He-3 beam (3HIXE). In the case of AUG samples, where 25 nm thick W marker layers had been deposited on unpolished and polished graphite substrates, the effect of surface roughness on local erosion and deposition was also investigated. The lateral distribution of W concentration showed that erosion is much more distinct in the case of polished samples and the resulting surface shows a "leopard" skin pattern of W accumulated on carbon aggregates left on the surface from polishing. The Pilot-PSI samples indicated preferential accumulation of deuterium a few mm off from the centre of the region affected by the plasma beam. This is connected with the largest surface modifications while the thick deposited layers at the centre do not favour deuterium retention per se. The results were cross correlated with those obtained using laser-induced breakdown spectroscopy (LIBS). With its quantitative abilities, micro-NRA provided essential calibration data for in situ LIES operation, as well as for complementary post mortem Secondary Ion Mass Spectroscopy (SIMS). (C) 2017 EURATOM. Published by Elsevier B.V. All rights reserved.Peer reviewe

Reduction of deuterium content in carbon targets for 12C+12C reaction studies of astrophysical interest

The 12C(12C,p)23Na and 12C(12C,$\alpha$)20Ne fusion reactions are among the most important in stellar evolution since they determine the destiny of massive ($M \simeq 8-10 M_{\odot}$) stars. However, experimental low-energy investigations of such reactions are significantly hampered by ubiquitous natural hydrogen and deuterium contaminants in the carbon targets. The associated beam-induced background completely masks the reaction products of interest thus preventing cross-section measurements at the relevant energies of astrophysical interest, $E_{\mathrm{cm}} < 2$ MeV. In this work, we report about an investigation aimed at assessing possible deuterium reductions on both natural graphite and Highly Ordered Pyrolytic Graphite targets as a function of target temperature. Our results indicate that reductions up to about 80% can be attained on both targets in the temperature range investigated, $T \simeq 200-1200 {}^{\circ}$C. A further reduction by a factor of 2.5 in absolute deuterium content is observed when the scattering chamber is surrounded by a dry nitrogen atmosphere so as to minimise light-particles uptake within the chamber rest gas (and thus on target) through air leaks. The results from this study will inform the choice of optimal experimental conditions and procedures for improved measurements of the 12C + 12C reactions cross-sections at the low energies of astrophysical interest

Cadastral Surveys Using Terrestrial Laser Scanning - Accuracy and Economy

Due to the wide range of possible applications, terrestrial laser scanning (TLS) has long since found its way into monument protection and construction. The advantage of this technology is the resulting 3D image of the surroundings including all visible objects as a 3D point cloud. At the office, all necessary information can be derived from the data set using specialized software. In order to test the suitability of TLS to create partition plans according to the Ordinance on Cadastral Surveying (VermV), a case study was conducted using both, a total station and a TLS. Both recordings were evaluated and compared for accuracy, cost-effectiveness, and legality. The accuracy of a TLS mainly depends on the care taken in the evaluation. Deviations occur if geometries are misinterpreted, however, such mistakes can be corrected in the office. From an economic point of view, the TLS requires 14% less total working time. The workload is shifted from the field to the office, with a 68% reduction in field work time. This leads to a reduction in costs and less disruption in the surveyed area. The use of a TLS is legally covered by the VermV. A major advantage of the technology is the recording of the entire environment suited for documentation, preservation of evidence, and added value for planners. However, the decision on the use of a TLS needs to be done case-based