Liste des documents
Theoretical model and experimental investigation of current density boundary condition for welding arc study
This paper presents results of theoretical and experimental investigation of the welding arc in Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding (GMAW) processes. A theoretical model consisting in simultaneous resolution of the set of conservation equations for mass, momentum, energy and current, Ohm’s law and Maxwell equation is used to predict temperatures and current density distribution in argon welding arcs. A current density profile had to be assumed over the surface of the cathode as a boundary condition in order to make the theoretical calculations possible. In stationary GTAW process, this assumption leads to fair agreement with experimental results reported in literature with maximum arc temperatures of ∼21 000 K. In contrast to the GTAW process, in GMAW process, the electrode is consumable and non-thermionic, and a realistic boundary condition of the current density is lacking. For establishing this crucial boundary condition which is the current density in the anode melting electrode, an original method is setup to enable the current density to be determined experimentally. High-speed camera (3000 images/s) is used to get geometrical dimensions of the welding wire used as anode. The total area of the melting anode covered by the arc plasma being determined, the current density at the anode surface can be calculated. For a 330 A arc, the current density at the melting anode surface is found to be of 5 × 107 Am−2 for a 1.2 mm diameter welding electrode. Voir les détails
Mots clés : Gas Tungsten Arc Welding, GTAW, Gas Metal Arc Welding, GMAW, spectroscopie
LTE Experimental Validation in a Gas Metal Arc Welding Plasma Column
During gas metal arc welding (GMAW), the plasma obtained has a rich composition and some hypothesis are often made for modelling, in particular the local thermodynamical equilibrium (LTE) state of the plasma. It is then important to study its validity domain, as it is also needed for plasma parameters determination.The plasma was investigated using optical emission spectroscopy. The electron temperature and density were determined from Stark width measurements, independently of the plasma equilibrium state, and compared to the excitation temperature obtained using the Boltzmann plot (BP) method. The welding experiments were made at arc current of 330 A, with pure argon as shielding gases. The LTE is verified in the core region of the plasma, for about one half of the column radius in the arc lower part. Voir les détails
Mots clés : GMAW, Optical Emission Spectroscopy, Boltzmann Plot, Sola method, LTE
Texture, microstructure and anisotropic properties in annealed 2205 duplex stainless steel welds
The effect of welding and subsequent annealing on the evolution of the crystallographic texture and the anisotropic properties in a 2205 duplex stainless steel was studied. A strong texture was found in the base metal for both austenite and ferrite phases. The austenite texture is composed of rolling (copper and brass) and recrystallization (cube) components whereas the ferrite texture contains mainly a marked < 001>//RD α fibre with a major rotated cube component. The texture evolution during welding and annealing was characterized in the base metal, heat affected zone and weld metal; the corresponding phase proportions and misorientation values were also calculated. The analysis of all these data allows improving the understanding of the recovery, recrystallization and grain growth mechanisms occurring during the various steps of the treatment. Finally, a micromechanical model was used to calculate some mechanical properties from the measured texture, and to confirm that the optimal annealing treatment is about 1050 °C. Voir les détails
Mots clés : duplex stainless steel, TIG welding, texture, anisotropy of mechanical properties, EBSD
Dual-Frequency Behavior of Stacked High Tc Superconducting Microstrip Patches
The dual-frequency behavior of stacked high Tc superconducting rectangular microstrip patches fabricated on a two-layered substrate is investigated using a full-wave spectral analysis in conjunction with the complex resistive boundary condition. Using a matrix representation of each layer, the dyadic Green's functions of the problem are efficiently determined in the vector Fourier transform domain. The stationary phase method is used for computing the radiation electric field of the antenna. The proposed approach is validated by comparison of the computed results with previously published data. Variations of the lower and upper resonant frequencies, bandwidth and quality factor with the operating temperature are given. Results showing the effects of the bottom patch thickness as well as the top patch thickness on the dual-frequency behavior of the stacked configuration are also presented and discussed. Finally, for a better comprehension of the dual-frequency operation, a comparison between the characteristics of the lower and upper resonances is given Voir les détails
Mots clés : Dual-frequency operation, Stacked patches, Superconducting microstrip patches
Application of Hybrid Wavelet-Fractal Compression Algorithm for Radiographic Images of Weld Defects
Based on the standard fractal transformation in spatial domain, simple relations may be found relating coefficients in detail subbands in the wavelet domain. In this work we evaluate a hybrid wavelet-fractal image coder, and we test its ability to compress radiographic images of weld defects. A comparative study between the hybrid coder and standard fractal compression technique have been made in order to investigate the compression ratio and corresponding quality of the image using peak signal to noise ratio. Numerical experiments using radiographic images of weld defects illustrate the superior performance of the hybrid coder compared to standard fractal algorithm. Voir les détails
Mots clés : Fractal Compression, Discrete wavelet transform, Wavelet-Fractal coder, Radiographic images of weld defects, Compression ratio, Peak signal to noise ratio
A Region-Based Model and Binary Level Set Function Applied to Weld Defects Detection in Radiographic Images
In this paper, we propose a model for active contours to detect boundaries’ objects in given image. The curve evolution is based on Chan-Vese model implemented via binary variational level set formulation. The particularity of this model is the capacity to detect boundaries’ objects without need to use gradient of the image, this property gives its several advantages: it allows detecting both contours with or without gradient, it has ability to detect automatically interior contours, and it is robust in the presence of noise. For increasing the performance of model, we introduce the level sets function to describe the active contour, the more important advantage to use level set is the ability to change topology. Experiments on synthetic and real (weld radiographic) images show both efficiency and accuracy of implemented model. Voir les détails
Mots clés : image segmentation, Curve evolution, Chan-Vese Model, EDPs, Level set, radiographic images
Maximum Likelihood Curves for Multiple Objects Extraction: Application to Radiographic Inspection for Weld Defects Detection
This paper presents an adaptive probabilistic region-based deformable model using an explicit representation that aims to extract automatically defects from a radiographic film. To deal with the height computation cost of such model, an adaptive polygonal representation is used and the search space for the greedy-based model evolution is reduced. Furthermore, we adapt this explicit model to handle topological changes in presence of multiple defects. Voir les détails
Mots clés : Explicit deformable model, adaptive contour representation, Maximum likelihood criterion
Adaptive and Statistical Polygonal Curve forMultiple Weld Defects Detection inRadiographic Images
With the advances in computer science and artificial intelligencetechniques, the opportunity to develop computer aided techniquefor radiographic inspection in Non Destructive Testing arose. This paperpresents an adaptive probabilistic region-based deformable model usingan explicit representation that aims to extract automatically defects froma radiographic film. To deal with the height computation cost of suchmodel, an adaptive polygonal representation is used and the search spacefor the greedy-based model evolution is reduced. Furthermore, we adaptthis explicit model to handle topological changes in presence of multipledefects. Voir les détails
Mots clés : Radiographic inspection, Explicit deformable model, adaptive contour representation, Maximum likelihood criterion, Multiple contours
Aluminium-induced crystallization ofamorphous silicon films deposited byDC magnetron sputtering on glasses
Amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) films were deposited by DC magnetron sputtering technique with argon and hydrogen plasma mixture on Al deposited by ther- mal evaporation on glass substrates. The a-Si/Al and a-Si:H/Al thin films were annealed at different temperatures ranging from 250 to 550 ◦C during 4 h in vacuum-sealed bulb. The effects of annealing temperature on optical, structural and morphological properties of as-grown as well as the vacuum- annealed a-Si/Al and a-Si:H/Al thin films are presented in this contribution. The averaged transmittance of a-Si:H/Al film increases upon increasing the annealing temperature. XRD measurements clearly evi-dence that crystallization is initiated at 450°C. The number and intensity of diffraction peaks appearing in the diffraction patterns are more important in a-Si:H/Al than that in a-Si/Al layers. Results show that a-Si:H films deposited on Al/glass crystallize above 450 ◦C and present better crystallization than the a-Si layers. The presence of hydrogen induces an improvement of structural properties of poly-Si prepared by aluminium-induced crystallization (AIC). Voir les détails
Mots clés : Crystallization, Thin films, Hydrogenated amorphous silicon, AIC, Raman, XRD
Effect of the heat treatment on the microstructural evolution of the Nickel based superlloy
The effect of heat treatment on the microstructure of cobalt-rich nickel based superalloy was studied applying scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The aim of the present work was to investigate the formation and evolution of different phases during the heat treatment of superalloy similar to Udimet 500. The presence of a relatively high volume fraction of γ’ particles in the γ matrix suggests on inefficient cooling rate during oil quenching from 1150°C. Carbides such as MC primary carbides of the type TiC and MoC as well as secondary carbides M23C6 (Cr23C6) were found in grains and at grain boundaries. Voir les détails
Mots clés : superalloy; heat treatment;, SEM and TEM; γ' particles; carbides .