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Dive into the research topics where Flavien Valensi is active.

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Featured researches published by Flavien Valensi.


Plasma Sources Science and Technology | 2007

Investigations of GMAW plasma by optical emission spectroscopy

Sylwia Zielinska; Karol Musiol; Krzysztof Dzierzega; Stéphane Pellerin; Flavien Valensi; Charles de Izarra; Francis Briand

We report on investigations of gas metal arc welding plasma operated in pure argon and in a mixture of argon and CO2 at a dc current of 326?A. The spatially resolved electron densities and temperatures were directly obtained by measuring the Stark widths of the Ar?I 695.5?nm and Fe?I 538.3?nm spectral lines.Our experimental results show a reduction of the plasma conductivity and transfer from spray arc to globular arc operation with increasing CO2 concentration. Although the electron density ne increases while approaching the core of the plasma in the spray-arc mode, a drop in the electron temperature Te is observed. Moreover, the maximum Te that we measure is about 13?000?K. Our experimental results differ from the Haidar model where Te is always maximum on the arc axis and its values exceed 20?000?K. These discrepancies can be explained as a result of underestimation of the amount of metal vapours in the plasma core and of the assumption of local thermal equilibrium plasma in the model.


Journal of Physics D | 2010

Plasma diagnostics in gas metal arc welding by optical emission spectroscopy

Flavien Valensi; Stéphane Pellerin; Amar Boutaghane; Krzysztof Dzierzega; Sylwia Zielinska; Nadia Pellerin; Francis Briand

The plasma column in a metal inert gas welding process is investigated by optical emission spectroscopy and high-speed imaging. The concentration and repartition of iron vapours are measured and correlated with the plasma and electrode geometric configuration. Plasma temperatures and electron densities are also measured for each studied position in the plasma. The temperatures are calculated using two different methods, allowing validation of the local thermodynamic equilibrium state of the plasma. The results show a maximum temperature of 12 500 K in the upper part of the arc, away from the arc axis. The iron concentration reaches a maximum of 0.3% close to the anode and strongly decreases along both the vertical and radial directions.The plasma thermophysical properties, calculated from this plasma composition, are then discussed regarding the metal transfer mode.


Science and Technology of Welding and Joining | 2012

TIG and A-TIG welding experimental investigations and comparison to simulation Part 1: Identification of Marangoni effect

Anne Berthier; Pascal Paillard; Muriel Carin; Flavien Valensi; Stéphane Pellerin

Abstract In the present work, the chemical mechanism was shown for tungsten inert gas (TIG) and active TIG (A-TIG) welding. The results obtained with a two-dimensional axial symmetric model developed to simulate the flow behaviour in the meting pool on a stainless steel disc (304L) melted by a stationary heat source were presented to show the influence of Marangoni convection combined with Lorentz forces. This study shows the influence of the addition of an activating flux on the geometric characteristics of the weld beads in A-TIG welding.


Journal of Physics D | 2010

Measurement of atomic Stark parameters of many Mn I and Fe I spectral lines using GMAW process

Sylwia Zielinska; Stéphane Pellerin; Krzysztof Dzierzega; Flavien Valensi; Karol Musiol; Francis Briand

The particular character of the welding arc working in pure argon, whose emission spectrum consists of many spectral lines strongly broadened by the Stark effect, has allowed measurement, sometimes for the first time, of the Stark parameters of 15 Mn I and 10 Fe I atomic spectral lines, and determination of the dependence on temperature of normalized Stark broadening in Ne = 1023 m−3 of the 542.4 nm atomic iron line. These results show that special properties of the MIG plasma may be useful in this domain because composition of the wire-electrode may be easily adapted to the needs of an experiment.


Science and Technology of Welding and Joining | 2012

TIG and A-TIG welding experimental investigations and comparison with simulation

Anne Berthier; Pascal Paillard; Muriel Carin; Flavien Valensi; Stéphane Pellerin

Abstract In this part II, the comparison of physical mechanisms between tungsten inert gas (TIG) and active TIG (A-TIG) welding is shown. The plasma was monitored by a high speed camera to present the arc constriction phenomena while passing from TIG to A-TIG. The elemental analysis and the arc temperature measured by optical emission spectroscopy were performed according to the type of welding and the different fluxes in A-TIG welding. The two-dimensional axial symmetric model presented in part I was used to simulate the flow behaviour in the melting pool realised on a stainless steel disc (304L) melted by a stationary heat source and to study the influence of energy density.


Journal of Physics D | 2013

Study of the spray to globular transition in gas metal arc welding: a spectroscopic investigation

Flavien Valensi; Stéphane Pellerin; Quentin Castillon; Amar Boutaghane; Krzysztof Dzierzega; Sylwia Zielinska; Nadia Pellerin; Francis Briand

The gas metal arc welding (GMAW) process is strongly influenced by the composition of the shielding gas. In particular, addition of CO2 increases the threshold current for the transition from unstable globular to more stable spray transfer mode. We report on the diagnostics—using optical emission spectroscopy—of a GMAW plasma in pure argon and in mixtures of argon, CO2 and N2 while operated in spray and globular transfer modes. The spatially resolved plasma parameters are obtained by applying the Abel transformation to laterally integrated emission data. The Stark widths of some iron lines are used to determine both electron density and temperature, and line intensities yield relative contents of neutral and ionized iron to argon.Our experimental results indicate a temperature drop on the arc axis in the case of spray arc transfer. This drop reduces with addition of N2 and disappears in globular transfer mode when CO2 is added. Despite the temperature increase, the electron density decreases with CO2 concentration. The highest concentration of iron is observed in the plasma column upper part (close to the anode) and for GMAW with CO2.Our results are compared with recently published works where the effect of non-homogeneous metal vapour concentration has been taken into account.


PLASMA 2005: Int. Conf. on Research and Applications of Plasmas; 3rd German-Polish Conf.on Plasma Diagnostics for Fusion and Applications; 5th French-Polish Seminar on Thermal Plasma in Space and Laboratory | 2006

Experimental investigations of the arc MIG‐MAG welding

Nadia Pellerin; Sylwia Zielinska; Stéphane Pellerin; Flavien Valensi; Karol Musiol; Charles de Izarra; Francis Briand; Frédéric Richard

The type of the applied shielding gas has a strong influence on quality of the welding process. In particular, increase of the percentage of carbon dioxide in argon, causes increase of the transition current value from the globular to spray mode of metal transfer. Observations by fast camera allows to better characterize the arc column shape in the different working modes. The spectroscopic diagnostic of the welding arc is also necessary to understand the observed changes in the mode of droplet transfer. The use of an original diagnostic method allows to estimate the temperature and the electronic density distributions in the plasma without hypothesis on its equilibrium state. Results of this work seem to show that the observed effects could be linked to the microstructural modifications of the anode tip during the MIG‐MAG welding process as a function of the gas composition, and especially to the existence and disappearance of an insulating oxide “gangue” at the wire extremity.


Nondestructive Testing and Evaluation | 2011

Explorative approach of the spectral analysis tools to the detection of welding defects in lap welding

David Groslier; Stéphane Pellerin; Flavien Valensi; Frédéric Richard; Francis Briand

The study of the voltage and current signals during welding can give information on the process, especially in case of defects. A signal processing software using Fourier and Wavelet transforms has been used to analyse the signals produced during welding under normal conditions and during artificially created defects. Then the fundamental frequency shift, or pitch, has been measured and the frequency variation for each case has been studied. Results showed specific features for each defect type.


Plasma Science & Technology | 2016

Synthesis of (B-C-N) Nanomaterials by Arc Discharge Using Heterogeneous Anodes

Djamel Eddine Gourari; Manitra Razafinimanana; Marc Monthioux; Raul Arenal; Flavien Valensi; Sébastien Joulié; Virginie Serin

In spite of the current prevalence of the CVD-based processes, the electric arc remains an interesting process for the synthesis of carbon nanoforms, thanks to its versatility, robustness and easiness. It also allows performing in-situ substitution of carbon atoms by hetero-elements in the graphene lattice. Our work aims to establish a correlation between the plasma properties, type and chemical composition (and the substitution rate) of the obtained single-wall carbon nanotubes. The plasma was characterized by optical emission spectroscopy and the products were analyzed by high resolution transmission electron microscopy and core level Electron Energy-Loss Spectroscopy (EELS). Results show that a high boron content leads to a plasma temperature decrease and hinders the formation of nanotubes. This effect can be compensated by increasing the arc current and/or yttrium content. The optimal conditions for the synthesis of boron- and/or nitrogen-substituted nanotubes correspond to a high axial plasma temperature associated to a strong radial gradient. EELS analysis confirmed that the boron incorporates into the graphenic lattice.


nanotechnology materials and devices conference | 2016

Enhancement of electrical insulation properties of epoxy nanocomposites with fullerenes

Rado H. Rabarison; Flavien Valensi; Sombel Diaham; Manitra Razafinimanana; Michel Baltas; Isabelle Fabing

In this paper, we propose to study the synthesis of fullerene nanoparticles (C60-C80) by electric arc method and to show the impact of very low filler content (0.005 wt.%) of those nanofillers on the electrical conductivity versus electric field of an epoxy nanocomposite. The results highlight a decrease in the DC electrical conductivity of the epoxy/fullerenes compared to that of neat epoxy in a large range of electric field from 1 to 6 kV/mm. This shows an improvement of the charge trapping efficiency.

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