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Joining dissimilar metals is very difficult due to the formation of brittle intermetallic phases which may be detrimental to mechanical properties. The present work aims to investigate the transport phenomenon in the weld bead and to understand the materials mixing during laser welding process of dissimilar Aluminum-Magnesium alloys. A three-dimensional transient model based on fluid flow, heat and mass transfer has been developed to predict the formation of the weld and to study numerically and experimentally the diffusion of alloying elements in the melted zone. SEM analysis of chemical composition has been realized to map elements distribution in the melted zone. The results of simulation show the formation of a heterogeneous mixture in the melt pool. The elements distribution map and the presence of brittle intermetallic phases in the fusion zone were analysed. The formation of intermetallic compounds, comprising Al3Mg2 and Al12Mg17 phases were predicted by studying the chemical elements distribution in the weld pool. A good tendency between experimental and numerical results is noticed for the weld.

The weld geometry is an important factor which determines the quality of the weld. This work proposes a transitional and three-dimensional numerical model performed by the hydrodynamic software Ansys-Fluent. We are interested in the heterogeneous assembly of Al alloy and Mg alloy by laser beam welding. Aluminum and Magnesium plates are joined in butt configuration. The objective of this work is to simulate the temperature field, velocity field, fluid flow during the interaction between the laser beam and the material and to predict the dimensional characteristics of the laser welded joint. This makes it possible to better understand different physical phenomena (e.g. heat transfer, mass transport) generated in the molten pool. The results show that the simulated weld bead profile is in good agreement with those obtained by experiment.

This paper investigates the laser welding process of dissimilar Aluminum-Magnesium materials. A three-dimensional transient model has been developed using the 'Ansys-fluent' software to study the transport phenomenon in the weld bead. The results show the formation of a heterogeneous mixture in the molten pool. The transport phenomenon is strongly influenced by convection, thermal and mass diffusivities during laser beam welding. Microhardness tests show that the mechanical properties of welded joint are affected by the formation of brittle intermetallic phases in the Al-Mg transition zone, which is the main cause of weld cracking. The formation of intermetallic phases in the melting zone was highlighted. The layer thicknesses of intermetallic compounds, comprising Al3Mg2 and Al12Mg17 phases were determined numerically by studying the distribution of chemical elements in the weld pool. The results of the numerical simulation are in good agreement with the corresponding experimental results.

A pulse laser (Nd:YAG) interaction with an AZ91 magnesium alloy has been experimentally and numerically studied. A two-dimensional (2D) axisymmetric model of a molten pool created by a laser heat source has been developed. The elaborated model solves the coupled equations of a laminar fluid flow and heat transfer to demonstrate the flow behavior in the pool. This model takes into account the coupled effects of buoyancy and Marangoni forces, the thermophysic variation properties with temperature, and the radiation and convection heat losses. Concerning numerical results, the molten temperature distribution, velocity field and molten shape were discussed. It was noted that the Marangoni flow significantly alters the characteristics of the thawing and solidifying processes, and makes the molten pool wider and shallower. On the other hand, the experimental results showed that the material thermal properties have significant effects on the transport phenomena which takes place in the molten pool, and consequently on the formation as well as the shape of the pool. Finally, a comparison between the numerical and experimental results exhibited a good agreement.

Magnesium alloys have attracted the interest of the modern manufacturing industry owing to their higher strength-to-weight ratios and heat conductivity than other alloys. Laser welding will be an important joining technique for magnesium alloys with their increasing applications in aerospace, aircraft, automotive, electronics, and other industries. The Nd:YAG laser beam has the reputation to be a rapid, precise, and easy process for welding of magnesium alloys. In this study, the use of an Nd:YAG laser for thin-plate AZ91 magnesium alloy welding was optimized. The welding cross section geometries were determined by optical microscopy analysis. Some important laser processing parameters (laser power, welding speed, and shielding gas flow) and their effects on the welding quality are discussed.

Heat transfer from the laser keyhole to the weld pool and details of the fluid flow play an important role in determining weld shape and size. In this work, both the experiment and the finite volume method (FVM) are utilized to investigate the thermal phenomena during continuous laser keyhole welding. Firstly, the experiments are carried out to examine the effects of laser welding parameters on the pool dimensions. Secondly, 3D FVM model is developed using the hydrodynamic software FLUENT to simulate the keyhole and pool formation during the interaction between continuous laser and AZ91 magnesium alloy sheet. In this simulation, based on a control volume method, a volumetric heat source is used to model the laser beam. Then, the equations of species, mass, momentum and energy are solved. Laser power and welding speed have a strong effect. Using the developed model, it has been found that the shape and size of the molten pool in the workpiece are affected by welding parameters such as welding speed and the incident laser power. Furthermore, it has been observed that the surface tension temperature coefficient, Marangoni convection, is sensitive to the active elements in the magnesium alloy composition. These coefficients also affect the pattern of the fluid flow in the molten pool. Finally, the simulation results were compared with the experimental data. The effectiveness of the developed computational procedure has been confirmed.

Laser welding is an important joining process for magnesium alloys. These materials are being increasingly used in different applications such as in aerospace, aircraft, automotive, electronics, etc. To date, carbon dioxide (CO 2 ) neodymium-doped yttrium aluminum garnet (Nd:YAG) and the high power diode laser have been extensively used to investigate the weldability of magnesium alloys. The present work describes an analytical thermal model for the weldability of magnesium alloys (WE43) using an industrial (CO 2 ) laser source. The main target of the project is to present to the industrial community a simple and rapid tool for the determination of the penetration depth and the bead width as a function of both the incident laser power and welding speed. The proposed model is based on the Davis thermal approach, largely considered for the characterization of the average radius of the liquid zone, aiming at predicting the joint shape. Moreover, since during the welding process considered in this study, a protecting gas is used to avoid joint oxidation, both thermal convection and radiation phenomena in the welding area have been estimated and introduced in our model for a better characterization of the welding process. The obtained results have been compared to the experimental ones and a satisfactory correlation has been observed, indicating the reliability of the model developed in this study.

The 125-fs laser ablation behaviour (800 nm) of aluminium and anodic oxide coatings on an AlMgSi1 alloy was investigated. The multi-pulse ablation threshold of aluminium at 1.2 J cm −2 was less than that of the oxides of 2–3 J cm −2 . Aluminium exhibited a single pulse modification (melting) threshold of 0.3 J cm −2 . These values derived from an evaluation of the crater geometry coincided with on-line acoustic measurements. The detected microphone voltage amplitude increased linearly with the laser fluence. The morphology of the ablation craters on aluminium indicated melt formation and displacement of a homogeneous melt phase due to the recoil action of the expanding metal vapour. The laser-processed ceramic oxide phases, on the other hand, showed a spongy resolidified melt layer, which denotes a collocated in-depth formation both of a melt and a gas phase. These phenomena are discussed in terms of the relative dominance of penetration depth of laser light and heat affected zones in the investigated materials with strongly varying optical and thermodynamical properties.

The authors wish to thank Mr. Henri ANDRZEJEWSKI for his technical assistance in laser experiments. The authors wish to place their sincere thanks to Professor Philippe BOURNOT and Dr. Eric VALERIO for helpful discussions.; International audience; Dissimilar laser beam welding between A5754 Al alloy and AZ31 Mg alloy with the plate thickness of 2 mm was investigated. Complex flow pattern characterized by a large volume of intermetallic compounds Al12Mg17 and Al3Mg2 is formed in the fusion zone. Microhardness measurement of the dissimilar welds presents an uneven distribution due to the complicated microstructure of the weld, and the maximum value of microhardness in the fusion zone is much higher than of the base materials.

The authors report results of experimental and theoretical studies of the soliton nature of laser‐induced nonequilibrium wave processes of energy transfer in solid bodies 1 μsec–10 sec after exposure to pulsed laser radiation. These results can serve as a key to the understanding of a new phenomenon that has been observed in investigation of the effect of IR laser radiation on various classes of solid bodies, namely, a wave of change in the optical reflection and electrical conduction. Investigation of this phenomenon pertains to a fundamental problem of solid‐state physics and thermal physics related to investigation of the nonequilibrium process of transfer of energy absorbed in a substance under pulsed action of high‐power electromagnetic, heat, or corpuscular fluxes.

International audience; The results of numerical simulation of pulsed laser ablation both in vacuum and into a background gas are presented. The influences of different processes, such as time evolution of the surface temperature, interspecies interactions (elastic collisions, recombination-dissociation reaction), interaction with an ambient gas, and excitations-relaxation processes on time-of-flight (TOF) distributions are examined. Experimentally obtained time-of-flight distributions are further analyzed, based on the results of numerical simulation. It is found that with the aid of numerical results one can explain not only the shape of the TOF distribution, but also the distance dependency of its maximum position (mean delay time). In addition, the mechanisms leading to the appearance of bimodal time-of-flight distribution are revealed. The study presents particular interest for the analysis of experimental results obtained during pulsed laser ablation.

Standard anodized aluminium samples with about a 20-μm-thick oxide layer were investigated for cleaning study by means of laser pulses. The removal caused by overheated melting and mechanical stress was studied at wavelengths of 1064 nm and 532 nm. The samples were examined with scanning electron and optical microscopy. We investigated the dependence of the laser cleaning efficiency on energy densities and the number of pulses. The layer removal mechanisms were compared on natural and black anodized aluminium samples. Due to the different absorptiveness of both layers, the ablation threshold varies. Furthermore, the ablation mechanism investigated using microscopy and photoacoustic measurements, respectively, varies with the absorptivity.

Les processus hydrodynamiques existant lors des phases d'irradiation de materiaux solides regissent en fait le couplage rayonnement - matiere et donc, les effets resultants sur la surface. Determiner quels processus interviennent dans telles ou telles applications permet de comprendre comment l'energie laser est couplee au materiau et, eventuellement, d'ajuster les conditions experimentales afin d'optimiser les procedes. Caracteriser les regimes hydrodynamiques, c'est determiner l'instant de formation du plasma, son evolution spatio-temporelle, la structure des ondes de choc et des ondes d'absorption creees dans le milieu ambiant et leur evolution. Ces differents regimes se caracterisent par des methodes optiques de visualisation de type ombroscopic, strioscopie et interferometrie.

Surface modifications on aluminium nitride (AlN) induced by an UV laser treatment have been investigated using advanced surface techniques (scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS), Auger electron spectroscopy (AES), X-ray diffraction (XRD)…) and electrical resistance measurements. AES and XRD analysis have revealed the presence of an Al metallic film upon the laser irradiated zone while SEM and EDS analysis have shown the formation of a halo rich in oxygen around it. Electrical measurements have confirmed the metallic nature of the laser-exposed surface, where a conductive path can be drawn upon the insulator AlN ceramic. Irradiation parameters (energy density and number of pulses) have been determined regarding to the obtaining of a metallic thin film by this one-step process.

International audience; A three-dimensional Monte Carlo simulation of the laser desorbed flow of particles was performed. In addition to the gas-phase collisions, the chemical processes, i.e., recombination and dissociation are considered. The width of the angular distributions as a function of the radius of the evaporated area is analyzed. The comparison between the results obtained in the reactionless flow and in the flow with chemical reactions shows that in the presence of chemical processes the focusing effect of collisions is partly concealed and more wide angular distributions can be observed.

International audience; Laser ablation from one- and two-component targets into a diluted gas background is simulated by combined direct simulation-test particle Monte Carlo method. The spatial and velocity distributions of particles deposited at the plane substrate are calculated. The approach developed has allowed us to consider the influence of the collisions both among the ablated particles and between the ablated and ambient gas particles on the uniformity of film stoichiometry. It is found that the increase of the background gas pressure results in the more uniform distribution of the stoichiometrical ratio of the deposited particles with different masses.

Cet article presente des resultats experimentaux concernant la dynamique d'une bulle de vapeur generee par laser pulse au voisinage d'une paroi solide. Le comportement d'une telle cavite gazeuse est identique a celui d'une bulle de cavitation. Le principal phenomene associe aux pulsations d'une structure cavitante est l'erosion de surface. Afin de contribuer a la comprehension de ce mecanisme, nous avons etudie les differentes sollicitations mecaniques que subit la paroi durant les oscillations de la bulle. Ces contraintes sont celles dues aux impacts de jet liquide et a l'interaction du materiaux avec des ondes de choc. Des visualisations ultra-rapides nous ont permis de caracteriser le jet en evaluant sa pression d'impact a partir de sa vitesse. Des mesures de pression ont ete realisees afin de connaitre l'amplitude et l'energie de l'onde de choc emise en fin de phase d'implosion (appelee encore phase de collapse).

The use of lasers in many applications requires an understanding and control of the fundamental processes involved during the laser radiation-material interaction. The importance and the duration of the phenomena involved (optical, thermal, mechanical, electromagnetic, radiative) depend on parameters such as the power density of the laser radiation, the nature and the surface morphology of the material, the surrounding gas and the wavelength of the radiation. A part of the incident energy is reflected by the surface, while the remaining quantity of this energy is absorbed by the material. This incident energy contributes to heating, melting, vaporization and plasma formation if the laser intensity is sufficiently high. The present study is devoted to the ceramic material irradiation in air by an excimer laser at a wavelength of 248 nm and a pulse duration of 20 ns. The objective is to understand the different phenomena induced by radiation and to improve mechanical properties of the surface. Presented results concern the dynamics of plasmas and shock waves, plus surface modifications (porosity, roughness, composition changes) on silicon carbide samples (SiC). A part of the study has resulted in the characterization of the flow created in front of the sample revealing different hydrodynamical regimes. Visualizations of the luminous plasma front and shock waves have been obtained using a fast electronic camera IMACON 790 and a ICCD camera. These optical devices have permitted us to observe simultaneously the formation and expansion of different fronts (plasma and shock waves) propagating into the surrounding gas. The results have been compared to the theory. Complex structures inside the plume have been observed inducing a turbulence phenomena some milliseconds after the irradiation. On the other hand, the surface morphology has been characterized by observations with a scanning electron microscope (SEM) showing important modifications of the morphology according to the energy density and number of pulses applied. A decrease of surface porosity is revealed using image processing technique. This improvement is accompanied with an increase of roughness which has been measured by mechanical profilometry and might be detrimental for friction applications. In addition, an analysis of the material in depth by Auger electron spectroscopy (AES) has shown chemical composition changes (an increase in the percentage of silicon) in the zone irradiated.

Pulsed laser ablation of a NbTe2 target was performed by an excimer laser in helium atmosphere at different fluences. Ablation plume characterization was realized by optical Time-of-Flight spectroscopy. Velocities and kinetic energies of the ablated particles were determined as a function of the fluence and of the background pressure. We observed an increase of the velocity of Te2 molecules when increasing the helium pressure. We show that this effect is due to three bodies associative reactions between monoatomic Te species and background gas particles.

International audience; Structure and properties of laser beam welding zone of dissimilar materials, AZ31 magnesium alloy and A5754 Aluminum alloy, are investigated. The microstructure and quality of the Mg/Al weld were studied by metallography, microhardness and optical microscopy. Differences in physical and mechanical properties of both materials, magnesium and aluminum, affect weldability and resistance of this combination, and lead to the formation of intermetallic compounds in the welded metal.

Pulsed laser deposition (PLD) of niobium telluride was performed by vaporisation of NbTe2 bulk targets with subsequent deposition of the ablation plume on heated silicon substrates. The laser energy density onto the target and the temperature of the substrate appear to play a major role in the quality of the deposited films. Correlations between these two parameters and the film composition on the one hand, and the structural properties on the other, were sought by systematic elemental and X-ray diffraction analysis. The deposition conditions for well-crystallized PLD films belonging to the Nb5Te4 (1D) structural type with strong (110) preferential orientation were demonstrated for the first time.

Laser Shock Processing is developing as a key technology for the improvement of surface mechanical and corrosion resistance properties of metals due to its ability to introduce intense compressive residual stresses fields into high elastic limit materials by means of an intense laser driven shock wave generated by laser with intensities exceeding the 109 W/cm2 threshold, pulse energies in the range of 1 Joule and interaction times in the range of several ns. However, because of the relatively difficult-to-describe physics of shock wave formation in plasma following laser-matter interaction in solid state, only limited knowledge is available in the way of full comprehension and predictive assessment of the characteristic physical processes and material transformations with a specific consideration of real material properties. In the present paper, an account of the physical issues dominating the development of LSP processes from a moderately high intensity laser-matter interaction point of view is presented along with the theoretical and computational methods developed by the authors for their predictive assessment and new experimental contrast results obtained at laboratory scale.

Resume : Les resultats presentes dans cette communication rendent compte d'experiences menees au moyen de laser a exciplexes exclusivement en vue de l'amelioration des proprietes d'usage de certains materiaux metalliques et ceramiques utilises dans l'industrie automobile, aeronautique ou de l'espace. Les principaux resultats concernent les modifications des caracteristiques mecaniques (rugosite, durete, tenacite, champ de contraintes,...), physico-chimiques (composition de surface, microstructure,...) et electrique (conductivite).

The interaction of high-intensity pulsed UV radiation with solid materials involves complex phenomena. Generally, they depend on different factors including laser wavelength, pulse duration, energy and power density, laser spot size, nature of the material and surface characteristics. The experiments presented in this paper have been performed using a 248 nm KrF laser which allowed aluminium and titanium alloys to be irradiated with an energy density from 0.3 up to 100 J/cm 2 and a power density from 1 up to 300 MW/cm 2 .

We would like to acknowledge the financial support of the ''Conseil regional de PACA'' and Protection des Metaux d'Arenc). The authors are grateful to PMA's engineers Ms. E. Castellan, Ms. A. Gonthier and Mr. F. Miretti for their help and assistance with the salt-spray tests; International audience; A laser surface melting treatment (LSMT) was performed on a ZE41 Mg-alloy using an excimer KrF laser. The laser-melted layer depth depends on the laser scan speed. The morphology and the microstructure of the laser-melted surface were characterized, thanks to the scanning electron microscopy (SEM). The melted Mg-alloy presented a homogenous distribution of the alloying elements in the magnesium matrix. The laser surface melting treatment increased the microhardness of the ZE41 Mg-alloy and improved its corrosion resistance.

This paper concerns the study of growth and characterizations of Co 2 YZ with Y an element of transition metal group and Z, a III-V group element deposited onto Si, GaAs and InAs substrates. Two PLD configurations have been explored, the conventional 1-Beam-PLD and the 2-Crossed-Beams-PLD one. We demonstrated that depending on the configuration we got Co 2 YZ polycrystalline structure with unwanted droplets or droplet-free, single crystalline oriented thin films at substrate temperature as low as 353 K. Optical conductivity and magnetic properties are presented.

This present work concerns the gr owth and the char acterization of Co 2 MnSi thin films deposited onto GaAs substrates. Two PLD configurations have been explored, the conventional 1-Beam-PLD and the 2-Crossed-Beams-PLD one. We demonstrated that, with 1B-PLD conditions, we got Co 2 MnSi polycrystalline structure with unwanted droplets. The 2 CB-PLD allowed us to get droplet-free, single crystalline th in films at substrate temperature as low as 353 K. Keywords: Pulsed Laser Deposition; Ferromagnetic a lloys; Crystallized Heusler thin films, spintronics 1. INTRODUCTION This study concerns the growth of high-quality ferromagnetic metallic cobalt-based Heusler thin films (predicted by band calculation to be half-metallic [1]) onto narrow gap se miconductors, quantum well structures or epilayer substrates. Up to now, half-metallic ferromagnets were pointed out as ideal candidates for spin injection devices due to their 100 % predicted spin polarization at Fermi level [2] and as alternativ e material to conventional Fe, Co or Ni ferromagnets (only 51 % spin polarization at low temperature. Various half-Heusler (C1

Investigation of the basic mechanisms taking place during pulsed laser cleaning of oxidized surfaces was carried out using different laser parameters in terms of wavelength, pulse duration and energy density. Working parameters were optimized for a reliable cleaning strategy comprising the total removal of the oxide layer, the minimization of laser effects onto the bulk and the beneficial laser effects onto the underlying base metal. The influence of the laser irradiation on the smaple and on the resulting mechanisms was investigated by in-situ real-time mass losses measurements and by post scanning electron microscopy and energy dispersive spectroscopy analysis. Two types of oxide layers were studied; the first one, transparent to the wavelength and the other one, absorbent to λ.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The experimental researches on excitation in Plexiglas of slow components of a solitonic-type Wave of change of reflection and conductivity (WCRC) are continued. Such discrete velocity excitations by a pulse of the CO2 laser (as first for ceramic HT SC samples) were found out by us in 1992. The results of soliton study in experiments with the various real condensed materials represent fundamental interest. We undertook the experiments with the use of the thermocouple as the detector with the purpose of increase of reliability of results received earlier by other methods, especially in relation to the sign of parameter variation in WCRC. Thus the assumption proves to be true stated by us before, that the arrival of a studied in Plexiglas results in local decreasing of temperature. The records of thermal processes in a copper foil sample for times in hundreds seconds after a stimulating pulse have allowed to register repeated passage through the foil several WCRC components with velocity of the order of μ/s. It is 104 times slower speed than in the first experiments of 1992.

Summary form only given. Aluminium nitride ceramic (AlN) is a very attractive material for advanced technological applications. It has a heat conductivity (180 Wm/sup -1/ K/sup -1/) comparable to most conductive metals and much greater than typical ceramics at room temperature. Its high electrical resistivity (>10/sup 14/ /spl Omega/cm), good dielectric strength, thermal expansion (5/spl middot/10/sup -6/ K/sup -1/) closely match to silicon and lack of toxicity are ideal for microelectronic substrate applications. After a KrF excimer laser irradiation of AlN, the surface becomes electrically conductive. This effect could be interesting for a microelectronic application to produce small conductive lines in one individual operation.

Femtosecond and nanosecond laser ablation and delamination of anodic oxide layers on an aluminium alloy were investigated. Laser wavelengths of 800 nm (125 fs pulse duration) and 532 nm (7 ns), respectively, and two oxide layers with thicknesses of 70 micrometers (opaque) and 30 micrometers (transparent) were considered. Laser-induced shock waves measured by means of a microphone provided an in-situ diagnostic tool for the monitoring of the coating removal process. The transparent anodic oxides showed contrasting ablation mechanisms in the nanosecond and femtosecond pulse duration regime. Nanosecond pulses led to spallation, whereas femtosecond treatment resulted in ablation.

Unusual solitonic-type excitations were found by our group in 1992. As it was shown since that time this waves travel trough different condensed matter with discrete velocities in the range of 9 orders of value (between km/s and less then μs). These waves can carry heat or cold as it can be seen from the sign of local parameters variation. Explanation to this facts one can find in suggestion that this waves could be as compression as expansion ones (the physical mechanism of all phenomenon not known yet). This work contains short review of experimental results, which show characteristically sign variation for different materials. This work was sponsored as projects 00-02-17249-(a) and 02-02-26611-(3) by RFBR, its presentation - by SPIE and KIE.

Experimental results of investigation in plexiglas support previously published our groups data of JR laser and e-beam excitation in solids of solitonic Wave of Change in Reflection and Conduction (WCRC). Method of single IR detector WCRC registration was applied in addition to JR thermovision method used before (there were used also -5other different methods). Results of slow WCRC components investigations showed that solitonic heat transfer play additional role to usual diffusive heat transfer. This role could be important at some conditions. As the control, the measured in this work WCRC component velocity ~ 0.26 cm/s does not depend from sample thickness which was equal 1.5 and 2.0 mm. Analysis of records treatment by help of (x-t) diagram permit to show many features of studied solitonic excitations lived in the solid sample during many minutes. Irradiation of the front surface by single laser pulse produced WCRC components; in experimental conditions of this work one can see pair components of the same velocity which were born as on front as on rear sample surface. They moved in different directions and reflects from sample surfaces many times with small energy losses. This work was sponsored by RFBR (Russia), project 97-02-17954-a) and 00-02-17249 a).

High-energy electron beams have been used to produce HF chemical lasers operating either on the chain reaction with high specific energy (100 J/I) or on the nonchain reaction with lower specific energy (10 J/I). This initiation is usually performed by injection of an electron beam along the optical cavity axis or perpendicular to this axis.

Standard anodized aluminum samples with about 20 micrometer thick oxide layer were investigated for cleaning study by means of laser pulses. The removal mechanism was studied for short laser pulses at 1064 nm, 532 nm and 248 nm wavelength in air and water. The samples were examined using scanning electron, optical microscopy. Photoacoustic, shock wave measurements and high-speed visualization have been set-up. The experiments were carried out in a new cleaning set-up, which allows a liquid surrounding medium. We investigate the influence of the surrounding medium and the dependence of the laser cleaning efficiency on energy densities and number of pulses. The ablation threshold and ablation mechanisms vary due to the different absorptance of the oxide and aluminum at different wavelength. The surroundings influence has been shown.

High-energy electrons beams have been used to produce HF chemical lasers operating either on the chain reaction with high specific energy (100 J/l) or on the nonchain reaction with lower specific energy (10 J/l). This initiation is usually performed by injection of an electron beam along the optical cavity axis or perpendicular to this axis. In the present experiment an intense electron beam (400 keV, 28 kA, 500 ns, 7*84 cm2) was injected into the laser mixture through a 15 cm high, 100 cm long, 7.5 * 10-3 cm thick aluminized kapton foil supported by a steel grid whose geometric transparency was approximately 90%. The laser cell had an over-all length of 120 cm and a cross section of 20 * 20 cm. The knowledge of the atomic quantity of fluorine produced allows to quantify the e-beam efficiency to dissociate the gas and the chemical efficiency of the laser. An estimate of atomic fluorine produced can be made by using the great solubility of the gas hydrofluoric acid to transform it into hydrofluoric acid by dissolution in water and making an acido-basic titration by pH-metry. The laser cell is lengthily passivated before each series of measurements. The dissociation rate of the SF6, CF4, C2F6 is presented and discussed for various experimental conditions. For each gases the laser performance is presented.

Pulsed laser ablation appears as a promising technique for depositing thin films. A large variety of successful experimental results were obtained in this field, including the growth of high-temperature superconducting films, ferroelectric films, oxides, semiconductors, diamonds, etc. One of the main advantages of this technology is the relative simplicity of the experimental set-up and the possibility to get good homogeneity, complex stoichiometry materials and well adhesive dense layers. The main drawback seems to be the production of macroparticles, their transfer to the growing film inducing inhomogeneity and roughness onto the surface, lowering the properties of the thin films. In a common configuration, the laser-generated flux is collected on a planar substrate positioned parallel to the irradiated surface. In order to improve the stoichiometry and the quality of the films, several modifications, like simultaneous generation of two plumes from different targets with different targets with different laser beams, were proposed. In this paper, we present some results concerning the production of cryolite thin films using the conventional pulsed laser deposition technique and the dual crossed beams pulsed laser deposition technique. Plasma plumes expanding in vacuum and interacting together are visualized. The different species ejected in the plumes are detected through narrow-band filters in order to determine their kinetic energies. The morphology and the composition of the films are compared with the thermal evaporation technique.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Excimer laser treatments of Si3N4 and A1N have been investigated. The Si3N4 ablation threshold in air is around 1.0J/cm2. With energy densities below 3.0 J/cm2, the irradiated surfaces seem to be rougher than the untreated material, andeven exhibit a cone-type structure depending on the number of laser pulses. X-EDS analysis of this uneven structure showsthat the top of the cones is crystalline silicon. With energy densities above 3.0 J/cm2, the irradiated surfaces seem to becomesmoother and no more cone-type structure is detected. As for A1N, the behavior of Al layer created under excimer laserirradiation was studied. Accelerated ageing tests (more than 1 10 h at 40°C in an humid atmosphere 60% RH) also show thatthe treated sample is still conductive with time. The first results concerning experiments in air and vacuum are reported too.Keywords: Si3N4, A1N, excimer laser, surface processing 1. INTRODUCTION Since last decades, excimer laser technology is developing in many industrial applications as micro machining or insurface treatment. The combination of the short laser emission wavelength (in the nm range) with the brief pulse duration(in the ns range) then allows a great accuracy in surface processing with very low thermal effects"2'3.Technical bulk ceramics are very interesting materials due to their low specific weight, their hardness and high-temperature properties. But their engineering applications are still limited, essentially because of their low fracturetoughness. In this context, the use of excimer lasers, which proved to be a promising tool4 in surface treatment, wasconsidered in the case of ceramics.In this paper, KrF surface modification of silicon nitride Si3N4 and aluminum nitride A1N are presented. The Si3N4main results concern the ablation behavior. As for A1N, we have demonstrated in a previous publication5 the creation of anAl layer. Accelerated ageing tests of this layer and A1N treatments under controlled atmosphere are here studied.The aim of this work is to improve or even change the ceramics surface properties in order to develop new technologicalapplications.

KrF excimer laser has been used for physical and chemical transformations of metallic materials and coated metal samples. Aluminum alloys, steels and chromium coated mild steel have been treated under excimer laser radiation in order to improve their mechanical properties and their corrosion and oxidation resistance. The laser surface treatment leads, after surface remelting process, to important changes in the topography, the microstructure, the phases and the chemical composition of the near-surface region resulting in different hardness, wear properties and corrosion/oxidation behavior. We focus this paper on aluminum alloys (2000 and 6000 type) and steels irradiated using a krypton fluoride laser (20 ns, 0.5 - 10 J/cm2, up to 200 Hz) in laboratory air. The analysis were carried out by means of scanning electron microscopy, energy dispersive spectroscopy, coupled and low incidence angle X-ray diffraction, microhardness tester and electrochemical test equipments.

Metallic alloys with a low mass density can be considered to be basic materials in aeronautic and automotive industry. Magnesium alloys have better properties than aluminum alloys in respect of their low density and high resistance to traction. The main problems of magnesium alloy welding are the inflammability, the crack formation and the appearance of porosity during the solidification. The laser tool is efficient to overcome the difficulties of manufacturing by conventional processing. Besides, the laser processing mainly using shielding gases allows an effective protection of the metal against the action of oxygen and a small heat affected zone. In this paper, we present experimental results about 5 kW CO2 laser welding of 4 mm-thick magnesium alloy plates provided by Eurocopter France. The focused laser beam has about 0.15 mm of diameter. We have investigated the following sample: WE43, alloy recommended in aeronautic and space applications, is constituted with Mg, Y, Zr, rare earth. More ductile, it can be used at high temperatures until 250 degrees Celsius for times longer than 5000 hours without effects on its mechanical properties. A sample of RZ5 (French Norm: GZ4TR, United States Norm ZE41) is composed of Mg, Zn, Zr, La, rare earth. This alloy has excellent properties of foundry and it allows to the realization of components with complex form. Also, it has a good resistance and important properties of tightness. The parameters of the process were optimized in the following fields: laser power: 2 to 5 kW, welding speed: 1 to 4.5 m/min, focal position: -3 mm to +3 mm below or on the top of the metal surface, shielding gas: helium with a flow of 10 to 60 l/min at 4 bars. Metallurgical analyses and mechanical control are made (macroscopic structure, microscopic structure, interpretations of the structures and localization of possible defects, analyse phases, chemical composition, hardness, tensile test etc.) to understand the parameters influence of welding on the obtained beads. For a given laser power, we considered that the welding speed as well as the focal position strongly influence the macroscopic and microscopic welding aspect, whereas the dependence with the flow of the protection gas is weak. For WE43, the bead appears correct in the macroscopic scale for a laser power of 2 kW, a speed of 2 m/min, a focal position on the metal surface or 1 mm under; and an output helium gas of 50 l/min. For RZ5, a correct weld is obtained with a 3 kW laser power, a welding speed of 2 m/min, a focal position of 1.5 mm under the surface and a 50 l/min output helium gas. The microscopic examination showed that the size of the grains has clearly reduced (reduction factor can be up to 35) without formation of porosities, neither cracks nor inclusions; indeed the measured Vickers microhardness of the weld bead is slightly higher than the basic metal. Experiments show that we obtained adequate parameters for high quality welding without using filler material. In future, we plan to weld at higher speed by optimizing the various parameters of the laser welding (power, focal position welding speed and gas flow, ...). Furthermore, we will try to weld samples with a thickness superior than 4 mm.

The application of the pulsed laser deposition technology requires a better understanding of the material evaporation mechanisms, of the gas and plasma plume formation, of its expansion and of its deposition onto the substrate. Experimental and numerical studies revealed the importance of the gas-phase collisions on the formation of a peaked angular distribution of particles described by cos p (theta) . Numerical calculations based on a Monte Carlo simulation of the desorption process were performed showing that the focusing effect of the angular distribution of particles increases with the number of collisions in the plume. This paper presents a 3D Monte Carlo simulation performed in vacuum and ambient gas in order to investigate the angular distribution of particles. Besides elastic collisions, the recombination-dissociation reactions proceed in the flow.

International audience; Numerical modelling of non-steady state laser-ablated flows is presented. Two simulation methods, the Monte Carlo technique and the large-particle (LP) method are used to investigate pulsed laser evaporation for different laser fluences under vacuum. Time-evolutions of the flow parameters are related to the evaporated material properties and laser pulse parameters. Non-Maxwellian time-of-flight (TOF) distributions are attributed to both non-stationary surface evaporation and inter-particle interactions in the laser plume. A radial surface temperature distribution was found to influence the plume shape. The influences of the experimental parameters on the TOF signal and on the plume shape are analyzed based on the Knudsen layer — unsteady adiabatic expansion model. The role of the evaporation yield, laser beam radius, initial plume length, and evaporation barrier in the formation of the angular distribution of the ablated material are discussed. The study concentrates on a better understanding of pulsed laser deposition (PLD).

Wave of Change of Reflection and Conduction (WCRC) is the new reversible IR laser induced phenomenon, which can present solitonic mechanism of heat transfer in solids. WCRC appears in answer to one exciting pulse as the consequence of the following each another pulses (wave-components) and could be important in many laser-surface interactions. One of the most interesting features of WCRC is the observed discrete (in 2 times) change of the velocity of each following component. Present work gives one more experimental proof of this statement.

Pulsed laser ablation appears as a promising technique for depositing thin films. A large variety of successful experimental results were obtained in this field, including the growth of high- temperature superconducting films, ferroelectric films, oxides, semiconductors, diamonds, etc. One of the main advantages of this technology is the relative simplicity of the experimental set-up and the possibility to get good homogeneity, complex stoichiometry materials and well adhesive dense layers. The main drawback seems to be the production of macroparticles, their transfer to the growing film inducing inhomogeneity and roughness onto the surface, lowering the properties of the thin films. In a common configuration, the laser-generated flux is collected on a planar substrate positioned parallel to the irradiated surface. In order to improve the stoichiometry and the quality of the films (particle free) several modifications were proposed like simultaneous generation of two plumes from different targets (double ablation) with different laser beams. In this paper, we present some results concerning the production of cryolite thin films using the conventional pulsed laser deposition technique (C-PLD) and the dual crossed beam pulsed laser deposition technique DCB-PLD). Plasma plumes expanding in vacuum and interacting together are visualized. The different species ejected in the plumes are detected through narrow-band filters in order to determine their kinetic energies. The morphology and the composition of the films are compared with the thermal evaporation technique.

A phenomenological laser-cleaning study of pure oxidized iron samples has been performed in order to model the influence of wavelength in air and the role of environment in the IR domain. Two sets of experiments were performed: (i) cleaning of oxidized specimens at different laser spot sizes and pulse numbers at wavelengths of 1064 nm and 532 nm in air and (ii) cleaning in air with electrolytes at 1064 nm for low energy densities.