Titanium dioxide thin films have been deposited by reactive magnetron sputtering on glass and subsequently irradiated by UV radiation using a KrF excimer laser. The influence of the laser fluence (F) on the constitution and microstructure of the deposited films is studied for 0.05<F<0.40 J/cm2. The diffraction data reveal that as deposited films are amorphous, while irradiated films present an anatase structure. Additional Raman spectroscopy study shows better crystal quality for the films irradiated with F<0.13 J/cm2. The film morphology appears to be strongly modified after laser treatment. Atomic force microscopy and scanning electron microscopy measurements reveal fractally textured films presenting characteristics of high porosity and high specific surface area. Finally, contact angle analysis suggests hydrophobic or wetting behavior depending on F. In order to explain the laser-induced structuration mechanisms, we have successfully applied a fractal as well as the nucleation theories. We propose that electronics effects could be responsible for the observed crystallization.
In this article we are interested in a moving mechanism occurring in the melted bath that is produced during the deep penetration laser welding process. It concerns the displacement of the melted metal induced by the friction effect due to the interaction with the metallic vapor when it flows toward the keyhole exit. Boundary conditions resulting from a ray-tracing procedure are used in the numerical solving of the unsteady Navier–Stokes conservation equation for both liquid and gaseous phases. The melted metal is considered for the liquid phase, whereas the metallic vapor and the environing air are taken as gaseous phases. The volume of fluid approach used in our modeling allows the tracking of the vapor–liquid wall interface evolution. We will present sequences of the melted bath profiles evolution as the vapor flows and interacts with it. An estimation of the velocity of the displaced liquid boundary layer allows a comparison with experimental results concerning the expelled melted metal velocity.
Laser cutting of thick metal workpieces requires the use of low operating speeds which, however, will have an influence on the cut quality following the used laser wavelength. In this study, we develop a model which takes into account the Fresnel coefficients in order to determine the local laser absorbed energy as a function of the beam incidence angle on the surface workpiece. The deposited laser energy occurs on the metal/air interface which evolution is tracked by the volume-of-fluid multiphase model. In the simulation, in order to find the difference between the patterns obtained on the kerf walls when different laser wavelengths are used, an important number of cells must be used at the regions of interest, which are characterized by high gradients, but a larger number of cells will have the consequence to complete the calculation with much less computer time consumption. Therefore, a gradient adaption method was implemented in order to control the number of cells, by multiplying or reducing them when it is necessary, depending on the importance of the temperature gradients. It was found out in our results that processing with higher wavelength (λCO2 = 10.6 μm) results in lower roughness on the kerf walls, compared to the surface quality obtained by using lower wavelength (λNd-YAG = 1.06 μm). A good accordance with the experimental observations is concluded.
In this paper, we study the oxidation process during the heating of a titanium metallic surface by a Nd-YAG fiber pulsed laser beam under air environment. For this, we adopted an approach that considers a three-dimensional heat diffusion model coupled with an oxidation parabolic law (oxidation kinetics). The heat diffusion equation solved numerically, gives the temperature field. The oxide film growth is simulated by implementing a dynamic mesh technique. We developed computational procedures UDFs (User Defined Function) running interactively with the Fluent fluid dynamics software [ that implements the finite volume method. These UDFs are developed to insert the oxidation law, the temperature field, the specific boundary conditions and the mesh deformation into the calculation.
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