J.M. Jouvard

Continuous wave Nd:YAG laser cladding modeling: A physical study of track creation during low power processing

This paper concerns the modeling of cladding using an Nd:YAG laser operating at low powers typically less than 800 W. Experimental observation of the evolution of the mass of the clads shows two power thresholds. The theoretical study relies on a calculation of the fluence provided to the substrate and on a model of heat transfer into the substrate. We suggest that the first threshold is the power required for substrate melting. The second power is the threshold when the powder is directly melted by the beam and is therefore a liquid when contacting the substrate.

Laser plasma plume structure and dynamics in the ambient air: The early stage of expansion

Laser ablation plasma plume expanding into the ambient atmosphere may be an efficient way to produce nanoparticles. From that reason it would be interesting to study the properties of these laser induced plasmas formed under conditions that are known to be favorable for nanoparticles production. In general, plume behavior can be described as a two-stage process: a “violent” plume expansion due to the absorption of the laser beam energy (during the laser pulse) followed by a fast adiabatic expansion in the ambient gas (after the end of the laser pulse). Plasma plume may last a few microseconds and may have densities 10−6 times lower than the solid densities at temperatures close to the ambient temperature. Expansion of the plasma plume induced by the impact of a nanosecond laser beam (λ = 1064 nm) on the surface of metallic samples in the open air has been investigated by means of fast photography. Spatio-temporal evolution of the plume at the early stage of its expansion (first 330 ns) has been recorded. ...

In-situ small-angle x-ray scattering study of nanoparticles in the plasma plume induced by pulsed laser irradiation of metallic targets

Small angle x-ray scattering was used to probe in-situ the formation of nanoparticles in the plasma plume generated by pulsed laser irradiation of a titanium metal surface under atmospheric conditions. The size and morphology of the nanoparticles were characterized as function of laser irradiance. Two families of nanoparticles were identified with sizes on the order of 10 and 70 nm, respectively. These results were confirmed by ex-situ transmission electron microscopy experiments.

Preliminary analysis of the pentad of 13CH4 from Raman and infrared spectra

Abstract Preliminary results on the simultaneous analysis of infrared and Raman data of 13 CH 4 in the 3-μm region ( ν 1 , ν 3 , 2 ν 2 , ν 2 + ν 4 , and 2 ν 4 ) are presented. The infrared spectrum of 13 CH 4 (90% enriched) has been recorded with the Fourier transform spectrometer at Kitt Peak National Observatory. Line positions have been measured with a relative accuracy of 0.0001 cm −1 (for well-isolated lines) using 0.0118-cm −1 resolution spectra. In order to compensate for the lack of infrared information about low J transitions of vibrational bands forbidden in infrared, two spectra of the ν 1 ( A 1 ) and 2 ν 2 ( A 1 ) Q branches have been recorded in Dijon by inverse Raman spectroscopy with an instrumental resolution of 0.0022 cm −1 . Line positions have been measured with a precision of 0.001 cm −1 using a profile fitting procedure. Raman and infrared data were combined in a weighted least-squares fit to determine vibration-rotation constants. We used an effective tensorial Hamiltonian taking into account all interactions within the pentad up to the fourth order of approximation. The preliminary analysis has been carried out throughout J = 13 involving 1200 data reproduced with a standard deviation of 0.0007 cm −1 , approximately 30 times better than the most recent results published on the 12 CH 4 pentad.

Keyhole formation and power deposition in Nd:YAG laser spot welding

The purpose of this work is to understand keyhole formation observed during pulse Nd:YAG laser welding. An analysis of the evolution of the keyhole geometry during a laser pulse was carried out by radiography x-ray flash, for tantalum and a titanium alloy target. From the images obtained by x-ray radiography, we determined the evolution of the geometry of the keyhole (depth and width) during the laser pulse. The keyhole appears after a delay, and increases linearly. We compare the measured creation delay with a value obtained by an analytical thermal model. We present a calculation of the energy deposit law by Fresnel absorption; the aim is to calculate the ray trajectory in the keyhole to deduce the local absorption. The absorption increases strongly during the formation of the keyhole, and tends towards a limit value. Further, we compare the mean absorption during a pulse with a measurement using a differential microcalorimeter.

Influence of laser–target interaction regime on composition and properties of surface layers grown by laser treatment of Ti plates

Surface laser treatment of commercially pure titanium plates was performed in air using two different Nd : YAG sources delivering pulses of 5 and 35 ns. The laser fluence conditions were set to obtain with each source either yellow or blue surface layers. Nuclear reaction analysis (NRA) was used to quantify the amount of light elements in the formed layers. Titanium oxinitrides, containing different amounts of oxygen and nitrogen, were mainly found, except in the case of long pulses and high laser fluence, which led to the growth of titanium dioxide. The structure of the layers was studied by x-ray diffraction and Raman spectroscopy. In addition, reflectance spectra showed the transition from a metal-like behaviour to an insulating TiO2-like behaviour as a function of the treatment conditions.Modelling of the laser–target interaction on the basis of the Semak model was performed to understand the different compositions and properties of the layers. Numerical calculations showed that vaporization dominates in the case of short pulses, whereas a liquid-ablation regime is achieved in the case of 35 ns long pulses.

29SiH4 and 30SiH4: Dipole moment parameters of the dyad from Stark effect observations with laser sidebands

Abstract The linear Stark effect within the ν 2 ν 4 dyad of 29SiH4 and 30SiH4 has been investigated by applying the infrared sideband technique at microwave modulation frequencies. Two vibration-induced dipole moment parameters have been measured for each isotopomer on natural abundance samples.

STUDY OF VOLUMINAL DEFECTS OBSERVED IN LASER SPOT WELDING OF TANTALUM

Significant problems in deep laser spot welding are the formation of welding defects, and particularly porosities in the case of tantalum joining. In this study, we investigate and model porosity formation and trapping. Two types of porosity are observed. (1)?Small porosities are round shape bubbles of less than 250??m diameter. These defects may come from gas bubbles generated following hydrogen rejection during solidification or they may come from bubbles induced by both an intense evaporation inside the keyhole and a turbulent flow in the molten pool. (2)?Large porosities are voids generated in the bottom of the welded zone. They arise from a lack of matter inside the weld as if the molten metal had not had enough time to fill up the keyhole completely before it solidified. We elaborated a numerical model for the comprehension of small porosities trapping by comparing the calculated spot weld solidification time with the rise time of bubbles in the liquid phase. We also studied the melt flow back into the keyhole in order to explain the formation of the large voids observed. These two models are in good agreement with experimental observations, in particular with x-ray radiographs achieved during the solidification of tantalum spot welds.

The modeling of dissimilar welding of immiscible materials by using a phase field method

A multiphysical model of high power beam welding of immiscible materials is developed to explain the influence of operational parameters and materials properties on resulting morphology by simultaneous solving of heat transfer, fluid flow and mass transfer problems. The introduction of phase field description of the interface motion between two immiscible liquids allows obtaining the cartography of melted zone in function of two key-parameters: the position of heat source relatively to joint line and the welding speed. Due to the short thermal cycle limiting mass transfer, high power beam welding techniques may result in very inhomogeneous melted zones. In this study, the interest is paid to copper to stainless steel Nd:YAG laser and electron beam joints, which often present the repetitive heterogeneous patterns. The modeling results are validated by the comparison with experimental data on weld shape, composition and morphology.

Voluminal defects observed in laser spot welding of tantalum

Significant problems in deep laser spot welding are formation of welding defects, and particularly porosities in the case of tantalum joining. In this study, we investigate and model porosities forming and trapping. Two types of porosity are observed: (1) Small porosities are round shape bubbles of less than 250 micrometer diameter. These defects may come from gas bubbles generated following hydrogen rejection during solidification or they may come from bubbles induced by both an intense evaporation inside the keyhole and a turbulent flow in the molten pool. (2) Large porosities are voids generated in the bottom of the welded zone. They arise from a lack of matter inside the weld as if the molten metal have not had enough time to fill up the keyhole completely before it solidifies. We elaborated a numerical model for the comprehension of little porosities trapping by comparing the calculated spot weld solidification time to the rising time of bubbles in the liquid phase. We also studied the melt flow back into the keyhole in order to explain the formation of the observed large voids. These two models are in good agreement with experimental observations, in particular with X-ray radiographs achieved during the solidification of tantalum spot welds.