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Synthesis of nanoscale particles as a result of vapor condensation under laser vaporisation of materials into high-pressure surrounding atmosphere has been studied. A 2D-hydrodynamic model is developed for a simulation of nanoscale particles formation in a laser-produced erosive flow propagating from the irradiated surface into the ambient gas. The analysis is based on a combined solution of the set of hydrodynamic equations describing the erosive flow and equations of nonequilibrium kinetics of particles nucleation and condensation growth. The detailed structure of the erosive jet is studied: It is shown that nucleation and condensation fronts are separated in space and are formed between the erosive vapor–gas flow and cold ambient gas. The model predicts bimodal particle-size distribution function. The particle size is shown to be independent on the evaporation rate and to increase with the laser beam radius and the ambient gas pressure. more...

A 2D-hydrodynamical model is developed for simulation of ultrafine particle elaboration by means of the laser-evaporation technique. The general picture of the process can be predicted as follows. An erosive flow of the submerged jet type is formed under laser evaporation of a target material into ambient gas atmosphere. Due to the interaction with surrounding gas, the erosive flow is cooled and the vapour condenses generating nanoscale particles. A thin condensation front is formed between the erosive vapour–gas jet and cold ambient inert gas. The gas flow streamlines intersect the condensation surface twice and accordingly the condensation process is divided into two stages. The initial stage corresponds to the inlet part of the front near the laser irradiation spot, where streamlines are directed inward the erosive jet, homogeneous nucleation and condensational growth of nucleation centres take place here. The formed particles of a condensate are transferred by the inert gas into internal flow area, where the condensation is suspended. At the final stage, the particles intersect the condensation front from within the vapour–gas mixture at an outlet section where they continue to grow. The resulting particle size distribution depends both on ambient gas pressure and laser irradiation conditions. more...

Real time temperature monitoring is carried out by high speed high spatial resolution pyrometers. The obtained thermocycles are compared with a 2D transient heat transfer model, which permits calculation of the evolution (depth, volume, shape, etc.) of the molten pool as well as the solid state (β→α) phase transformation induced by the pulsed laser action. As a result, the correlation between surface temperature dynamics measured by pyrometer and parameters of phase transformations are studied. A phenomenological relation between molten pool evolution (obtained by simulation) and duration of solidification (measured by pyrometer) is presented for different values of the density energy flux. more...

The correlation between laser processing conditions and microstructure of a solidified alloy is studied, namely, the dendrite growth during directional solidification of alloys induced by pulsed laser irradiation. A model including the thermal and solute diffusion processes as well as capillary effects at the solid/liquid interface is proposed. The dependence of dendrite tip radius on power density and pulse duration is obtained for relatively `soft' regimes of laser processing, when evaporation and melt removal are negligible. The parameters of process conditions are found, when non-equilibrium conditions at the solid/liquid interface influence significantly the dendrite formation. The conditions when the dendrite growth is replaced by the planar one are determined. more...

The main characteristics of the developed two-dimensional resolution pyrometer are the following: temperature range 1000–4700°C; sampling time (17 μs for each channel) suitable for temperature image monitoring both for continuous wave and pulsed (millisecond range) laser action; spatial resolution 250 μm (10×10 points for measured area 2.5×2.5 mm) compatible with the typical size of the laser focus spot. The developed instrument has been tested under conditions of free-running Nd:YAG pulsed laser action on Ti, Al, Cu, and stainless steel (pulse duration 1–20 ms, pulse energy 10–40 J). more...

Melting and solidification of two-component ceramics induced by pulsed laser action are studied together with diffusion mass transfer. The position and temperature of the phase change front are determined from numerical solution of the coupled heat and diffusion mass transfer equations. The temperature of the solid/liquid interface is a function of the phase-change front velocity and composition. The final distribution of concentration is obtained for different values of the energy flux density and laser pulse duration. more...

By making use of the collision-radiative model that describes the phenomenon of optical breakdown in a metal vapour, the influence of the electron structure of atoms shells on the threshold intensity and breakdown period is analyzed. Two metals to be examined, aluminium and copper, are close in their thermo-physical properties but differ in the configuration of the electron shells of their atoms. General characteristics of optical breakdown, the influence of excitation energies of the neutral atom electron shells, as well as the ionisation potential, and the influence of higher excited states on threshold intensity and breakdown period are analyzed. more...

A set of pyrometers was developed for monitoring of real-time thermocycles in laser materials processing. The combination of high speed (sampling time 20–50 μs), wide temperature range (800–4700 K) and spatial resolution (300–400 μm) provides the means for precise analysis of surface temperature dynamics. The advantages and the problems of pyrometry in pulsed laser applications are illustrated on the example of Nd:YAG-laser action on metallic and ceramic materials. As a result, quantities such as the maximum value of temperature during whole thermocycle, the time of liquid phase existence, the duration of an allotropic transformation in a solid state, the heating and cooling rates, etc. may be determined. more...

Synthesis of ultrafine particles as a result of condensation in an erosive jet at laser vaporization of materials (metals, metal oxides, carbon) in gases (hydrogen, oxygen, helium, argon, xenon and air) at high pressures is investigated. The granulometric, phase, and chemical composition of the condensate particles is analyzed in relation to the elaboration conditions. It is found that the mean dimensions of the condensate particles increase with increasing the ambient gas pressure. A particle nucleation and growth theory is used to describe the formation of the dispersed condensate in the erosive plasma. It is shown that the resulting particle size distribution is determined by the cooling rate dependence of the vapor-gas mixture on the pressure of the surrounding gas. more...

The simulation is based on a set of two-dimensional transient equations of gas dynamics with two-dimensional radiation transfer equation completed by heat transfer equation for condensed phase. The influence of the laser radiation wavelength and the choice of the diameter of the initial high-temperature region in the gas phase (i.e. the region of optical breakdown) on the laser plasma dynamics and the transfer of the laser and plasma radiation energy to the target are analyzed. more...

The temperature gradient G, the solidification velocity R, and the G/R ratio are used in solid-liquid interface analysis. The critical G/R values characterizing the transitions from a planar front to a cellular structure, and from a cellular to cellular-dendritic structure during solidification are determined. The regularities of the rapid solidification process are shown on the example of the titanium alloy subjected to boriding with a CW-laser. The G/R ratio and cooling rate values versus molted zone depth are analyzed. The dependence of the relative transition depth from cellular to cellular-dendritic structure on laser beam velocity is determined. more...

The mechanism of laser sublimation of polymers with low absorption coefficient and, as a result, with a volume nature of energy release is simulated. It is shown that the volume character of absorption of laser radiation together with the phase transformation leads to the existence of the overheated metastable state of material. The peculiarities of transient sublimation induced by laser action with the parameters similar for the formation of optical disks, are analysed. more...