Yuri N. Zavalov

Turbulent flow structure of FAF CO2 laser active medium at nonuniform temperature and heat generation

The isobaric fluctuations of temperature and density, attending with turbulent adiabatic fluctuations of velocity, have been evaluated under spatial non-uniformity of temperature and at heat generation in the incompressible flow. The character of their quantity dependence on spatial scale has been revealed. It has been shown that the temperature gradient and specific power of heat generation strongly affect the isobaric fluctuations of temperature.

Diagnostics of turbulent flow parmeters of active mixture in fast axial flow CO2 laser

The characteristics of turbulent flow of laser mixture in high-power industrial CO2 lasers with fast axial flow have an influence on energy parameters of laser beam. In particular the transverse size of DC discharge and discharge stability depend on coefficient of turbulent diffusion. Moreover, turbulent pulsations cause small-scale optical inhomogeneities in the active medium. By the method of intra-resonator four-wave mixing in the active mixture of industrial CO2 laser with fast axial flow the coefficient of turbulent diffusion was determined. The nonlinearity of gain coefficient was used, and dissipation of amplitude diffraction grating due to turbulent diffusion was taken into consideration.

The multichannel pyrometer of the spectral ratio for on-line monitoring in the powder bed additive technologies

Optical diagnostic methods were used to study the physical processes occurring on the surface of the melt in the technology of selective laser melting (SLM) of metallic powders. Independent registration of the fraction of reflected laser radiation and thermal radiation from several points of the surface of the melt were carried out simultaneously. This made it possible to distinguish changes in the surface relief and subsurface processes of heat and mass transfer during laser heating. It is shown that the frequency and amplitude characteristics of signals obtained by optical diagnostics make it possible to identify the moments of intensification of convective heat and mass transfer. The results of the research can be used to develop methods and tools for on-line monitoring and control of the SLM process.

Temperature distribution of gas powder jet formed by coaxial nozzle in laser metal deposition

Results of the experimental study of the effect of the laser radiation on the jet of a gas-powder mixture are presented. The flow of the gas-powder mixture (GPM) was formed by the cone-slit nozzle of the set-up of laser metal deposition (LMD). Spatial-temporal distributions of the temperature of the powder phase of the GPM are obtained. Three granulometric compositions of stainless-steel powder Ch18N9 (PR-X18H9) was used apart in the experiment. The weight-average diameters d50 of powder particles of their compositions were 114, 63 and 36 μm, respectively. The characteristic distance of the temperature rise of particles in the gas-powder jet and the maximum temperature of particles are obtained experimentally and amount, respectively: 9.4 mm and 2200 K for the coarse powder; 6.3 mm and 2250 K for the medium-sized powder; 4.6 mm and 2700 K for the fine powder. The heating rate increased from 0.4*10^6 K/s for the coarse powder to 0.68*10^6 K/s for fine powder. The results of the study can be used to develop methods and tools for monitoring and control the LMD process. The revealed features of the dynamics of the temperature of the powder phase in the LMD process must be taken into account in modeling the processes of the effect of laser radiation on a gas-powder medium.

Features of the estimation of temperature distribution on the bead formed by the laser aided metal powder deposition

The estimation of temperature variations in five regions of layer track is fulfilled in two different cases of layer track formation: laser aided metal powder deposition and selective laser melting with a pre-deposited layer of metal powder. The peculiarities of the multichannel pyrometer have been described, and the advantages of the application of spectralratio pyrometer in diagnostics in the additive laser technologies have been demonstrated. A set of measurements of the temperature distribution in the region of exposure to laser radiation during a bead formation in technology of laser aided metal powder deposition with powder injection has been performed at the specific energy input of J=90 MJ/kg. The dependences of the temperature on scanning velocity, laser power and powder thickness have been derived in the technology of selective laser melting. It has been shown that the maximum temperature of the track surface is little changed in the wide range of variation of the calculated parameter of specific energy input J=4…20 MJ/kg, the bead width is here increased as J grows. The increase of J above a certain value J0 leads to a rise in the temperature at the bead axis. For the experimental conditions with the track width around 1 mm, the character value is estimated as J0 = 24 MJ/kg. It has been shown that J0 depends inversely on the track width.

Evaluation of thermal behavior during laser metal deposition using optical pyrometry and numerical simulation

3D mathematical model of non-stationary processes of heat and mass transfer was developed for additive manufacturing of materials by direct laser metal deposition. The model takes into account self-consistent dynamics of free surface, temperature fields, and melt flow speeds. Evolution of free surface is modelled using combined Volume of Fluid and Level-Set method. Article presents experimental results of the measurement of temperature distribution in the area of bead formation by direct laser metal deposition, using multi-channel pyrometer, that is based on two-color sensors line. A comparison of experimental data with the results of numerical modeling was carried out. Features of thermal dynamics on the surface of melt pool have been detected, which were caused by thermo-capillary convection.

On-line diagnostics of the melt surface dynamics during layerwise melting of the metal powder

The high-speed imaging method was used to study the dynamics of the melt surface during layerwise laser melting of the metal powder. We used the experimental equipment for selective laser melting process. CO2-laser radiation with TEM01 ∗ mode was focused on the surface of the powder in a spot with a diameter of 0.4 mm. The following alloy powders were used: VG-159, Inconel-718, and VKNA-1VR with a grain size distribution of 10…50 microns. The powder melting dynamics was investigated using set at an angle high frame rate video camera in a one-pulse mode and in a continuous scanning exposure mode with melted track formation.

The technique of measuring the velocity of melt removal in gas-laser cutting technology using multi-channel pyrometer

Multichannel pyrometer was used to measure fluctuations of temperature on the front of action of laser radiation on steel in technology of laser cutting with an assisted gas. Time-resolved measurements were made of local temperature in the five areas with spot size of about 100 mkm each, spaced approximately 0.6 mm apart in the upper part of the front cut. The technique of measuring the velocity of melt removal using the cross-correlation of data of multichannel pyrometer is described. It is shown that the reject filtering allows emphasizing the speed of so-called “fast” and “slow” waves, as the interface velocity. in which they take into account the generation of surface waves in thin film blowing a jet of gas. Obtained experimental data of the velocity of the melt and film thickness are consistent with the results of the model, Chen (1999), which takes into account the gas- jet generation of surface waves on the film.

The investigation of turbulence in FAF CO 2 laser by phase-conjugation method

The spectra of pulsation of signal beam of intracavity four-wave mixing in 1 kW fast-axial-flow CO2 laser were measured. The information about turbulent parameters of laser-active medium flow was obtained.

Estimation of Cn2 of nonequilibrium turbulent gas flow in CO2 laser on data of luminescence diagnostics

The method of estimation of Cn2 of laser mixture turbulent flow is proposed based on experimental dependencies of amplitude of gas pulsation on spatial pulsation scale. The connection between degree of nonequilibrium and C,2 magnitude is shown. The boundaries of inertial and viscosity ranges are determined. The estimation of Cn2 is within (3 .. . 14). 10 15 cm2/3 as dependence of gas- discharge current.