R. V. Grishaev

Numerical investigation of the microstructure of a clad layer produced via laser cladding with coaxial metal powder injection

The microstructure of a clad layer produced via selective laser cladding with coaxial metal powder injection is studied numerically. The Johnson–Mehl–Avrami–Kolmogorov equation for condensed systems with inhomogeneous rates of nucleation is used to model the phase change kinetics. The impact of the substrate boundary along with interconnected heat transfer and phase change processes on the final microstructure of a built-up layer is demonstrated. The qualitative difference between the behavior of the temperature on the built-up layer’s surface and at the depth of the substrate is established, revealing the inhomogeneous microstructure of the final layer.

Multipass modes in an open resonator

The papers dealing with multipass modes in open stable resonators are reviewed. A numerical model of an axially symmetric resonator is described that permits the properties of multipass modes to be studied in wave approximation. It is shown that for Fresnel numbers larger than unity, multiple reflections of radiation from the stable resonator mirrors lead to sustained quasi-stationary field oscillations in the resonator which are indicative of the essential contribution of multipass modes. The calculations in filling the resonator with the active medium have been performed for two types of lasers: with the on-axis and off-axis gain maxima. Resonator designs are suggested, trajectory selection techniques are considered to provide high-quality radiation at large Fresnel numbers. The confocal resonator properties are analyzed in the context of competition between single-pass and multipass modes. Relying on the results of calculations for multipass mode generation, a critical analysis of the experimental works is presented suggesting a logical explanation for the peculiarities which had not been physically interpreted in the original publications. An advanced understanding of the physics of multipass mode formations allowed us to put forward a special design of stable resonators for the generation of high quality radiation in high-power lasers.

Estimation of output beam characteristics of laser with unstable resonator and turbulent active medium

In this paper we present calculations of the three-dimensional mode structure and output beam characteristics of unstable resonator with turbulent active medium. The calculations of the output beam are carried out in the Fresnel approximation using the well-known iterative method [1]. The active medium is modeled as a series of phase screens with Kolmogorov statistics. The dependence of output radiation parameters of the gas flow laser with unstable resonator on the strength of turbulence in active medium and parameters of resonator is under consideration.

The development of turbulence in the active medium of a fast-flow gas-discharge laser

The effect of pump nonequilibrium on the heat generation and the parameters of turbulent flow in the active medium of a fast-flow gas-discharge laser is studied. It is shown that the amplitudes of the turbulent pulsations of the density and refractive index of the active medium depend on the pump method and the degree of saturation by laser radiation and grow with the pump level. Criteria for estimating the effect of turbulence on the quality of the active medium are presented. A model is proposed that relates the build-up of the turbulent pulsations of the gas density and refraction to the heterogeneous heat release in the nonequilibrium active medium. The effect is determined by both the growth of the isobar local temperature pulsations and the ionization-overheating instability. In the case of dc-discharge pumping, the optical inhomogeneity of the active medium may substantially exceed that for the high-frequency discharge pumping.

Three dimensional model of melting and crystallization kinetics during laser cladding process

Unsteady heat transfer with simultaneous melting and crystallization at laser cladding process with coaxial metal powder injection is investigated numerically. Numerical modeling determined that the main parameters that govern melt pool dynamics and system maximum temperature are mass feed rate, laser power and scanning speed. Also it is determined that taking in to account the kinetics of phase change results in melt pool boundary and melting temperature mismatch. Dimensions of melted zone and cladding height are compared with experimental data.

Numerical investigation of unstable resonator for generating radially or azimuthally polarized beams

We present the calculations of intracavity radiation for a confocal positive-branch unstable resonator with a radially or azimuthally selective mirror. We carried out beam propagation in the resonator by the fox-li iterative method using jones vector calculus for the polarization state. The optimization of the resonator for generating a high-power radially or azimuthally polarized beam is discussed.

Mechanism of plasma generation under focused beam effect on metallic surface in laser welding

Numerical modeling of the process of thermal ionization associated with the interaction of laser radiation and metal target was performed and the temperature dynamics and the plasma absorptance were obtained. Threshold values of temperature for the appearance of the thermal instability and near-surface optical plasma generation have been derived. The processes in near-surface laser induced plasma in specific conditions of laser welding are under consideration.

Influence of nonuniform heat-releasing on optical inhomogeneities of active medium turbulent flow in high power CO 2 laser

The enhancement of optical inhomogeneities of active medium of high-power fast-axial flow CO2 laser is coupled with increasing of the isobaric fluctuations of temperature and density in turbulent flow due to nonuniform heat-releasing.