Dynamics of radiation scattering by particles of condensed phase in quasicontinuous laser irradiation of metals

Abstract

Radiation scattering by particles of condensed phase in an ablation plasma plume has been experimentally studied during quasicontinuous laser irradiation (λ = 1.06 µm, q = 0.1-9 MW/cm 2 , τ , 1.5 msec) of duraluminum D16T, aluminum A99, and bismuth. The particle size distribution and the nature of their dispersal during irradiation was studied in scattered light (λ = 0.69 µm) from individual particles that could be visually observed on photographs. It was found that under the pressure developed in the plume, large particles ejected from the irradiated zone can move backward and return to the target (D16T). The plume (Bi) becomes brighter due to ablation of particles in the path of the laser beam. The directional scattering coefficients for scattering from the local zone on the axis of the plume, measured during the laser pulse, were used to study the relationship between the dynamics of entry of condensed phase into the plume, shielding of the target by the particles, and brightening of the plume under the action of the incident laser radiation.Introduction. Among the conditions for pulsed laser irradiation of metals in solving some scientific and applied problems (laser welding, laser cladding, etc.), there is interest in conditions where the laser pulse hitting the target has a quasicontinuous shape (peakless lasing conditions) and the pulse duration is on the order of a few milliseconds [1]. In this case, the relatively uniform spatial structure of the radiation, the sufficient duration of action, and the smooth rise in the intensity of the radiation at the beginning of the quasicontinuous pulse make possible, for relatively low radiation flux densities, "soft" heating and melting of the metal without substantial vaporization of the metal, which cannot be achieved with a pulse with a peak structure (free lasing conditions) or are more difficult to realize (in the initial stage) with a pulse having a steep leading edge. However, for high radiation flux densities, irradiation of the metal is accompanied by development of rapid vaporization and ejection of ablation products of the material from the irradiated zone, which considerably complicates the laser irradiation conditions and the nature of the irradiation itself.Refining our ideas about the physics of processes accompanying laser irradiation of materials under such conditions is important for practical applications, and also is of independent scientific interest. Some early papers are devoted to individual topics (see, for example, [2][3][4][5][6]). In [7][8][9][10][11], in which the dynamics of near-surface plasma formation was studied for quasicontinuous laser irradiation, some data were presented on the active role of particles of condensed phase in the complex pattern of interaction of the radiation with the surface and the plume. Early results of our own studies of radiation scattering by particles in the plume were published in [7,12]. The studies have been continued, and separate data on scattering and absorption of laser radiation by the particles a...