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.
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.
For a full-fledged application laser additive manufacturing technologies of parts from metal powder and for maximum disclosure of their potential, it is necessary to provide automated construction of an optimal synthesis strategy with determination of the process parameters to ensure the specified properties and geometry of the product. In this paper, in the framework of the thermo-hydrodynamic model, the influence of the geometric boundaries of the workpiece on the processes accompanying laser metal deposition is numerically investigated. The geometric characteristics of the melt pool and the forming bead are investigated: length, width, height and dilution, as well as features of the formation of the vortex structure of the flows in the molten pool caused by thermocapillary forces and injection of powder. Modeling of the process of formation of two adjacent tracks in the technology of selective laser melting is carried out. It is shown that in the case of scanning two adjacent tracks with alternating directions, the volume of the melt region increases. To maintain the parameters in the required range, a variation in the laser radiation power or the scanning speed can be used.
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