Alexander V. Dubrov

Diagnostics of laser radiance penetration into material by multi-channel pyrometer

Investigations of the melt removing in the gasjet-assisted CO2 laser cutting by pyrometer have been performed for steel plate 3, 6 mm and 10 mm thick are reported. The measurements of local brightness and brightness temperature were conducted for different values of cutting speed and assisted gas pressure.

Spectrum of temperature pulsations of the melt in gas-assisted cutting with fiber laser

Abstract. Measurements of the temperature behavior in the zone of action of the laser-radiation on the molten metal have been performed using multichannel pyrometer. Measurements were carried out for test cutting of a 3-mm mild-steel plate with several values of cutting speed and pressure of assist gas (oxygen), using an 1800-watt Ytterbium fiber laser. It is shown that fluctuations of temperature are related to local melt’s surface deformations due to unequal radiation absorption; thus the noise spectrum of temperature fluctuations reflects turbulent surface deformation caused by gas jet and capillary waves. The maximum density of turbulent energy dissipation ε depends on cutting conditions: its value rises with increasing cutting velocity and oxygen pressure in a described range of parameters. The maximum of ε is localized near depth of (1.2…1.5)  mm along the cutting front. We can distinguish the specific radiation pulsation spectrum of laser cutting from other processes of radiation affection to the sample, including unwanted degrading of the quality of technological operations. The spectrum of capillary waves on the melt’s surface is formed under the effect of assisted gas jet and has a function of ω−3, ω is cycle frequency. The results of this investigation can be useful for the development of monitoring and quality-control systems for the laser-cutting process.

Heat Transfer and Thermocapillary Convection during the Laser Deposition of Metal Powders Implemented in Additive Technologies

Heat-transfer- and thermocapillary-convection macroprocesses observed during direct laser metal deposition (DLMD) with coaxial powder injection are examined. The study is performed using the 3D mathematical model incorporating self-consistent equations for free surface evolution, heat transfer, and hydrodynamics, which allow for powder-particle embedding into the thermocapillary convection zone under DLMD. The processes under consideration refer to the main ones underlying additive laser technologies, which determine the microstructural properties and quality of synthesized parts. The convection-diffusion equations are numerically solved using the final volume method. Calculations are carried out for the thermocapillary convection of H13 steel powder. The influence of laser-radiation characteristics (power, scanning rate, intensity distribution in the beam) and the powder-mass flow velocity on temperature fields, the structure of convective melt flow (including a maximum melt velocity), and the geometric characteristics (height and width) of the object formed is investigated.

Using optical diagnostics to determine the melt temperature field in layer-by-layer laser alloying of metal powder

The results of application of optical diagnostics in the estimation of the temperature field at the melt surface in layer-by-layer laser alloying of metal powder are presented. It is demonstrated that surface concavity induced by the thermocapillary effect upon nonuniform heating may distort pyrometry data considerably. The use of external illumination provides an opportunity to determine the shape of the melt surface. The obtained minimum estimate of the temperature gradient in the metal region affected by laser radiation is 2.8 × 104 K/cm.

Pyrometry diagnostic in laser cutting technology

The measurement data of temporal temperature fluctuation on the cut front while laser cutting of sheet metal (3 mm, 6 mm and 10 mm thickness) are obtained using two-color multi-channel pyrometer. Measurements were carried out for several values of cutting speed and pressure of assisted gas (oxygen), using a 1500W CO2 laser radiation and 1800W Ytterbium fiber laser. The relationship between the temperature fluctuations and the deformation of the melt flow surface on the cutting front in the frequency range above 3 kHz was proven. It is shown that in the case of CO2 laser cutting the temperature fluctuations RMS is greater than 10 K in the range of sub-millimeter capillary waves, but in the case of Fiber Laser cutting the temperature fluctuations RMS is less than 3 K. The spectrum of capillary waves in case of Fiber- Laser cutting is formed under the effect of forced surface deformation at lower frequencies, in particular related to the assisted gas jet. It is shown, that thermo-capillary effect with capillary-wave turbulence generation can be observed in the locations, where exposition intensity of melt surface to CO2 laser radiation exceeds 1 MW/cm2. Thus, an additional mechanism of the anomalous absorption on the front of cutting can compensate the low absorption of the metal in case of 10.6 ìm laser in comparison with the absorption of the metal in the near infrared range.

Thermocapillary effects in CO 2 laser cutting of metals

The measurements of local pulsations of brightness temperature T of melt radiance along the front of the cut of metal sheet by CO2 laser are carried out. Both spectrum regions of hydrodynamic and capillary turbulence of temperature fluctuations are shown with experimental estimation of threshold of thermocapillary effect of absorption as 1 MW/cm2.

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.

On numerical modeling of heat transfer and fluid flow in selective laser melting of metal powder bed

Selective laser melting (SLM) of the powder bed is one of the promising techniques for additive manufacturing of metals. Laser powder bed fusion is an inherently multiscale process and calls for an approach using multiple coupled models. In this work we developed the macroscopic thermodynamic model of SLM involving sequential deposition of powder layers on the plate followed by their melting. The accompanying processes of heat transfer, Marangoni convection and evolution of the melt free surface are included in the model. This model gives self-consistent consideration to the distributions of temperature and melt velocities during SLM. Modeling of the free surface evolution is performed by the VOF method. For numerical calculations the program software has been developed and tested. Its realization involved the C++ class library of numerical modeling OpenFOAM 2.4. Thermal flows, melt velocities and resulting profiles of sintered layers depending on the SLM parameters (beam power, scanning speed, powder layer) have been obtained. The calculated distributions demonstrate the development of widespread defects in SLM, e.g. residual porosity inside the solidified metal in the form of gas bubbles, incomplete penetration and bonding of the substrate metal and the particles. The results show that capillary effects play an important role in the liquid phase dynamics and, correspondingly, in the formation of final profile and structure of the deposed layer. The macro-level data (heat removal rate, cooling rate) can be used as the input parameters in boundary condition formulation for solving the microstructure evolution and the residual stress formation problems during SLM.

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.

Numerical simulation of thermal behavior for process parameters optimization in laser additive manufacturing

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.