In this paper, the processes accompanying multitrack and multilayer selective laser melting of metal powder are studied by methods of multiscale numerical simulation. The model includes the coupled macro-scale balance equations of energy and momentum, describing heat transfer, fluid flow and phase transformations, while modeling the structure of the deposited layer by the discrete elements method. The features of the processes of heat transfer and the formation of the melt pool are studied, the surface profile and the relative density (porosity) of the synthesized sample of stainless steel 304 are determined. The calculated values of the final porosity coincide with the experimental data. On the micro-scale, a phase field method characterized by a self-consistent thermodynamic approach and universality is used to describe phase transitions and structure formation dynamics. The numerical implementation of the microstructure evolution micromodel is carried out, the dynamics of morphology and growth of columnar dendritic microstructures in the process of selective laser melting is analyzed. The influence of the governing parameters of the model and the process on the formation of stress fields in dendritic crystal structures is studied.
This paper formulates a general thermodynamically consistent theory of the coupled solute transport and large strain to describe the transformation kinetics of precipitation in a supersaturated matrix during laser additive manufacturing (AM) by selective laser melting of powder bed using phase field method. The structure of the constitutive equations is derived utilizing multiplicative kinematic decomposition of the total deformation gradient into elastic and eigen transformation parts. Expressions for the first Piola–Kirchhoff stress and the Cauchy stress are derived. The stress-dependent diffusion potential accounts for nonlinear, finite deformation. A high nonlinear Ginsburg–Landau kinetic equations coupled to Cahn-Hilliard type of diffusion-drift equation for solute atom concentration are derived. The Voigt/Taylor homogenization theory is used to model the dependence of total stress on the phase-field, which assumes equal elastic strains in the different phases at the diffuse interface region. To describe the effects of temperature and fluid velocity distributions and thermal history on the precipitate growth mechanisms a linking of microscale model with the macroscopic AM processing conditions is discussed. To describe the effects of temperature gradient and fluid velocity distributions and thermal history on the precipitate growth mechanisms a linking of microscale model with the macroscopic AM processing conditions is discussed. Then the proposed model is applied to cylindrical precipitate growth to explore the stress evolution with taking account of finite deformation and plastic deformation.
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.
A numerical study of the influence of macroscopic processes of heat and mass transfer on the dendrites formation during laser metal deposition is carried out in the paper. The algorithm used to calculate the processes realizes the concept of multiscale modeling of crystallization and provides the interaction of models of different structural levels. Simulation of macro-level processes is carried out on the basis of 3D self-consistent equations for the dynamics of the free surface, temperature, and melt flow velocities. The microlevel problems are related to the modeling of the formation of dendrites during the crystallization of the melt and are solved using the phase field equation conjugate with the of heat conduction equation. The profiles of the phase field and the temperature gradient in different spatial regions of the object being formed are calculated. The orientation of the dendritic structures strongly depends on the thermal prehistory and the shape of the melt pool and varies in different areas. In the near-surface layers, the dendrites have a pronounced orientation along the plane of the part. In the process of depositing the material over the previously treated areas, the microstructural properties of the crystallized material change partially. After solidification in areas that have undergone repeated remelting, dendrites oriented vertically appear. With the growth of the temperature gradient, the morphology of the dendrites transforms with the formation of columnar structures.
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.
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.
Laser metal deposition (LMD) by powder injection is an attractive and innovative additive manufacturing of metals. The key to predict material properties is the state of microstructure. In this paper, we develop a thermodynamically consistent temperature-displacement phase field model for grain growth during the LMD process. The governing equations that follow from the balance laws involve the phase variable, the displacement field, and the temperature field, with significant couplings between all equations. The model includes thermal expansion, transformation dilatation, strain dependency on phase transformation and local mechanical equilibrium conditions. Extensions to plastic models are discussed. Temperature dependencies of material properties (Young's modulus, Poisson's ratio, thermal expansion coefficient) are also included in the model formulation.
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.
We present a phase-field model for predicting elastic effects on microstructure evolution at the process of
laser sintering with powder injection. We derive a system of governing equations describing coupling
effects among phase variable, concentration, thermal and elastic displacement fields based on the principle
of entropy production positiveness, in which thermal and concentration expansions, mechanical anisotropy
effects, transformation dilatation, and strain dependency on phase transformation are considered. The
microstructure model is coupled with a macroscopic thermodynamic model. Effects of thermo-capillary and
thermo-gravitation convections are included. The possibility to describe the process of structure formation
at the phase interface during the melt crystallization is discussed. This model enables prediction and
visualization of grain structures during and after the laser sintering process.
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.
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.
The surface stress effects on the Rayleigh wave propargation characteristics in elastic solids with a
distribution of laser induced atomic point defects are studied. The frequency equations for waves
are obtained and analized. It is found that the Rayleigh waves are generally dispersive; and in the
case of low frequency with residual surface tension, a critical wave length exists, below which the
propagation of Rayleigh waves is not possible. This critical wave length depends on both the
residual stress and the defect distribution.
Direct laser sintering of a mixture of two metal powders with significantly different melting points is
investigated by numerical simulation. The model is based on self-consistent non-linear continuity equations for volume
fractions of components and on energy transfer equations for the powder mixture. It includes the movement of the solid
particles due to shrinkage because of the density change of the powder mixture and the liquid flow driven by the
capillary and gravity forces. The liquid flow is determined by Darcy's law. The effect of surface settlement of the
powder is obtained. The width increasing rate of the melting zone depend both on the parameters of the laser radiation
(on the power of the beam) and on the physical parameters of the particle's material, and it increases with the increasing
of the penetrability or the increasing of the phase-transition heat. The increasing of the laser power under other factors
being equal results in the acceleration of the melting front propagation.
The processes of heat transfer in microparticles in the gas atmosphere due to kinetics of phase
change (melting) under laser radiation have been numerically investigated. The temperature dependence of thermal and
optical parameters, irradiative cooling and heat exchange with the gas atmosphere are considered. The dynamics of
temperature distribution of particles of different sizes subject to melting has been determined. The dependences of the
time of full melting on the properties of particles and on laser radiation have been obtained. This will help to choose the
right regime of laser treatment in selective laser sintering.
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