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The processes of melt crystallization and conductive heat transfer during selective laser melting of a metal alloy are considered in a conjugate formulation. A mathematical model is proposed for describing the inhomogeneous temperature field for calculating the fraction of the crystalline phase during laser treatment of the surface of a metal alloy. A numerical study of laser amorphization of bulk metallic glass (BMG) by melting the surface of samples of finite dimensions has been carried out. A comparison with the known experimental data is carried out, and a qualitative agreement is obtained for the calculations on the depth of the melt pool, the homogeneity of the amorphous layer, and its location for a multicomponent alloy based on zirconium: Zr 57 Cu 15 , 4 Ni 12 , 6 Al 10 Nb 5 . Potentially suitable laser scanning regimes are identified, in which the fraction of the crystalline phase in the treated layer does not increase compared to the initial one in the initial amorphous-crystalline alloy. It was found that when using the calculated modes of laser scanning, it is possible to reduce the fraction of the crystalline phase in the treated surface layer of the BMG alloy by orders of magnitude. [ABSTRACT FROM AUTHOR]

A thermal model of nanosecond laser ablation considering kinetics of surface evaporation is proposed. Equations concerning heat transfer in the target and associated gas dynamics are coupled by mass and energy balances at the surface and Knudsen layer conditions. Rigorous analysis of gas-dynamics related to condensation at the target surface is introduced in this model. Laser energy absorbed by the target is partly spent for evaporation and partly dissipated in the target by thermal conduction. The sum of thermal and kinetic energies of the gas phase is, usually, less than the energy of evaporation. The fraction of energy lost for target heating increases with decrease in laser fluence and attains 100% at the ablation threshold. The dependence of ablated depth on fluence is, thus, determined by energy partition between the solid and gas phases. The gas-dynamic flow accompanying ablation consists of a layer of compressed high-temperature vapor adjacent to the target that expands and pushes the ambient gas from the surface to generate a strong shock wave. Ablation of Al and Au by laser with 193 nm wavelength, 12 ns full width at half maximum (FWHM) pulses, and 5.3 J/cm2 incident fluence and that of Au by laser with 266 nm wavelength, 6 ns FWHM, and 3.5 J/cm2 incident fluence is analyzed utilizing the present thermal model. It is concluded that optical breakdown does not occur at the considered conditions. The present model can be applied when the target surface temperature is less than the critical temperature. In case of nanosecond laser ablation of metals this, normally, restricts the value of absorbed fluence by the maximum of several J/cm2. [ABSTRACT FROM AUTHOR]

Proposes a two-stage two-dimensional gas-dynamic model of laser ablation in an ambient gas atmosphere. Compression of the ambient gas around the expanding plume of the laser-evaporated material and a shock front propagating through the undisturbed ambient gas found; Relation between the initial state of laser ablation and the ambient atmosphere.

One of the most perspective methods of additive manufacturing is selective laser melting. It allows producing the parts directly from 3D-model to 3D-object from metallic powders and alloys. Nowadays the method has very low productivity, which limits its extensive use and possible application to solve a modern design problem by introducing direct and fast metal production. The solution of its productivity would allow receiving metallic 3D-objects with complex geometry in short production period. The method was improved by the installation of laser beam profiling and online monitoring systems on the developed experimental setup. The profiler contributes to obtaining the alternative power density distributions of the laser beam. The CoCrMo powder was chosen because of its excellent melting qualities, and the initial powder was pre-treated with the purpose to get the diameter of the particles less than 20 μm. During the experiments, 3D-samples were obtained by the improved method of selective laser melting with parameters of two process windows for each of the laser beam spot. The 3D-samples were studied for the revelation of common material defects of the microstructure. The chemicalanalyses of the samples were implemented by scanning electronic microscopy. The analyses showed that the samples of each laser beam spot had defects related to the formation of a solid solution. Application of Inverse Gaussian laser beam spot into the SLM-machine for production of 3D-objects allows producing the pieces with the values of the parameters exceeding the value for Gaussian laser beam spot in several times. The typically recommended parameters for production of the piece on the modern SLM-machine are less than 100 W for laser source power and less than 30 mm/s for scanning speed. The analytical data, presented in the article demonstrated the field of process parameters, which can give a possibility to obtain 3D-object with the parameters up to 1 kW for laser source power and up to 0.3 m/s for scanning speed. The analyses of microstructure give a possibility to conclude about no visible difference between the formation of the objects with different laser beam spots. The online video-monitoring shows that with the application of laser beam profiling system the negative effects of the selective laser melting reduces visibly. [ABSTRACT FROM AUTHOR]

Abstract: This study aims at the theoretical examination of the anisotropy of thermal conductivity F = κ ||/κ ⊥ that could be engineered in hard multilayer coatings, prepared using arc-evaporation. High values of F and thereby high lateral heat dissipation can reduce detrimental thermal gradients that emerge during cutting and friction processes in hard coatings. As coating deposition is widely done using arc-evaporation that leads to inclusion of droplets of different shapes and material in the coating, it is further evaluated to which extent F is be affected by these inclusions. The ideal continuous anisotropic medium with inclusions can be described using the effective medium Maxwell Garnett Approximation (MGA). A deposited multilayer structure presents a particular case with a limited number of layers inducing anisotropy and is simulated using the Finite Element Method (FEM) and compared to the idealized MGA predictions. The results show that the effect of droplets on the anisotropy F is strongly dependent on droplet shape, material and orientation. For spherical droplets at concentrations that are usually observed in arc-evaporation coatings, the value of F decreases linearly, proportional to the droplet concentration and almost independent of droplet thermal conductivity for regularly experimentally encountered droplet materials. When ellipsoidal droplets are considered, F depends strongly on the material and orientation of the droplets. The effects of finite thermal interface resistance between individual layers and around the droplet inclusions are evaluated separately and are found to be generally beneficial. This study shows that creating multilayers with high anisotropy of thermal conductivity should be possible even in the presence of unavoidable droplet inclusions. Furthermore, controlling metal droplet formation upon arc-evaporation of hard coatings can be used as a tool to engineer the anisotropy of thermal conductivity in arc-deposited multilayer coatings. [Copyright &y& Elsevier]

Kinetic equations for ions and neutrals are numerically solved in the plasma sheath formed at a condensed phase when strong evaporation is taking place. The Boltzmann distribution is assumed for electrons. A weakly ionized vapor with the Debye length much shorter than the mean free path is considered. This is typical for laser evaporation of metals. Under these conditions, the sheath consists of a Knudsen layer and a thin charge separation layer between the Knudsen layer and the condensed phase. The self-consistent electrostatic field in the Knudsen layer is obtained from the quasineutrality condition. The potential barrier in the charge separation layer is determined by the charge balance. Kinetic boundary conditions for neutrals and charges are estimated by the detailed balance principle from the parameters of the saturated vapor. The transport of charges in the sheath is controlled by ions and depends on ion-neutral collisions and the self-consistent electrostatic field. Ionization degree in the vapor formed by strong evaporation increases with the Mach number and can attain values about 30% higher than the ionization degree in the saturated vapor. Two factors contribute to this increase. The first is the drop of the potential barrier in the charge separation layer and the second is the strengthening of the field in the Knudsen layer. The ionization equilibrium may be disturbed by a considerable excess of charges. [ABSTRACT FROM AUTHOR]

The gas-dynamic Euler equations require two boundary conditions to be specified at the surface of evaporated condensed phase and one condition at the surface of condensation. In the commonly considered three-parameter space of the temperature and pressure ratios and the Mach number this corresponds to a three-dimensional curve in the case of evaporation and to a surface in the case of condensation. To obtain the conditions of evaporation and condensation the steady-state Knudsen layer is numerically studied by the discrete velocity method applied to a Boltzmann equation with a relaxation collision term. Simple models of Mott-Smith type based on the conservation laws and analytical approximations of the velocity distribution function in the Knudsen layer may give satisfactory description of the gas-dynamic evaporation and condensation conditions while in general they inadequately represent the detailed structure of the distribution function. One of the reasons why the models deviate from the calculations is that they do not allow different parallel and perpendicular temperatures of the velocity distribution. Under evaporation, the Knudsen layer thickness increases with the Mach number M. Under condensation, it is inversely proportional to M when M is low. Numerical results are obtained and an analytical model is proposed for the vapor temperature considerably less than the condensed phase one (up to 10 times) what is typical for back condensation under pulsed laser ablation. [ABSTRACT FROM AUTHOR]

• Carbon fibers are in-plane preferentially oriented. • A fiber consists of non-textured graphite nanocrystals and amorphous carbon. • Radiative thermal conductivity is not sensitive to the reflective properties of fiber surface. • Reflectance of the carbon fiber material is highly anisotropic. • 90% of the incident radiation is absorbed in ∼200–300 μm thick outermost surface layer. Highly porous carbon-fiber materials are used in ablative heat shields for atmospheric re-entry. These carbon fiber based materials perform well against heat convection and conduction, and have high radiation absorption. During re-entry, spacecrafts are exposed to shock layer radiation, which provides a considerable share of the heat load faced by their shields. In this work, models for reflectance and heat transfer through FiberForm, a porous carbon-fiber material, are described. The models are based on the multiphase approach and were validated by comparison with experimental data on reflectance and on effective high-temperature thermal conductivity. The morphology of FiberForm was reconstructed from X-ray tomography data and was used as an input for the models. Transmission electron microscopy and diffraction analyses of FiberForm revealed the presence of an amorphous phase and graphite nanocrystalline grains smaller than 10 nm, with no preferential crystallographic orientation. A strong influence of anisotropy in fiber orientation was observed in measured reflectance and has been included in modeling. The porosity and the extinction coefficient were obtained from X-ray tomography images. The radiative transfer modeling indicates that 90% of the incident radiation at wavelengths around 1 μm is absorbed in the 200–300 μm thick outermost surface layer. [ABSTRACT FROM AUTHOR]

Phase composition and microstructure of initial WC, BK8 (powder alloy 92 wt.% WC-8 wt.% Co), Co powders, ball-milled powders with four different compositions (1) 25 wt.% WC-75 wt.% Co, (2) 30 wt.% BK8-70 wt.% Co, (3) 50 wt.% WC-50 wt.% Co, (4) 94 wt.% WC-6 wt.% Co, and bulk alloys obtained by selective laser melting (SLM) from as-milled powders in as-melted state and after heat treatment were investigated by scanning electron microscopy and X-ray diffraction analysis. Initial and ball-milled powders consist of WC, hexagonal a-Co and face-centered cubic b-Co. The SLM leads to the formation of major new phases W3Co3C, W4Co2C and face-centered cubic b-Co-based solid solution. During the heat treatment, there occurs partial decomposition of the face-centered cubic b-Co-based solid solution with the formation of W2C and hexagonal a-Co solid solution. The microstructure of obtained bulk samples, in general, corresponds to the observed phase composition. [ABSTRACT FROM AUTHOR]

Phase composition and microstructure of initial WC, BK8 (powder alloy 92 wt.% WC-8 wt.% Co), Co powders, ball-milled powders with four different compositions (1) 25 wt.% WC-75 wt.% Co, (2) 30 wt.% BK8-70 wt.% Co, (3) 50 wt.% WC-50 wt.% Co, (4) 94 wt.% WC-6 wt.% Co, and bulk alloys obtained by selective laser melting (SLM) from as-milled powders in as-melted state and after heat treatment were investigated by scanning electron microscopy and X-ray diffraction analysis. Initial and ball-milled powders consist of WC, hexagonal α-Co and face-centered cubic β-Co. The SLM leads to the formation of major new phases W3Co3C, W4Co2C and face-centered cubic β-Co-based solid solution. During the heat treatment, there occurs partial decomposition of the face-centered cubic β-Co-based solid solution with the formation of W2C and hexagonal α-Co solid solution. The microstructure of obtained bulk samples, in general, corresponds to the observed phase composition. [ABSTRACT FROM AUTHOR]

Metals and alloys in amorphous state are promising for the use as structural and functional materials due to their superior properties related to the absence of such defects as grain boundaries and dislocations. Obtaining the amorphous state requires quenching from liquid state with a high cooling rate. In conventional technologies, this is the reason for a considerable size limitation hindering application of amorphous alloys. Additive manufacturing (AM) is free of the size limitation. The so-called bulk metallic glass (BMG) alloys have extremely low critical cooling rates and can be used in AM. Recent studies on AM from Zr-based BMGs by selective laser melting (SLM) has proved the possibility of attaining the amorphous state and revealed partial crystallization to be the principal drawback of this process. The present work aims to analyze the conditions for partial crystallization and attempts to control it by optimizing the process parameters. A comprehensive parametric analysis is accomplished in single-track SLM experiments with Zr-based BMG alloy Vit 106. The observed microstructures are related to the temperature fields and thermal cycles estimated by an analytic heat-transfer model in the laser-impact zone. In the cross section of a laser track, a central bright domain is identified as the remelted zone. An annular darker crystallization zone encircles the remelted zone. The model fits the experimentally obtained dependencies of the remelted zone size versus laser power and scanning speed and indicates that 43 ± 2% of the incident laser energy is transferred into the substrate thermal energy. The principal energy losses are reflection of the incident laser radiation and material evaporation. Primary crystalline inclusions existing in the substrate before laser processing dissolve at the laser melting. The mean cooling rate in the remelted zone is up to 4 orders of magnitude greater than the critical cooling rate. Therefore, homogeneous nucleation is not expected. Nevertheless, the theoretically estimated crystallization times are sufficient for a considerable crystal growth in the heat-affected zone where primary crystalline inclusions and nuclei are not completely dissolved. [ABSTRACT FROM AUTHOR]

Photonic additives have been investigated as a means to enhance the efficiency of thermal protection systems (TPS) against the adverse effects of thermal radiation. State-of-the-art TPS consist of carbon fibers embedded in a phenolic resin matrix. During operation, the TPS is consumed because it is exposed to an excess heat flux, a large fraction of which is due to thermal radiation. Here, we show that a properly modeled and designed additive-impregnated TPS can block a considerable part of this heat influx and quantify how different control parameters, in particular the additives' amount, placement and alignment, influence the achieved photonic enhancement. More specifically, the intrinsic reflectivity of 8.5% of a conventional TPS can been improved to values exceeding 85% by controllably inserting additives, consisting of a Ta/[SiO 2 /TiO 2 ] 6 heterostructure, here referred to as Type 1, an ideal, optimized, high and broadband reflector. Nevertheless, even simple, commercially available additives composed of TiO 2 /Al 2 O 3 /TiO 2 , here referred to as Type 2, provide a high reflectivity enhancement with values of up to 76%, when used in larger quantities. The simulations of this work are based on the Monte Carlo Ray Tracing (MCRT) method. The MCRT simulation method has been validated against experiment, using the structure and experiments from a literature reference. Our analysis method allows one to design and model the performance of photonically enhanced TPS that operate in high-flux, radiative conditions, like those expected in future aerospace re-entry missions or next-generation, gas turbines and thermophotovoltaic plants and provides a viable option for efficiently enhancing a TPS. • Impregnation with photonic additives enhance the innate low reflectivity < R > of TPS. • Optimized, heterostructure additives achieve a high, broadband 85% reflectivity. • Simple, commercially available additives offer similar reflectivity response (~76%). • The additives' quantity, position and alignment are crucial parameters for tuning R. [ABSTRACT FROM AUTHOR]

The microstructural and optical reflectivity response of photonic SiO2/TiO2 nanomultilayers have been investigated as a function of temperature and up to the material system's melting point. The nanomultilayers exhibit high, broadband reflectivities up to 1350 °C with values that exceed 75% for a 1 μm broad wavelength range (600–1600 nm). The optimized nanometer sized, dielectric multilayers undergo phase transformations from anatase TiO2 and amorphous SiO2 to the thermodynamically stable phases, rutile and cristobalite, respectively, that alter their structural morphology from the initial multilayers to that of a scatterer. Nonetheless, they retain their photonic characteristics, when characterized on top of selected substrate foils. The thermal behavior of the nanometer sized multilayers has been investigated by differential thermal analysis (DTA) and compared to that of commercially available, mm-sized, annealed powders. The same melting reactions were observed, but the temperatures were lower for the nm-sized samples. The samples were characterized using X-ray powder diffraction before DTA and after annealing at temperatures of 1350 and 1700 °C. The microstructural evolution and phase compositions were investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy measurements. The limited mutual solubility of one material to another, in combination with the preservation of their optical reflectivity response even after annealing, makes them an interesting material system for high-temperature, photonic coatings, such as photovoltaics, aerospace re-entry and gas turbines, where ultra-high temperatures and intense thermal radiation are present. [ABSTRACT FROM AUTHOR]