Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

Description of a Eulerian–Lagrangian Approach for the Modeling of Cooling Water Droplets

The present paper describes a tool developed in-house for the modeling of free-falling water droplet cooling processes. A two-way coupling model is employed to account for the interactions between the droplets and the carrier fluid, following a Eulerian-Lagrangian approach. In addition, a stochastic separated flow technique is employed, involving random sampling of the fluctuating fluid velocity. In physical modeling, two empirical correlations are considered for determining the heat and mass transfer coefficients, with the possibility of accounting for vibrations. The numerical results indicate the preponderance of the interactions between droplet and carrier fluid at various humidity ratios.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio

ATOMIC SCALE SIMULATION OF ACCIDENT TOLERANT FUEL MATERIALS FOR FUTURE NUCLEAR REACTORS

The 2011 accident at the Fukushima-Daiichi power station following the earthquake and tsunami in Japan put renewed emphasis on increasing the accident tolerance of nuclear fuels. Although the main concern in this incident was the loss of coolant and the Zr cladding reacting with water to form hydrogen, the fuel element is an integral part of any accident tolerant fuel (ATF) concept. Therefore, to license a new commercial nuclear fuel, the prediction of fuel behavior during operation becomes a necessity. This requires knowledge of its properties as a function of temperature, pressure, initial fuel microstructure and irradiation history, or more precisely the changes in microstructure due to irradiation and/or oxidation. Amongst other nuclear fuels, uranium diboride (UB2) and uranium silicide (U3Si2) are considered as potential fuels for the next generation of nuclear reactors due to their high uranium density and high thermal conductivity compared to uranium dioxide (UO2). However, the thermophysical properties and behavior of these fuels under extreme conditions are not well known, neither are they readily available in the literature. Therefore, in this thesis, density functional theory (DFT) and classical molecular dynamic (MD) simulations were used to investigate the thermophysical properties, radiation tolerance and oxidation behavior of UB2 and U3Si2 as potential fuels or burnable absorbers for the next generation of nuclear reactors. UB2 was studied in order to understand its thermophysical properties as a function of temperature. The phonon-assisted thermal conductivity (kph) exhibits large directional anisotropy with larger thermal conductivity parallel to the crystal direction. This has implications for the even dissipation of heat. The increase in thermal conductivity with temperature is justified by the electronic contribution to the thermal transport, especially at high temperatures. This shows that UB2 is a potential ATF candidate. In terms of radiation tolerance, Zr is more soluble in UB2 than Xe, while uranium vacancy is the most stable solution site. Furthermore, as the concentration of Zr fission product (FP) increases, there is a contraction in the volume of UB2, while an increase in Xe results in swelling of the fuel matrix. In terms of diffusion, the presence of an FP in the neighboring U site increases the migration of U in UB2, making U migrate more readily than B as observed in the ideal system. The thermophysical properties of U3Si2 as a possible ATF were studied and discussed considering the neutronic penalty of using a SiC cladding in a reactor. The calculated molar heat capacity and experimental data are in reasonable agreement. Due to the anisotropy in lattice expansion, a directional dependence in the linear thermal expansion coefficient was noticed, which has also been experimentally observed. The thermal conductivity of U3Si2 increases with temperature due to the electronic contribution while the phonon contribution decreases with increasing temperature. A comparison of the thermal conductivity in two different crystallographic directions sheds light on the spatial anisotropy in U3Si2 fuel material. The inherent anisotropic thermophysical properties can be used to parametrize phase field models by incorporating anisotropic thermal conductivity and thermal expansion. This allows for a more accurate description of microstructural changes under variable temperature and irradiation conditions. Due to the metallic nature of U3Si2, the oxidation mechanism is of special interest and has to be investigated. Oxidation in O2 and H2O was investigated using experimental and theoretical methods. The presence of oxide signatures was established from X-ray diffraction (XRD) and Raman spectroscopy after oxidation of the solid U3Si2 sample in oxygen. Surface oxidation of U3Si2 can be linked to the significant charge transfer from surface uranium ions to water and/or oxygen molecules. Detailed charge transfer and bond length analysis revealed the preferential formation of mixed oxides of U-O and Si-O on the U3Si2 (001) surface as well as UO2 alone on the U3Si2 (110) and (111) surfaces. Formation of elongated O−O bonds (peroxo) confirmed the dissociation of molecular oxygen before U3Si2 oxidation. Experimental analysis by Raman spectroscopy and XRD of the oxidized U3Si2 samples has revealed the formation of higher uranium oxides such as UO3 and U3O8. Overall, this work serves as a step towards understanding the complex anisotropic behavior of the thermophysical properties of metallic UB2 and U3Si2 considered as potential accident tolerant nuclear fuel. The calculated anisotropy of thermophysical properties can be used to parametrize phase field model and to incorporate in it anisotropic thermal conductivity and thermal expansion

Molecular modelling and simulation of fluids, surfaces and their interactions in reservoir settings

Fluid interactions within porous media are of vital importance in petrol-chemical processes. Problems encountered macroscopically, e.g., contact line tracking, confined fluid, and wetting alternation motivates investigations at a smaller scale. Nanoscale molecular modeling techniques such as molecular dynamics (MD) are a suitable tool; recent development of the Statistical Associating Fluid Theory (SAFT) force fields used in MD makes coarse-grained (CG) representation a viable option to describe large and complex systems. The work starts by assessing the predictive and correlative capability of the SAFT CG force field as applied to pure components and mixtures of CO2 with n-decane and n-hexadecane and 2,2,4-trimethylhexane, a lubricant. By using the principle of corresponding states to obtain the SAFT force field parameters for chemicals of limited experimental measurements and are selected as representative components based on 2D gas chromatography data of three different stock tank crude oils, the density, viscosity and surface tension as functions of temperature are simulated and compared to experimental data, observing good overall agreement. This work also provides a strategy for converting the topography of a natural rock surface measured by atomic force microscopy (AFM) into a series of synthetic surfaces of nanometer-scale amplitudes and wavelengths. The surface chemistry is characterised as calcite by fitting the water-surface interactions to atomistic water density profiles and the oil-surface interactions to experimental contact angle measurements on a smoothly cleaved calcite. The wettability can be examined systematically via the apparent contact angles and curvatures of the droplet at various surface roughness. The dependence of the contact angle with roughness is found to be in qualitative agreement with macroscopic Wenzel and Cassie-Baxter relations. Overall, the flow of this work provides a plausible strategy of assessing nanoscale wetting in reservoir settings; future information on surface adsorption of specific components would facilitate more quantitative evaluations.Open Acces

Study of CO2 desublimation during cryogenic carbon capture using the lattice Boltzmann method

Cryogenic carbon capture (CCC) can preferentially desublimate CO2 out of the flue gas. A widespread application of CCC requires a comprehensive understanding of CO2 desublimation properties. This is, however, highly challenging due to the multiphysics behind it. This study proposes a lattice Boltzmann (LB) model to study CO2 desublimation on a cooled cylinder surface during CCC. In two-dimensional (2-D) simulations, various CO2 desublimation and capture behaviours are produced in response to different operation conditions, namely, gas velocity (Péclet number Pe) and cylinder temperature (subcooling degree Tsub). As Pe increases or Tsub decreases, the desublimation rate gradually becomes insufficient compared with the CO2 supply via convection/diffusion. Correspondingly, the desublimated solid CO2 layer (SCL) transforms from a loose (i.e. cluster-like, dendritic or incomplete) structure to a dense one. Four desublimation regimes are thus classified as diffusion-controlled, joint-controlled, convection-controlled and desublimation-controlled regimes. The joint-controlled regime shows quantitatively a desirable CO2 capture performance: fast desublimation rate, high capture capacity, and full cylinder utilization. Regime distributions are summarized on a Pe–Tsub space to determine operation parameters for the joint-controlled regime. Moreover, three-dimensional simulations demonstrate four similar desublimation regimes, verifying the reliability of 2-D results. Under regimes with loose SCLs, however, the desublimation process shows an improved CO2 capture performance in three dimensions. This is attributed to the enhanced availability of gas–solid interface and flow paths. This work develops a reliable LB model to study CO2 desublimation, which can facilitate applications of CCC for mitigating climate change

Simulation of Microstructural Evolution of Selective Laser Melting of Metal Powders

Selective Laser Melting (SLM) is an Additive Manufacturing (AM) process used to create 3D objects by laser melting pre-deposited powdered feedstock. During SLM, powdered material is fused layer upon layer, the scanning laser melts regions of the powder bed that corresponds to the geometry of the final component. During SLM the component undergoes rapid temperature cycles and steep temperature gradients. These processing conditions generate a specific microstructure for SLM components. Understanding the mechanism by which these generated microstructures evolve can assist in controlling and optimising the process. The present research develops a two dimensional Cellular Automata – Finite Element (CA-FE) coupled model in order to predict the microstructure formed during the melting process of a powdered AA-2024 feedstock using the AM process SLM. The presented CA model is coupled with a detailed thermal FE model which computes the heat flow characteristics of the SLM process. The developed model takes into account the powder-to-liquid-to-solid transformation, tracks the interaction between several melt pools within a melted track, and several tracks within various layers. It was found that the simulated temperature profiles as well as the predicted microstructures bared a close resemblance with manufactured AA-2024 SLM samples. The developed model predicts the final microstructure obtained from components manufactured via SLM, as well as is capable of predicting melt pool cooling and solidification rates, the type of microstructure obtained, the size of the melt pool and heat affected zone, level of porosity and the growth competition present in microstructures of components manufactured via SLM. The developed models are an important part in understanding the SLM process, and can be used as a tool to further improve consistency of part properties and further enhance their properties