Yazar "To, Albert C." seçeneğine göre listele

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    A process-microstructure finite element simulation framework for predicting phase transformations and microhardness for directed energy deposition of Ti6Al4V
    (Elsevier, 2020) Baykasoğlu, Cengiz; Akyıldız, Öncü; Tunay, Merve; To, Albert C.
    This paper presents a process-microstructure finite element modeling framework for predicting the evolution of volumetric phase fractions and microhardness during laser directed energy deposition (DED) additive manufacturing of Ti6Al4V. Based on recent experimental observations, the present microstructure evolution model is formulated to combine the formation and dissolution kinetics of grain boundary, Widmanstatten colony/basketweave, massive/martensitic alpha and beta phases of Ti6Al4V. The microstructure evolution algorithm is verified and embedded into a three-dimensional finite element process simulation model to simulate thermally driven phase transformations during DED processing of a Ti6Al4V thin-walled rectangular sample. The microhardness values of different locations of the part, which experience different thermal histories, are computed based on the simulated fractions and hardness values of different phases in the final microstructure. The simulated volumetric phase fractions and related microhardness distribution agree reasonably well with experimental measurements performed on the sample. Thus the proposed simulation model could be useful for designers to understand and control process-microstructure-property relationships in a DED-processed part.
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    Effects of lithium doping on hydrogen storage properties of heat welded random CNT network structures
    (Elsevier Ltd, 2016) Baykasoğlu, Cengiz; Öztürk, Zeynel; Kırca, Mesut; Çelebi, Alper Tunga; Muğan, Ata; To, Albert C.
    This paper presents the effects of lithium doping on the hydrogen storage capability of heat welded random carbon nanotube (CNT) network structures having different cross-link densities at room temperature. Cluster based and atom based doping strategies are taken into consideration in the current simulations. Moreover, different doping ratios are used in our calculations to clarify the effect of doping ratio on hydrogen uptake behavior of CNT networks. The network structures are generated by using a cyclic stochastic algorithm and covalently bonded couplings are created by applying the heat welding method via molecular dynamic simulations. Hydrogen storage capacity of the Li-doped CNT networks is investigated using Grand Canonical Monte Carlo (GCMC) simulations. The simulation results show that hydrogen storage capacity is appreciably enhanced with the presence of lithium atoms and increases as the doping ratio increases. Besides, atom based doping method is found to be more efficient in hydrogen uptake than cluster based doping method under the same doping ratio. Furthermore, our results also show that the cross-link density is a critical parameter and hydrogen storage capability of Li-doped CNT network material can be improved with the appropriate choice of cross-link density. © 2016 Hydrogen Energy Publications LLC.
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    Hydrogen storage in heat welded random CNT network structures
    (Elsevier Ltd, 2015) Öztürk, Zeynel; Baykasoğlu, Cengiz; Çelebi, Alper Tunga; Kırca, Mesut; Muğan, Ata; To, Albert C.
    The objective of this study is to investigate hydrogen storage capability of heat welded random carbon nanotube (CNT) network structures. To achieve this objective, different three-dimensional random CNT network structures are generated by using a stochastic algorithm and molecular dynamic simulations. The interaction of CNT networks with hydrogen molecules is then examined via grand canonical Monte Carlo calculations. Hydrogen adsorption capacity of CNT networks having an arbitrarily natured morphology, adjustable porous structure and large surface ratio is investigated. The results show that if cross link density of random CNT networks decreases, hydrogen storage capability of CNT networks increases in terms of the gravimetric capacity. It is observed that random CNT networks could uptake 8.85 wt.% hydrogen at 77 K and this result is very comparable with the results reported in literature where generally ideal ordered nanostructures having no topological irregularities are considered. © 2014 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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    Predicting microstructure evolution during directed energy deposition additive manufacturing of Ti-6Al-4V
    (American Society of Mechanical Engineers (ASME), 2018) Baykasoğlu, Cengiz; Akyıldız, Öncü; Candemir, Duygu; Yang, Qingcheng; To, Albert C.
    Laser engineering net shaping (LENS) is one of the representative processes of directed energy deposition (DED) in which a moving heat source having high-intensity melts and fuses metal powders together to print parts. The complex and nonuniform thermal gradients during the laser heating and cooling cycles in the LENS process directly affect the microstructural characteristics, and thereby the ultimate mechanical properties of fabricated parts. Therefore, prediction of microstructure evolution during the LENS process is of paramount importance. The objective of this study is to present a thermomicrostructural model for predicting microstructure evolution during the LENS process of Ti-6Al-4V. First, a detailed transient thermal finite element (FE) model is developed and validated for a sample LENS process. Then, a density type microstructural model which enables calculation of the a-phase fractions (i.e., Widmanstatten colony and basketweave a-phase fractions), b-phase fraction, and alpha lath widths during LENS process is developed and coupled to the thermal model. The microstructural algorithm is first verified by comparing the phase fraction results with the results presented in the literature for a given thermal history data. Second, the average lath width values calculated using the model are compared with the experimentally measured counterparts, where a reasonable agreement is achieved in both cases. Copyright © 2018 by ASME.
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    Tensile behavior of heat welded CNT network structures
    (Elsevier, 2014) Çelebi, Alper Tunga; Kırca, Mesut; Baykasoğlu, Cengiz; Muğan, Ata; To, Albert C.
    In this study, a computational investigation on the mechanical properties of random carbon nanotube (CNT) networks is reported. For this purpose, atomistic models of 3-D CNT network structures are generated by an automated stochastic algorithm that is capable of randomly inserting CNTs into a design space while controlling several topological parameters including cross-link density, distance between cross-links, as well as length and chirality of CNTs. During the atomistic model generation process, heat welding is applied to the close contacts to form a welded CNT network with covalently-bonded junctions. Following the generation of atomistic models, molecular dynamics (MD) simulations of uniaxial tensile loading experiments are performed to investigate the basic deformation mechanisms of CNT networks as well as to obtain mechanical properties such as the Young's modulus, yield strength and ultimate strain values. The effects of cross-link density on the mechanical performance of CNT network materials are obtained by employing network specimens with different cross-link densities. © 2014 Elsevier B.V. All rights reserved.