• Archives of Metallurgy and Materials
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• v.64 no.3
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• pp.1175-1182
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• 2019

In this study, we present a new method for obtaining the parameters of the Johnson-Mehl-Avrami-Kolmogorov equation for dynamic recrystallization grain size. The method consists of finite-element analysis and optimization techniques. An optimization tool iteratively minimizes the error between experimental values and corresponding finite-element solutions. Isothermal backward extrusion of the AA6060 aluminum alloy was used to acquire the main parameters of the equation for predicting DRX grain size. We compared grain sizes predicted using optimized and reference parameters with experimental values from the literature and found better agreement when the optimized parameters were applied.

• Korean Chemical Engineering Research
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• v.54 no.3
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• pp.404-409
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• 2016

Thermal Characteristics and kinetic parameters of $LaMnO_3$ synthesis reaction were investigated by means of TGA (Thermogravimetric analysis) at non-isothermal heating conditions (5.0, 10.0, 15.0 and 20.0 K/min). The reaction was occurred rapidly at 450~600K (X=0.4~0.7) depending on the heating rate. Activation energy for the synthesis of $LaMnO_3$ from the precursor, which was determined by different method such as Friedman, Ozawa-Flynn-Wall and Vyazovkin methods, was in the range of 23~243 kJ/g-mol depending on the fractional conversion level and estimation method. The reaction order decreased with increasing heating rate and fractional conversional level. The average reaction order was 4.50 in case of X=0.1~0.3, while it was 1.87 in case of X=0.7~0.9, respectively. The value of frequency factor of reaction rate increased with inceasing heating rate and fractional conversion level. The aveage value of frequency factor was 205.6 ($min^{-1}$) when X=0.1~0.3, while it was 475.2 ($min^{-1}$) when X=0.7~0.9, respectively.

• Journal of the Korean Wood Science and Technology
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• v.34 no.2
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• pp.37-45
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• 2006

The viscoelastic properties of blends of melamine-formaldehyde (MF) resin and poly(vinyl acetate) (PVAc) for engineered flooring used on the Korean traditional ONDOL house floor heating system were investigated by dynamic mechanical thermal analysis (DMTA). Because MF resin is a thermosetting adhesive, the effect of MF rein was shown across all thermal behaviors. The addition of PVAc reduced the curing temperature. The DMTA thermogram of MF resin showed that the storage modulus (E') increased as the temperature was further increased as a result of the cross-linking induced by the curing reaction of the resin. The storage modulus (E') of MF resin increased both as a function of increasing temperature and with increasing heating rate. From isothermal DMTA results, peak $T_{tan{\delta}}$ values, maximum value of loss modulus (E") and the rigidities (${\Delta}E$) of MF/PVAc blends at room temperature as a function of open time, peak $T_{tan{\delta}}$ and maximum loss modulus (E") values were found to increase with blend MF content. Moreover, the rigidities of the 70:30 and 50:50 MF/PVAc blends were higher than those of the other blends, especially of 100% PVAc or MF. We concluded that blends the MF/PVAc blend ratios correlate during the adhesion process.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2005.04a
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• pp.137-140
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• 2005

Resin transfer molding (RTM) is one of the most popular processes for producing fiber reinforced polymer composites. In the manufacture of complex thick composite structures, analysis on flow front advancement on the resin impregnating the multi-layered fiber preform is helpful for the optimization of the process. In this study, three-dimensional mold filling simulation of RTM is carried out by using CVFEM (Control Volume Finite Element Method). On the assumption of isothermal flow of Newtonian fluid, Darcy’s law and continuity equation are used as governing equations. Different permeability tensors employed in each layer are obtained by experiments. Numerically predicted flow front is compared with experimental one in order to validate the numerical results. Flow simulations are conducted in the two mold geometries, rectangular plate and hollow cylinder. Permeability tensor of each layer preform in Cartesian coordinate system is transformed to cylinder coordinates system so that the flow within the multi-layered preforms of the hollow cylinder can be calculated exactly. Our emphasis is on the three dimensional flow analysis for circular three-dimensional braided preform, which shows outstanding mechanical properties such as high impact strength and toughness compared with other conventional two-dimensional laminar-structured preforms.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.10
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• pp.2601-2610
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• 1993

A numerical study on the melting process inside an isothermal spherical capsule is made. It is assumed that the phase change medium of its solid phase is heavier than the liquid phase and therefore the unmelted solid core is continuously moving downward on account of gravity forces. Such a gravity-assisted melting is commonly characterized by the existence of a thin liquid film below the solid core. The present study is motivated to present a full-equation-based analysis of the influences of the initial subcooling and the natural convection on the fluid flow and heat transfer characteristics associated with the gravity-assisted melting. In the light of the solution strategy, the present study is substantially distinguished from the existing works in that the complete set of governing equations in both the melted and unmelted regions are resolved without subdivision of the solution domains. For example, the liquid film region and the upper melted region are treated here as one domain and thus obviating laborious efforts to couple them. Numerical results are obtained by varying the Rayleigh numbers and the degree of subcooling. For the range of parameters examined, the presence of subcooling was found to impede the melting rate. The dropping velocity of the unmelted solid core was observed to affect the natural convection in the liquid significantly. When compared with the available experimental data, much improved prediction was achieved.

• Journal of Powder Materials
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• v.18 no.6
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• pp.510-516
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• 2011

In the present study, potassium and caesium doped Ag/$Al_2O_3$ catalysts were synthesized by simple wet impregnation method and evaluated for selective catalytic reduction (SCR) of NOx using methane. TEM analysis and diffraction patterns demonstrated the finely dispersed Ag particles. BET surface measurements reveal that the prepared materials have moderate to high surface area and the metal amount found from ICP analysis was well matching with the theoretical loadings. The synthesized K-Ag/$Al_2O_3$ and Cs-Ag/$Al_2O_3$ catalysts exhibited a promotional effect on deNOx activity in the presence of $SO_2$ and $H_2O$. The long-term isothermal studies at $550^{\circ}C$ under oxygen rich condition showed the superior catalytic properties of the both alkali promoted samples. The crucial catalytic properties of materials are attributed to NO adsorption properties detected by the NO TPD.

• Journal of the Korean Ceramic Society
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• v.55 no.2
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• pp.178-184
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• 2018

In the present study, $TiB_2-Co$ composite coatings were thermally sprayed onto the surface of a 304 stainless steel substrate using an atmospheric plasma spray (APS). The phase analysis of the powders and plasma-sprayed coatings was performed using X-ray diffractometry analysis. The microstructures of the coatings were studied by a scanning electron microscope (SEM). The average particle size and flowability of the feedstocks were also measured. Both $TiB_2-32Co$ and $TiB_2-45Co$ (wt.%) coatings possessed typical dense lamellar structures and high-quality adhesion to the substrate. The oxidation behaviors of the coatings were studied at $900^{\circ}C$ in an atmospheric environment. In addition, the cross-sectional images of the oxidized coatings were analyzed by SEM. A thin and well-adhered layer was formed on the surface of both $TiB_2-Co$ coatings, confirming satisfactory high-temperature oxidation resistance. The kinetic curves corresponding to the isothermal oxidation of the coatings illustrated a short transient stage from rapid to slow oxidation during the early portion of the oxidation experiment.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.5
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• pp.735-745
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• 1997

In this study, the compressibility of resin was considered in filling analysis to account for the possible packing type flow. A numerical simulation program employing a hybrid finite element/finite difference scheme was developed to solve Hele-Shaw flow of the compressible viscous fluid at non-isothermal conditions. To advance the melt front, a control volume approach was adopted. Thin complex 3-D shapes of cavities, runners, and sprues were discretized by employing triangular, cylindrical and/or rectangular strip elements. Mass conservation was applied to each control volume to solve for the pressure distribution. Directly applying a constant mass flow rate at the inlet removes calculation of the apparent pressure boundary conditions, resulting in better simulation condition. The Cross model was used to model viscosity and the Tait equation was employed to represent density as a function of temperature and pressure. The validity of the developed program was verified through comparisons with available data in the literature and the effect of compressibility on the pressure distribution was discussed. To reduce computation time, 1-D and 2-D elements were used instead of applying triangular elements and the numerical results were compared to each other.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.7 s.166
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• pp.1075-1084
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• 1999

The family of unidirectional continuous fiber reinforced polymeric composites are currently used in automotive bumper beams and load floors. The material properties and mechanical characteristics of the compression molded parts are determined by the curing behavior, fiber orientation and formation of knit lines, which are in turn determined by the mold filling parameters. In this paper, a new model is presented which can be used to predict the 3-dimensional flow under consideration of the slip of mold-composites and anisotropic viscosity of composites during compression molding of unidirectional fiber reinforced thermoplastics for isothermal state. The composites is treated as an incompressible Newtonian fluid. The effects of longitudinal/transverse viscosity ratio A and slip parameter $\alpha$ on the buldging phenomenon and mold filling patterns are also discussed.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.04a
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• pp.805-810
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• 1997

In this paper the technique to predict tool were theoretically in the sheet metal shearing process is suggested. The were in sheet metal tool affects the tolerances of final parts, metal flows and costs of processes. In order to predict the tool were the deformation of workpiece during the process is analyzed by using non-isothermal finite element program. The ductile fracture criterion and the element kill method are also used to estimate if and where a fracture will occur and to investigate the features of the sheared surface in shearing process. Results obtained form finite element simulation such as node velocities and node forces are transformed into sliding velocity and normal pressure on tool monitoring points respectively. The monitoring points are automatically generated and the were rates on these points are accumulated during a process. It is assumed that the wear depth on the tool surface are linear function of the lot sizes based upon the known experimental results. The influence of clearance between die and punch upon tool wear is were is also discussed during the process.