• Journal of Welding and Joining
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• v.16 no.5
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• pp.56-66
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• 1998

During solidification or welding of alloys, the solute redistribution brings out microsegregation. The microsegregation causes the formation of non-equilibrium second phases, shrinkage and porosity degrading mechanical/chemical properties Therefore, it has been required to predict microsegregation quantitatively. To predict the degree of microsegregation, more exact and appropriate computer simulation technique has been actively used during last two decades. To predict the degree of microsegregation in weld metal, an advanced two dimensional model was suggested. In the new model, both primary and secondary arm regions were defined for the analysis region. The growth in the primary arm regina was assumed to be a planar for effective calculation. Especially, for the growth of a secondary arm, a simple and effective mathematical function was established to show the growing pattern, the solute diffusion in the solid phase was calculated by finite difference method (FDM). The solid-liquid interface movement was considered to be in local equilibrium state. The experiments for welding of 310S stainless steel were carried out in order to examined the reasonability and feasibility of this model. The concentration profiles of the solute predicted by this model were compared with those obtained from experimental works.

• Journal of Welding and Joining
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• v.33 no.6
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• pp.21-26
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• 2015

Recently, application of UHSS(Ultra High Strength Steels) whose tensile strength is over 1000MPa to car body structure are growing due to great needs for light weighting and improved crash worthiness. However, their poor weldability is one of obstacles to expand selecting to car body. In this study, effect of Phosphorus contents on resistance spot weldability of high elongation DP980 steel whose Si content is over 1% was investigated. The cross tension strength (CTS) was decreased showing partial interface fracture as Phosphorus content increase because of solidification segregation of Phosphorus. In order to improve resistance spot weldability by modification of welding condition, in-situ post-weld heating pulse was introduced after main pulse. The optimum cooling time between main and post pulse and post-pulse current condtion were determined through FEM welding simulation and DOE tests. The CTS was increased about 1.5 time showing plug fracture. The decrease of Phosphorus segregation was found to be a major reason for weld ductility and CTS improvement.

• Proceedings of the Korean Reliability Society Conference
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• 2004.07a
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• pp.147-151
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• 2004

In this study, an optical fiber assembly directly coupled with a laser diode or a photo diode is designed to confirm high reliable optical coupling efficiency of optical transmitter(Tx) and receiver(Rx). The optical fiber assembly is fabricated by soldering an optical fiber and a Kovar ferrule using a glass solder after inserting an optical fiber through a Kovar ferrule. The Kovar which has good welding characteristics is applied to introduce laser welding technique. The glass solder has excellent thermal characteristics such as thermal shift delamination compared with PbSn, AuSn solder previously used usually. Furthermore, the glass solder doesn't need fiber metalization and this enables low cost fabrication. However, the glass soldering is high temperature process over 35$0^{\circ}C$ and the convex shape after solidification due to surface tension causes the stress concentration on optical fiber. The stress concentration on the optical fiber increases the optical insertion loss and possibility of crack formation. The shape of glass solder was designed referring to 2-D Axi-symmetric FEM simulation. To test the mechanical reliability, mechanical vibration test and shock test were done according to Telcorida GR-468-Core protocol. After each test, the optical loss of the stress distributed fiber assembly didn't exceed 0.5 dB, which passes the test.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.2 no.2
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• pp.53-62
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• 1992

In Zone Melting Recrystallization(ZMR) of SOl structure, thin silicon films have been recrystallized by artificial control of beam intensity profile which was obtained by tilting of upper elliptical reflector. Temperature profiles and gradients near solidification interface were calculated by numerical simulation for analysis of asymmetric line heating effect. The larger the tilting angle of the upper reflector, the larger the degree of supercooling at liquid and the interdefect spacing in thin silicon films. Major defects were continuous subgrainboundaries. Isolated threading dislocations were observed in the case of the film having low defect density. We have found that the thin silicon films were recrystallized into (100) textured single crystals by cross-sectional TEM and thin film X-ray diffraction analysis.

• Korean Journal of Materials Research
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• v.17 no.1
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• pp.11-17
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• 2007

Turbulent in-situ mixing process is a new material process technology to get dispersed phase in nanometer size by controlling reaction of liquid/solid, liquid/gas, flow ana solidification speed simultaneously. In this study, mixing which is the key technology to this synthesis method was studied by computational fluid dynamics. For the simulation of mixing of liquid metal, static mixers investigated. Two inlets for different liquid metal meet ana merge like 'Y' shape tube having various shapes and radios of curve. The performance of mixer was evaluated with quantitative analysis with coefficient of variance of mass fraction. Also, detailed plots of intersection were presented to understand effect of mixer shape on mixing. The simulations show that the Reynolds number (Re) is the important factor to mixing and dispersion of $TiB_2$ particles. Mixer was designed according to the simulation, and $Cu-TiB_2$ nano composites were evaluated. $TiB_2$ nano particles were uniformly dispersed when Re was 1000, and cluster formation and reduction in volume fraction of $TiB_2$ were found at higher Re.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.10
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• pp.1549-1554
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• 2010

Recently, plastic screws have replaced metal screws because of the former's light weight, thermal and electrical insulating properties, and anticorrosion characteristics. Plastic screws are usually produced by injection molding, which involves material shrinkage during the solidification of the polymer. This shrinkage results in the degeneration of the dimensional accuracy. In the present study, the effect of injection-molding conditions on the dimensional accuracy of plastic screws was investigated through a numerical simulation of injection molding; on the basis of this simulation, we could determine the mold-design parameters. The design of experiment was applied in accordance with the numerical analysis in order to optimize the injection-molding conditions with a view to improving the dimensional accuracy of the precision plastic screw.

• Journal of the Korean Society of Industry Convergence
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• v.24 no.5
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• pp.589-596
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• 2021

In this study, the optimum gating system was designed, and the two zinc alloy worm gears were manufactured in single process by applying a symmetrical gating system with 2 runners. The SRG ratio is set to 1 : 0.9 : 0.6, and the cross-sectional shapes such as sprue, runner and gate are designed. In order to determine whether the design of the gating system is appropriate, casting analysis was carried out. It takes 4.380 s to charge the casting 100%, 0.55 to 0.6 m/s at the gates and solidification begins after the casting is fully charged. The amount of air entrapment is 2% in the left gear and 6% in the right gear. Hot spots occurred in the center hole of the gear, and pores were found to occur around the upper part of the hole. Therefore, the design of the casting method is suitable for worm gears. CT analysis showed that all parts of worm gear were distributed with fine pores and some coarse pores were distributed around the central hole of worm gear. The yield strength and tensile strength were 220 MPa, 285 MPa, and the elongation rate was 8%. Vickers hardness is 82 HV.

• Design & Manufacturing
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• v.15 no.2
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• pp.30-35
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• 2021

The casting method uses a mold to solidify a liquid metal to make a solid metal. Since it uses a liquid metal with the least deformation resistance, it has the characteristic that it can easily manufacture even a complex shape. However, the process of solidifying a liquid metal into a solid metal inevitably involves a volume change and contains internal defects such as shrinkage holes. Therefore, in the design of the casting plan, an excess volume called a pressurization compensates for the volume shrinkage. in the product, and it induces the shrinkage hole defects to occur in parts other than the product1). In this study, casting analysis was performed using casting analysis software (anycasting) in order to optimize the design of the tilting gravity casting method for automobile brackets. In particular, the filling and solidification analysis according to the shape and volume of the pressurized metal was conducted, and applied to the actual product to study the effect of the pressurized metal on the shrinkage defect. Through this study, it is possible to understand the effect of the pressure metal on shrinkage defects in the actual product and propose a design of the pressure metal that improves reliability and productivity.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.4
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• pp.433-440
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• 2004

Since the numerical analysis was adopted in the mold design, lots of computational methods have been proposed for the simulations of casting processes for the various shaped molds. Today, it is possible to simulate the filling and solidification processes of most casts using the VOF technique. Though the three-dimensional numerical model based on the Cartesian coordinate system can be applied to any shape of cast, it becomes very inefficient when the three-dimensional model is applied to the cast of axi-symmetrical shape since the control volume includes at least 11 of the physical model. In addition, the more meshes should be distributed along the circumferential boundaries of curved shape in the Cartesian coordinate system fur the better results, while such curved circumferential boundary does not need to be considered in the two-dimensional cylindrical coordinate system. This motivates the present study i.e. developing a two-dimensional numerical model for the axi-symmetrically shaped casts. The SIMPLER algorithm, the VOF method, and the equivalent specific heat method have been adopted in the combined algorithm for the flow calculation, the free surface tracking, and the phase change heat transfer, respectively. The numerical model has been applied to the casting process of a pulley, and it was proven that the mesh and time effective calculation was accomplished comparing to the calculation using three-dimensional model.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.3
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• pp.245-253
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• 2008

A precise description of the material is a key point to obtain reliable results when using wave propagation codes. In the case of multipass welds, the material is very difficult to describe due to its anisotropic and heterogeneous properties. Two main advances are presented in the following. The first advance is a model which describes the anisotropy resulting from the metal solidification and thus the model reproduces an anisotropy that is correlated with the grain orientation. The model is called MINA for modelling anisotropy from Notebook of Arc welding. With this kind of material model1ing a good description of the behaviour of the wave propagation is obtained, such as beam deviation or even beam division. But another advance is also necessary to have a good amplitude prediction: a good quantification of the attenuation, particularly due to grain scattering, is also required as far as attenuation exhibits a strong anisotropic behaviour too. Measurement of attenuation is difficult to achieve in anisotropic materials. An experimental approach has been based both on the decomposition of experimental beams into plane waves angular spectra and on the propagation modelling through the anisotropic material via transmission coefficients computed in generally triclinic case. Various examples of results are showed and also some prospects to continue refining numerical simulation of wave propagation.