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Pohang, South Korea

Ning X.-J.,Beijing Institute of Technology | Wang Q.-S.,Beijing Institute of Technology | Ma Z.,Beijing Institute of Technology | Kim H.-J.,Welding Research Center
Journal of Thermal Spray Technology | Year: 2010

A 2-D model of the low-pressure cold spray with a radial powder feeding was established using CFD software in this study. The flow field was simulated for both propellant gases of nitrogen and helium. To predict the in-flight particle velocity and temperature, discrete phase model was introduced to simulate the interaction of particle and the supersonic gas jet. The experimental velocity of copper powder with different sizes was used to validate the calculated one for low-pressure cold spray process. The results show that the computational model can provide a satisfactory prediction of the supersonic gas flow, which is consistent with the experimental Schlieren photos. It was found that similar velocity was obtained with the drag coefficient formula of Henderson and with that of Morsi and Alexander. As the shape factor was estimated, the reasonable prediction of velocity for non-spherical particle can be obtained, to compare with the experimental results. © 2010 ASM International.

Lee M.-Y.,Welding Research Center
29th International Congress on Applications of Lasers and Electro-Optics, ICALEO 2010 - Congress Proceedings | Year: 2010

Bicycle is historical device for transportation. Also it is environmental friendly and good for a health. The higher the strength is the better for material of bicycle, because the weight of it is lower and lower. Magnesium alloy have higher potential for bicycle cause its high strength but low density. But magnesium alloy have poor formability at room temperature and hardly welded with conventional welding processes. The laser welding was effective way to joint magnesium alloys because it had high weld strength and productivity compare with other welding processes. In this study, the punch and die set was designed and machined according to the results of FEM simulation. The forming of magnesium sheet with tubular shape was done at elevated temperature. The seam butted both end face of magnesium sheet was welded with laser. The tensile strength of magnesium tube prepared on this study was higher than conventional extrusion tube. Consequently, the bicycle frame was assembled successfully with magnesium alloy tube.

Kim J.-D.,Korea University | Lee S.-J.,Korea University | Park S.-J.,Welding Research Center | Lee J.-H.,Korea Institute of Machinery and Materials
Metals and Materials International | Year: 2011

Ceramics have attractive properties to metals and polymers, and they are consequently useful for specific applications. Silicon nitride has been studied extensively and has been found to possess promising thermal and mechanical properties at high temperatures. However, it has drawbacks such as brittleness and large scatter in its mechanical properties. As a result, it is difficult to fabricate complex shapes using traditional methods because of high cost and difficulties to machine the components. Today, ceramic parts have limited use for production of simple shaped parts and low quantities. This study explores the possibility of employing silicon nitride for more diverse applications using laser-assisted machining (LAM). Because the surface of the workpiece is locally heated by an intense laser source prior to material removal, softening and damage of the workpiece surface make machining of ceramics easy. Among the parameters of LAM, laser power is one of the decisive factors during the process. In this study, fractured cross sections were observed to examine how the microstructure of silicon nitride was changed by laser power. The deformation of microstructure near the surface of the workpiece increases when the laser power increases. It is found that increasing the laser power facilitates cutting of silicon nitride but results in detrimental heat effects on the surface of the workpiece. © KIM and Springer.

Lee K.-A.,Andong National University | Kwon S.-H.,Welding Research Center | Kim H.-J.,Welding Research Center
Materials Transactions | Year: 2010

Copper coating was manufactured by super-sonic flow deposition method and especially two kinds of feedstock powders with different powder particle size distribution (A: 9-53 μm, B: 4-23 μn) and grain characteristic in the powder (A: fine and inhomogeneous, B: coarse) were used for the deposition. It was found that the use of small particle distributed feedstock powder B could decrease the surface roughness and porosity of the coating layer. After annealing, the micro-hardness of the super-sonic flow deposited copper coating decreased significantly with increasing annealing temperature. The abrupt decrease in hardness exactly corresponded to the starting annealing temperature of primary recrystallization. It was also suggested that the hardness of Cu coating layer mainly attributed to grain size, shape and the bonding of particles but not a change in the porosity of the coating layer. The electrical conductivity and thermal conductivity of powder B coating were higher than those of powder A. The superiority of electrical and thermal conductivities of B coating layer could be related not only to the large size and homogeneity of grains, and strong bonding of particle-particle interface but also to a decrease of porosity content, caused by small size and homogeneous distribution of powder B. © 2010 The Japan Institute of Metals.

Cho D.W.,KAIST | Na S.J.,KAIST | Cho M.H.,POSCO | Lee M.Y.,Welding Research Center
Proc. of the Int. Conf. on Advances in Welding Science and Technology for Construction, Energy and Transportation, AWST 2010, held in Conj. with the 63rd Annual Assembly of IIW 2010 | Year: 2010

Previous simulations of Gas Metal Arc (GMA) Vgroove welding assumed that the shape of the arc is axissymmetric, which is not always correct. In this study, however, the elliptic arc heat flux model was adopted to conduct 3D numerical simulations by applying Abel inversion to the arc images captured with a CCD camera. Heat transfer and fluid flow in the weld pool were analyzed to examine the temperature distribution, flow velocity fields and the solidified weld bead geometry during GMA V-groove welding of pipes. The model solved the equations of conservation of mass, momentum and energy with the VOF (Volume of Fluid) method which was adopted to calculate the shape of free surfaces. For various welding positions, different molten pool flows and bead shapes caused mainly by the different gravity force directions in the numerical simulations were compared with data from experiments, and showed a good agreement.

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