Advanced Materials Research Vols. 26-28

Abstract: Alloy 600 is used as a material for a steam generator tubing in pressurized water reactors(PWR) due to its high corrosion resistance under a PWR environment. In spite of its corrosion resistance, a stress corrosion cracking(SCC) has occurred on the primary side as well as the secondary side of a tubing. It is known that a SCC is related to the electrochemical behaviors of an anodic dissolution and a passivation of a bare surface of metals and alloys. Therefore in the present work, the passive oxide films on Alloy 600 have been investigated as a function of the solution temperature by using a potentiodynamic polarization, electrochemical impedance spectroscopy and a TEM, equipped with EDS. Moreover the semiconductive property was evaluated by using the Mott-Schottky relation. It was found that the passivity depends on the chemical composition and the densification of the oxide film rather than the oxide thickness. As the solution temperature of 0.5M H3BO3 increased, the thickness of the passive film increased but the oxide resistance of the passive film was decreased, indicating that the measured current in the passive region of the potentiodynamic curve is closely related to the stability of the passive film rather than the oxide thickness. It was found that the oxide films were composed of an outer oxide layer with a lower resistance and an inner oxide layer with a relatively higher resistance. From the Mott-Schottky relation, the oxide formed at 300oC showed a p-type semiconductor property unlike the n-type oxide films up to 250oC.

Abstract: Vibration in a driveline is presented in this paper. In the experiment, the rear subframe and propeller shafts and axle were composed and mounted with rubber each other. For applying the vibration input instead of the torsional vibration effect of an engine, the shaker was taken. In particular, torsional vibration due to fluctuating forced vibration excitation across the joint between driveline and rear subframe was carefully examined. Accordingly, the joint response was checked from experiments and the FE-simulation using FRF (frequency response function) analysis was performed. All test results were signal processed and validated against numerical simulations. In present study, the new test bench for measuring the vibration signal and simulating the vehicle chassis system was proposed. The modal value and the mode shape of components were analyzed using the model to identify the important components affecting driveline noise and vibration. It could be reached that the simplified test bench could be well established and be used for design guide and development of the vehicle chassis components.

Abstract: A mathematical model for three-dimensional simulation of unidirectional solidification process and microstructure evolution of Ni-based superalloy investment castings was developed based on CA-FD method. The modified ray tracing method was used to solve the complicated heat radiation transfer among the multiple blades and outer space during withdrawal process. Various withdrawal rates were used. During one process high withdrawal rate was used first before the platform approached the baffle. Then the low withdrawal rate was used to reduce the temperature difference of the platform in horizontal section and avoid the defects formed in the corner of the platform. The experimental cooling curves of different positions in the blades and microstructure were compared with the simulation results. Both the results showed that the various withdrawal rates process was effective to reduce the temperature difference of the platform and avoid the formation of stray grains. This process could be helpful to increase the productivity.

Abstract: This paper presents our experimentation and modeling efforts to study the pattern of material flow in the extrusion and deposition stages of the Contour Crafting (CC) process. Specifically, we performed a preliminary finite element analysis (FEA) of extrusion and deposition mechanisms with clay as the fabrication material. Using the FEA simulations, we derived certain basic understandings of the effect of extrusion orifice geometry on the performance of CC. We found that a square orifice is most aptly suited, both in terms of delivering the optimal fusion between layers as well as creating the desired external surface profile. Our experiments validate these results.

Abstract: A modified cellular automaton model was proposed to simulate the dendrite growth of alloy. Different from previous models, this model used neither an analytical equation(such as KGT model) nor an interface solute gradient equation to solve the velocity of solid-liquid interface, but used the interface solute and energy conservation and thermodynamic equilibrium condition to describe the solid/liquid interface growth kinetics process. In present model, once the temperature field and solute field were solved by finite different method in the entire domain, the material thermodynamic properties can be substituted into four algebraic equations to easily determine the variation of solid fraction, interface temperature and solute concentration, instead of calculating interface moving velocity. As a result, the complexity of the calculation can be largely reduced. The simulated dendrite growth was in a good agreement with the Lipton–Glicksman–Kurz (LGK) model for free dendritic growth in undercooled melts.

Abstract: The assembled tube was made of beryllium tube and tungsten tube, and was produced by over fitting. The residual stresses in the assembled tube were very comprehensive after lasers braze fusion welding, not only including the welding residual stresses, but also the initial stresses from over fitting. Both of the dimension and strength were affected by the complex residual stress distribution in the assembled tube. MSC.MARC software was used to simulate the stress fields during lasers braze fusion welding of the assembled tube. The stress fields during lasers braze fusion welding and the residual stresses after welding have been obtained, and the effect of over fitting on welding residual stress of the assembled tube have also been studied. The residual stresses at outer surface of the assembled tube have been measured by the X-ray diffraction method. A comparison of the residual stresses by finite element method (FEM) to that by experiment indicates an identical stress change trend and thus validates the FEM model.

Abstract: The effect of molybdenum substitution in electronic structures of manganese dioxide has been calculated by the DV-Xα molecular orbital method. The molybdenum atom substituted for manganese atom located in center-sited of (Mn15O56)-52 cluster model. By the molybdenum substitution the energy band gap between the highest occupied molecular orbital (HOMO) of valence band and the lowest unoccupied molecular orbital (LUMO) was decreased, also the interaction of Mn-3d and O-2p was weakened but that of Mo-4d and O-2p was intensified. Both bonds of (Mn15O56)-52 and (Mn14MoO56)-52 was composed of not only ionic but also partially covalent.

Abstract: The solidification behavior of AZ31 magnesium alloy plate during horizontal continuous casting (HCC) with constant velocity was analyzed by three-dimensional CFD simulation. At steady state, the temperature profile in the direction of width was slightly inhomogeneous due to wide width compared to thickness. The temperature profile in the direction of thickness was symmetrical although the temperature at lower part was slightly higher than that at upper part. The region of high temperature increased rapidly with increasing the withdrawal speed, and then the finishing point of solidification increased dramatically. The shape of temperature profile was similar regardless of melt temperature but the overall temperature of plate and the finishing point of solidification increased with increasing the melt temperature. The finishing point of solidification decreased reciprocally with the coefficient of heat transfer.

Abstract: The welding process, a conventional production method of gate valves, has advantages such as light weight, but it also has disadvantages such high production price. However, the forging process, has economic merits and mass production capability. The main focus of this paper is the optimization of the preform in the forging process. This paper proposes an optimal design of the preform to improve the mechanical efficiency of the gate valve made by the forging method. The design of the preform is optimized by the use of the real response model to the Kriging model by computer simulation. Also, the optimized results were used to verify the response model. The verified response model confirmed the usefulness and reliability of the Kriging method in optimum structural design of preform achieved by finite element analysis and Kriging equations.

Abstract: Flame bending has been widely used for forming hull plates of marine vessels, Since it is difficult to estimated the amount of residual deformation after applying the flame heat, flame bending has been carried out by skilled workers without automatic facility. A mechanical bending such as multiple-piston pressing forming has been studied as an alternative to the flame bending. However, springback is a major problem in the press forming process. Present paper presents a combination of FEM (Finite Element Method) and springback compensation algorithm to calculate the compensated stroke of pressing points. In order to calculate the springback, the process is modeled by an elastic plastic material and shell elements. Combination of global scale factor and local scale factors is suggested to adjust the amount of strokes through an iterated numerical calculation. In each iteration, shape deviation between object surface and processed shape is minimized to reach the designed shape. The shape deviations due to springback are compensated using the residual shape estimated by FEM.