Papers by Author: Yeong Moo Yi

Abstract: The objective of this study is to fabricate lightweight structural panels and pressurized tanks by blow forming of solid-state bonded Ti-6Al-4V sheets. The solid-state bonding process was conducted in selective areas of the sheets and gas pressure was applied to the core sheet in order to form center cells. The microstructure of bonded region shows no indication of any discontinuous or heterogeneous microstructure at interface. It is shown that the blow forming of a lightweight sandwich structural panel and a sphere vessel was successfully performed with solid-state bonded multiple sheets of titanium alloy. The result of configuration and thickness distribution measurement indicate that the FEM analysis can predict the forming behavior during blow forming process of solid-state bonded sheets.

Abstract: In this paper, mould configurations are studied by finite element simulation for superplastic blow forming of combustion chamber outer jacket. One concave and two convex mould configurations are basically considered to determine which type would be advantageous in terms of forming time and thickness distribution. For the simulation, the flow stress equation of duplex stainless steel was determined from free bulging test. The simulation results showed that the concave type was quit a bit different from the other two cases. The concave mould configuration produced shorter forming time and thicker thickness distribution than the others, and it seems to be more effective for superplastic forming of outer jacket. The obtained pressure profile for the concave mould configuration was employed in actual forming and the thickness distribution was compared for the verification.

Abstract: Titanium alloys have been widely used in aeronautics and aerospace industries due to their high strength, good corrosion resistance and low density. Since many aerospace vehicle systems require high performance lightweight pressurized vessel for storage of propellant, nitrogen, oxygen, or other medium, the titanium alloy is one of the excellent candidates for this purpose. Conventionally spin forming and TIG welding process have been applied to manufacture titanium spherical vessel. In this work, an innovational method of blow forming and solid state bonding technology has been developed to save manufacturing cost and reduce weight of titanium vessel. High temperature behavior of titanium alloy was characterized and according to this result, solid state bonding process was established with demonstration of manufacturing spherical and hollow cylinder pressure vessel. The optimum condition for solid state bonding of this alloy was obtained by applying hydrostatic pressure of 4MPa at 1148K for 1 hour. For blow forming, the pressure profile was developed using MARC software and the maximum pressure of 30MPa was applied. The structural integrity of the vessel was demonstrated by performing a hydraulic pressurization test.

Abstract: It is well known that the utilization of superplastic characteristics in manufacturing process makes many of aerospace components lighter and stiffer. The weight saving is vitally important especially for aerospace application and even more weight saving is possible when the superplastic forming is carried out with diffusion bonding. In this study, the lightweight sandwich structure was fabricated with superplastic forming(SPF) process from diffusion bonded(DB) Ti-6Al-4V sheets. The solid state diffusion bonding process was conducted in non-vacuum environment under a pressure of 4MPa for 60 minutes at 875°C and the superplastic forming process was followed for another 40 minutes. Good solid state bonding interface have been observed in microstructure observation and the sandwich structure was successfully manufactured. It is important to note that the forming conditions of present study are more practical for application than the previously published conditions, which require vacuum environment, higher temperature and/or pressure.

Abstract: Composite materials are used in aerospace structures due to their considerable bending stiffness and strength-to-weight ratio. A composite sandwich is composed of a face-sheet and an aluminum core. The face-sheet of the sandwich takes the bending stress and core of sandwich takes the shear stress. A compression test and FEM analysis accomplished about composite sandwich panels that have curvature. The FEM analysis was performed using a commercial code, ANSYS and the compression test was performed until failure occurred in the sandwich panel. A strain gauge and a displacement gauge were used to acquire the data. In this paper, the failure strength and failure mode was checked. Additionally, the results of the test and analysis were compared.

Abstract: It is known that Ti-6Al-4V alloy is one of the excellent candidates for aerospace structure due to their high specific strength.However, its higher cost and low formability relative to other materials tend to limit the wide usage of the material.The purpose of this study is to characterize the superplasticity of this alloy so to obtain materials and process parameters for superplastic forming and diffusion bonding for industrial application. High temperature tensile tests was carried out at the strain rate range of 10-4 to 10-2 s-1 and temperature range of 1123°C to 1223°C. According to the results of the experiment, the optimum diffusion bonding condition was obtained at 1148°C, applying pressure of 4MPa for 1 hour in argon gas environment, which condition is more practical than expensive vacuum condition. It is shown that at the optimum condition for diffusion bonding with parent metal, the oxide film becomes unstable and the oxygen is diffused into the bulk. At this condition, the mechanical and microstructural integrity at the bonding interface was observed in a sandwich structure and a heavy block of titanium part from massive diffusion bonding process.

Abstract: Solid State diffusion bonding is obtained by applying heat, well below the melting temperature of the metals, a static pressure which does not cause a macroscopic plastic deformation in the material, and a time required to form a metallurgical bond with atomic diffusion process. This process is used for aluminum alloys, high strength steels and titanium alloys in the aerospace industry to produce complex and inaccessible joints without localized distortion. Ability to diffusion bond titanium alloys is strongly needed to promote the use of superplastic forming technology. In the present work, the solid state diffusion bonding was carried out using specimens in Ti-6Al-4V and Ti-15V-3Cr-3Sn-3Al. The microstructure of the bonded interface indicates the diffusion bonding process is successful for both alloys. It is also shown that the diffusion bonding of a superplastic Ti-6Al-4V alloy is possible at the optimum superplastic condition so that two processes can be performed simultaneously. The structural integrity of diffusion bonding was evaluated with a hydraulic test of diffusion bonded part.

Abstract: Superplastic blow forming with diffusion bonded sheet is an effective forming technology for the production of multi-cell structures which should have light weight and high stiffness for aerospace purpose. In the current study, finite element analysis on superplastic blow forming process has been carried out in order to improve the forming process when manufacturing axi-symmetric multi-cell structures using diffusion bonded Ti-6Al-4V multi-sheets. The simulation focused on the reduction of forming time and obtaining finally required shape throughout investigating the deformation mode of sheet according to the forming conditions, which are diffusion bonding pattern and die geometry. To reduce forming time, a preforming die was required, and to obtain the final shape the bonding pattern should be also modified within allowable geometrical margin, so that the sheet is easy to deform. Moreover, an intermediate simulation result, which was forming pressure profile, was employed in real forming test to check if the prediction was reasonably on progress. In the future, a study on the thickness ratio between each sheet should be followed to obtain optimum process parameters.

Abstract: The surface oxidation behavior was investigated over a range of solid state bonding condition of the Ti-6Al-4V ELI alloy. Since the oxides at the bonding interface may prevent the materials from complete bonding, it is important to understand the oxidation behavior at solid state bonding condition. The activation energy of oxidation of Ti-6Al-4V ELI is estimated to be 318 KJ/mol in an environment of solid state bonding process. For Ti-6Al-4V ELI alloy, strucutral integrity of bonding interface without oxides have been obtained at 850°C applying pressure of 3MPa for 1 hour. Solid state diffusion bonding of Ti-15V-3Cr-3Sn-3Al alloy was also obtained under a pressure of 6MPa for 3 hours at 925°C.

Abstract: Postbuckling failure behaviors and strength were investigated in compression tests of the hat stiffened composite panels manufactured with different bonding methods and different stiffener section shapes. The skin-stiffener separation loads, separation failure behaviors and final collapse loads were different with respect to bonding methods and stiffener shapes. As the separation failure was initiated early and propagated larger area, collapse loads and structural performance of the panels decreased. The test results showed that the co-curing with or without adhesive film and open type stiffeners were most beneficial for the postbuckling strength.