Jong Hoon Yoon

Solid State Diffusion Bonding of Titanium Alloys

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.

Formation Mechanism of Diffusion Welded Joint

In this study, solid state diffusion welding behavior of titanium alloys was investigated. Formation mechanism of diffusion welding process in six stages is proposed, which is based on cavitation process. The specimens were diffusion welded at elevated temperature under a hydrostatic pressure in an inert environment and the results confirmed the formation mechanism at each corresponding stage. This includes clear evidence of void closure and grain boundary migration at the final stage.

Manufacturing of Titanium Spherical and Hollow Cylinder Vessel Using Blow Forming

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.

Superplastic Characteristics and Diffusion Bonding of Ti-6Al-4V Alloy

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.

An Experimental Study on Elevated Temperature Biaxial Bulge Test

By using biaxial bulge test, it is possible to predict sheet metal forming behavior during hot forming process. The purpose of this study is to obtain materials parameters for elevated temperature forming condition during biaxial bulge test of a nickel base superalloy in hemispherical die. At constant gas pressure, the strain rate in which the metal sheet experiences varies and therefore the strain rate sensitivity can be obtained in a single loading. Biaxial bulge tests on superalloy metal sheet were performed and results are in satisfactory agreement with uniaxial test results at elevated temperature.

Manufacturing of Aerospace Parts with Diffusion Bonding Technology

The objective of this study is to manufacture aerospace components with diffusion bonding technology. Examples produced with this technology consist of thin-sheet diffusion bonding and massive diffusion bonding. The mechanism of diffusion bonding process was presented with schematic microstructure development. Aerospace parts include titanium tanks and a scaled combustion chamber with bonded steel and copper. The microstructure of bonded region shows no indication of heterogeneous phases at interface. It is shown that the diffusion bonding of aerospace materials was successfully performed to manufacture lightweight aerospace parts.

Superplastic Forming of Combustion Chamber

The major advantage of superplastic forming (SPF) technology is that it can form integral and complex components in simple operation, since it is possible to form one or more sheets of superplastic grade metal into single surface tools by relatively low gas pressure. Duplex steel contains two phases in nearly equal proportions which can suppress grain growth at a high temperature, like superplastic Ti-6Al-4V, and many duplex stainless steels with fine grained microstructures show superplastic behavior. In this study, superplastic forming technology was developed to fabricate a duplex stainless steel sheet for the outer surface of liquid propellant combustion chamber. Superplasticity of this alloy was investigated and forming methodology was analyzed and developed. The experimental results show a complex configuration of aerospace component was successfully fabricated by superpalstic forming of a sheet of duplex stainless steel.

Biaxial Gas-Pressure Forming of a Duplex Stainless Steel

By using biaxial gas-pressure forming method, it is possible to predict sheet metal forming behavior during hot forming process. The purpose of this study is to investigate the deformation behavior during equi-biaxial forming of a duplex stainless steel using conical and hemispherical dies. The present study constructs an analysis model to predict biaxial sheet forming behavior of duplex steel from results of tension tests. The results demonstrate that the developed technology to process design of high temperature gas-pressure forming by the finite element method can be applied to various types of components.

Solid State Bonding of CuCrZr to Duplex Stainless Steel

In the solid state bonding, joint are made by pressing surfaces together at high temperature so that a bond grows across the interface by atomic diffusion. In order to satisfy both requirements of thermal and mechanical properties of aerospace vehicle, conductive CuCrZr alloy was bonded to duplex steel with high strength. Solid state bonding was performed at 3 different pressure conditions and at temperatures of 850°C and 950°C. Microstructural and mechanical evaluation was performed to obtain the optimum joining condition.

Finite Element Simulation for Superplastic Blow Forming of Toroidal Ti-6Al-4V Fuel Tank

In the current study, the finite element simulation for superplastic blow forming of a toroidal Ti-6Al-4V fuel tank is discussed. 3 types of preforms are investigated in order to obtain defect free final shape with desirable thickness distribution. From the simulation result, forming tool is designed so that the hydraulic pressure is not used. The forming test is carried out using forming pressure profile obtained from the simulation, and the validity of the selected perform is investigated in terms of thickness distribution and deformed shape.