Papers by Author: Jong Hoon Yoon

Abstract: This study presents manufacturing lightweight aerospace components by solid state joining technologies. The advantages of solid state joining are due to the lack of hot cracking from solidification, since there is no liquid phase involved in joining process. This produces a high quality joint as compared to that from conventional fusion welding process. In diffusion bonding process, two different surfaces are matched together at elevated temperature under a low pressure without macroscopic plastic deformation in the interface. In friction stir welding process, the rotating shoulder of the tool generates frictional heat on the surface. As the pin rotates it forces the plastic material to mix mechanically in the vicinity of the pin and produces a heavily deformed microstructure around the pin. In this study, solid state joining processes of diffusion bonding and friction welding, are applied to manufacture several launcher components with lightweight, efficient and cost saving.

Abstract: This study presents manufacturing cryogenic tanks for aerospace applications. Since most high strength aerospace alloys like titanium alloys and Al-Li alloys exhibit low formability due to low ductility and work hardening, superplastic forming technology is applied to manufacture hemispherical shapes. Superplasticity is the ability of materials to deform plastically to show very large amount of strains. Advantages of superplastic forming technology include its design flexibility, low tooling cost and short leading time to produce. In this study, various manufacturing processes, like superplastic forming, diffusion bonding, laser beam welding and friction stir welding, are applied to manufacture titanium and aluminum cryogenic tanks. Using these technologies in manufacturing process makes the aerospace components lighter and stiffer, with efficient energy and cost saving.

Abstract: It is known that Al-Li alloys show high specific strength and have been used in space vehicles with Friction stir welding (FSW). FSW has many advantages including the absence of porosity, low distortion and reduced residual stresses which are typical defects of the fusion welding processes. The process uses a rotating tool with a profiled pin that penetrates the parts to be welded. The tool starts to travel along the welding line and the softened material due to the frictional heat is stirred and mechanically mixed together by the rotating pin forming a weld in solid state without melting. Welding parameters such as tool rotational speed, travelling speed, and tool geometry are the main parameters which affect the material flow and the heat generation rate. The important tool geometry includes pin size and shape, pin tread and pitch, tool materials, and shoulder size and shape. The present work is to study the effect of tool geometry on the microstructure and mechanical properties of friction stir welded aluminum alloy 2195. Five different tool profiles have been used to investigate the effects of tool geometry on mechanical properties. The experimental results show that aluminum alloy 2195-T8 can be welded using FSW process with maximum welding efficiency of 75% using threaded cylindrical with concave shoulder at rotation speed, 600 RPM and welding speed, 300 mm/min.

Abstract: Since solid state welding is formed from an intimate contact between two metals at temperatures below the melting point of the base materials, the structural integrity of welded zone is maintained without presence of foreign materials or temporary liquid phase. This paper provides some of examples for solid state joining of aerospace materials. Diffusion bonding process was developed for a titanium alloy for lightweight sandwich panels. Diffusion bonding of copper and stainless steel was also demonstrated to manufacture a combustion chamber. HIP (Hot Isostatic Press) bonding and friction stir welding process of aluminum alloy was developed in order to study possible application for a large launcher fuel tank. It is shown that solid state joining processes can be successfully applied to various aerospace materials and provide innovative solution for lightweight structures.

Abstract: The object of this study was to study mechanical properties of friction stir welded joints of AA2219 and AA2195. AA2219 has been used as an aerospace materials for many years primarily due to its high weldability and high specific strength in addition to the excellent cryogenic property so to be successfully used for manufacturing of cryogenic fuel tank for space launcher. Relatively new Aluminum-Lithium alloy, AA2195 provides significant saving on weight and manufacturing cost with application of friction stir welding. Friction stir welding is a solid-state joining process, which use a spinning tool to produce frictional heat in the work piece. To investigate the effect of the rotation direction of the tool, the joining was performed by switching the positions of the two dissimilar alloys. The welding parameters include the travelling speed, rotation speed and rotation direction of the tool, and the experiment was conducted under the condition that the travelling speed of the tool was 120-300 mm/min and the rotation speed of the tool was 400-800 rpm. Tensile tests were conducted to study the strength of friction stir welded joints and microhardness were measured with microstructural analysis. The results indicate the failure occurred in the relatively weaker TMAZ/HAZ interface of AA2219. The optimum process condition was obtained at the rotation speed of 600-800 rpm and the travelling speed of 180-240 mm/min.

Abstract: It is well known that the addition of Li to aluminum alloys offers an attractive combination of low density and high modulus, which are useful for lightweight structures of aerospace vehicles. However, microstructure of Al-Li alloys are complex, which consist of a number of equilibrium and metastable phases. In addition, Al-Li alloys are weldable but the weldability is not as good as that of other aerospace alloys. This is due to the reactive property of element Li during melting and causes porosity, cracking and low joint efficiency. In friction stir welding (FSW), rotating welding tool generates frictional heat and by keeping the tool rotating and moving speed, the heat from friction causes the plate to soften without melting. Therefore, this solid state welding is adequate to Al-Li alloys. The friction stir welded joint was divided into 9 regions and each microstructure was investigated in detail to present the microstructure evolution and material flows during friction stir welding process. The recrystallized structure is observed in nugget zone and the evidence of initiation of dynamic recrystallization is found around the boundary between thermo-mechanically affected zone (TMAZ) and nugget region. This paper describes the results of a study to investigate the microstructure change of Al-Cu-Li alloy during the friction stir welding process.

Abstract: Aluminum-copper-lithium alloy is a light weight metal that has been used as substitute for conventional aerospace aluminum alloys. With addition of Li element, it has lower density but higher strength. However these aluminum alloys are hard to weld by conventional fusion welding, since they often produce porosities and cracking in the weld zone. It is known that solid state welding like friction stir welding is appropriate for joining of this alloy. In this study, friction stir welding was performed on AA2195 sheets, in butt joint configuration in order to understand effects of process parameters on microstructure and mechanical properties in the weld zone. The results include the microstructural change after friction stir welding with electron microscopic analysis of precipitates.

Abstract: It is well known that the significant weight reduction and increased strength have placed advanced aluminum-lithium alloys at the forefront of aerospace materials research. For example the use of aluminum-lithium based alloys for rocket fuel tank domes can reduce weight because aluminum-lithium alloys have lower density and higher strength than Al-Cu alloy 2219. However, Al-Li alloys have been shown the inherent low formability characteristic that make them susceptible to cracking during the spinning operations. In this study a novel heat treatment process on the formability of friction stir welded Al-Li alloy blanks are presented. It is shown that the successful heat treatment process has been developed with superior mechanical properties and currently the patent is applied.

Abstract: Alloy IN718 is a nickel based precipitation hardened material and it has the necessary strength over a range of temperatures which includes the operating range for many rocket propulsion systems and jet engines. This performance is accomplished by a combination of solid-solution strengthening, precipitation strengthening and grain-boundary strengthening. However, it is common for precipitation hardened nickel based superalloys to have a problem of post-weld cracking. In this study, several welding processes are investigated to obtain the optimum welding method of IN718 for elevated temperature forming. These include LBW(Laser Beam Welding), EBW(Electron Beam Welding), HIP(hot isostatic pressing), and solid state diffusion bonding. The result shows that the LBW specimen performs the highest formability at 980°C so that this process can be applied to superplastic forming of IN718 sheet. It is demonstrated that the elevated temperature superplastic forming of nozzle extension with internal cooling channels was possible with laser beam welded IN718 sheet.

Abstract: Many metals, such as titanium and superalloys, are used for a wide range of aerospace applications, which include aircraft gas turbine engine and space launcher propulsion engine. In order to manufacture a stiffened extension with superplastic blow forming at elevated temperature, the structural integrity of the joint part was investigated since the welded or bonded joint of internal channels should maintain its strength during superplastic blow forming process. Various types of joint methods were performed in order to investigate microstructural and mechanical properties of the bonded specimen at elevated temperature. In this paper, the possibility of manufacturing combustion chamber and other aerospace components with superplastic blow forming of titanium and superalloy was demonstrated. An innovative manufacturing method to produce complex configuration from titanium multi-sheets by superplastic forming with low hydrostatic pressure was presented. The result also shows that the manufacturing method with superplastic blow forming of multi-sheets of IN718 alloy has been successfully demonstrated for near net shape forming of subscale nozzle extension cone with internal channels.