Kyung Ju Min

Development of Diffusion Welding Process for High Temperature Materials

Aerospace vehicle requires structures to withstand intense aerodynamic heating and propulsion system to provide very high effective heat exchange. In order to satisfy both requirements of effective thermal exchange and mechanical properties, high temperature materials, or sometimes dissimilar metals with different properties must be joined together. In this study, it is demonstrated that diffusion welding process can be applied to manufacturing lightweight integral structures of aerospace structural parts with titanium and stainless steel. The results show that the technology to design and develop the diffusion welding process of high temperature metals can be applied for manufacturing of aerospace components for high temperature application.

Composite Materials Characterization for Aircraft Application

Various composite materials have been developed for many of aircrafts feature and it is important to establish the composite materials characterization procedure based on the airworthiness requirement. Since properties of composites are a function of the properties of the constituent phases, their relative amounts, and processing methods, the overall processing must be carefully evaluated. This paper presents a composite materials evaluation procedure for composite aircraft and provide the effective statistical allowable to be approved by airworthiness certification agency. All material, specimens, fixtures and test results contained within this study were traceable and approved by the agency. In this study 8 different mechanical properties are obtained for 4 environmental conditions from 5 batches for robust sampling. By using modified coefficient of variation, the B-basis value of 0°tensile strength of carbon/epoxy unidirectional composite for elevated temperature/wet condition increases from 2,291MPa to 2,331MPa. The result would be applied to qualify domestic composite materials for aircraft within the level of a global standard.

Microstructure and Mechanical Properties of Friction Stir Welded AA2195-T0

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.

Influence of Tool Rotational Speed on the Mechanical Properties of Friction Stir Welded Al-Cu-Li Alloy

Aluminum-Lithium alloys have been found to exhibit superior mechanical properties as compared to the conventional aerospace aluminum alloys in terms of high strength, high modulus, low density, good corrosion resistance and fracture toughness at cryogenic temperatures. Even though they do not form low-melting eutectics during fusion welding, there are still problems like porosity, solidification cracking, and loss of lithium. This is why solid state friction stir welding is important in this alloy. It is known that using Al-Cu-Li alloy and friction stir welding to super lightweight external tank for space shuttle, significant weight reduction has been achieved. The objective of this paper is to investigate the effect of friction stir tool rotation speed on mechanical and microstructural properties of Al-Cu-Li alloy. The plates were joined with friction stir welding process using different tool rotation speeds (300-800 rpm) and welding speeds (120-420 mm/min), which are the two prime welding parameters in this process.

A Study on Precipitation Kinetics of an Al-Cu-Li Alloy

Aluminum-Copper-Lithium alloys are of strategic important [1] in lightweight aerospace structures due to their high specific modulus and specific strength and are under consideration for one of the potential choices for Korea Space Launch Vehicle structure. However, this is one of the most complicated alloys due to the precipitation of a large number of second phase particles. In this study, the precipitation kinetics of an Al-4.3%Cu-1.2%Li alloy was studied by differential scanning calorimetry (DSC) technique since precipitation plays an important role for hardening of this alloy. Several models including Kissinger’s, Starink’s and Chaturvedi’s were used to obtain the activation energy and precipitation kinetics.