Papers by Keyword: AA2195

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: 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: The influence of larger pre-strains, than are currently used in industry, has been investigated on the ageing behaviour of the Al-Cu-Li alloy 2195 - in the context of producing near-net-shape, taper-rolled, thickness-tailored, plates for aerospace applications. FE modelling and simulations have demonstrated it is viable to stretch plates with rolled tapers of up to 1:1.6 in thickness. Increasing the pre-strain level at the thin end of the plate, up to 15%, resulted in a continued increase in microstructure refinement and yield strength, which rose to ~ 670 MPa without an unacceptable loss of ductility. It is shown that, even with such high pre-strains, a relatively low level of recovery occurs after artificial ageing and increasing the pre-strain is predicted to result in a reduction in strengthening from the T₁ phase, due to precipitate refinement, in favour of a higher contribution from strain hardening.

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: 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.

Abstract: For aerospace applications, Al-Cu-Li alloys are more attractive than conventional aluminum alloys due to their low density, high modulus and high strength. AA2195 is a third generation Al-Li alloy, developed with improved weldability. In this study, AA2195 alloy of 5mm thick sheets were welded by friction stir welding process (FSW). Tool rotational speed was varied from 400 rpm to 1000 rpm at constant travel speed of 60mm/min. Optimum tool rotation speed was identified and defect free weld coupons were processed with optimized parameter. Mechanical properties and micro structural characterization have been conducted on FSW welds.

Abstract: Aluminum-Copper-Lithium alloys are used as substitute for conventional aerospace Al alloys in cryogenic tank of liquid rocket engines, aircraft wing box and satellite systems due to their high specific modulus and specific strength. For this reason they are currently under consideration for one of the potential choices for a large structure of Korea Space Launch Vehicle. In this study, friction stir welding and electron beam welding were conducted on AA2195 sheets, in butt joint configuration in order to compare the two processes and to evaluate mechanical properties. The results provide valuable information for the optimal condition of joining AA2195 sheets for a large tankage structure of the space launcher.

Abstract: A novel technique of pure Lithium addition has been adopted for the processing of Al-Cu-Li alloy AA2195 cast ingots (7-8 kg each) in VIM under dynamic inert atmosphere, which gives more than 95% recovery of Lithium. The cast billets were homogenized, forged and converted into 12mm diameter rods by caliber rolling in the temperature range of 250°C, 300°C, 350°C and 400°C. The caliber rolled rods were treated to T8 (Solution Treatment+WQ+CW+Aging) condition. Mechanical properties were evaluated for T8 tempered bars at room temperature and correlated with microstructural observations. Highest mechanical properties in T87 temper have been obtained for rods caliber rolled at 350°C temperature.

Abstract: For 2195-T8 plate, design properties are based on the mechanical properties at the near surface location, corresponding to the load carrying thin membrane location in machined integrally stiffened structure. Mechanical properties at the near surface location are lower than those at the t/4 and t/2 locations. This work examined the effect of alternate temper and hot rolling practices on near surface strength levels. Results showed that alternate hot rolling practice were very effective in raising near surface strengths and improving property uniformity, and were well correlated with crystallographic texture measurements.