Author : Jannik Entringer
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (115 download)
Book Synopsis Microstructure Evolution and Stress Corrosion Cracking Behavior in Short-term Thermomechanically Processed Al-Cu-Li Alloys by : Jannik Entringer
Download or read book Microstructure Evolution and Stress Corrosion Cracking Behavior in Short-term Thermomechanically Processed Al-Cu-Li Alloys written by Jannik Entringer and published by . This book was released on 2019 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Increasing access to space leaves human footprints in orbits closer to the Earth. Today, space debris poses a major risk for in-service missions and on-ground casualties. Reducing the amount of debris is the only way to preserve key orbits. One strategy to avoid future debris is to maneuver spacecraft after mission into the atmosphere where the structure burns up. This strategy is taken into account at early design stages of modern spacecraft. Therefore, material choices must be made to ensure high structural demisability. Titanium, currently used in propellant tanks, is often a large portion, by mass, of the total structure and does not completely ablate during re-entry. Aluminum-copper-lithium alloys are being considered as a substitute for titanium in propellant tanks because they deliver comparable specific material properties at a higher demisability rate. Because the technology of aluminum-copper-lithium alloys in space structures is not as mature as that of titanium, new joining methods must be developed. Friction stir welding features the unique characteristic of joining in the solid state; thus, it is especially attractive for hard-to-weld alloys such as aluminum-copper-lithium. The development of space structures requires a fundamental knowledge of the material behavior, both during processing and during the subsequent exposure to the environment. In recent years, aluminum copper-lithium alloys have rarely been welded by bobbin tool friction stir welding. The underlying microstructure evolution has been described as complex, certain aspects are not fully understood and contradictory results have been reported. Current knowledge on the behavior of welded joints on this specific alloy under stress and exposed to corrosive environments is limited. Therefore, for the possible use in spacecraft structures, a scientific analysis of the stress corrosion behavior of welded joints is necessary. As part of the present work, two modern aluminum-copper lithium alloys were successfully joined by semi-stationary bobbin tool friction stir welding. Identical parameters allowed a detailed comparison of the process response. The welding process imposes a short time, thermomechanical exposure in the base material that leads to severe microstructure modification. Based on the chemical composition of the two alloys, the microstructural evolution is explained, and a precipitation sequence is proposed. Overaged strengthening precipitates and equilibrium phases of several types were identified to form during welding. These modifications led to a reduced mechanical performance of 78 % of the ultimate tensile strength. Stress corrosion analyses were performed on pre-, as- and post-welded samples and were correlated with the modified microstructure. Stress corrosion cracking phenomena were found to result from the short-time, thermomechanical effect induced by the welding process. The mechanism leading to stress corrosion lies in the modified microstructure, where coarse, precipitates accumulate at the grain boundaries. These particles are observed to promote local galvanic reactions, which promote dissolution and the consequent development of a cracking network under stress.
