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Microstructure evolution and stress corrosion cracking behavior in short-term thermomechanically processed Al-Cu-Li alloys
Citation Link: https://doi.org/10.15480/882.2390
Publikationstyp
Doctoral Thesis
Date Issued
2019-08
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2019-08-13
TORE-DOI
TORE-URI
Citation
Technische Universität Hamburg (2019)
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.
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.
Subjects
Spannungsrisscorrosion
Aluminium Legierung
Reibrührschweißen
Mikrostruktur
Phasenumwandlung
DDC Class
530: Physik
600: Technik
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