Airbus Defence and Space (ADS), together with TWI Ltd, NLR, ALCOA and Constellium, has undertaken to investigate the possibility of applying the friction stir welding (FSW) process to third-generation aluminium–lithium (Al–Li) alloys (eg AA2050, AA2060, AA2099, AA2195 and AA2055) for the manufacture of demisable spacecraft propellant tanks.
Why Al–Li?
Spacecraft propellant tanks are commonly made from titanium and its alloys which, due to the high melting point of these materials (1604–1660°C), are not demisable. Demisability is a key aspect of current clean space initiatives and constitutes an important part of space debris mitigation strategies.
Due to their lower melting point and good strength-to-density and stiffness-to-density ratios, Al–Li alloys could replace titanium alloys for demisable propellant tank manufacture while providing an overall cost saving.
Why FSW?
Friction stir welding has been proven to be highly efficient in the welding of aluminium alloys. Common flaws associated with fusion welding of aluminium alloys such as porosity and solidification cracking are avoided. As FSW is a solid-state process the level of distortion and residual stress is lower than in fusion welding.
Project summary
During the initial phases of the project ADS and the project partners undertook a comprehensive literature review to identify potential Al–Li alloy candidates and carried out a trade-off study on the Al–Li alloy candidates against several welding processes, in order to select the particular Al–Li alloy(s) for FSW in the project.
Following completion of the materials trade-off study, AA2060 alloy in the T3 and the T8 conditions was procured for welding trials. It is worth mentioning that the selection of this specific alloy was driven by its availability, and the results in terms of preliminary weld development will not be too sensitive with respect to the specific Al–Li alloy. This is an important point because, as of today, the investigation on Al–Li alloy as replacement of titanium alloy for the use on propellant tank applications is not mature enough to perform a definitive selection.
It was agreed that electron beam welding (EBW) trials would also be undertaken with the AA2060 T3 and T8 alloy in order to provide a comparison of metallurgical and mechanical properties achieved using FSW.
Friction stir welding trials
Friction stir welding trials were undertaken by TWI on AA2060 T3 and T8 alloy and the metallurgical and mechanical properties of the weldments assessed. Welding parameter optimisation resulted in high weld efficiency relative to the parent material ultimate tensile strength (UTS) of up to 97% in T3 condition and 92% in T8. No flaws were detected in the welds
Electron beam welding trials
TWI carried out a series of EBW trials on AA2060 T3 and T8 alloy. The trials achieved weld efficiencies of up to 82% in T3 condition relative to the parent material UTS. T8 melt runs were found to be 75% efficient relative to parent material UTS. Non-destructive testing and metallographic examination detected no weld flaws.
Conclusions
Friction stir welding and EBW trials on AA2060 alloy have achieved welding efficiency in excess of 91% and 84% respectively, exceeding typical values achieved with these techniques. Inspection of welds did not reveal evidence of flaws.
Friction stir welding and EBW are both suitable for the closure of propellant tanks, although there are some challenges in using FSW, especially in relation to the closure of the tanks. This, however, may be addressed by utilising advanced tooling and fixture designs.
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