This project aimed at developing an aluminium alloy, primarily for fuselage skin application, that offers a step change in terms of corrosion resistance versus conventional aluminium alloys used for this application.
As the overall purpose of Clean Sky is to offer more eco-friendly solutions, the proposed alloy must also offer properties that allow some weight saving versus the incumbent alloys. This is achieved through mechanical properties, which determine the component thickness, and through density reduction.
The option proposed was to develop an AlMgLi alloy that inherently offers a 10% weight saving by density versus conventional alloys. Further improvements versus the currently flying aircraft can be achieved by design optimization.
Alcan has developed this type of alloy to a Technology Readiness Level 3. Following lab trials, two campaigns at industrial scale were already performed, that demonstrate the capabilities of this alloy type to fulfil the above requirements.
To bring the alloy to TRL6, the proposition was first to roll and heat treat an existing slab to determine the properties according to the precise requirements of the airframers issuing the call. Corrosion tests relevant to the application, with and without protection, fatigue before and after corrosion protection, and large flat panels damage tolerance tests were performed.
A second step consisted of casting, converting and characterising a slightly modified composition, to be determined according to the results obtained in the first step. Minor elements could be tuned in order to obtain the optimum microstructure. In that second step, properties pertaining to the fabrication of components were assessed in addition to the material properties as described above.
Capitalising on Alcan’s experience in airframe technology, the next step was to support the build and testing of a demonstrator.
The project aimed at starting in September 2011. This date had been fixed in agreement with the topic manager due to project planning and casting facility availability (scheduled month n°11).
Aircraft manufacturers were looking for new materials offering lower operating and maintenance costs. For metallic aerostructures, weight reduction solutions and improvement of corrosion resistance are two strategic orientations. Al-Mg-Li type alloys were good candidates to meet these requirements as they are weldable (avoiding riveted structures that constitute corrosion initiation sites), present a low density and intrinsic good performance in corrosion resistance, in particular compared to 2024 alloy. The three years project “ECEFA” permitted to prove the interest of using such type of material for environmental benefit:
- During operating phases :
Light weighting estimated at least at 7.9 % in direct relation with the density reduction offered by AlMgLi.
- Out of operating phases :
Corrosion resistance of AlMgLi alloy studied was significantly improved compared to incumbent 2024 T351. This improvement had permitted to allow the use of green treatment (CrVI free) with the same or better performance as existing CrVI protections. The development of more resistant alloys to corrosion like AlMgLi offers then the possibility to introduce green surface treatment more easily and should permit to fulfil the REACH compliance. Moreover, the better fatigue propagation compared to 2024 T351 also offered an opportunity for increasing inspection time interval.
These achievements fulfilled the expected outcome of Clean Sky program to offer more eco-friendly solutions: the product should offer, through design adaptation, an excellent corrosion resistant product with the possibility to use green surface treatment and permitting a weight loss and a lower maintenance cost. However, some investigations were still needed to industrialise the product. AlMgLi alloy in comparison with 2024 exhibited a better performance in terms of corrosion and fatigue propagation, similar performance on tensile properties, fatigue initiation with however some limitations with regards to toughness (lower value in T-L) and impact of surface treatment on fatigue.