The material distribution on an Airbus aircraft structure predominantly remains on aluminum based alloys. The example on the A380 super sized aircraft shows that 61% of the structure is made of aluminum alloys, 22% in composites, 10% in titanium and steel and 3% in fiber metal laminate. Nevertheless, the use of composite materials is continuously growing and the new A380 contains 22% of composite structures compared to 12% on the A340.
The competition between metals and composites for the aircraft structures is open for the future and the target compared to a standard metal technology (baseline 1990) is 20 to 30% in terms of weight and 20 to 40% in terms of cost. This competition is managed by a step-by-step approach regarding metal or composite design.
The large size of the A380 aircraft, the corresponding loads and the targeted structural weight led to significantly higher requirements for alloy properties. This meant that improvements had to be made in the two major design axes, static performance and/or damage tolerance. To achieve these goals, the Alcan-Airbus Integrated Project Teams worked to both extend and qualify existing alloys, and to develop new dedicated alloys.
On April 27th 2005, the giant Airbus A380 took off from Toulouse airport for a successful first flight. Seven years earlier, in April 1998, discussions had started between Alcan Aerospace (at that time Pechiney Aerospace) and Airbus for the development of advanced alloys and innovative solutions for the A3xx, as it was then designated. Seven years were needed to develop, qualify and produce a full set of new alloys for wing and fuselage structures, as well as the equipment to fabricate such large structures.
When the first meetings were held with Airbus in April 1998, participants agreed to form integrated product teams (IPT) who would select and develop appropriate alloys and fabrication methods for the various structures of the giant aircraft. Two main objectives were assigned to these IPT teams:
- Structures: The objective for Alcan was to provide all the A380 metallic parts. This goal required investing in equipment that could fabricate large structures compatible with the large dimensions of the aircraft, particularly for the wings. The objective for Airbus was to define maximum part dimensions as a function of what Alcan was able to propose.
- Alloys: The objective for Alcan was to follow as closely as possible the need for new alloys as well as the need to extend availability of existing alloys (particularly in the high gauge ranges). The objective for Airbus was to follow the alloy development activities, and adapt when needed their design allowable to Alcan capabilities.
Most of the work in the Issuer plant was dedicated to extending the maximum length to the required 36 meters. This included the following, along with the processing route:
- Developing hardware and know-how to cast very large ingots in advanced alloys needed for the volume of metal related to the upper and lower inner wing panels, as well as to the upper integrally machined outer wing panel.
- Installing or upgrading the cast-house handling equipment, because larger ingots meant heavier weights.
- Lengthening all necessary equipment in the plate department. The hot-mill table, heat-solution furnaces, the stretcher, the U.S. inspection tank, and the contouring machine are examples of major equipment that was upgraded.
- Installing or upgrading the plate handling equipment. Safe handling of 36-meter plates required other means than those previously available for shorter lengths. This included dedicated turn-over equipment for skimming both sides of the wing panels.
As a result of these investments, ingots of up to 20 tons were successfully produced in advanced 2xxx and 7xxx alloys. Also, long 36-meter wing panels were processed very rapidly through the plate department, after optimization of the plate flow through the plant.
To supply aluminum for the large spars, a significantly larger ingot had to be cast in Ravenswood. In the late 1990s, the largest production-scale 7xxx alloy cast ingot weighed 22,000 lb. Over the next several years, with the combined resources of the Ravenswood cast house and the R&D Casting Research Team located in France, ingots weighing over 37,000 lb were successfully produced. Further process improvements led to excellent recovery and reliability in casting such a challenging geometry in an advanced 7xxx alloy.
As a result of these investments, very large ingot sections and lengths were successfully produced in advanced 7xxx alloys. Also, new plate equipment quickly proved its efficiency in the processing of long and wide wing spars and ribs.
Wing Alloy Development
Considering the increased design values of the wing structural parts necessary to fulfill the higher criteria, new alloys had to be developed for all wing major parts, such as panels, stringers, spars, and ribs.
- For spars, an improvement in both static and fracture toughness levels versus the incumbent 7010/50-T7651 solution was required. Also, spar alloys should display good cold expansion ability and machining behavior. Development loops in the Ravenswood plant led to the qualification of a 7040-T7651 high static, high toughness, LRS (low residual stress) and cold expandable alloy quality. Thanks to a collaborative effort with the machining subcontractors, 7040-T7651 was selected for the two largest spars in the world, the inner front and inner center spars.
- For ribs, mostly governed by static strength and modulus, higher strength was required for weight reduction. The 7449 alloy, initially developed and industrially produced for very high-strength wing panels for A340-500/600, was tested in higher gauges up to 100 mm, in an over-aged T7651 temper (Fig. 2). Alloy 7449-T7651 was selected to fly on all low-gauge (thickness [left arrow] 100mm) A380 wing ribs, as well as for the rib caps of the few composite ribs.
- For lower wing stringers, higher strength was needed. Alloy development work was run by the R&D teams, leading to the definition of a zirconium-containing 2xxx alloy, registered at the Aluminum Association as 2027, with increased fracture toughness and fatigue strength. The 2027-T3511 alloy was then selected for the A380 lower wing stringers.
- For upper wing covers, A380-800F offered the opportunity to qualify and produce a new alloy, AA7056. This was because the design requirements for freighters are slightly different from those of the passenger version. Wing cover alloys required much improved fracture toughness, associated with possibly a slight reduction in the required static level. Alloy 7056-T7951 was qualified with an impressive 40% improvement in fracture toughness over 7449, and was selected as baseline for these A380-800F upper wing panels.
- For the lower cover, alloys needed high fracture toughness. Therefore, Alcan developed the 2024A-T351 plate solution, which is produced for various structural items, and has been extended recently for service as A330 lower wing panels (Fig. 4). The zirconium-containing 2027 alloy was also developed at a later stage with improved static strength and toughness versus 2024A-T351; it found application on the lower outer wing panel of A380-800F, as well as on the lower structure of the A340-600 center wing box.
- For lower wing structures, third-generation Al-Li alloys were approved for A380-800 and A380-800F. The alloy of interest for lower wing structures was 2050-T84, cast in the Alcan Dubach (Canada) dedicated foundry; it was recently qualified and has entered production in the Issuer plant.
Alcan had to develop a full series of very different alloys for the fuselage structure. The fuselage is a combination of many different parts and product forms that are subjected to many different types of load. Airbus has chosen the Laser Beam Welding (LBW) technology for welding stiffeners to skin on several panels.
- The 7040-T7451 plate alloy was selected for several fuselage applications, such as integrally machined main frames, cockpit window frames, beams, and fittings. It offers significantly improved static strength and toughness properties versus the incumbent 7010/7050-T74 solution. Improved properties are due to the lower (Cu, Mg) solute content that is optimized at a level just below the solubility limit, thus making it compatible with high strength and good fracture toughness. Furthermore, the alloy is processed by a technology that results in low residual stress, which means minimized machining distortion. It also offers a low-cost alternative to forgings. Alloy 7040-T7451 has been developed and qualified in both the Ravenswood and Issuer plants for thickness up to 220 mm.
- A 6xxx alloy weldable by LBW was required by Airbus for various lower shell fuselage panels, as well as for a pressurized bulkhead located under the cockpit in the front nose. Such LBW concepts were selected for their benefit in both cost and weight.
- Alloy 6156 was developed for the lower shell fuselage application. Damage tolerance behavior of the 6056 chemistry was too short for the design criteria, and an HDT version had to be developed: the result was 6156. Because the alloy needed high strength, a T6 temper was required; therefore, it had to be clad in order to avoid intergranular corrosion.
- Alloy 2024- T432: Extruded sections were chosen for many fuselage frames, due to their weight and material usage efficiencies. Keeping the appropriate level of strength over the required forming sequence was a challenge that Alcan took up with a dedicated new temper 2024- T432, enabling about 100% strength benefit over incumbent 2024 while providing a very satisfactory bend- forming behavior. This solution is currently produced by the Montreuil-Juigne extrusion plant.
After several years of research and development on this new family of alloys, the third generation of Al-Li alloys are now ready to be implemented on aircraft as the disadvantages encountered on the first generations (reduced ductility and fracture toughness in the short--transverse direction and reduced thermal stability) have been solved by an improvement of the chemical composition of the alloys and optimized thermo-mechanical treatments.
As mentioned above, concerning the A380 wing, the selection of improved 2xxx and 7xxx alloys has achieved important weight saving .The challenge for this new aircraft was not only to reduce the weight but also to deal with the particular large sizes of the components. The A380 wing spars in 7085 alloy are the world's largest die forgings today. Airbus is working on a new long-range aircraft the A350, to complement the existing A330 and A340 product line.
The future aluminum developments are focusing on advanced alloys (new 2xxx, 7xxx, Al Mg Sc, new Al-Li....), always looking for improved properties: increase of strength and damage tolerance properties, better corrosion behavior, lower density, etc. Associated technologies (welding, casting, extruded panels ...) have always to be combined with the intrinsic improvement of the performance of the alloys to optimize the cost and weight savings on the aircraft structures.