Advanced hybrid aluminum structures cut weight and costs 15 to 20 percent and promise to provide stiff competition to plastic composites for the next round of single-aisle and other commercial planes. The new concepts also reduce maintenance intervals by a factor of two to three.
The new systems feature recently developed high-performance alloys and structures, including a matrix that provides stiffness and lift for the lower wing.
“Modern aluminum alloys in conjunction with fiber metal laminates provide higher static strength, damage tolerance and corrosion resistance resulting in significantly higher weight savings over state of the art,” Dr. John Liu, Alcoa’s director of aerospace and materials technology, told Design News in a briefing at the Alcoa Technical Center near Pittsburgh, PA. There are also several innovations in fastening technology, including friction stir welding, laser welding and adhesive bonding. Alcan is also pursuing the development of a new range of products from aluminum-lithium in addition to its existing aluminum-lithium alloys, out of which four are flying on aircraft today.
The developments are being driven by aircraft manufacturers’ requirements for reduced fuel burn, greater payload, extended range and reduced maintenance costs. Soaring costs for jet fuel and demands for more environmentally friendly aircraft are feeding the trend. Another factor, of course, is the spectacular success of Boeing’s new 787 Dreamliner, which features a dramatic increase in composites. Aluminum manufacturers hope that Boeing may hedge its bets with use of the new aluminum approaches for its next single-aisle aircraft. Airbus, which is not using composites to the same extent as Boeing, is already working with some of the advanced metals’ concepts, including for use in the fuselage of the A350. The big advantage of sticking with aluminum, according to Airbus, is the ability to use standard repair procedures in an open hanger.
Underlying the engineering is the requirement that metallic structures must operate at higher stress levels and must match the static load requirements of composites. The new approaches must also reduce crack growth and improve residual strength.
“In the old days, planes were only made of one material, aluminum,” comments Dan Goodman, Alcoa’s marketing director of aerospace. “Because of the nature of materials available today and the design capabilities, you now have a mélange of materials.”
New aluminum-lithium alloys alone provide about a 5-percent improvement in weight savings. New-generation materials also provide excellent corrosion resistance, good spectrum fatigue crack growth performance and a good strength-and-toughness combination. Two examples include 2099 extrusions for internal structure and 2199 sheet and plate for fuselage and lower wing applications, respectively. Alcoa has worked particularly closely with the aircraft OEM Bombardier to develop the new materials for given programs and applications.
Extrusions for Bombardier
Alcoa and Bombardier began work on the new alloys in 2005 for the C-Series aircraft, which may also make wider use of composites for the wing and fuselage. Target alloy applications to date on the C-Series are lower wing skin and stringers as well as fuselage skin because they provide the most potential for performance improvement and weight savings. The 2099 extrusions are being studied for the lower wing stringers and 2199 sheet for the fuselage skin and lower wing skin.
"The C-Series plan includes international partnerships, and discussions are progressing,” comments Pierre Beaudoin, president, Bombardier Aerospace. “We continue to see the lower end of the 100- to 149-seat market as a segment with a solid potential.” The target date for market entry is 2013. The projected market for 100- to 149-seat aircraft is 5,800 over the next 20 years. One of the interested customers is China Aviation Corp., which wants to participate in development of five-abreast, 90- to 149-seat commercial aircraft. Based in Montreal, Bombardier is a $14.8 (US) billion company.
Some iterations of these new materials are already commercially available and in use. For example, 2099-T83 extrusions in the thickness range of 1.27 to 76.2 mm (0.05 to 3 inches) are used in the Airbus A380's fuselage and in floor applications. For the sheet products, 2199-T8E74 is sold in thicknesses greater than 3.2 mm (0.125 inch). This grade of sheet is also being developed in a thermally stable temper known as T8Prime, which has higher toughness, modulus, tensile and compressive yield strength as well as improved fatigue crack resistance over a previous alloy. Another advantage of the 2199 sheet tested for Bombardier is that it no longer requires cladding for corrosion protection.
Wing Box Design
The silver bullet in the new approach for better stiffness and reinforcement in the new wing box concept is the new matrix composite, which consist of four elements:
• The upper wing, which uses advanced aluminum.
• Ribs, which can be machined as single pieces from advanced alloy plate or extruded as integral pieces.
• Spars, which are multiple pieces, and
• The lower wing would use an advanced hybrid structure called a fiber metal laminate (FML).
The centerpiece is the FML, which is a composite structure. But it’s a metal composite, not a plastic composite. Alternating bonded layers of metal sheet and fiber reinforced adhesive is called a prepeg — the same term that’s used to describe a thermoset plastic structure. The FML is a five-layer structure: Aluminum is at the center and on both sides. In between the sheets of aluminum are layers of glass fiber bonded with adhesive, specifically a special grade of epoxy.
One member of the family is an Alcoa material branded as Glare. The sheets are made of alloy such as 2024 and 7475, and are 0.3 to 0.4 mm thick. The prepregs consist of unit-directional S2-glass with a matrix of Fm94 epoxy. Each prepreg layer is 0.125 mm thick.
“This is not just a materials solution,” comments Liu. “We have had to develop advanced manufacturing processes to make the structures.” Glare is being used by Airbus Germany to make certain structures.
Here’s what’s in the super alloys:
Source: Proceedings of the Light Metals Technology Conference 2007
New Alloys Are Set to take Off
Use of aluminum-lithium alloys dates all the way back to the 1950s. Second generation alloys were developed in the 1980s, but never gained wide acceptance due to property issues and high manufacturing costs. New alloys show property improvements and the capability to be manufactured with standard techniques.
Use of the second-generation alloys has been primarily limited to military and space applications, such as the external fuel tank on the NASA Space Shuttle. One exception has been the EH101 medium-lift helicopter, a commercial and naval helicopter developed jointly by English and Italian companies. The weight savings of second-generation alloys allowed delivery of higher loads to the International Space Station. The new alloys can cost three to five times more than the alloys they replace. Part of the higher cost is due to lithium. Efforts are under way to continue to reduce increased manufacturing costs.
There is renewed interest in the new alloys for commercial aircraft because of increased performance demands and competition form plastic composites. It’s the goal of the aluminum industry to demonstrate that the new alloys to show equivalent performance at a lower cost. The case for aluminum: lower risk and ability to use existing tools and workforce. Repair and maintenance issues with aluminum are also well known. Another boost for the green crowd: the aluminum parts are more easily recyclable.