ADVANCED MATERIALS & PROCESSES | MARCH 2024 26 segregation. The materials are then sent to remelting facilities for recycling and casting new ingots. WING STRUCTURE The wings of commercial aircraft are extremely complex. The internal structure of the wing is reinforced by spars (laterally) and ribs that join the upper and lower wing skins and provide stiffness to the wing box (Fig. 4). Stringers run in the long direction of the wing parallel to the front and rear spars and help provide stiffness to the wing structure. Wing spars are bulky structures that attach to the fuselage and provide the main support for the wing. Stringers and wing spars are typically produced from thick extrusions or forgings (Fig. 5). Airbus A380 wings weigh 30 tons each and contain approximately 500,000 separate parts[3]. The insides of the wings also serve as fuel storage tanks for the A380 and other commercial aircraft. The first step in wing construction is to machine a large aluminum plate; a numerically controlled milling operation removes more than half the weight of the plate. In some areas, as much as 90% of the plate thickness is milled away. The next step is to set the shape. The machined plate is fastened to an aluminum mold and baked for hours at 160°C to set the final wing contour. The assembly portion of wing manufacturing is labor-intensive, requiring the positioning and fastening of plates, spars, ribs, and stringers to create the final wing structure. FUSELAGE The fuselage, or skin, of the aircraft is usually made from sheet. The 2x24 alloys in the T351 temper are often used for skins due to their superior fracture toughness and fatigue properties. The skins are attached to the circular frames that form the barrel of the fuselage. Longitudinal stringers (or longerons) support the skin. The stringers are made from either rollformed sheet or thin extruded profiles. The 7xxx alloys in a high strength, corrosion-resistant temper are typically used. AEROSPACE ALLOYS Alloy 24S (2024) was the mainstay of aircraft construction during the 1930s. Virtually all of the structures of World War II airplanes were produced from this alloy, which was a direct descendant of Alfred Wilm’s original Duralumin composition. In fact, it was dubbed “Super Duralumin.” The Japanese were the first to use 7xxx alloys during World War II on their famous Zero fighter aircraft. Near the end of the war, Alcoa was able to supply alloy 75S (7075) to save weight and increase the range of the B-29 bomber[4-7]. Both 2xxx and 7xxx alloys were supplied as sheet and extrusions. Often the sheet was supplied with a cladding of nearly pure aluminum to protect the stronger alloy from corrosion. This Alclad product was used on many military aircraft built for service in World War II. Although Alclad remains a standard product offering in the aircraft industry, its use has diminished due to improvements in corrosion-resistant coatings, alloys, and tempers. The aircraft industry boomed as the world recovered from World War II. The aluminum companies had produced almost 300,000 aircraft, requiring 3.5 billion pounds of metal during the war years[8]. After the war, they devoted their research and development to improving alloys for a new generation of higher-performance aircraft. Alcoa, Alcan, Reynolds, and Pechiney partnered with Boeing, McDonnell Douglas, Lockheed, and (much later) Airbus to develop improved combinations of properties in wrought aircraft products. As designs became more sophisticated, different alloys and tempers were optimized for different parts of the airplanes. Property improvements included corrosion resistance, fracture toughness, and fatigue performance, all of which contributed to lighter weight aircraft with higher performance and durability[1,2]. Until the 1950s, alloy 7075-T6 was considered the strongest commercial aluminum wrought alloy and the benchmark against which other alloys were judged. The development of T7xx tempers brought clear improvements in corrosion performance, and the 7050/7010 alloys were distinctly superior to 7075 in thick product forms. Modifications of 7050 alloy, 7150, and subsequently 7055 were used commercially in the 1980-2000 timeframe for the Boeing 757, 767, and 777 aircraft[9,10]. Fracture toughness became an important attribute for the F/A-18 fighter and later commercial aircraft models. This requirement drove the development of alloy 7475. Aircraft manufacturers were demanding higher toughness for both wing and fuselage products, and meeting those needs required a higher purity (low iron and silicon) base alloy. Note that the more recently registered 2xxx and 7xxx alloys feature lower impurity limits. IMPORTANCE OF T7X TEMPERS Although the conversation about aluminum aerospace materials most often focuses on alloys and their chemical compositions, the temper is equally Fig. 4 — Cross-section of typical wing structure. Fig. 5 — A380 wing spar forging.
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