ADVANCED MATERIALS & PROCESSES | MARCH 2024 25 commuter aircraft, regional jets use a high percentage of aluminum alloys in their structure. Narrow-body jets: The most common airliners in the industry are narrow-body or single-aisle aircraft and are generally used for medium-haul flights with 100-200 passengers. The early jet airliners in the 1950s were in this category, including the de Havilland Comet and Boeing 707. Later competition came from the Douglas DC-8, DC-9, and other McDonnell Douglas derivatives. Today’s most popular single-aisle planes are the Airbus A220 and A320, Boeing 737, and the Embraer E-Jet family. They could be joined in the near future by the Comac C919 developed in China and put into commercial passenger service in May 2023. Wide-body jets: Wide-body or twinaisle aircraft have one key function— hauling large numbers of passengers over long distances. Since the first Boeing 747 entered service in 1970, improvements in aircraft design and engine technology have made these planes more reliable, comfortable, and fuel efficient. Several models of Boeing and Airbus aircraft are capable of flying more than 300 passengers for distances of 12,000 km or more. MODERN AIRCRAFT MANUFACTURING Early aircraft manufacturing was essentially a one-at-a-time process, with workers adding thousands of parts individually. Today’s aircraft are produced on assembly lines with the number of parts and assembly time reduced by using subassemblies. Subcomponents can be produced by Tier 1 suppliers in factories located remotely from the final assembly plant. The aluminum alloy/temper and part dimensions are specified for the various parts of the aircraft structure: wing, fuselage, and empennage (Fig. 2). The menu of aluminum products used for commercial airliners consists of various wrought 2xxx, 7xxx, and some 6xxx alloy products in different tempers: sheet (typically 3-6 mm thick); plate (typically 6-150 mm); extruded shapes (thick and thin) in standard or custom profiles; forged shapes according to specification; and shaped castings. Shaped castings, large and small, are used in many aircraft applications. Most are component parts of hydraulic and power generation systems. For example, pumps as well as gear, brake, and air duct housings are typical. Some structural parts are made of sand cast high-strength 2xx alloys. Die cast parts of A355-T71 are also common when larger quantities of parts are needed. Because the total weight of cast alloys is typically small, the following discussion focuses on wrought alloys and products that comprise the bulk of aluminum aircraft structure. BUY-TO-FLY RATIO Aircraft manufacturers are conscious of the amount of aluminum they purchase to make a finished part. High-speed machining is used to manufacture efficient lightweight structural parts with tight tolerances (Fig. 3). The buy-to-fly ratio is the weight of metal bought divided by the final part weight. For example, if 10 kg of plate is needed to make a 1 kg part, the buy-to-fly ratio is 10:1. In other words, 90% of the plate is turned into machining chips. For parts machined directly from plate, buy-to-fly ratios of 20:1 are not unusual. The use of extruded profiles and forged shapes can increase material recovery and lower costs by providing a nearer-net shape that requires less machining. For example, the end frame for a B777/737 tail rib weighs just 0.92 kg after machining from an extrusion. However, extruded or forged parts may still require extensive machining. The deformation processes do not inherently have the dimensional or metallurgical tolerances required for using a nonmachined part. The thin webs on ribs, for instance, can be less than 1 mm thick, and the metallurgical structure of the part may vary through the cross-section of the wrought part. Coarse grains or other nonuniform features are eliminated by removing sufficient surface material during the machining process. Special attention to alloy selection and extrusion processing can help improve microstructural uniformity, thus reducing the buy-tofly ratio. At first, high buy-to-fly ratios may suggest high material wastage, but in most cases, machined chips and other off-cuts are collected with alloy Fig. 2 — Basic structure of a modern airliner. Fig. 3 — High-speed milling of an aircra wing structure.
RkJQdWJsaXNoZXIy MTYyMzk3NQ==