ADVANCED MATERIALS & PROCESSES | MARCH 2024 27 important in meeting application requirements. The T6 temper is the maximum strength condition for a given alloy. However, the corrosion resistance of high-strength 7xxx-T6 alloys is notoriously bad. Aluminum cladding, or Alcladding, alleviates some of the issue for sheet, but metallurgists found that overaging the alloy in a second step past the peak strength level could greatly improve corrosion resistance of thick products with only a modest reduction in strength. The appropriate T7 temper can be selected based on the strength and corrosion properties required for the application. Note that the 2xxx-T6 and T8 alloys do not suffer from the same corrosion issues as 7xxx high-strength alloys. LITHIUM-CONTAINING ALLOYS Aircraft designers have always realized the benefits of higher specific strength (e.g., yield strength/density) in aluminum alloys. Because lithium additions can effectively reduce the density of aluminum alloys, the industry has pursued several generations of Al-Li-x alloys[11-15]. Aluminum suppliers spent millions of dollars to develop, evaluate, and scale up these alloys for production. Notably, the molten metal handling and casting processes were much more difficult for this new alloy family. Although these production issues were addressed, their cost remained at a multiple of conventional alloys. More than 20 new Al-Li-x alloys were registered between 1980 and 2010[4]. Reception of Al-Li products by the aircraft companies was mixed. They liked the low density, fatigue, and corrosion resistance, but not the cost or scrap problems created by the lithium addition. Research on conventional 7xxx and 2xxx alloys continued as designers and metallurgists worked together to squeeze incremental improvements from familiar materials. To recoup their research and development costs, aluminum companies aggressively protected their alloy developments with patents. Aircraft companies were faced with more expensive proprietary alloys and sometimes a single source of supply for a critical component. Adoption of new materials is an arduous process with detailed laboratory tests, followed by increasingly complex testing of subcomponents and components and, finally, flight tests. Collaboration between the aluminum producer and aircraft manufacturer was critical in the years-long process. The Airbus A350 XWB was a key test and ultimately a successful application for these alloys. Unfortunately for aluminum fans, the wing and fuselage of the plane was specified to be composite. The A350 used Al-Li alloy for the wing ribs and crossbeams, as well as seat rails and cargo floors in the fuselage. These applications constituted about 10% of the weight of the plane. Composites, by weight, represented approximately 50% of the plane[16]. As some key aluminum products became more costly, carbon fiber composites that were previously considered too expensive began to look more feasible. Figure 6 illustrates the trends in aluminum’s share of aircraft over time. The reduction is directly connected to the increased share of high-performance composites, which have excellent specific strength and high resistance to fatigue and corrosion. Composite use allows the airlines to keep planes in the air longer between routine structural inspections. The Boeing 787 Dreamliner, introduced in 2005, marked the first widespread use of composite materials by Boeing. Aluminum use on the 787 was only 20%, compared to 70-80% on previous Boeing airliners[17]. However, one key point from Fig. 6 should be appreciated. The airliners introduced since 2000 are wide-body aircraft designed for longer routes. Composites and Al-Li alloy solutions are more important in these aircraft compared with the more economical single-aisle airliners where weight is not as critical. Manufacturers will continue to use more economical aluminum mill products to build smaller airliners with the same streamlined assembly processes that have proven successful in earlier generation aircraft. SUMMARY During the 20th century, the use of aluminum alloys helped the Allied Powers win World War II and made modern global air travel possible. Continuous improvements in engine technology, alloys, and manufacturing methods enabled the development of practical and efficient aircraft with varying passenger capacity and range capability. Conventional wrought aluminum alloys make up 70-80% of the weight of single-aisle airliners. Aluminum sheet, plate, forgings, extrusions, and castings all continue to be utilized in modern aircraft construction. ~AM&P Fig. 6 — Aluminum percentage by weight for major jet airliners[17].
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