Metals Knowledge: Recycling Aluminum Alloys

Abstract: The growth in aluminum usage in transportation applications, relative decline in aluminum beverage can recycling, and increasing reliance of the fabrication industry on secondary aluminum have combined to create new needs in both the materials design and processing space. To most economically utilize these scrap streams, new approaches to developing acceptable materials processed to control properties suitable for an expanded range of applications are needed. Recycling aluminum alloys has been shown to provide major economic benefits. As a result, it is appropriate for the aluminum industry to identify, develop, and implement all technologies that will optimize the benefits of recycling. Aluminum recycling in North America and Europe is a mature, well-developed economy. It gained momentum after World War II following rapid economic and industrial growth, and especially after the introduction of the aluminum beverage can with its easy-open end. While today’s recycling metals markets also include ferrous metals like iron and steel, and nonferrous metals like copper and brass, aluminum recycling is the engine of recycling economics. The growth in aluminum usage in transportation applications, relative decline in aluminum beverage can recycling, and increasing reliance of the fabrication industry on secondary aluminum have combined to create new needs in both the materials design and processing space. To most economically utilize these scrap streams, new approaches to developing acceptable materials processed to control properties suitable for an expanded range of applications are needed. There are a number of challenges to be met to create a “recycling-friendly” world. Among the key challenges are the following: Maximize recovery of used aluminum products and components for recycling Automate and optimize pre-sorting, shredding, and separation technologies, and make them broadly available Identify more useful by-products to handle elemental residual unsuitable for reuse in recycled metal, e.g. Fe Broaden the number of available aluminum alloys whose specifications will readily directly accept recycled metal and will perform well in high-quality, value added products Aluminum remains the most economically attractive material from which to make aircraft and space vehicles, and new construction proceeds at a prodigious rate. However, the development of newer aircraft structures has proceeded at such a pace that thousands of obsolete civil and military aircraft stand idle in “graveyards”, especially in the USA. Yet it has been impractical to reuse the metal in these planes because of the combination of the differences in compositions of older obsolete aircraft and those of new aircraft, often having special performance requirements requiring specialized alloy compositions. Since the demand for recycled aluminum continues to increase, the discarded aircraft provide a large source of valuable metal. However cost-effective recycling of aircraft alloys is complex because aircraft alloys are: (a) typically relatively high in alloying elements and (b) contain very low levels of impurities to optimize toughness and other performance characteristics. Thus recycling of aluminum aerospace alloys represents a major challenge to both the aluminum and aerospace industries. While the recycling of high percentages of aluminum from packaging and automotive applications has been commercialized and become economically attractive, the unique compositions and performance requirements of aerospace alloys have resulted in delaying directly addressing techniques for cost-effectively recycling those alloys. To a large extent, aircraft alloys fall into two series, the Al-Cu or 2xxx series and the Al-Zn-Mg or 7xxx series. While automated sorting techniques applied after shredding will unquestionably work, anything that can be readily done to pre-sort those alloys would be helpful. One technique that seems practical would be to dismantle aircraft into certain logical component groups, as these typically are made of similar alloys of the same series. As example, landing gears, engine nacelles, tail sections, and flaps could be presorted, and wings separated from fuselages. Such separations may be desirable anyway to permit removal of non-aluminum components before shredding. There are several basic facts that we can be certain of. The metal from recycled 2xxx alloys will be high in Cu, Mg, Mn and Si and the metal from 7xxx alloys will be high in Zn, Cu, and Mg. In older aircraft structures, 2024 has been for many years the most widely used 2xxx alloy, and 7075 the most widely used of the 7xxx series. Newer aircraft have more high-purity alloys like 2124, 2324, 7050, 7175, and 7475. As noted above, an ideal component of maximization of resources in aircraft recycling would be the availability of several new aluminum alloys that would take advantage of the unique characteristics of recycled aircraft metal. Such an approach may call for some “tailored” alloys, enabling broader specification limits on alloying elements likely to be found in recycled aircraft metal, notably the high Cu in 2xxx alloys and Zn in 7xxx alloys. Six preliminary candidate wrought alloy compositions that might be reasonably made from recycled and shred-sorted wrought products with at minimum the addition of some alloying elements are shown in Table 1 below: Table 1: Wrought alloy compositions ALLOY Si Fe Cu Mn Mg Zn Others A(2xxx) 0.7 0.6 5.5-7.0 0.2-0.4 0.7 0.5 0.3 B(3xxx) 0.7 0.6 0.4 1.0-1.5 0.8-1.5 0.5 0.3 C(4xxx) 10.0-14.0 1 0.5-1.5 0.3 0.8-1.5 0.5 0.3 D(5xxx) 0.7 0.6 0.3 0.05-0.35 2.0-3.0 0.5 0.3 E(6xxx) 0.3-1.0 0.6 0.3 0.3 0.4-1.0 0.5 0.3 F(7xxx) 0.5 0.6 0.5-1.2 0.3 2.0-2.8 4.0-6.0 0.3 In this initial list, one composition has been selected from each major alloy series. Other candidates might well be devised by adjustments in the major alloying elements and/or the addition of other minor alloying elements. These representative compositions illustrate several of the fundamental complications in directly reusing scrap aluminum: Even when wrought scrap has been segregated, individual lots can have relatively widely varying compositions; for example, wrought B has higher Cu (possibly from more auto body alloy 2036 alloy) and higher Zn (possibly from more bumper alloy 7029) in the mix. Some lots of wrought recycled metal match existing wrought alloys reasonably well, e.g. 3005, 3104, 3105, and 6061, and can be readily reused; others like Wrought do not and will be more difficult to use directly. Cast alloy scrap can vary greatly in composition, and is likely to differ significantly in composition from wrought alloy scrap. Note the differences in Cu, Si, and Zn. Very significant economic and ecological advantages of maximizing the rate of recycling and reusing aluminum alloys lead to a number of important conclusions for the aluminum industry throughout the world. Among these conclusions are the following: Methods for the recovery of aluminum scrap from as many products as possible should continue to be exploited. Strategies for the most cost-effective remelting processes should be pursued, including technologies to facilitate separation of undesired elements such as Fe, Ni, and/or V. The development of alternative products such as Al- Fe de-oxidizing agents should be pursued to utilize that part of recycled aluminum that cannot cost-effectively be used in the production of new aluminum alloys. Serious consideration and study should be given to the development of new aluminum alloys designed for application directly from recycled aluminum, and still providing performance criteria required for a wide variety of applications, when produced directly from recycled metal. A study should be carried out to explore the potential of adding to the number of alloys available for direct recycling. This study should identify more precisely and with higher probability the sources and expected ranges of compositions of current and future recycled metal content. There are a number of more detailed challenges facing any effort to increase the number of aluminum alloys and applications suitable for direct production from recycled metal, among them the following: With the exception of recycled beverage cans, most recycled aluminum involves a mixture of alloys from a fairly wide variety of applications, including a selection of castings containing rather high percentages of silicon (Si). While there is generally no problem recycling most of this metal as castings, there is a significant challenge in shredding, sorting, and, in some cases, further refinement of the metal to achieve acceptable impurity levels for products other than castings, including sheet, plate, forgings, and extrusions. Many premium alloys utilized today, especially in the aerospace industry where requirements for exceptionally high ductility and toughness are common, call for very tight composition controls on both Fe and Si. Impurity levels above 0.10-0.15% Fe or 0.15-0.25% Si are unacceptable, for example, in premium high toughness aerospace alloys. High performance automotive alloys generally restrict both Si and Fe to 0.40% maximum. Both of these elements (Fe and Si) are difficult to control in recycled metal, and tend to increase modestly the more often the metal has been recycled. Elements other than Fe may be expected to gradually increase with time and may require special attention; magnesium, nickel and Vanadium are three examples.