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.
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:
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:
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 | 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:
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:
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:
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