Recycling Aluminum Alloys

---

Introduction to Aluminum Recycling Economics

Recycling aluminum alloys delivers substantial economic benefits, making it imperative for the industry to identify, develop, and implement technologies that optimize recycling outcomes. In North America and Europe, aluminum recycling represents a mature economy that gained significant momentum following World War II and the introduction of aluminum beverage cans with easy-open ends. While today's recycling markets include various metals, aluminum recycling remains the primary economic driver of the recycling industry.

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 utilize these scrap streams most economically, new approaches are needed to develop acceptable materials with controlled properties suitable for an expanded range of applications.

Key Challenges in Creating a "Recycling-Friendly" Aluminum Industry

There are several significant challenges in creating a more recycling-friendly aluminum ecosystem:

• Maximizing recovery of used aluminum products and components
• Automating and optimizing pre-sorting, shredding, and separation technologies, and making them widely available
• Identifying useful by-products to handle elemental residuals unsuitable for reuse in recycled metal (e.g., Fe)
• Broadening the number of available aluminum alloys whose specifications readily accept recycled metal while performing well in high-quality, value-added products

Aircraft Aluminum Recycling: A Special Case

Aluminum remains the most economically attractive material for aircraft and space vehicles, with production continuing at a rapid pace. However, thousands of obsolete civil and military aircraft sit idle in "graveyards," particularly in the USA. Reusing this metal has been impractical due to compositional differences between older aircraft alloys and those required for newer aircraft with specialized performance requirements.

The increasing demand for recycled aluminum makes these discarded aircraft a valuable metal source. However, cost-effective recycling of aircraft alloys presents complex challenges because these alloys typically contain:

• Relatively high levels of alloying elements
• Very low levels of impurities to optimize toughness and other performance characteristics

These unique compositions and performance requirements have delayed the development of cost-effective techniques for recycling aerospace alloys, despite the successful commercialization of aluminum recycling in packaging and automotive applications.

Strategies for Aircraft Alloy Recycling

Aircraft alloys primarily fall into two series: the Al-Cu (2xxx) series and the Al-Zn-Mg (7xxx) series. While automated sorting techniques can be applied after shredding, pre-sorting would significantly enhance the process. A practical approach would be to dismantle aircraft into logical component groups typically made of similar alloys within the same series. For example, landing gears, engine nacelles, tail sections, and flaps could be presorted, and wings separated from fuselages. Such separations may also facilitate the removal of non-aluminum components before shredding.

Key characteristics of recycled aircraft aluminum include:

• Metal from recycled 2xxx alloys will be high in Cu, Mg, Mn, and Si
• Metal from 7xxx alloys will be high in Zn, Cu, and Mg
• Older aircraft structures predominantly used 2024 (2xxx) and 7075 (7xxx) alloys
• Newer aircraft incorporate more high-purity alloys like 2124, 2324, 7050, 7175, and 7475

Proposed Alloys for Recycled Aircraft Aluminum

An ideal approach to maximizing resources in aircraft recycling would be developing several new aluminum alloys that leverage the unique characteristics of recycled aircraft metal. This might require "tailored" alloys with broader specification limits on alloying elements commonly found in recycled aircraft metal, notably high Cu in 2xxx alloys and Zn in 7xxx alloys.

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	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

These representative compositions illustrate several fundamental complications in directly reusing scrap aluminum:

• Even when wrought scrap has been segregated, individual lots can have widely varying compositions
• Some lots of wrought recycled metal match existing wrought alloys reasonably well and can be readily reused; others do not and will be more difficult to use directly
• Cast alloy scrap can vary greatly in composition and likely differs significantly from wrought alloy scrap, particularly in Cu, Si, and Zn content

Recommendations for Improving Aluminum Recycling

The significant economic and ecological advantages of maximizing aluminum alloy recycling lead to several important recommendations:

• Continue exploiting methods for recovering aluminum scrap from as many products as possible
• Pursue cost-effective remelting strategies, including technologies to facilitate separation of undesired elements such as Fe, Ni, and/or V
• Develop alternative products such as Al-Fe deoxidizing agents to utilize recycled aluminum that cannot cost-effectively be used in new aluminum alloy production
• Study and develop new aluminum alloys designed for direct application from recycled aluminum while maintaining required performance criteria
• Explore the potential for expanding the number of alloys available for direct recycling, identifying more precisely the sources and expected composition ranges of current and future recycled metal

Ongoing Challenges in Expanding Recycled Aluminum Applications

Several detailed challenges remain in increasing the number of aluminum alloys and applications suitable for direct production from recycled metal:

• Most recycled aluminum (except beverage cans) involves mixtures of alloys from various applications, including castings with high silicon content. While recycling this metal as castings is straightforward, significant challenges exist in shredding, sorting, and refining to achieve acceptable impurity levels for other products like sheet, plate, forgings, and extrusions.
• Premium aerospace alloys require tight composition controls, particularly for Fe and Si. Impurity levels above 0.10-0.15% Fe or 0.15-0.25% Si are unacceptable in high-toughness aerospace alloys, while high-performance automotive alloys generally restrict both elements to 0.40% maximum. These elements are difficult to control in recycled metal and tend to increase slightly with each recycling cycle.
• Elements other than Fe may gradually increase over time and require special attention, including magnesium, nickel, and vanadium.