AMAG CrossAlloy® - the new generation of recycled aluminium
Tapping into new potential
Aluminum is a cornerstone of modern engineering, offering unmatched lightweight properties, high recyclability, and versatility across industries. In transportation and aerospace, it is a key enabler of energy efficiency and reduced carbon footprints. However, the production of aluminum from ores is highly energy-intensive and contributing significantly to greenhouse gas emissions. Recycling aluminum addresses these challenges, requiring only 5 % of the energy needed for primary production and drastically reducing emissions. [1]
The aluminum industry has undergone significant transformation in recent decades with nearly one-third of global aluminum production sourced from recycling [2]. Yet, achieving sustainable recycling is fraught with challenges. Contaminants such as Fe and Si in scrap streams degrade alloy performance, while inconsistent scrap quality complicates processing. Despite these obstacles, advancements in recycling technology, particularly for high-value applications, are reshaping the landscape. [3] The demand for aluminum alloys in sectors like automotive and aerospace continues to grow, driven by electrification trends and sustainability goals. Automotive manufacturers are increasingly relying on aluminum to reduce vehicle weight and enhance fuel efficiency, while aerospace demands remain focused on high-strength, corrosion and fatigue resistant alloys for critical structural components. This demands underscores the necessity for innovative solutions that balance performance and recyclability. [4-6] Traditional recycling methods, however, often fall short in meeting requested requirements. Downcycling practices dilute high-grade scrap into lower-quality alloys, squandering the potential of valuable materials. To counter this, the development of crossover alloys marks a pivotal innovation. By bridging the gap between scrap-derived materials and high-performance demands, these alloys enable the production of advanced aluminum solutions with substantial recycled content. [7,8]
Alloys in Automotive and Aerospace Applications
The utilization of aluminum alloys in automotive and aerospace sectors exemplifies the material's versatility and critical role in advancing technological performance. In the automotive industry, aluminum alloys like the 5xxx and 6xxx series dominate due to their lightweight properties, excellent corrosion resistance, and favorable cost-performance ratio. The average EU Vehicle contains 180 kg of Al, which is split into roughly 44 % wrought and 56 % cast alloys which can be further categorized into 26 individual alloys for specific purposes. [4,9] In contrast, the aerospace sector prioritizes high-strength, low-density alloys that can withstand extreme operational environments. The 2xxx and 7xxx series, characterized by their high copper and zinc content respectively, are frequently used for critical components such as airframe structures, wing skins, stringers or stabilisers. The development in this sector is driven by a fatigue-strength trade-off. Additionally, alloys are often susceptible to corrosion, necessitating protective coatings and treatments, inflicting challenges to future recycling processes. [5,6,10] Recycling practices in these industries present significant challenges. Automotive scrap, derived from end-of-life vehicles (ELVs), typically consists of mixed alloys contaminated by joining methods and impurities such as Fe and Si. Effective sorting technologies like Laser Induced Breakdown Spectroscopy (LIBS), X-ray tomography (XRT) and advanced eddy current separators are crucial for improving scrap quality. Aerospace material recycling grapples with coatings containing hazardous elements like Cr(VI), which complicate remelting and refining processes. Despite these barriers, initiatives to enhance separation techniques and refine recycling processes continue to make progress, particularly in the adoption of green technologies for alloy purification. [11-13]
| Alloy class | Max. (mean) content Fe [Gew.-%] | Max. (mean) content Si [Gew.-%] | Max. (mean) content Cu [Gew.-%] |
|---|---|---|---|
| Average composition of scrap predicted for 2030-2050 | ≈0,7 | 4-6 | ≈0,2 |
| 2xxx | 1,4 (0,5) | 1,2 (0,5) | 6,8 (4,3) |
| 3xxx | 0,8 (0,7) | 0,6 (0,5) | 0,3 (0,2) |
| 4xxx | 0,8 (0,7) | 13 (7) | 0,5 (0,3) |
| 5xxx | 0,7 (0,4) | 0,45 (0,3) | 0,5 (0,2) |
| 6xxx | 0,8 (0,4) | 1,5 (0,7) | 1,1 (0,3) |
| 7xxx | 0,5 (0,2) | 0,5 (0,2) | 2,6 (1,4) |
| 8xxx | 1,3 (0,9) | 0,8 (0,4) | 0,1 (0,075) |
As the demand for lightweight materials in electric vehicles and advanced aerospace designs increases, so does the urgency to optimize recycling pathways. Forecasts indicate that the transition to electric drivetrains will significantly reduce the consumption of cast alloys, further emphasizing the need for high-performance wrought alloys derived from recycled content. Table 1 shows the predicted impurity content in scrap for 2030 to 2050 and the currently allowed concentration in wrought aluminum alloys classes. The mismatch for certain grades is obvious. [3]
In this context, the development of crossover alloys represents a necessary advancement, enabling the effective utilization of recycled aluminum while meeting stringent performance requirements.
Introducing aluminum crossover alloys
Crossover alloys represent a novel approach in aluminum alloy development, bridging the gap between the properties of traditionally distinct alloys series while incorporating higher amounts of tramp elements. Designed to address the challenges of increasing sustainability in high-performance applications, these alloys are uniquely positioned to meet the dual demands of increased mix-scrap utilization and mechanical excellence. The performance of certain members of the AMAG CrossAlloy® family in different heat treatment conditions is displayed in Figure 1 and compared to traditional wrought alloys. The concept of crossover alloys revolves around merging the beneficial traits of established alloy series while allowing input of various scrap streams. Beyond the 5xxx/7xxx (AMAG CrossAlloy®.57) and 6xxx/8xxx (AMAG CrossAlloy®.68) combinations, research efforts on 2xxx/7xxx and 6xxx/7xxx combinations are also increasing throughout the scientific community. A 2xxx/7xxx crossover allows strengthening by multiphase precipitation, therefore, reducing the lengthy production process of 2xxx series alloys [18,19]. On the other hand, in 6xxx/7xxx crossover alloys the introduction of Zn allows higher strengths [20] as it can enter the structure of all phases, without altering the precipitation sequence, yet it does result in precipitate disorders. [20,21].
The role of zinc (Zn) in these alloys is key, especially in balancing strength and corrosion resistance. However, if not carefully controlled, it can exacerbate localized corrosion as it leads to a widening of the precipitation free zones (PFZ) around the grain boundaries. This was found in 2xxx/7xxx [22,23] as well as 6xxx/7xxx [24] crossover alloys and is displayed in Figure 2. Further investigations and improvements are needed.
Applications of crossover alloys could extend across automotive structural components, the aerospace sector and even consumer electronics and specialty products. Their adaptability makes them a preferred choice for industries prioritizing lightweight designs with high environmental sustainability.
Challenges coming along with increasing impurity levels
As the aluminum industry moves toward greater sustainability, the transformation of recycled materials into high-performance alloys remains a significant challenge. As crossover alloys are not bound to any classification system, they offer the unique opportunity for increased impurity content, thus increased scrap content in their production. Therefore, these alloys can further reduce the carbon footprint of consumer goods as they can reduce the reliance on primary aluminum. However, increasing recycled content in crossover alloys necessitates a thorough understanding of phase formation and morphology. Impurity elements like Fe and Si, abundant in recycled aluminum, are known to form complex intermetallic phases during casting. These phases, depending on their size and morphology, can significantly impact mechanical properties, such as ductility and strength. Advanced characterization and processing techniques are critical for tailoring these phases to enhance recyclability while maintaining or even improving alloy performance. [9,25,26]
Especially iron-rich intermetallic phases (IMPs), such as ß-AlFeSi and α-AlFeSi, require increased attention in that regard. These particles are present in aluminum alloys containing elevated levels of Si and Fe. The ß-phase, characterized by a plate-like morphology, tends to reduce ductility and act as crack initiation sites. Conversely, the more compact α-phase has a less detrimental impact and is generally preferred in wrought aluminum applications. Mn additions can modify the ß-phase into the α-phase, while fast solidification rates can suppress ß-phase formation altogether, promoting a fine distribution of α-phase particles. [25,26]
Therefore, Fe and Si content must be managed carefully to control phase formation in crossover alloys. Studies on 5xxx/7xxx crossover alloys highlight that the morphology of primary Fe-rich phase - such as Al6(Fe,Mn) or Al(Fe,Mn)Si - can be transformed during optimized homogenization to mitigate their detrimental effects. [8]
Advanced solidification techniques, such as near-rapid cooling, have shown potential in refining intermetallic phase morphology. For instance, solidification rates above 10 K/s can promote the formation of fine α-AlFeSi particles while suppressing ß-phase formation. Direct chill casting with optimized cooling rates has also been demonstrated to lead to finer, more evenly distributed particles. These refined particles act as nucleation sites during recrystallization, significantly enhancing grain refinement and mechanical properties. [7]
Optimized thermomechanical treatments are also capable of influencing the phase distribution of primary and secondary particles. For example, homogenization at temperatures between 530 °C and 600 °C can transform plate-like α-particles into globular ß-particles, improving formability and reducing crack initiation risks [9,25]. These processing strategies are important in leveraging recycled aluminum scrap with higher impurity content, enabling the production of high-performance crossover alloys. For AMAG CrossAlloy®.68 coarse, primary IMC´s are fragmented by optimized thermomechanical treatment thus not only to mitigating their detrimental impact but also to improving the mechanical properties of the alloy. [25]
Summary
Aluminum is an essential material in modern technology, known for its lightness, high recyclability, and versatility. Particularly in the transportation and aviation industries, aluminum contributes to energy efficiency and the reduction of CO2 emissions. Although recycled aluminum is beneficial regarding energy consumption, there are challenges such as contamination and inconsistent scrap quality that affect both processing and performance of the alloys. Therefore, innovative solutions that balance performance and recyclability are crucial to cope with the increasing demand. Based on the current outlook crossover alloys have potential to address this issue by enabling the effective use of recycled aluminum while meeting stringent performance requirements.
Customer advantages
The AMAG CrossAlloy® family offers an attractive mix of mechanical and technological features while addressing the need for materials with increased scrap tolerance and reduced CO2 footprint. Our innovative crossover alloys are particularly suitable for customers that prioritize sustainable lightweighting and are open for “out-of-the-box” solutions.
References::
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