Sustainable aluminium production
Metallurgical challenges and potential solutions
Aluminium can be made “greener” through recycling, energy efficiency, durability and systematic application of lightweight design concepts
As Figure 1 illustrates, the issues engineers face in relation to the green, sustainable use of aluminium can best be divided into four areas:
Energy efficiency
- How can we make processes energy-efficient?
- How does the switch from fossil fuels to electricity affect the physical properties of metals?
Durability
- How can we optimize the corrosion resistance of materials and damage tolerance to make products more durable?
Resource conservation
- How can we create high-strength alloys in order to use less material?
- How can we reduce alloy diversity to facilitate more efficient scrap cycles?
Recycling
- How can we separate scrap efficiently?
- How can we make alloys easier to recycle?
AMAG encounters these issues in many different areas of its activities. This article explores the development of recycling-friendly alloys.Recycling offers immense potential to produce goods in a way that conserves materials. As carbon-neutral pre-material is not widely available and the processing of scrap only requires approx. 5-10% of the energy needed to produce primary aluminium. However, due to the rising use of impure scrap, growing quantities of impurities are being introduced to the cycle, which can negatively impact the performance profiles of different alloys. There are different ways of producing recycling-friendly alloys. Widening alloy tolerance ranges makes it possible to use higher amounts of scrap. The process parameters can then be adjusted as needed to compensate for any variations. AMAG’s Science of Recycled Alloys research program uses common scrap compositions as the basis for developing new alloys. Its “uni-alloy” concept aims to replace a number of highly specialized alloys with a small number of versatile alloys. Process adjustments optimize these alloys to give them characteristics that serve a wide range of uses. The alloys in the AMAG CrossAlloy® family are one example. Regardless of the concept used, it is vital to understand the influence that alloying elements have on the development and properties of materials’ microstructure. These technical adjustments to production processes can prevent the use of scrap from negatively impacting material quality. Products’ properties can then be tailored to customers’ broad requirements, balancing material performance and sustainability. Let’s look at an example that illustrates how we can leverage this potential.
Variations in alloy composition can shift process windows - so processes must be optimized based on composition
The influence of magnesium and silicon as alloying elements in the production of 6xxx alloys provides a useful example. Sheets made from Al-Mg-Si alloys typically undergo recrystallization at least once during the production process to create a grain structure and phase structure with good formability. This recrystallization is determined by multiple factors, including the number of nucleation sites. Primary and secondary phases in the order of 1-10 µm can serve as suitable sites. If we compare a low-alloyed 6016 alloy with a recycling-friendly 6016 (RF), which in this case contains noticeably higher quantities of magnesium and silicone, the composition’s influence on the selected process parameters becomes evident.Figure 2 shows CALPHAD simulations in equilibrium using Pandat. We can see that higher magnesium and silicone contents shift the solution of Si and Mg phases at higher temperatures. In the relevant temperature range for hot rolling, namely 400°C to 550°C, this means that different intermetallic phases occur in varying proportions depending on the alloy composition. This is evident in the material’s microstructure. Following two hours of lab-based annealing at a potential hot rolling temperature of 480°C and subsequent quenching in water, the low-alloyed 6016 has significantly lower secondary phase content. In the RF variant in particular, the intermetallic phases are of a scale that is relevant to particle-stimulated recrystallization. Depending on the selected temperature window, this can be used to influence microstructure development and, consequently, the product’s character profile. If the proportion of scrap is increased without appropriate adjustment of process parameters, this usually has a negative impact. It is therefore vital to make careful process management adjustments based on the alloy composition.
Thanks to its sophisticated casting and rolling processes for recycled materials, combined with its extensive knowledge of the factors that affect the physical properties of aluminium alloys, AMAG is the perfect partner for innovative, sustainable alloy development.
Benefits for customers:
Our customers benefit from our outstanding recycling expertise and detailed understanding of materials and processes, which has allowed us to introduce recycling-friendly alloys. In addition, our material experts can assist with technical measures to optimize the processing of these alloys.We encourage our customers to structure their value chains as effectively as possible - from materials recycling through to processing, in the interests of promoting sustainability.
Sources
[1] D. Raabe et al., Nature 2019 575, 64-74 [2] D. Raabe, Chemical Reviews 2023 123 (5), 2436-2608
