Green Alumunium and the Science of 'Dirty Alloys'
Aluminium has two faces when it comes to sustainability
Aluminium has two sides when it comes to sustainability. It helps saving fuel due to its low mass density (called indirect sustainability), yet, it is a very carbon-intensive metal to produce from ores (called direct sustainability). Recycling shifts this balance towards higher sustainability since even when using hydro power for primary synthesis, the energy needed for melting aluminium from scraps is only about 5% of that consumed for reducing ores.
What is the role of scrap for green Aluminium?
Recycling shifts this balance towards higher sustainability since even when using hydro power for primary synthesis, the energy needed for melting aluminium from scraps is only about 5% of that consumed for reducing ores. The amount of aluminium available for recycling is estimated to double by 2050, offering opportunities to bring the metallurgical sector closer to the global goal of a circular economy. The ‘green aluminium’ trend has even triggered a new trading platform in 2020 for low-carbon aluminium at the London Metal Exchange. A consequence might be that less sustainable materials might experience limitations for use in future products.
Which type of research is required for green Aluminium?
Identifying pathways towards Sustainability Alloy Design requires to understand and utilize how multiple scrap-related contaminant elements act on aluminium alloys, coining the term ‘Science of Dirty Alloys’, and how alloys can be upfront designed to become ‘scrap-compatible’, for maximized use of scrap, a property that equips these materials with the ‘Gene of Recyclability’. This paper addresses the metallurgical fundamentals behind the aim to develop, process and optimize aluminium alloys for performance, using higher scrap fractions. We aim at developing design guidelines for alloys with more built-in recyclability, which targets a shift from primary synthesis (reducing ores) to secondary synthesis (melting scraps). Exploring suited pathways towards Sustainability Alloy Design requires to identify, understand and exploit the underlying metallurgical mechanisms behind the use of high scrap fractions. This includes topics such as the influence of scrap-related impurity ingress on thermodynamics and kinetics of precipitation reactions as well as their mechanical and electrochemical effects; precipitation free zones around grain boundaries; casting microstructures; adjustment of processing parameters, as well as the resulting mechanical, functional and chemical properties. For this we also review advanced experimental probing methods to identify the partitioning of tramp elements between the lattice defects, the precipitate phases and the matrix and the associated effects on precipitation kinetics and thermodynamics and mechanical response. The goal of the article is to guide the design and production of aluminium alloys from highest possible scrap fractions, low quality scraps and scrap types with only few matching target alloys they can serve in when recycled. Discovering the underlying foundations of this field can guide the direction for the development of a generation of more sustainable metallic materials. This can be a fundamental contribution not only to the metallurgical sciences but also to a more sustainable society and a circular economy. Translation of such ambitions into successful, safe and high performance products that can meet highest quality standards requires deep understanding of the basic science behind sustainable metallurgy and recycled ‘green’ alloys, establishing a new field of the Science of Dirty Alloys.
Sustainable aluminum by recycling scrap [...]
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