Fractional Crystallization Model of Multicomponent Aluminum Alloys: A Case Study of Aircraft Recycling
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OSPACE industry is facing an increasing waste generation coming from the decommissioning of retired aircraft.[1] It has been estimated that over the next twenty years approximately 12,000 aircraft will be at their end of life (EOL), reaching a peak of annual 1000 aircraft disposal by 2023.[2,3] In order to reduce the environmental footprint generated by the growing decommissioned aircraft, research initiatives aiming to improve the management of aircraft at their end of service life have been carried out.[2,4,5] The Airbus project ‘‘Process for Advanced Management of End-ofLife of Aircraft (PAMELA)’’ reported that around 85 pct (in weight) of the aircraft under study can be either reused (15 pct) or recycled (70 pct), where Al components from the 2XXX and 7XXX series account for the highest recycling volume.[4] Recycling these Al components would correspond to the saving of around 95 pct of the embodied energy and greenhouse gas emissions associated with the primary Al production for the same volume.[6,7] One of the main problems for the recycling of aerospace Al alloys, in addition to their high amount of alloying elements, is the pickup of detrimental impurities such as Fe and Si during handling the EOL aircraft fuselage.[6] If it remains in the recycled products, the presence of these impurities negatively impacts the fracture toughness of JOSE ALBERTO MUN˜IZ LERMA, IN-HO JUNG, and MATHIEU BROCHU are with the Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC H3A 0C5, Canada. MANAS PALIWAL is with the Material science and Engineering Department, Indian Institute of Technology, Gandhinagar, India. Contact e-mail: [email protected] Manuscript submitted September 6, 2016. Article published online January 9, 2017. 1024—VOLUME 48B, APRIL 2017
the alloys.[8] Up to now, conventional refining methods such as fluxing and filtration were not found successful for the removal of Fe and Si during the production of secondary alloys.[9] So far, the dilution of aerospace Al alloys with primary Al to produce cast products, downgrading the alloy value, is typically carried out as a recycling process. Therefore, development of the recycling technology able to produce high value-added alloys from aircraft scrap in a closed-loop recycling process is important and critical issue. A promising technology that can be envisaged as a refining method for aircraft Al scrap is fractional crystallization.[10–15] In this technique, the impure material is processed between the solidus and liquidus temperature, where solid and liquid phases coexist. A controlled cooling in this region allows the material to slowly solidify with the impurities segregating at the solid–liquid interface. As the solidification proceeds, a pure solid phase forms along one end of the ingot, whereas a high impurity concentration section moves to the outer end which can be discarded. Essentially, the fractional crystallization process is governed by solidification kinetics such as partitioning of impurities between so
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