ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2024 16 The conservation of critical materials is an increasing area of focus in the United States. In 2023, aluminum was added to the U.S. Department of Energy’s (DOE) Final Critical Materials List, which has spurred public and private research into sustainable management of these resources[1]. Additionally, efficiency in manufacturing and the conservation of natural resources are growing concerns to be addressed by lowering global carbon emissions. This is especially pertinent for primary aluminum alloy production, which is energy intensive, requiring 96 MJ of electricity per kg of primary aluminum[2]. Because of these factors, it is essential that more sustainable manufacturing methods for aluminum alloys are developed going forward. A technology with a large potential impact on carbon emissions is the recycling of post-consumer scrap because it reduces or eliminates the use of primary aluminum. Primary aluminum production requires environmentally damaging mining, and its reduction from ore to metal is energy intensive and has high carbon emissions compared to recycling. To this end, many aluminum production companies are moving toward higher post-consumer scrap use. For example, Hydro launched a wrought aluminum alloy called CIRCAL made with up to 100% post-consumer scrap through advanced sorting of 6060[3]. Rio Tinto has also invested $700 million for a 50% stake in Matalco, an aluminum production company that uses advanced remelting tech- nology to increase the amount of post-consumer scrap in wrought products[4]. And Emirates Global Aluminum recently acquired German recycling giant Leichtmetall as a move toward circularity and increased scrap content in extruded products[5]. Research on the topic of more efficient aluminum utilization and recovery is ongoing. Even considering recent developments in the recycling of Al scrap, there is still a large amount of post- consumer scrap that is underutilized because of its high impurity content. A challenge to be addressed before the coming scrap wave can be fully utilized is that the tolerance of manufacturing techniques to impurities or off-spec alloy compositions must be increased. Particularly, in 5000- and 6000-series alloys (the most common wrought alloys in durable products), excess iron, copper, and silicon create brittle intermetallics during casting that remain in the extruded microstructure. These intermetallics limit the formability, ductility, and corrosion resistance of the alloy. Concerningly, many of the highest-volume post-consumer aluminum scrap streams such as automotive shredder scrap contain a mix of alloys including both wrought and cast alloys[6]. Their compositions can vary widely depending on geography and the time of year. Because they are mixed, they often contain a high content of multiple alloying elements such as Si and Cu in greater concentrations than are found in typical wrought alloys. They may also be contaminated with non-Al alloys from fasteners and often have a high amount of unwanted elements such as Fe. An emerging extrusion technology, called Shear Assisted Processing and Extrusion (ShAPE) is being developed at the Pacific Northwest National Laboratory (PNNL) with one application being to shift beyond today’s recycling paradigm to upcycle 100% post-consumer aluminum scrap directly into extruded components without the addition of primary aluminum. This new manufacturing approach may allow manufacturers to reach deeper into lower-value scrap streams, to effectively convert scrap that is high in tramp elements into high-performance finished and semi-finished products. Sometimes referred to as Twitch or Tweak, these scrap streams result from the shredding and sorting of automobiles, building materials, appliances, and consumer goods[6]. ShAPE combines the linear axis of conventional extrusion with a rotating extrusion die or billet. The rotation applies a large strain to the material during extrusion, which breaks up large impurity-containing inter- metallic particles, reducing their deleterious effects. This has been demonstrated for 6063 machining scrap spiked with excess Fe and for Twitch scrap high in Fe, Si, and Cu, where the strength and ductility were retained for both feedstock compositions. Additionally, the extreme plastic deformation during ShAPE enables the extrusion of billets with a high Si content that are too brittle for processing by conventional extrusion. By using 100% post-consumer shedder scrap as feedstock, ShAPE has the potential to slash embodied energy and carbon in extruded components by >80% compared to the conventional extrusion of primary aluminum alloys. ShAPE II machine. Image courtesy of Andrea Starr/PNNL.
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