By Nidhi DhullReviewed by Lexie CornerDec 19 2024
A recent article published in Nature Communications described a simultaneous solid-phase recycling and alloying method to address the limitations of traditional alloy recycling through melting. The technique was used to alloy 6063 aluminum scrap with Cu, Zn, and Mg, producing a nanocluster-strengthened aluminum alloy with properties comparable to 7075 aluminum alloy.
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Background
Aluminum is the most widely used non-ferrous metal, and its production accounts for 3 % of global greenhouse gas emissions, largely due to the energy-intensive Hall-Héroult process for primary aluminum fabrication. Recycling aluminum scrap significantly reduces emissions, but impurities in the scrap often require the addition of primary aluminum for purification. Traditional secondary aluminum production also involves energy-intensive processes.
Friction extrusion, a solid-phase technique originally developed for metal matrix composites, has been created as a promising alternative. It offers energy efficiency and produces materials with homogeneously dispersed secondary phases. This study used friction extrusion to simultaneously recycle and alloy aluminum scrap in a single step, creating a high-performance material.
Methods
The feedstock for the study included 6063 aluminum alloy scrap, Zn powder, Cu powder, and ZK60 Mg alloy ribbons. Friction extrusion tooling, made of H-13 tool steel, included spiral grooves on the die face to facilitate material flow into a 5-mm extrusion orifice.
Two types of products were fabricated:
Recycled Alloy: 6063 scrap was cold-compacted and friction extruded into void-free rods. Upcycled Alloy: 6063 scrap and alloying elements were friction extruded under similar conditions to produce a void-free rod with a composition comparable to 7075 aluminum alloy.
The proportion of alloying elements in the mix for the upcycled product was determined based on the chemical composition differences between standard 7075 aluminum alloy and 6063 scrap. The microstructures of the products were analyzed using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and three-dimensional atom probe (3DAP) analysis.
Material hardness was assessed using a Vickers microhardness tester with a 200-gram load and a dwell time of 12 seconds. Tensile tests were conducted on a mechanical testing machine to evaluate deformation strain.
Results and Discussion
The upcycled alloy demonstrated significant grain refinement, with an average grain size of 7.7 µm compared to 43.1 µm for the recycled alloy. This refinement was attributed to the lower extrusion temperature during upcycling. Stress-strain curves revealed a more than 200% increase in yield and ultimate tensile strength for the upcycled alloy compared to the recycled material.
The microstructural analysis identified four processing zones in the upcycled material:
Zone A: Unprocessed scrap and alloy additions. Zone B: Transition zone. Zone C: Processed zone. Zone D: Extruded rod.
As the upcycled material progressed from Zone A to Zone D, the alloying elements (Cu, Zn, and Mg) became more evenly distributed throughout the microstructure. During the friction extrusion process, these elements were incorporated into the aluminum matrix while the aluminum scrap was consolidated into the extrudate.
XRD analysis showed that Mg2Si was the primary precipitate phase in the recycled material. In contrast, the upcycled material contained η'/Mg(CuZn)2 in addition to Mg2Si. TEM images further revealed that the new phases included amorphous Mg(ZnCu) and a crystalline Mg(ZnCu), with a crystalline η' phase/Mg(CuZn)2 embedded within the amorphous phase. These findings confirm that solid-phase alloying occurred during the upcycling process.
Conclusion
The researchers successfully synthesized a high-performance aluminum alloy by upcycling 6063 scraps with Cu, Zn, and Mg using a single-step, solid-phase alloying process. The upcycled product demonstrated yield and ultimate tensile strengths more than 200 % higher than those of the recycled material, largely due to the formation of Guinier-Preston (GP) zones.
This approach demonstrates that low-strength, low-cost aluminum scrap can be transformed into high-strength, high-value aluminum alloy products using scalable solid-phase processing, such as friction extrusion, without melting the precursor materials. This method reduces the energy demands and environmental impact of metal production and enables the creation of novel alloys that cannot be produced through conventional melt-based techniques.
Journal Reference
Wang, T., et al. (2024). Upcycled high-strength aluminum alloys from scrap through solid-phase alloying. Nature Communications. DOI: 10.1038/s41467-024-53062-2, https://www.nature.com/articles/s41467-024-53062-2
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