Metal scrap upcycled into high-value alloys with solid phase manufacturing

The research study, published this week in the journal Nature Communications, demonstrates that scrap aluminum from industrial waste streams can produce high-performance metal alloys. The upcycled aluminum performs on par with identical materials produced from primary aluminum, indicating that this approach can provide a low-cost pathway to bringing more high-quality recycled metal products to the marketplace. By converting waste into high-performance aluminum products, the new method, called solid phase alloying, not only enhances material properties but also contributes to environmental sustainability.

“The novelty of our work here is that by adding a precise amount of metal elements into the mix with aluminum chips as a precursor, you can actually transform it from a low-cost waste to a high-cost product,” said Xiao Li, a PNNL materials scientist and lead author of the research study. “We do this in just a single step, where everything is alloyed in five minutes or less.”

The innovative solid phase alloying process converts aluminum scrap blended with copper, zinc and magnesium into a precisely designed high-strength aluminum alloy product in a matter of minutes, compared to the days required to produce the same product utilizing conventional melting, casting and extrusion. The research team used a PNNL-patented technique called Shear Assisted Processing and Extrusion, or ShAPE, to achieve their results. However, the researchers noted that the findings should be reproducible with other solid phase manufacturing processes.

Within the ShAPE process, high-speed rotating die create friction and heat that disperses the chunky starting ingredients into a uniform alloy with the same characteristics as a newly manufactured aluminum wrought product. The solid phase approach eliminates the need for energy-intensive bulk melting, which combined with the low-cost feedstocks originating from scrap, has the potential to sharply reduce the cost of manufacturing these materials. For consumers, this means recycled aluminum products will have a longer lifespan and better performance at a lower cost, whether they are part of a vehicle, a construction material, or a household appliance.

Metal alloy that is strong to the core

The scientific team used both mechanical testing and advanced imagery to examine the internal structure of the upcycled materials produced by solid phase alloying. Their results showed that the ShAPE upcycled alloy imparts a unique nanostructure at the atomic level. During ShAPE, atomic-scale features called Guinier-Preston zones form within the alloy. These features are well known to improve strength in metal alloys. Compared to conventional recycled aluminum, the upcycled alloy is 200 percent stronger and has increased ultimate tensile strength. These characteristics could translate into longer-lasting and better-performing products for consumers.

“Our ability to upcycle scrap is exciting, but the thing that excites me the most about this research is that solid phase alloying is not just limited to aluminum alloys and junk feedstocks,” said Cindy Powell, the chief science and technology officer for energy and environment at PNNL and a coauthor of this study. “Solid phase alloying is theoretically applicable to any metal combination that you can imagine, and the fact that manufacturing occurs wholly in the solid state means you can begin to consider totally new alloys that we’ve not been able to make before.”

The solid phase alloying process could be used to create custom metal wire alloys for various 3D printing technologies, Li said. For example, wire arc additive manufacturing, or “WAAM,” is used to 3D print or repair metal parts. In this process, a roll of wire feeds into a robotic welding torch, which melts it to build 3D parts.

“It’s difficult to obtain feed wires with customized compositions for wire-based additive manufacturing,” said Li. “Solid phase alloying is a fantastic way to produce tailored alloys with exact compositions such as 2 percent copper or 5 percent copper.”

The research study was supported by the Laboratory Directed Research and Development program at PNNL, as part of the Solid Phase Processing Science Initiative. PNNL researchers Tianhao Wang, Zehao Li, Tingkun Liu, Xiang Wang, Arun Devaraj, Cindy Powell and Jorge F. dos Santos also contributed to the research.