Researchers from the Pacific Northwest National Laboratory have demonstrated an advanced friction extrusion manufacturing process to produce nanostructured rods and tubes directly from high-performance aluminum alloy powder in a single step.
Using a novel Solid Phase Processing (SPP) approach, the research team eliminated several steps that are required during conventional extrusion processing of aluminum alloy powders, while also achieving a significant increase in product ductility.
Extrudates exhibited 450 MPa ultimate tensile strength, 380 MPa yield strength, and 15.7% elongation at ambient temperature. Twice the elongation was achieved compared to conventional direct extrusion of the same material, with similar ultimate and yield strengths, and is attributed to extensive reduction of the matrix grain size and refinement and redistribution of nanoscale second phases.—Whalen et al.
(SPP is a high-strain process applied to materials during fabrication to produce high-performance alloys, semi-finished products, and engineered assemblies without the requirement to melt the constitutive materials. This emerging manufacturing platform delivers extraordinary performance in metal alloys, relative to identical materials produced by conventional manufacturing routes.)
This is good news for sectors such as the automotive industry, where the high cost of manufacturing has historically limited the use of high-strength aluminum alloys made from powders.
The team’s research is described in the paper “High Ductility Aluminum Alloy Made from Powder by Friction Extrusion,” published in the June 2019 issue of Materialia.
High-performance aluminum alloys made from powder have long been used in lightweight components for specialized aerospace applications, where cost is not a limiting factor. However, these alloys have typically been too expensive for the automotive industry.
A typical extrusion process for aluminum alloy powders is energy- and process-intensive, requiring multiple steps to mass-produce the material. First, the loose powder must be loaded into a can and gases removed using a vacuum (degassing). he can is then sealed, hot pressed, pre-heated, and placed into the extrusion press. After extrusion, the can is removed (decanned) to reveal the extruded part made from consolidated powder.
In this study, the team eliminated many of these steps, extruding nanostructured aluminum rods directly from powder in a single step, using PNNL’s Shear Assisted Processing and Extrusion technology, or ShAPE.
ShAPE allows creation of wire, bar, and tubular extrusions that show significant improvement in material properties—for example, magnesium extrusions have been manufactured with unprecedented ductility (how far the material can stretch before it breaks) and energy absorption (how much energy can be absorbed during compression of a tubular extrusion) over conventional methods.
In the ShAPE process, a powder—in this case, an Al-12.4 aluminum alloy powder provided by SCM Metal Products, Inc., a division of Kymera International—is poured into an open container. A rotating extrusion die is then forced into the powder, which generates heat at the interface between the powder and die. The material softens and easily extrudes, eliminating the need for canning, degassing, hot pressing, pre-heating, and decanning.
This is the first published instance of an aluminum alloy powder being consolidated into nanostructured extrusions using a single-step process like ShAPE.
The elimination of both the processing steps and the need for pre-heating could dramatically reduce production time as well as lower the cost and overall embedded energy within the product, which could be beneficial for automotive manufacturers who want to make passenger vehicles more affordable, lighter, and fuel-efficient for the consumer.—PNNL materials scientist Scott Whalen, who led the study
Besides providing the Al-12.4 powder, SCM Metals Products performed mechanical testing to validate the resulting material’s performance. PNNL and SCM Metal Products, Inc. are now collaborating on a project for DOE’s Office of Technology Transitions to scale up the process for larger diameter extrusions.
Ductility. Elimination of processing steps and reduced heating weren’t the only successful findings by the team.
While high-performance aluminum alloys have historically shown excellent strength, they have typically been hampered by poor ductility. However, the team found dramatic improvements in the ductility of the extrusion produced by ShAPE, measuring ductility that is two to three times that found in conventional extrusion products, and with equivalent strength.
To understand the reason for the substantial increase in ductility, transmission electron microscopy was used to evaluate the microstructures of the powder and the extruded materials.
The results indicated that the ShAPE method refined the second phases in the powder—tiny strengthening particles of non-aluminum materials. ShAPE reduces the particles to nanoscale sizes and evenly distributes them throughout the aluminum matrix, increasing ductility.
ShAPE is part of PNNL’s growing suite of capabilities in Solid Phase Processing—a disruptive approach to metals manufacturing that can be better, cheaper, and greener than melt-based methods typically associated with metals manufacturing.
The research was supported by the Materials Synthesis and Simulation Across Scales Initiative, a Laboratory Directed Research and Development project at PNNL. In addition to Kymera International, researchers from the University of California, Riverside collaborated on this project.
Scott Whalen, Matthew Olszta, Christian Roach, Jens Darsell, Daniel Graff, Md. Reza-E-Rabby, Timothy Roosendaal, Wayne Daye, Tom Pelletiers, Suveen Mathaudhu, Nicole Overman (2019) “High ductility aluminum alloy made from powder by friction extrusion,” Materialia, Volume 6, doi: 10.1016/j.mtla.2019.100260