The Shear Assisted Processing and Extrusion (ShAPE^TM) project shows the potential to increase throughput and reduce manufacturing costs and energy use for aluminum alloy 7075 extrusions.

Aluminum alloy 7075 (AA7075) is a high-strength, low-weight metal alloy that can handle heavy mechanical stresses. These properties make AA7075 attractive for automotive, aviation, aerospace, defense and marine applications. Considering its suitability for harsh service conditions, there is additional demand from other sectors for this lightweight structural material.

AA7075 has not been adopted more widely, however, because it is expensive to manufacture and difficult to extrude. Relevant challenges include an inherently slow extrusion speed, high energy use, high ram pressure, narrow process window, high flow stress and sensitivity to incipient melting. Alternatives or improvements to conventional extrusion methods can address these challenges.

One viable alternative extrusion method for AA7075 is solid-phase processing (SPP) – a family of emerging techniques that process bulk metals under severe plastic deformation without melting. SPP methods are notable because they offer a way to manufacture metals and alloys with potential for enhanced performance at lower cost. New research funded by the U.S. Department of Energy’s Advanced Manufacturing Office (AMO) is exploring the use of a new SPP approach called Shear Assisted Processing and Extrusion (ShAPE^TM) for the manufacture of AA7075 extrusions.

In ShAPE, a rotating die is rammed against a metal feedstock, which results in heating due to deformation and friction. The metal softens as a result of this heat, while spiral scroll features on the face of a rotating die force material between the mandrel and the die to form hollow profile extrusions (Fig. 1). This combination of linear and rotational shear, which is unique to ShAPE, enables extensive grain refinement, uniform dispersion of secondary phases and alignment of crystalline structures.

Due to the high shear deformation and mixing that occurs during ShAPE, AA7075 can be fed into the process as either a homogenized billet or a direct-chill cast billet and yield a uniform internal composition after extrusion. This is a clear advantage over conventional extrusion techniques, which are unable to process cast billets.

Those billets are too brittle for conventional machines to readily extrude and have a microstructure that is highly segregated and non-uniform, which results in a poor-quality extruded material during conventional extrusion. The use of cast billets avoids the need to homogenize billets, eliminating an energy-intensive, 24-hour-long thermal-treatment process involving temperatures ranging from 392-896°F (200-480°C). ShAPE’s compatibility with cast billets alone is estimated to save 5% on the cost of extruded product.

Awards and Patents for ShAPE

ShAPE is an emerging manufacturing method that has processed a variety of materials, including aluminum, magnesium, steel, copper, magnets and even semiconductors. The process has been recognized through the following awards and patents:

2020 R&D 100 award winner – Process/Prototyping category¹

Two patents (U.S. 10,189,063 “System and Process for Formation of Extrusion Products” and U.S. 10,695,811 “Functionality Graded Coatings and Claddings”)

Seven patents pending

Rapid Extrusion of AA7075 with ShAPE

To demonstrate the capabilities of ShAPE, researchers at Pacific Northwest National Laboratory (PNNL) manufactured over 100 meters of 12-mm-diameter tubular extrusions with 1-mm wall thickness from both homogenized and cast billets. One goal of this project was to increase the speed of extrusion compared to conventional techniques, which are limited to speeds of 1-2 meters/minute with homogenized billets.²

When these speeds are exceeded during conventional extrusion, downstream issues such as surface tearing and cracking occur due to abnormal grain growth toward the outer diameter. Project results show that ShAPE was able to easily surpass the project’s goal by achieving extrusion rates up to 12.2 meters/minute and 6.2 meters/minute for homogenized and cast billets, respectively (Fig. 2). Faster speeds translate to lower production costs and better thermal efficiency for AA7075 extrusion.

Figure 3 shows the surface finish for homogenized billets extruded at 12.2 meters/minute and 13.9 meters/minute. The onset of surface tearing and cracking can be seen in the 13.9 meters/minute sample compared to the smooth, defect-free surface resulting from processing at 12.2 meters/minute. Notably, the surface markings for the 12.2 meters/minute extrusion are helical due to the rotational shear component of ShAPE. These markings contrast with those produced in conventional extrusion, where surface markings are strictly linear. Using cast billets, extrusion at 6.2 meters/minute was achieved while maintaining smooth surfaces with tearing onset not occurring until 7.4 meters/minute.

Comparable Strength, Improved Elongation for AA7075 through Novel Microstructures

With ShAPE, AA7075 achieved the ultimate strength, yield strength and elongation seen in Table 1 for the project’s tubular extrusions made from homogenized and cast billets. Each sample was heat treated to the T6 condition after extrusion. This table also compares the mechanical properties measured for the ShAPE specimens to the corresponding ASTM standard³ and ASM typical values⁴ for extruded AA7075-T6 tubing.

Ultimate and yield strength of ShAPE extrusions are on par with the standards for homogenized billets and are substantially higher for cast (unhomogenized billets). For both types of billets, elongation is significantly improved compared to the standards. The fact that ShAPE obtained such high performance from cast billets, which completely avoid the thermally intensive homogenization step, is a major project achievement that underscores the key benefits of ShAPE.

ShAPE achieved the strong material properties for AA7075 shown in Table 1 due to the novel microstructures formed during processing. Figure 4 shows the overall grain structure of the cast and homogenized billets that undergo significant refinement during ShAPE extrusion. Due to the high degree of mixing in ShAPE, the resulting grain structures were not particularly sensitive to the starting microstructure of the billet.

Figure 5 shows SEM images in the longitudinal and transverse planes (relative to the extrusion direction) after T6 heat treatment for ShAPE extrusions made from homogenized billets. Grain size is seen to be uniform across the entire wall thickness. Compared to conventional extrusion, where second-phase particles tend to form “stringers” in the extrusion direction, second-phase particles are more refined and evenly distributed with ShAPE, which is the source of such dramatic improvement in elongation.

Energy Use and Manufacturing Analysis of ShAPE Process

As part of this AMO project, National Renewable Energy Laboratory is performing a techno-economic analysis (TEA) and manufacturing analysis on ShAPE. These analyses will evaluate the cost- and energy-reduction potential of this manufacturing technology. Models for ShAPE and conventional extrusion, based on materials and energy inputs, have been developed for both AA7075 billets and high-performance aluminum-powder feedstocks.

Initial findings from the economic and energy analysis indicate that AA7075 tubes created with ShAPE use less energy than tubes that are conventionally heated and extruded. The reduction in energy use is possible because preheating the billet in a furnace is not required, the billet homogenization step is eliminated and faster extrusion speeds improve thermal efficiency.

Additional energy savings are possible because the post-extrusion solution heat-treat step may be unnecessary with ShAPE. Future TEA will quantify the potential cost and energy implications as a function of different factors (e.g., raw-materials price fluctuations).

Technology Scaling: ShAPE 2.0

ShAPE has been used to extrude numerous magnesium and aluminum alloys at laboratory scale with diameters up to 2 inches and lengths up to 15 feet. One of the advantages of the ShAPE process, compared to other severe plastic deformation techniques, is its potential to scale up to industrial manufacturing. With support from the U.S. Department of Energy and others, PNNL plans to build and test a larger machine in a new project: ShAPE 2.0.

This new machine will enable extrusion at industrially relevant diameters and lengths using the general principles established in the existing project, which is scheduled to end September 2021. The new project will achieve industrial specifications using significantly higher ram force and spindle torque capacity.

ShAPE 2.0 will also enable extrusion of more difficult materials such as steel, titanium and intermetallics. Since the ShAPE 2.0 machine will demonstrate the industrial scalability of this process, this project will also enable increased collaboration with industrial partners.

Conclusion

Researchers at PNNL selected aluminum alloy AA7075 for this project because of its extrusion challenges and potential for processing cost and energy savings. The results of this project indicate that ShAPE can provide a pathway to improve the manufacture of AA7075 so it can be used in a range of new applications. ShAPE has shown it is ready to be scaled up and tested with many more metals as it moves toward industrial production of AA7075.

All images and graphics were contributed by the AMO at the DOE.

References

1. Heney, Paul, “R&D 100 Award winners announced in Process/Prototyping and Software/Services categories” R&D World, Oct. 2020. https://www.rdworldonline.com/rd-100-award-winners-announced-in-process-prototyping-and-software-services-categories/.
2. M. Bauser, Historic Development of Extrusion, Extrusion: Second Edition, edited by M. Bauser, G. Sauer, K. Siegert, ASM International, 2006, pg. 2-8
3. ASTM B221-14, “Standard Specification for Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wires, Profiles, and Tubes,” vol. 02.02, ASTM International
4. ASM Handbook Volume 2b: Properties and Selection of Aluminum Alloys, edited by K. Anderson, J. Weritz, G. Kaufman, ASM International, 2019, pg. 432-438