Lighter vehicles can travel farther on less energy, hence driving demand for lighter automotive components. High-performance aluminum alloys, such as alloy 7075, are among the lightest and strongest options, but they require energy-intensive production that raises costs and therefore limits their use.

Now, researchers from the Pacific Northwest National Laboratory (PNNL) have shown that their Shear Assisted Processing and Extrusion (ShAPE) technology (earlier post) can eliminate heat treatment steps in the production process, resulting in significant energy savings and reduced emissions. An open-access paper on the study is published in the journal Materials & Design.

Conventional metal production uses heat to melt individual metals and alloying elements together—such as aluminum, copper, or magnesium—to create alloys that are lighter, stronger, or easier to form. If these elements aren’t well-mixed, cracks and fractures can form during processing that compromise the properties of the final product. In metals production, heat is used to ensure that individual metal elements in an alloy are well-mixed during a step called homogenization.

During homogenization, large metal castings called billets are heated to nearly 500 degrees Celsius—about 900 degrees Fahrenheit—for up to 24 hours. This heat treatment step dissolves alloy aggregates in the billet to ensure that all metal elements are evenly distributed or homogenized. This improves the performance of the final product. After homogenization, the metal rods undergo further heating and forming in a step called extrusion.

Homogenization is the biggest energy-consuming step in the entire metals extrusion process.—Scott Whalen, PNNL chief materials scientist and co-developer of ShAPE

PNNL’s ShAPE process uses a machine to spin billets or chunks of bulk metal alloy, creating just enough heat through friction to soften the material so it can be easily extruded through a die to form tubes, rods, and channels. The simultaneous linear and rotational forces use only 10% of the force typically needed to push the material through the die in conventional processes.

PNNL researchers have now found that the ShAPE machine eliminates the need for separate homogenization and extrusion steps by combining heating and deformation—the change in the shape of the metal itself. In the ShAPE machine, the metal billet is simultaneously pushed through a small opening in a die which rotates. Together, the rotational movement and deformation thoroughly mixes the metal elements as they are being extruded. Essentially, the ShAPE process homogenizes the metal billet in a few seconds, immediately before it is extruded. This eliminates the need for a day-long, pre-heating homogenization step and means that no additional energy is used to heat the billet during extrusion. Together, this results in an energy savings of up to 50% using ShAPE.

Extrusion of unhomogenized castings of the 7075 aluminum (Al) alloy has been accomplished using shear assisted processing and extrusion (ShAPE). The simultaneous plastic deformation and heat generation during ShAPE rapidly fracture and dissolve interdendritic and intragranular secondary phases of Al-Zn-Mg-Cu, accomplishing homogenization in seconds rather than many hours in a furnace before extrusion. ShAPE thereby eliminates the energy-intensive and time-consuming homogenization step required to prepare as-cast microstructures for conventional extrusion.

Concurrently, extensive grain refinement occurs due to gradient activation of dynamic recrystallization during ShAPE, which facilitates a threefold increase in extrusion speed compared to the conventional extrusion method. Evident enhancement of the mechanical properties of ShAPE + T6 samples is achieved, compared to the ASTM standard values for conventional extrusion products. —Wang et al.

Images of aluminum alloy 7075 taken with a scanning electron microscope before (A), during (B), and after (C) going through the ShAPE machine show how the microstructure of the alloy changes during extrusion. The shearing effect of the ShAPE machine breaks up particles, into much smaller pieces to create a more uniform microstructure. (Image by Joshua Silverstein | Pacific Northwest National Laboratory)

Not only is ShAPE a more energy efficient and quicker process, but it also improves how well the individual alloying elements are mixed, leading to a better final product. Performance testing showed that components made of aluminum alloys processed with ShAPE exceeded current American Society for Testing and Materials standards for strength and elongation.

Schematics of (a) the ShAPE process and (b) the corresponding material response in the remnant billet after extrusion (longitudinal view). Wang et al.

We took a closer look using an electron microscope and saw that ShAPE breaks apart the alloy aggregates and dissolves them into the aluminum matrix prior to extrusion, making it more extrudable. This translates to better performance—our aluminum 7075 alloys are stronger and stretch farther before breaking.—Tianhao Wang, PNNL materials scientist and lead author

The most high-performance aluminum alloys are time- and energy-intensive to manufacture, pricing them out of many markets, such as applications in passenger vehicles. The ShAPE process removes a major hurdle in the production of high-performance aluminum alloys by significantly reducing energy consumption and greenhouse gas emissions during manufacturing.

This research was supported by the Department of Energy’s Advanced Manufacturing Office and performed using a purpose-built ShAPE machine manufactured by BOND Technologies, Inc. The ShAPE process, tooling, and techniques are available for licensing.

Tianhao Wang, Julian Escobar Atehortua, Miao Song, Md Reza-E-Rabby, Brandon Scott Taysom, Josh Silverstein, Timothy Roosendaal, Darrell Herling, Scott Whalen (2022) “Extrusion of Unhomogenized Castings of 7075 Aluminum via ShAPE,” Materials & Design, Volume 213, doi: 10.1016/j.matdes.2021.110374

Posted on 20 July 2022 in Manufacturing, Market Background, Materials | Permalink | Comments (0)

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