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Spray Rolling of Aluminum Strip-A Future Production Method?
12-27-02
About 70% of the U.S. production of aluminum flat rolled products is done through the conventional approach of casting large rectangular ingots and then rolling them on large integrated hot and cold mill facilities to the final product thickness. Continuous casting processes, such as twin roll casting and block casting, are in commercial use, but primarily for low solute alloys. What if there were a process that provided the cost and energy advantages of continuous casting and the ability to make higher solute alloys now currently produced commercially only by the ingot casting and rolling approach? Such a process may be on the horizon, based on development work in a DOE-OIT Aluminum Industries of the Future program on a process called "spray rolling".
Spray rolling combines elements of twin roll casting with another process called spray forming. Spray forming has been under development for nearly 30 years, and is commercially used for producing specialty aluminum products as well as other ferrous and non-ferrous alloys. It is characterized by a spray of metal droplets formed by atomizing, or breaking apart, a molten metal stream. Through careful control of droplet size and cooling conditions, the metal is deposited in a partially solidified state that allows a nearly fully dense metal product to be formed. Spray forming has been most effectively employed for "bulky" products such as extrusion stock or forging preforms, with production of flat products being hampered by challenges of maintaining strip flatness and eliminating porosity, especially at the deposit/substrate interface. The spray rolling project is an extension of earlier work funded by DOE to develop a spray forming process for aluminum sheet production.
The new project, led by Prof. Enrique Lavernia, now at the University of California, Davis, CA, seeks to overcome these barriers. A schematic view of the process, shown in the figure, illustrates that a patterned spray of atomized metal droplets is deposited into the gap of mill rolls, in a manner similar to that used for twin roll casting, for consolidation. By eliminating the typical deposition substrate, the issue of deposit/substrate porosity is eliminated. Strip flatness would be ensured by the control of strip shape through the rolls. The more rapid cooling of the metal droplets as compared to a fully liquid molten metal stream will enable higher solute and hence higher performance alloys to be produced economically in strip form.
This process won't be appearing in aluminum production plants anytime soon. The initial feasibility study is planned for a three-year study, primarily at the bench scale, to demonstrate feasibility. At the end of the project, the goal would be to transfer the technology to the industrial partners for further development and scale-up. A strength of this program, as with the other DOE-OIT Industry of the Future programs, is the direct involvement of aluminum industry participants in the work. In the case of the Spray Rolling Aluminum Strip project, Alcoa, Inc. (Pittsburgh, PA), and Pechiney Rolled Products (Ravenswood, WV) are the industry participants. A diversity of industry, National Lab, and university capabilities are provided by the other project team participants: Inductotherm Corp., Metals Technology, Inc., Idaho National Engineering and Environmental Laboratory (INEEL), and Colorado School of Mines (CSM). INEEL's role in the project involves equipment design, construction, and verification of the spray rolling concept at the bench-scale level. The spray forming step is being investigated in detail both experimentally and numerically in order to characterize the relationship between process parameters and the spray state (solid fraction, size distribution, dispersion, velocities) versus distance from the nozzle. The modeling efforts are also directed to optimizing the processing parameters to produce a stable rolling condition, predict deposition geometry on the rolls, and simulate the rolling temperature during spray rolling process.
Further information on the project can be obtained from the DOE-OIT Industries
of the Future page for Aluminum at www.eere.energy.gov/industry/aluminum.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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