|
|
|
Semi-Solid Aluminum R&D Produces
Industrially-Relevant Results
02-06-03
Semi-solid aluminum has created much interest in the parts manufacturing industry, as the accompanying article describes. Broader application of the semi-solid process depends on a better understanding of the fundamental issues concerning the constitutive behavior of the material during processing. With this goal in mind, a joint industry-university-National Lab project, supported by the Department of Energy's Office of Industrial Technologies, was begun. This project has now concluded, and a number of potentially valuable results have been documented and transferred to industry.
The Semi-Solid Aluminum Alloys project was one of the efforts funded through the Aluminum Industry of the Future program at DOE-OIT. The driving force from DOE's perspective was the potential energy savings for the semi-solid forming process compared to conventional casting. This energy saving is due to the lower metal temperatures required, as well as the potential for improved repeatability and productivity. While semi-solid forming is being used selectively in the auto industry, it has not seen widespread use due to cost and quality concerns.
The R&D team for this project was lead by the Aluminum Casting Research Center at Worcester Polytechnic Institute with support from the Massachusetts Institute of Technology and Oak Ridge National Laboratory. Tied in closely with the project were over twenty industrial partners from the Cast Metals Coalition providing guidance and opportunities for testing of the project results.
One area of focus in the project was the measurement and modeling of the fundamental rheological aspects of the material and process. MIT investigated flow behavior under dynamic conditions through the use of a drop forge viscometer apparatus that more closely simulates the conditions that occur in the casting process. These results were utilized in process models developed by WPI based on the Herschel-Bulkley fluid model. These models were used to study basic flow geometries in the semi-solid forming process, develop processing maps, and better understand the types of processing defects that can occur.
A second area of effort, not originally envisioned in the project, was the discovery and development at MIT of an alternative process for producing a semi-solid microstructure. It was found that the formation of the desired non-dendritic structure was developed during initial stirring of the melt when as little as 1% solid phase was present. Subsequent stirring and cooling had secondary effects on the structure. The importance of this observation is that it points to a potentially much lower cost feedstock production method.
Another important area addressed in this project was an increased understanding of the microstructure of semi-solid aluminum alloys on a quantitative basis. Using image analysis techniques, WPI studied the initial microstructures of semi-solid materials produced by a variety of methods. This was then correlated with changes that occurred in these microstructures with heating into the solid + liquid phase field as well as the effect of holding time in the semi-solid condition. One key difference observed was in the amount of entrapped liquid in the alpha phase of the alloy. An ideal semi-solid microstructure would contain no entrapped liquid, since this liquid does not contribute to enhancing the flow of the material. Notably, the semi-solid material produced by the new MIT process did not contain any entrapped liquid, while materials produced by other processes did. Also measured was the size and shape of the alpha aluminum particles, which evolved with reheating and holding time.
The number of aluminum alloys that are available for semi-solid forming is now still fairly small, and the objective of the ORNL portion of the program was to develop a fundamentally-based methodology to develop alloys especially suited for semi-solid forming. Specifically, ORNL utilized computational thermodynamics software to simulate the solidification behavior of commercial semi-solid alloys and identified the key solidification characteristics. They found potential for Al-Mg-Si alloys with 4-6% Mg and 0.5-2% Si that may be able to be processed at a higher solid fraction, which would produce advantages in the area of process robustness.
This initial project has just been completed, and the final report is still in
preparation. A follow-on effort is planned that will further develop the research
directions described here. Additional information can be obtained from Prof. Diran
Apelian, Director of the Metal Processing Institute at WPI, at dapelian@wpi.edu,
or at the DOE-OIT Aluminum Industries of the Future web site a www.eere.energy.gov/industry/aluminum.
Article provided courtesy of The Aluminum Association - www.aluminum.org
|
|
|
