A metals-forming process called semi-solid forming, developed 25 years ago by scientists at MIT, is now becoming industrially important, giving automobile components manufacturers a new way to produce highly durable, lightweight precision parts.
Although the process was discovered in 1971, it was not until this year that it really took off, mainly because of new automotive industry requirements for parts of long life, reliability and light weight that cannot be met by the traditional forming process of die casting. To date, the MIT process has been used primarily to make long-lasting aluminum components for high-stress or leak-tight systems such as suspensions and air conditioners in cars.
"The automotive industry pays a great deal of attention to making parts as low cost, light weight and reliable as possible," said Merton C. Flemings, Toyota Professor in the Department of Materials Science and Engineering. "So the automotive industry is now using much more aluminum than before. This is a real growth area for the process," which is formally called "rheocasting," or flow casting.
Rheocasting was discovered during the doctoral thesis work of David Spencer, one of his graduate students. Dr. Spencer was researching the effects of fluid flow during the solidification of metals. He found that agitation of a molten metal during solidification made it smooth and creamy when it was partly solid, like ice cream. It was immediately clear to the researchers that this flowable semi-solid material could be the basis of a wholly new metal forming process, one that would avoid the high cost of forging and machining but would produce parts that would be stronger and more reliable than those made by coventional casting processes.
In commercial practice today, the first step is to produce aluminum continuous casting "feedstock" with the correct "flowable" structure. These castings are then cut into small lengths and shipped to the parts producer, where they are heated until they are partly liquid. (This segment of the process is like buying very cold ice cream from a supermarket and heating it until it is soft enough to eat.) Next, the soft metal "glob" is moved from the heater to a press, where it is shaped into the final part. Rheocasting makes near net-shaped parts, or parts that need little if any additional shaping after they are formed, thus saving time and expense during manufacturing.
Metal working is a historic art that goes back at least 10,000 years, Professor Flemings explained. Until now, almost all shaping of metals has been done when the metal was fully liquid as in a casting process, or fully solid as in a forging process. A semi-solid forming, or rheocasting, process is an historic innovation, a major change in the way we think about the forming of metals, Professor Flemings said.
"We've discovered a wholly new way to make shapes out of metals that has opened up new possibilities both for the manufacturers and for the users of metal parts such as the automobile industry," he said.
PARTS IN `PROWLER'
MIT has licensed the technology to Alumax Inc. of St. Louis, a pioneer in using rheocasting to make air-conditioner, suspension and other components at its Alumax Engineered Metal Processes Inc. subsidiary. Alumax will use the process to supply suspension-system control arms and steering knuckles for Chrysler Corp.'s Plymouth Prowler roadster, according to a September 4, 1996 article in American Metal Market, which quoted Alumax director of market development Jim Courtois. The Prowler is an aluminum-intensive car that is to be built for the 1997 model year. A number of major aluminum companies, auto makers and auto parts makers are using the process or components made by the process in the United States, Japan and Europe.
"In 1996, more than 10 million parts will be made by the process and rapid further growth is expected next year," Professor Flemings said.
According to a June 1996 article in Light Metal Age, an industry publication, semi-solid forming of aluminum and magnesium components is gaining a foothold in the automotive industry because of several significant advantages over conventional casting: energy savings of about 35 percent, reductions of component weight, higher strength and integrity of components, and closer tolerances and improved surface finish.
Professor Flemings said it is possible to cast composite materials by the process-although it is not yet done on a production scale-to achieve qualities like improved resistance to abrasion, or to dilute expensive metals with inexpensive materials such as sand, mica and recycled glass.
The original work on rheo-casting was supported by grants from the Office of Naval Research and the Army Research Office. Subsequent work was funded by the Defense Advanced Research Projects Agency (now called the Advanced Research Projects Agency), and current work is funded by the Department of Energy and industry sources.
A version of this article appeared in MIT Tech Talk on December 1, 1996.