Metal’s Superplasticity Stretches Envelope
Superplasticity. The mere sound of the word makes you want to dance, right?
Well, maybe not, but chances are that manufacturers are going to be singing its praises more and more in the coming years.
Some metals, such as certain aluminum alloys, reach superplasticity when cooked to temperatures up to 1,800 degrees Fahrenheit. At that temperature, the metal can be stretched like a sheet of thin plastic and forced against a mold to form an extremely strong, intricately detailed part.
The technology has gained a toehold in industry, especially among aircraft manufacturers, but it has been slow to catch on for mass producers of consumer items such as automobiles. The problem is it takes too long to fabricate a part for a high-volume industry, and the required temperatures are too high for some applications.
But now there is some indication that that may be changing.
Researchers at UC Davis say they have been able to achieve superplasticity at temperatures as low as 450 degrees, and they claim to have opened the door to using the technology with a wide range of materials, including nickel and ceramics.
That has “an enormous potential for application as lightweight, high-strength structural materials,” said Amiya K. Mukherjee, professor of materials science at Davis. “A typical example would be the metallic neck braces worn by people with whiplash injuries, where the total weight of the device can be reduced by about 70% without sacrificing strength.”
Scientists have heard similar claims before, and many question whether superplasticity will ever reach its potential in manufacturing.
“In terms of research the field is very exciting, but in terms of application I’m very disappointed,” said Amit Ghosh, professor of materials science and engineering at the University of Michigan in Ann Arbor. “The economics are against it in most cases.”
However, one Southland firm uses superplasticity almost exclusively, it says, to manufacture a wide range of products for everything from missiles to sports cars.
“It enables you to make complex shapes in aluminum alloys that would normally be quite expensive and very difficult to achieve” by other means, said Anthony Barnes, vice president for technology at Superform USA Inc. in Riverside.
The Davis researchers have taken the technology a step beyond the way it is now used in industry. Sam McFadden, a doctoral candidate and the lead author of a report on the Davis work published in the April 21 issue of Nature, believes their research opens new doors for the technology.
Superplasticity is at its best when used to fabricate large, complex parts, like the ribs around an aircraft fuselage. Without superplasticity, McFadden said, “you would have to cut out lots of different parts and do a bunch of machining and then use fasteners to stick them together, or weld them together. Superplasticity can allow you to make a part like that all in one piece.”
Metals used in the current process are made up of tiny grains, called microcrystals, that are no bigger than about one-fourth the width of a human hair. The Davis group experimented with metals that have been reconstituted by Russian collaborators so that the microcrystals have been replaced by nanocrystals, which are 1,000 times smaller.
“In the case of superplasticity, really small grains can roll over each other like ball bearings,” said Cliff Bampton of Rocketdyne Propulsion & Power in Canoga Park. That makes it possible to stretch a sheet of metal by up to 1,000% before it fails, said Bampton, who pioneered the field while with Boeing, which uses the technology for some airframe components.
That would suggest that the smaller the grain, the easier it should be to deform the metal into the desired shape because it is easier for the smaller grains to move around.
“If you can go to a smaller grain size, they can rotate and slide over each other significantly faster for a given temperature and stress,” Bampton said. “So you can get a part made in a few minutes instead of a few hours.”
“The finer the grain size, the faster you can form parts,” said Superform’s Barnes, “but the price of those materials with nanosized grains is enormous because of the exotic ways in which they are produced.
“The pot of gold at the end of the rainbow would be a fast-forming, low-cost material so that you could form parts in seconds rather than tens of minutes,” Barnes said. But he doesn’t see that on the immediate horizon.
Furthermore, said Bampton, when nanocrystals are heated to the temperatures needed for superplasticity, they tend to grow and the material becomes unstable.
“So you’ve got to come up with some technique of stabilizing that fine grain size at the elevated temperatures where you are going to be doing your superplastic forming,” he said.
The UC Davis researchers believe their work shows that the temperature needed to achieve superplasticity can be lowered by reducing the grain size, and that should extend the technology to some materials that are very difficult to machine, such as ceramics.
The cost of the ultrafine-grained materials and the time required to form parts remain the major obstacles.
“When you are building airplanes, you can afford to take a couple of hours to form a part,” said Davis’ McFadden. “But when you are building automobiles, that will kill you.”
So at this point it remains an exciting technology that has the experts debating its future.
“I feel sad about that,” said Ghosh.
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Lee Dye can be reached at leedye@compuserve.com.
