A Cut above the Ordinary: Low-tech machining yields coveted nanostructure

Drills, lathes, and milling machines produce metal trimmings that machine shops discard as trash or melt down for reuse. A new study finds that these trimmings often end up with a fine-grained and especially hard microscopic structure equivalent to that of expensive high-tech materials.

STOP ACTION. The fine-grained chip of copper (at top) was created when a machine tool plowed into a piece of bulk copper (horizontal region at bottom). The chip’s finer structure makes it harder (higher numbers) than the piece from which it emerged. T. Brown et al./J. Mater. Res.

Because the presence of this hard structure endows materials with exceptional strength and wear resistance, materials scientists have long sought cheap and easy ways of inducing it in metals and alloys. Now it seems that companies could achieve this goal on industrial scales, and for as little as a hundredth the cost of current methods, says industrial engineer W. Dale Compton of Purdue University in West Lafayette, Ind.

That could open the door to widespread use of the improved metals in many demanding applications. Those could include gears and shafts for cars, aircraft, and agricultural machinery, as well as casings for armor-penetrating shells, Compton says. He and his Purdue colleagues report their findings in the October Journal of Materials Research.

Under a microscope, metals have a grainy look. Those granules are tiny metallic crystals in varied shapes and orientations. Scientists have known for at least 40 years that stretching a metal’s internal structure breaks up or otherwise shrinks those crystal grains. That shrinkage, in turn, hardens and strengthens the metal. These days, scientists often refer to the resulting structure as “nanocrystalline” because the diameters of the grains typically measure several hundred nanometers or less.

Suspecting that trimmings might be more interesting than metallurgists had believed, Compton and his colleagues polished, etched, and examined an assortment of machined scraps of various metals and alloys. “Lo and behold, we looked at them and saw they were nanocrystalline,” recalls research-team member Srinivasan Chandrasekar. The engineers also measured the hardness of the scraps to be up to three times that of their parent metal pieces.

It’s not new that machining transforms metals’ properties, comments Anthony G. Evans of the University of California, Santa Barbara. However, no one had made the connection to nanocrystalline structure. “It just needed someone clever enough to unearth it,” he says.

To create actual products from the trimmings, manufacturers would pulverize them and then sinter the resulting powder into parts, a process that would preserve the nanocrystalline structure. Besides potentially leading to new industrial processes, the Purdue researchers say, the findings may inspire factories and machine shops to seek new markets for their metal trimmings.

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