A photo showing the results of sinuous flow in which metal is deformed into folds while being cut

Discovery could improve metal machining efficiency

The discovery of a previously unknown type of metal deformation and a method to suppress it could lead to more efficient machining, say researchers.

Using high-speed microphotography, engineers from Purdue University have discovered that when ductile metals - those that can be formed into wires - are cut, they are deformed into folds. The process, dubbed 'sinuous flow' by the team, flies in the face of long-held assumptions that the metals are sheared uniformly.

More importantly the researchers found that simply painting the metal with a standard marking ink supressed the phenomenon, while at the same time reducing the required cutting force by 50 per cent, a finding that could allow faster and more efficient industrial machining by reducing the force and energy required to process metals.

"The fact that the metal can be cut easily with less pressure on the tool has significant implications," said professor W Dale Compton. "Machining efficiency is typically limited by force, so it is possible to machine at a much faster rate with the same power."

In a paper published in the Proceedings of the National Academy of Sciences yesterday the scientists describe how the folding resembles patterns created during the flow of highly viscous fluids such as honey, liquid polymers and even natural rock formations. They borrowed methods from the geophysics community to help analyse the fold properties in metals.

According to lead author professor Srinivasan Chandrasekar, painting the marking ink on the surface of the metal appears to reduce the formation of these folds due to the fact it had been designed to stick well to metals.

The discovery is intriguing to researchers because the ink was not added between the cutting tool and the metal; it was painted onto the free surface of the metal where it was not in direct contact with the tool, which was used to shave a layer from the top of the metal.

In one class of experiments the researchers inked only half of a sample. When the cutting tool reached the inked portion, the amount of force required dropped by half immediately.

"This may sound eerie, even ridiculous to people in the field because the cutting is not happening on the painted surface, it is occurring at some depth below," graduate student Koushik Viswanathan said.

According to the team, coatings with improved adhesion could potentially produce even greater suppression of sinuous flow and further reductions in cutting force, and Chandrasekar added that understanding sinuous flow and how to control it could lead to new opportunities in machining, stamping, forging and sheet-metal processes.

Deliberately enhancing sinuous flow could also be used to design materials with improved energy absorption for applications such as armour, vehicles and structures.

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