The observed folding in metal resembles patterns created during the flow of highly viscous fluids such as honey and liquid polymers. It also is similar to fold patterns observed in natural rock formations. The researchers borrowed methods from the geophysics community in their analysis of fold properties in metals.

They also found that sinuous flow can be controlled by suppressing this folding behavior by simply applying common marking ink remote from the cutting interface.

Our observations establish sinuous flow as another mesoscopic deformation mode, alongside mechanisms such as kinking and shear banding. Additionally, by suppressing the triggering surface undulations, sinuous flow can be eliminated, resulting in a drastic reduction of cutting forces. We demonstrate this suppression quite simply by the application of common marking ink on the free surface of the workpiece material before the cutting. Alternatively, prehardening a thin surface layer of the workpiece material shows similar results. Besides obvious implications to industrial machining and surface generation processes, our results also help unify a number of disparate observations in the cutting of metals, including the so-called Rehbinder effect.

—Yeung et al.

The conventional view. Schematic of idealized plane–strain cutting showing chip formation by smooth laminar flow, with simple shear. The deformation zone is highlighted in blue. Initial chip thickness h0 is measured from free surface to the surface of material separation. The workpiece material undergoes plastic shape transformation to form a chip with final thickness hc. The velocity of bulk material flow against the tool is V0. When the workpiece ia annealed, an unusually thick chip results and the forces in- volved in the process are very large. This well-known difficulty in cutting had eluded fundamental explanation. The high forces required were attributed to the thick chip developed in the process, without an explanation of the cause of such anomalous chip formation. Yeung et al. Click to enlarge.

Sinuous flow. The image, at left, shows a previously unknown type of metal deformation—sinuous flow—in which metal is deformed into folds while it is being cut. New research findings, in graph at right, reveal the cutting force can be reduced 50% simply by painting metal with a standard marking ink, suggesting that not only can energy consumption be reduced by 50% but also that machining metals can be achieved faster and more efficiently and with improved surface quality. (Purdue University image/ Ho Yeung and Koushik Viswanathan) Click to enlarge.

When the metal is sheared during a cutting process it forms these finely spaced folds, which we were able to see for the first time only because of direct observation in real time.

—postdoctoral research associate Ho Yeung

Findings showed the cutting force can be reduced 50% simply by painting metal with a standard marking ink. Because this painted layer was found to suppress sinuous flow, the implications are that not only can energy consumption be reduced by 50% but also that machining can be achieved faster and more efficiently and with improved surface quality, said Srinivasan Chandrasekar, a professor of industrial engineering.

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

—W. Dale Compton, the Lillian M. Gilbreth Distinguished Professor Emeritus of Industrial Engineering

Applying less force also generates less heat and vibration, reducing tool wear and damage to the part being machined, which would improve the accuracy of the process while reducing cost, Compton said.

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.

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

In one class of experiments, Yeung inked only half of a sample. When the cutting tool reached the inked portion, the amount of force dropped immediately by half. Yeung tested various coatings including the marking ink, nail polish, resins and commercial lubricants. He also tried first coating metal with a lubricant before adding the ink. Findings revealed that because the lubricant prevented the ink from sticking well to the surface, the suppression of the sinuous flow was less effective.

It seems that the ink used commercially to mark metal is very good at suppressing the sinuous flow, probably because it is designed to stick well to metals

—Srinivasan Chandrasekar

The discovery leaves open the possibility that coatings with improved adhesion might produce greater suppression of sinuous flow and further reductions in cutting force. Although the team made the discovery in metal-cutting experiments, Chandrasekar said understanding sinuous flow and its suppression and control could lead to new opportunities in a range of manufacturing applications that involve metal deformation such as in machining, stamping, forging and sheet-metal processes.

Another possibility is the design of new materials for energy absorption—by deliberately enhancing sinuous flow—for applications in armor, vehicles and structures.

Future research will include work to develop a model for sinuous flow, to learn more about the physical mechanisms in sinuous flow and its suppression and to investigate properties of coatings. The work was funded by the National Science Foundation and conducted through Purdue’s Center for Materials Processing and Tribology.

Resources

• Ho Yeung, Koushik Viswanathan, Walter Dale Compton, and Srinivasan Chandrasekar (2015) “Sinuous flow in metals” PNAS doi: 10.1073/pnas.1509165112