A Mechanochemical Route to Cutting Highly Strain-Hardening Metals

Abstract

Highly strain-hardening metals such as Al, Ni, and stainless steels, although relatively soft, are well known as being difficult to cut, because of an unsteady and highly redundant mode of plastic deformation—sinuous flow—prevailing during chip formation. This difficulty in cutting is greatly ameliorated, if the workpiece surface ahead of the chip formation region is coated with certain chemical media such as glues, inks, and alcohols that are quite benign. High-speed imaging shows that the media effect a change in the local plastic deformation mode, from sinuous flow to one characterized by periodic fracture—segmented flow. This flow transition, due to a mechanochemical effect, results in significant reduction of deformation forces and energy, often > 50%, thus facilitating the cutting. The effect is mostly pronounced at smaller undeformed chip thickness, typical of finish and semi-finish machining regimes. The quality of the cut surface, as measured by defect density and surface roughness, improves by an order of magnitude, when the media are applied. Furthermore, this surface is relatively strain free in contrast to conventionally machined surfaces. The mechanochemical effect, with a strong coupling to the flow mode, is controllable, with the media showing similar efficacy across different metal systems. The results suggest opportunities for improving performance of machining processes for many difficult-to-cut gummy metals.