On the Cutting of Metals: A Mechanics Viewpoint
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AbstractThe mechanics of large-strain deformation in cutting of metals is discussed, primarily from viewpoint of recent developments in in situ analysis of plastic flow and microstructure characterization. It is shown that a broad range of deformation parameters can be accessed in chip formationstrains of 110, strain rates of 10105/s, and temperatures up to 0.7Tmand controlled. This range is far wider than achievable by any other single-stage, severe plastic deformation (SPD) process. The resulting extreme deformation conditions produce a rich variety of microstructures in the chip. Four principal types of chip formationcontinuous, shear-localized, segmented, and mushroom-typeas elucidated first by Nakayama (1974, The Formation of Saw-Toothed Chip in Metal Cutting, Proceedings of International Conference on Production Engineering, Tokyo, pp. 572577) are utilized to emphasize the diverse plastic flow phenomena, especially unsteady deformation modes that prevail in cutting. These chip types are intimately connected with the underlying flow, each arising from a distinct mode and triggered by an instability phenomenon. The role of plastic flow instabilities such as shear banding, buckling, and fracture in mediating unsteady flow modes is expounded, along with consequences of the flow modes and chip types for the cutting. Sinuous flow is shown to be the reason why gummy (highly strain-hardening) metals, although relatively soft, are so difficult to cut. Synthesizing the various observations, a hypothesis is put forth that it is the stability of flow modes that determines the mechanics of cutting. This leads to a flow-stability phase diagram that could provide a framework for predicting chip types and process attributes.
