The cutting of metals via plastic buckling

Abstract

The cutting of metals has long been described as occurring by laminar plastic flow. Here we show that for metals with large strain-hardening capacity, laminar flow mode is unstable and cutting instead occurs by plastic buckling of a thin surface layer. High speed imaging confirms that the buckling results in a small bump on the surface which then evolves into a fold of large amplitude by rotation and stretching. The repeated occurrence of buckling and folding manifests itself at the mesoscopic scale as a new flow mode with significant vortex-like components-sinuous flow. The buckling model is validated by phenomenological observations of flow at the continuum level and microstructural characteristics of grain deformation and measurements of the folding. In addition to predicting the conditions for surface buckling, the model suggests various geometric flow control strategies that can be effectively implemented to promote laminar flow, and suppress sinuous flow in cutting, with implications for industrial manufacturing processes. The observations impinge on the foundations of metal cutting by pointing to the key role of stability of laminar flow in determining the mechanism of material removal, and the need to re-examine long-held notions of large strain deformation at surfaces.

Comment on ‘The cutting of metals by plastic buckling’ by Udupa et al .

“…For t/h less than ≈ 7 there is no separation (as is classically observed and as is the basis of many successful analyses of chip formation [3]). Beyond that, with increasing t/h, pre-compression increasingly modifies chip formation, towards the extreme form observed in [1].…”

“…Udupa et al [1] report observations of a bulging instability in the surface of work material ahead of the chip formation zone in metal cutting. It results in chip formation by a folding flow instead of by the steady shearing that is the basis of long-standing theory.…”

“…The main results in [1] are for machining an annealed copper, of mean grain size 0.5 mm with a tool of rake angle = 0° and edge radius less than 5 m, at a cutting speed v c = 0.5 mm/s and uncut chip thickness h = 0.05 mm. Subsidiary results are for the same conditions except = 45°, and for machining the copper with the = 0° tool after applying a pre-strain p  ≈ 2.5.…”

“…Figure 3b shows chip formation when a softer slice has just entered the pre-shear region. It is 0.5 mm wide to match the grain size in [1], is initially parallel-sided and its flow stress is 80% of that to either side (see Appendix A). The free surface ahead of the chip is now convex upward with an incipient bulge, with shear split into major and minor bands.…”

“…The comments here are made around these conditions. They are based on results from finite element simulations carried out in response to the discussion in [1], though with a tool of edge radius 10 m (for minimum mesh size reasons), using the bespoke for machining commercial finite element software AdvantEdge-2D, developed from [2], with its option that enables users to include their own material flow stress model. Details are in Appendix A.…”