Sinuous Flow in Cutting of Metals

Abstract

Using in situ high speed imaging, we unveil details of a highly unsteady plastic flow mode in cutting of annealed and highly strain hardening metals. This mesoscopic flow mode, termed sinuous flow, is characterized by repeated material folding, large rotation and energy dissipation. Sinuous flow effects a very large shape transformation, with local strains of 10 or more, and results in a characteristic mushroom-like surface morphology that is quite distinct from the well-known morphologies of metal cutting chips. Importantly, the attributes of this unsteady flow are also fundamentally different from other well-established unsteady plastic flows in large-strain deformation, like adiabatic shear bands. The nucleation and development of sinuous flow, its dependence on material properties, as well as its manifestation across material systems are demonstrated. Plastic buckling and grain-scale heterogeneity are found to play key roles in triggering this flow at surfaces. Implications for modeling and understanding flow stability in large strain plastic deformation, surface quality and preparation of near strain-free surfaces by cutting are discussed. The results point to the inadequacy of the widely used shear zone models, even for ductile metals.

Material-Independent Mechanochemical Effect in the Deformation of Highly-Strain-Hardening Metals

“…Sinuous plastic flow, an unsteady flow mode characterized by repeated material folding and large local strains, is the norm when cutting soft and highly strain hardening metals [2,6]. The highly redundant deformation, due to folding, results in large cutting forces, energy dissipation and a very thick chip [4,6]. This type of flow is nucleated by a plastic buckling instability on the material free surface [5], see Fig.…”

“…This difficulty is manifest as very large forces, thick chips and a profusion of defects on the surface [1,2]; hence, such soft metals are often called "gummy" [3]. It was shown recently that this difficulty is due to an unsteady mode of large-strain plastic deformation-sinuous flow, characterized by plastic buckling and large amplitude material folding-that prevails during cutting [2,[4][5][6]. If this sinuous flow mode can be disrupted and replaced by a more favorable deformation mode, with smaller forces and deformation strain, then cutting of these metals could be carried out efficiently.…”

Cutting of Al/Si bilayer systems: molecular dynamics study of twinning, phase transformation, and cracking

“…2-two families of twin boundaries are created; besides (111), we have also (111) twin planes. As Si flows under the indenter, a wavy Al/Si interface is created; such wavy interfaces are well-known in unsteady plastic flows during cutting [53,54]. Depending on the slope of this interface, (111) and (111) twin planes are generated.…”

Altering the Stability of Surface Plastic Flow via Mechanochemical Effects

“…1. The four flow modes result in distinct morphologies in the chip, a fact that has only recently begun to be appreciated based on combined in situ and ex situ analyses [11,12,33], even though chip morphologies themselves have been studied for a long time [5,13,[34][35][36]. The four modes are: 1) steady homogeneous deformation, henceforth referred to as laminar flow ( Fig.…”

“…This example of a homogeneous to non-homogeneous flow transition occurs in an array of material systems, from rocks on the geological-scale [6] to structural metals on the mesoscale [5,7,8] and to glasses on the nanoscale [9]. However, the fact that other such transitions may also exist, particularly in large strain deformation, resulting in flows hitherto poorly understood, is only now beginning to be appreciated [10][11][12]. It is quite natural to expect that these transitions can also be explained in terms of changes in stability, thereby predicting characteristics of the resulting flow.…”