Simulation of complex plastic flows in metal sliding and cutting

Abstract

Metal manufacturing encompasses a broad range of mechanical transformation processes that shape native metal into useful forms. These processes generally involve the interaction of a hard tool with a specimen / workpiece, and can be broadly classified into deformation processing and machining; The latter involves material removal in the form of a chip (e.g. milling, drilling), while the former does not (e.g. shear spinning, burnishing). FE simulations have been used extensively to analyze and design both deformation processing and machining. A vast majority of these analyses treat the work metal in the continuum / homogenized lengthscale. However, in the recent past, in situ imaging experiments have revealed the existence of complex plastic flows even in simple sliding systems (e.g. hard wedge sliding against a soft metal) in both the chip formation and sliding regimes. The occurrence of these complex flows has nontrivial consequences. For instance, repeated sliding has been explored as a way to produce ultra–fine-grained surfaces; however, a single sliding pass can produce severe surface damage due to the formation of surface folds and self-contacts. This is also quite unexpected from the perspective of conventional triboplasticity, which considers surface damage accumulation to occur over many sliding passes in wear. Similarly, the occurrence of highly “sinuous” flow in cutting of soft metals is associated with high cutting forces, very thick chips, and a damaged residual surface. In sinuous flow, streaklines of flow are highly undulating and very different from the rectilinear, “laminar” flow pattern associated with the widely used Merchant model of machining. It is important to note that these complex plastic flows (and, therefore, their consequences) are not captured in conventional homogenized FE simulations: It is necessary to incorporate material property inhomogeneity due to the polycrystalline aggregate nature of metals for the purpose of accurate analysis and design of these processes. Moreover, while state-of-the-art in situ imaging techniques can provide high fidelity kinematic information (e.g. velocity, strain, strain-rate), simulations can also provide dynamic information beyond cutting and sliding forces (e.g. hydrostatic stress and triaxiality fields), which are difficult to obtain experimentally. This thesis is an attempt to incorporate models of inhomogeneity in pure, soft FCC metals, and, thereby capture the complex flow physics in sliding and cutting in FE simulations at the hundred microns to several mm lengthscale.