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The deformation field, material flow, and mechanics of chip separation in cutting of metals with superimposed low-frequency modulation (

The flow dynamics, deformation fields and chipparticle formation in cutting and sliding of metals are analysed, in situ, using high-speed imaging and particle image velocimetry. The model system is a brass workpiece loaded against a wedge indenter at low speeds. At large negative rake angles, the flow is steady with a prow of material forming ahead of the indenter. There is no material removal and a uniformly strained layer develops on the workpiece surface--the pure sliding regime. When the rake angle is less negative, the flow becomes unsteady, triggered by formation of a crack on the prow free surface and material removal ensuing--the cutting regime. The strain on the prow surface at crack initiation is found to be constant. Chip morphologies, such as discrete particle, segmented chip and continuous chip with mesoscale roughness, are shown to arise from a universal mechanism involving propagation of the prow crack, but to different distances towards the indenter tip. The simple shear deformation in continuous chip formation shows small-angle oscillations also linked to the prow crack. Implications for material removal processes and ductile failure are discussed. [ABSTRACT FROM AUTHOR]

Large strain plastic flow in cutting of metals is studied at multiple length scales using high-speed imaging and marker techniques, complemented by particle image velocimetry and electron microscopy. Quantitative analysis of streak-lines, strain fields and microstructure, shows the flow to be often unsteady. Instabilities such as segmentation driven by ductile fracture, vortex-like flow in ductile metals, and shear banding in low-thermal diffusivity systems are elucidated using direct observations. A constrained-cutting process is demonstrated for suppressing the instabilities and unsteady flow. [ABSTRACT FROM AUTHOR]

Abstract  The effects of superimposed, low-frequency modulation on contact conditions at the tool–chip interface and mechanics of machining are analyzed. It is shown that modulation can disrupt the severity of the tool–chip contact, enabling enhanced lubrication of this contact; and discretize chip formation to small dimensions with controlled size and shape. The deformation strain in the chip is also reduced, suggesting a 40% reduction in energy of machining due to the modulation. Conditions of frequency and amplitude for achieving these effects are presented and confirmed using a compact modulation device that can be retro-fitted onto conventional machine platforms. Implications for enhancing efficiency of industrial machining processes are briefly discussed. [ABSTRACT FROM AUTHOR]

Plane-strain machining can be used to impart large plastic strains in alloys that are difficult to deform by other severe plastic deformation (SPD) processes. By cutting at low speeds, the heating caused by friction with the tool can be reduced to insignificant levels. The utility of this approach for characterizing microstructure development in SPD is demonstrated using a variety of commercial alloys that exhibit different deformation behaviors and strengthening mechanisms, including CP-titanium, aluminum alloy 6061-T6, nickel-base superalloy IN-718, and pearlitic plain-carbon steel. [ABSTRACT FROM AUTHOR]

Measurement of strain field in the primary deformation zone is of major interest for development of machining as an experimental technique for studying phenomena associated with large strain deformation. A study has been made of the primary deformation zone and tool-chip interface in planestrain (two-dimensional) machining of metals. The use of a high-speed, charge-coupled device (CCD) imaging system in conjunction with an optically transparent, sapphire cutting tool has enabled characteristics of the deformation field such as velocity, strain, and material flow, to be obtained at high spatial and temporal resolution. The velocity distributions in the primary deformation zone and along the tool rake face have been obtained by applying a particle image velocimetry (PIV) technique to sequences of high-speed images of the chip-tool interface taken through the transparent tool, and of the primary deformation zone recorded from a side of the workpiece. A procedure is presented and demonstrated for determining the strain and strain rate distributions in the primary deformation zone. The measurements have provided data about the variations of velocity, strain rate, and strain, in and around the cutting edge and primary deformation zone: confirmed the existence of a region of retarded sliding in the region of intimate contact between tool and chip; and highlighted the occurrence of a region of dead metal ahead of the cutting edge when cutting with a negative rake angle tool. [ABSTRACT FROM AUTHOR]

Large strain extrusion machining (LSEM) is presented as a method of severe plastic deformation for the creation of bulk nanostructured materials. This method combines inherent advantages afforded by large strain deformation in chip formation by machining, with simultaneous dimensional control of extrusion in a single step of deformation. Bulk nanostructured materials in the form of foils, plates, and bars of controlled dimensions are shown to result by appropriately controlling the geometric parameters of the deformation in large strain extrusion machining. [ABSTRACT FROM AUTHOR]

Widespread application of lightweight magnesium and titanium alloys sheet is limited mainly because of their poor-workability issues, both in primary processing by rolling and secondary sheet forming. This study describes a hybrid cutting–extrusion process, large-strain extrusion machining (LSEM), for producing sheet and foil. By utilizing a constraining edge placed across from the cutting tool edge, the usual cutting process is transformed into continuous shear-deformation process, wherein the thickness of the sheet at its exit from the deformation zone is directly controlled. The confinement of the deformation field in LSEM enables near-adiabatic heating in the deformation zone. Consequently, external workpiece heating, intrinsic to sheet manufacturing by multistage rolling in alloys of poor workability (e.g., hexagonal close packed (hep) alloys and cast materials), is minimized. Furthermore, the deformation parameters, such as strain, strain rate, and strain path, can be controlled to refine the microstructure and induce sheartype crystallographic textures that enable enhanced sheet mechanical properties (strength and formability). This application of LSEM is demonstrated using magnesium alloy AZ31B as a model system. Since LSEM is a single-stage process for sheet production, it is potentially attractive in terms of production economics and energy. Implications for process scale-up and control of plastic flow localization are briefly discussed. [ABSTRACT FROM AUTHOR]

Abstract: The deformation history and state of machined surfaces in low-speed cutting of metals with sharp, wedge-shaped tools have been characterized using high-speed image analysis, complemented by hardness and microstructure study. Large surface and subsurface strains are observed, and have been shown to arise from the severe plastic deformation intrinsic to chip formation. The deformation history of the chip and the near-surface region during the process of surface generation are found to be equivalent. The dependence of surface and subsurface deformation on parameters such as tool rake angle and undeformed chip thickness has been explored. The subsurface strain distribution is shown to scale with undeformed chip thickness (self-similarity), and to be influenced by the rake angle. Based on the observations, and process–deformation–microstructure correlations, a framework is developed for directly engineering surfaces with controlled deformation levels and microstructures by machining. The results also offer scope for enhanced validation of machining simulations and future development of multiscale models of machining. [Copyright &y& Elsevier]

Abstract: The specific energy in modulation assisted machining (MAM) – machining with superimposed low frequency (<1000Hz) modulation in the feed direction – is estimated from direct measurements of cutting forces. Reductions of up to 70% in the energy are observed relative to that in conventional machining, when cutting ductile metals such as copper and Al 6061T6. Evidence based on chip structures and strains, stored energy of cold work, recrystallization, and finite element simulation of chip formation, is presented to show that this reduction is due to smaller strain levels in chips created by MAM. A simple geometric ratio of the length to thickness of the ‘undeformed chip’, which can be estimated a priori from MAM and machining parameters, is shown to be a predictor of the transient chip formation conditions that result in the reduction in specific energy and deformation levels. [Copyright &y& Elsevier]

Abstract: An analysis of the mechanics of large strain extrusion machining (LSEM), a constrained chip formation process, is presented for deformation processing of bulk alloys. The deformation field is shown to be narrowly confined and controllable, with attributes ranging from conventional deformation processing to severe plastic deformation. Controllable deformation parameters include strain/strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of Mg AZ31B, an alloy of commercial significance but noted for its poor workability, into sheet and foil forms. Noteworthy features of the processing are suppression of segmentation, realization of a range of strains and deformation rates, engineering of microstructures ranging from conventional to ultrafine grained, and creation of sheet/foil from the bulk in a single step of deformation without pre-heating. Guidelines for realizing specific sheet attributes, and scalability of LSEM for production are analyzed and discussed. [Copyright &y& Elsevier]

A class of deformation processing applications based on the severe plastic deformation (SPD) inherent to chip formation in machining is described. The SPD can be controlled, in situ, to access a range of strains, strain rates, and temperatures. These parameters are tuned to engineer nanoscale microstructures (e.g., nanocrystalline, nanotwinned, and bimodal) by in situ control of the deformation rate. By constraining the chip formation, bulk forms (e.g., foil, sheet, and rod) with nanocrystalline and ultrafine grained microstructures are produced. Scaling down of the chip formation in the presence of a superimposed modulation enables production of nanostructured particulate with controlled particle shapes, including fiber, equiaxed, and platelet types. The SPD conditions also determine the deformation history of the machined surface, enabling microstructural engineering of surfaces. Application of the machining-based SPD to obtain deformation-microstructure maps is illustrated for a model material system—99.999% pure copper. Seemingly diverse, these unusual applications of machining are united by their common origins in the SPD phenomena prevailing in the deformation zone. Implications for large-scale manufacturing of nanostructured materials and optimization of SPD microstructures are briefly discussed. [ABSTRACT FROM AUTHOR]

Abstract: The deformation field in machining was controlled to access a range of deformation parameters—strains of 1–15, strain rates of 10–100,000s−1 and temperatures of up to 0.4 T m—in the severe plastic deformation (SPD) of copper. This range is far wider than has been accessed to date in conventional SPD methods, enabling a study of the interactive effects of the parameters on microstructure and strength properties. Nano-twinning was demonstrated at strain rates as small as 1000s−1 at −196°C and at strain rates of ⩾10,000s−1 even when the deformation temperature was well above room temperature. Bi-modal grain structures were produced in a single stage of deformation through in situ partial dynamic recrystallization. The SPD conditions for engineering specific microstructures by deformation rate control are presented in the form of maps, both in deformation parameter space and in terms of the Zener–Hollomon parameter. [Copyright &y& Elsevier]

Abstract: Characteristics of the deformation fields associated with chip formation in plane strain machining are described. The ability to impose very large strain deformation in a controlled manner is highlighted. The creation of nano- and ultra-fine grained structures by machining is demonstrated in a variety of metals and alloys. These results indicate that machining not only offers a simple method for large scale manufacturing of nanostructured materials, but also provides a unique experimental configuration for studying large strain deformation phenomena. [Copyright &y& Elsevier]

Abstract: We present the properties of aluminum 6061 alloys of various tempers, severely deformed in plane strain at room temperature, by machining. Various values of strain are introduced into the chip, in a single pass, by varying the rake angle of the tool. Chips cut from peak-aged 6061 (T6 condition) are composed of finer microstructures and possess higher hardness than peak-aged 6061 subjected to equal channel angular pressing at elevated temperatures. Thermal stability of the chips with different levels of strain is analyzed by studying evolution of Vickers micro-hardness and microstructure after different heat treatments. Chips produced from the peak-aged temper and over-aged temper soften following heat treatment while those from the solution-treated state first gain strength before softening. The results are rationalized based on prior studies of the characteristics and kinetics of precipitation and coarsening in Al–Mg–Si systems. The observations also suggest processing routes for consolidation of the chips into bulk forms while retaining the UFG microstructure with enhanced mechanical properties. [Copyright &y& Elsevier]