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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 in situ 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 bucklingmodel 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 reexamine long-held notions of large strain deformation at surfaces. [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]

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]

Taylor's theory of crystal plasticity is reformulated for dislocations in a couple stress medium. The divergence between Taylor's approach and an approach that includes the effects of couple stresses on dislocation interactions is demonstrated. It is shown that dislocations separated by a distance that is comparable to a characteristic material length scale, have mutual interaction somewhat weaker than that predicted by classical elasticity. [ABSTRACT FROM AUTHOR]

Shear-based deformation processing by hybrid cutting-extrusion and free machining are used to make continuous strip, of thickness up to 1 mm, from low-workability AA6013-T6 in a single deformation step. The intense shear can impose effective strains as large as 2 in the strip without pre-heating of the workpiece. The creation of strip in a single step is facilitated by three factors inherent to the cutting deformation zone: highly confined shear deformation, in situ plastic deformation-induced heating, and high hydrostatic pressure. The hybrid cutting-extrusion, which employs a second die located across from the primary cutting tool to constrain the chip geometry, is found to produce strip with smooth surfaces (Sa < 0.4 μm) that is similar to cold-rolled strip. The strips show an elongated grain microstructure that is inclined to the strip surfaces-a shear texture-that is quite different from rolled sheet. This shear texture (inclination) angle is determined by the deformation path. Through control of the deformation parameters such as strain and temperature, a range of microstructures and strengths could be achieved in the strip. When the cutting-based deformation was done at room temperature, without workpiece preheating, the starting T6 material was further strengthened by as much as 30% in a single step. In elevated-temperature cutting-extrusion, dynamic recrystallization was observed, resulting in a refined grain size in the strip. Implications for deformation processing of age-hardenable Al alloys into sheet form, and microstructure control therein, are discussed. [ABSTRACT FROM AUTHOR]

An experimental study has been made of the flow field in indentation of a model granular material. A granular ensemble composed of spherical sand particles with average size of 0.4 mm is indented with a flat ended punch under plane-strain conditions. The region around the indenter is imaged in situ using a high-speed charge-coupled device (CCD) imaging system. By applying a hybrid image analysis technique to image sequences of the indentation, flow parameters such as velocity, velocity gradient, and strain rate are measured at high resolution. The measurements have enabled characterization of the main features of the flow such as dead material zones, velocity jumps, localization of deformation, and regions of highly rotational flow resembling vortices. Implications for validation of theoretical analyses and applications are discussed. [ABSTRACT FROM AUTHOR]

Enhanced workability, as characterized by the magnitude and heterogeneity of accommodated plastic strains during sheet processing, is demonstrated in high Si content Fe-Si alloys containing 4 and 6.5 wt% Si using two single-step, simple-shear deformation techniques – peeling and large strain extrusion machining (LSEM). The model Fe-Si material system was selected for its intrinsically poor material workability, and well-known applications potential in next-generation electric machines. In a comparative study of the deformation characteristics of the shear processes with conventional rolling, two distinct manifestations of workability are observed. For rolling, the relatively diffuse and unconfined deformation zone geometry leads to cracking at low strains, with sheet structures characterized by extensive deformation twinning and banding. Workpiece pre-heating is required to improve the workability in rolling. In contrast, peeling and LSEM produce continuous sheet at large plastic strains without cracking, the result of more confined deformation geometries that enhances the workability. Peeling, however, results in heterogeneous, shear-banded microstructures, pointing to a second type of workability issue – flow localization – that limits sheet processing. This shear banding is to a large extent facilitated by unrestricted flow at the sheet surface, unavoidable in peeling. With additional confinement of this free surface deformation and appropriately designed deformation zone geometry, LSEM is shown to suppress shear banding, resulting in continuous sheet with homogeneous microstructure. Thus LSEM is shown to produce the greatest enhancement in process workability for producing sheet. These workability findings are explained and discussed based on differences in process mechanics and deformation zone geometry. [ABSTRACT FROM AUTHOR]

Surface folding in large-strain deformation of metal polycrystals, mediated by unsteady sinuous plastic flow, was recently uncovered by direct observations. Here, we examine microstructural origins and mechanics of the folding process in polycrystalline aggregates, using computational methods and in situ , high-speed imaging experiments. Our model loading system is an indenter contact that imposes large strain deformation typical of metal forming, sliding and cutting. Folding arises primarily from intrinsic, grain-level flow stress variation in the polycrystalline ensemble. This flow stress heterogeneity is incorporated, spatially, in a continuum Lagrangian finite element framework, by partitioning the metal surface into grain-like structures. This pseudograin model captures all key aspects of the folding as observed by direct imaging, from fold nucleation via microstructure heterogeneity through various stages of fold development on the surface; surface strain fields; and deformation parameter effects such as indenter geometry and friction. The folding phenomenon is quite general, and provides a direct route for formation of surface defects and delamination wear particles. The microstructure-based simulation capability, thus validated, can be used as a virtual tool for analyzing large-strain plastic flow at surfaces and its consequences. Besides demonstrating the importance of folding in surface plasticity, the study points to a critical need to consider microstructure effects on local plasticity for sliding wear and deformation processing. [ABSTRACT FROM AUTHOR]

Understanding transitions from homogeneous to localized flow, and mechanisms underlying flow localization, is of paramount importance for deformation processing of magnesium. In this study, a shear-based deformation method is utilized for imposing large strains ( ∼ 1 ), under controllable strain rates (10–10 5 /s) and temperatures (80–300 °C), in order to examine flow patterns in a magnesium alloy. Based on microstructure characterization, deformation twinning is suggested to contribute to the localized flow at temperatures below 200 °C and at low strain rates. The transition from the localized to homogeneous flow with increasing temperature is due to reduction in twinning activity, and enhanced strain-rate sensitivity. At constant temperature, an increase in the strain rate decreases the propensity for flow localization. A model is presented for characterizing the maximum uniform strain as a function of temperature and deformation state (simple shear, plane-strain compression). The model incorporates temperature-sensitive microstructural changes and flow properties of magnesium into a classical framework to capture the flow localization phenomena at low temperatures and strain rates. [ABSTRACT FROM AUTHOR]

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]

Constrained chip (sheet) formation by large strain extrusion machining is used to impose effective strains of ∼1 in Mg alloy AZ31B sheet in a single step of deformation. High-speed image analysis shows the deformation underlying sheet formation to be simple shear that is confined to a narrow zone. This confinement of the deformation limits the need for pre-heating of the workpiece to realize continuous sheet forms. Tilted-basal textures, wherein the basal poles are inclined from the sheet surface normal, are achieved by this processing. These textures are quite different from those prevalent in rolled sheet. By controlling the strain path, the basal pole inclination could be varied in the range 32–53°. The primary texture component is the B-fiber, indicating basal slip to be the main deformation mode over the temperature range 165–400°C. An additional C2-fiber component appears above 250°C due to the activation of pyramidal slip. In conjunction with these textures, microstructures ranging from ultrafine-grained (∼200nm) to fine-grained (∼2μm) could be obtained by controlling the deformation temperature. Implications of the results for production of Mg sheet are discussed. [ABSTRACT FROM AUTHOR]

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]

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: Severe plastic deformation of titanium at near-ambient temperature has been realized using large-strain machining. The material that received the highest levels of strain, the chip, is found to have a nanocrystalline structure with ∼100nm sized grains. This microstructure is finer than that observed in titanium following equal channel angular pressing at elevated temperatures. The machined surface of the workpiece is found to have a graded microstructure composed of ultrafine grains at and near the surface and a preponderance of twins further into the subsurface. The parameters of the large-strain deformation field, namely strain rate and strain, are obtained using an adaptation of a particle image velocimetry technique. The microstructure characteristics are shown to be a consequence of the deformation field parameters, which activate both twinning and dislocation-mediated mechanisms of plastic flow. [Copyright &y& Elsevier]