Walter Dale Compton

Low-frequency modulation-assisted drilling using linear drives

Abstract A new machine tool based on linear drive technology has been developed for low-frequency modulation-assisted drilling. The linear drive utilizes a novel design for generating continuous drill feed motion with a superimposed low-frequency modulation of up to 400 Hz without the use of secondary actuators. The frequency characteristics of the drive are such that a wide variety of drilling conditions can be systematically explored. Results pertaining to torque, thrust and controlled chip breakage when drilling ductile aluminium alloys are presented to demonstrate the attractive features of modulation-assisted drilling using linear drives. It has been shown that the application of a superimposed modulation of the appropriate frequency and amplitude produces consistent chip breakage, thereby facilitating the drilling process. Furthermore, under these conditions, the mean torque and mean thrust are decreased in comparison with conventional drilling. The frequency and amplitude conditions for which such beneficial effects are observed can be predicted using a simple model for chip formation and forces in modulation-assisted drilling.

Unusual Applications of Machining: Controlled Nanostructuring of Materials and Surfaces

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.

Sinuous flow in metals

Significance It is counterintuitive, yet well known, that cutting a soft metal, vis-à-vis a hardened one, involves significantly larger forces, with the formation of a thick chip. Using in situ imaging we show that this phenomenon results from a hitherto unidentified flow mode in metals, called sinuous flow due to its repeatedly folded nature, that resembles irreversible flows in geological rocks and some complex fluids. We also demonstrate how sinuous flow can be suppressed, by simply applying common marking ink remote from the cutting interface—the forces are reduced significantly and the thick chip is eliminated. Besides explaining some important decades-old phenomena in metal cutting, our work has broad implications for many natural and industrial cutting processes. Annealed metals are surprisingly difficult to cut, involving high forces and an unusually thick “chip.” This anomaly has long been explained, based on ex situ observations, using a model of smooth plastic flow with uniform shear to describe material removal by chip formation. Here we show that this phenomenon is actually the result of a fundamentally different collective deformation mode—sinuous flow. Using in situ imaging, we find that chip formation occurs via large-amplitude folding, triggered by surface undulations of a characteristic size. The resulting fold patterns resemble those observed in geophysics and complex fluids. Our observations establish sinuous flow as another mesoscopic deformation mode, alongside mechanisms such as kinking and shear banding. Additionally, by suppressing the triggering surface undulations, sinuous flow can be eliminated, resulting in a drastic reduction of cutting forces. We demonstrate this suppression quite simply by the application of common marking ink on the free surface of the workpiece material before the cutting. Alternatively, prehardening a thin surface layer of the workpiece material shows similar results. Besides obvious implications to industrial machining and surface generation processes, our results also help unify a number of disparate observations in the cutting of metals, including the so-called Rehbinder effect.

Vacancies, twins, and the thermal stability of ultrafine-grained copper

Ultrafine-grained metals have impressive strength but lack the thermal stability necessary for most applications. Nano-scale, deformation twinned copper microstructures exhibit a rare combination of strength and stability. While storing less energy in their interfaces than other nanostructured metals, they also exhibit lower vacancy supersaturations, reducing the driving force and mobility for microstructure evolution. From a thermal stability perspective, the nano-twinned microstructure may thus be preferred over the more commonly produced nano-scale equiaxed microstructures.

Exploration of contact conditions in machining

Abstract The contact conditions along the tool-chip and tool-work interfaces in the machining of metals are analysed and discussed. The principal experimental techniques used are direct optical measurements of the interfaces at visible and infrared wavelengths using transparent tools, measurements of the variation of forces with flank wear and microstructural changes produced in steel surfaces during machining and perturbation of the tool-chip interface using low-frequency modulation. The application of these techniques has provided new insights into the motion of the chip relative to the tool along the rake face, enabled measurement of the full-field temperature along the rake face and suggested avenues for modifying friction conditions along the tool rake and flank faces. It is shown that important differences as well as similarities exist between the rake face and flank face boundary conditions. The implications of these results for the theoretical analysis of machining are discussed.

Effect of Controlled Modulation on Chip Formation and Interface Tribology in Machining

Based on consideration of mechanics of chip formation, it is shown that the application of a controlled modulation fundamentally changes the nature of the extreme deformation underlying chip formation, and the severe contact conditions at the tool-chip interface. Important consequences are significant reduction in the energy of chip formation, and control of chip shape and size for improved chip management. Implementation of modulation-assisted machining for industrial machining processes is discussed.Copyright

Severe plastic deformation and the production of nanostructured alloys by machining

Abstract: This chapter describes the production of nanostructured materials using severe plastic deformation (SPD) inherent to machining. The SPD can be controlled, in situ, to access a range of strains, strain rates and temperatures, enabling deformation-microstructure maps to be created. By tuning the SPD parameters, various nanoscale microstructures (e.g. nanocrystalline, nano-twinned, bimodal) can be engineered; and by constraining the chip formation, bulk forms (e.g. foil, sheet and rod) with nanocrystalline and ultrafine-grained microstructures are produced. Chip formation in the presence of a superimposed modulation enables the production of nanostructured particulate with controlled shapes including fiber, equiaxed and platelet types. SPD conditions also determine the deformation history of the machined surface, enabling microstructural engineering of surfaces. These diverse nanostructuring characteristics of machining are united by their common origins in the SPD phenomena prevailing in the deformation zone. Implications for large-scale manufacturing of nanostructured alloys, optimization of SPD microstructures, and consolidation–recycling of industrial machining chips are also briefly discussed.

Nanocrystalline Materials from Aerospace Machining Chips

The creation of nanostructured materials with enhanced mechanical properties by controlled chip formation has been demonstrated. The present study examines the microstructure and mechanical properties of chips from various alloys -Waspaloy AMS 5704, Inconel 718, Al 6061-T6, and titanium - produced in aerospace machining operations. While the deformation conditions with respect to chip formation may be ‘less than controlled’ in these cases, it is nevertheless seen that the chips created are composed entirely of nanocrystalline structures of high hardness and strength. The microstructural characteristics and properties of these chips are compared and contrasted with those produced under controlled conditions of strain and temperature. The results suggest that aerospace machining chips can be up-scaled as high-performance structural materials with substantial cost benefits.

Microstructural Refinement in Single-Phase Copper Solid Solutions by Machining

A study has been made of the effect of solute (Mn, Al, Ni) additions on microstructure refinement due to large strain deformation in single phase, copper solid solutions. The solutes were specifically selected for their influence on stacking fault energy (SFE) of copper, and the large strain deformation was imposed by chip formation in machining. The microstructure of Cu- 0.7at%Ni chip consists of elongated, sub-micrometer sized grains while Cu-7at%Al chip is made up of long, thin microbands with twins. The microstructure of the chip changes as the SFE of the material varies. With all of the solid solutions studied, the hardness of the chips is found to be significantly greater than that of the bulk material. Recrystallization temperature of solid solution chips is found to be higher than those of pure copper chips.

Friction and Wear of a Polymer-Matrix Reinforced With Nanocrystalline Alloy

The tribological behavior of a polymer-matrix composite containing discontinuous nanocrystalline alloy reinforcements is studied using a model system composed of a bakelite matrix reinforced with unitary Al6061 nanostructures. Chip formation by machining is used to produce the unitary nanostructures with grain size in the range of 50–200 nm. The hardness and strength of the unitary nanostructures are seen to be substantially greater than those of micro-crystalline Al 6061. Polymer-matrix composites containing these nanocrystalline metal reinforcements in a bakelite matrix are prepared using standard polymer processing routes. The friction and wear characteristics of the composites are studied using block-on-cylinder and pin-on-disk configurations. The wear performance of the nanocrystalline, metal-reinforced polymer is compared with that of the unfilled polymer and also with that of the polymer reinforced with microcrystalline alloy reinforcements, and shown to be superior.Copyright