James B. Mann

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.

Deformation and Microstructure in Machining

Deformation history and state of chip and machined surface in low-speed cutting have been characterized using image analysis, complemented by microstructure and hardness. Fine scale details of the deformation field of relevance to machining modelling are highlighted. The severe plastic deformation inherent to chip formation results in microstructure changes which can be controlled through appropriate process parameters selected with the aid of machining simulations. Scaling of subsurface strain distribution is observed. Similarity in deformation history of chip and near-surface suggests opportunities for engineering surfaces with controlled deformation levels and microstructures, directly, by machining. The deformation characterization offers substantial scope for improvement and validation of machining models, and enhancement of machining process capability.

Modulation-Assisted Machining: A New Paradigm in Material Removal Processes

Modulation Assisted Machining (MAM), based on controlled superimposition of low-frequency modulation to conventional machining, effects discrete chip formation and disrupts the severe contact condition at the tool-chip interface. The underlying theory of discrete chip formation and its implications are briefly described and illustrated. Benefits such as improved chip management and lubrication, reduction of tool wear, enhanced material removal, particulate manufacturing and surface texturing are highlighted using case studies. MAM represents a new paradigm for machining in that it deliberately employs ‘good vibrations’ to enhance machining performance and capability.

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

Effects of Friction on Contact of Transverse Ground Surfaces

The two-dimensional plane-strain sliding contact of a smooth rigid roller on a transuerse ground rough surface is analyzed. The rough surface is idealized as an elastic half-space with periodic roughness modeled as cylindrical ridges oriented transverse to the sliding direction. The contact problem is solved using a numerical iterative method in which each asperity contact is treated as a micro-Hertz contact, and the exact treatment of asperity interaction is included. The subsurface stress field is calvulated using Westergaard stress fuuntions. The subsequent analysis compares the rough surface stress fields with the corresponding smooth Hertz contact to evaluate the influence of surface roughness and friction on the subsurface stress distributions

Piezo-Actuated Modulation-Assisted Drilling System With Integrated Force Sensing

A new electromechanical modulation system designed with piezoelectric stacks for both linear actuation and force sensing functions is described. The system can be adapted for modulationassisted machining (MAM) drilling processes where a lowfrequency (<1000 Hz) sinusoidal oscillation is superimposed directly onto the drilling process, such that the feedrate is modulated. A series of drilling experiments were conducted in Ti6Al4V, 17-4 steel, and Al6061 with the system installed on a CNC machine. The drill displacement, thrust force, and chip morphology were characterized across a range of conventional and MAM drilling conditions. The mechanical response (stiffness) of the system agrees with the design specifications. The system offers new capabilities to control the modulation frequency and amplitude in MAM drilling, while simultaneously measuring the drilling thrust force in real time. The force sensing function enables detection of the intermittent separations between the drill tip and the workpiece surface (occurrence of discrete cutting), providing a method to prescribe and control the modulation conditions necessary for effective MAM drilling. Opportunities for force feedback control and process monitoring in MAM drilling processes are discussed. While the system described emphasizes MAM drilling, the capabilities can be extended to other machining processes. [DOI: 10.1115/1.4033929]

Effect of Low-Frequency Modulation on Deformation and Material Flow in Cutting of Metals

The deformation field, material flow, and mechanics of chip separation in cutting of metals with superimposed low-frequency modulation (<1000 Hz) are characterized at the mesoscale using high-speed imaging and particle image velocimetry (PIV). The twodimensional (2D) system studied involves a sharp-wedge sliding against the workpiece to remove material, also reminiscent of asperity contacts in sliding. A unique feature of the study is in situ mapping of material flow at high resolution using strain fields and streaklines and simultaneous measurements of tool motions and forces, such that instantaneous forces and kinematics can be overlaid onto the chip formation process. The significant reductions in specific energy obtained when cutting with modulation are shown to be a consequence of discrete chip formation with reduced strain levels. This strain reduction is established by direct measurements of deformation fields. The results have implications for enhancing sustainability of machining processes and understanding surface deformation and material removal in wear processes. [DOI: 10.1115/1.4031140]

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.

Mechanics of Modulation Assisted Machining

The controlled application of low-frequency modulation to machining — Modulation Assisted Machining (MAM) — effects discrete chip formation and disrupts the severe contact condition at the tool-chip interface. The role of modulation in reducing the specific energy of machining with ductile alloys is demonstrated using direct force measurements. The observed changes in energy dissipation are analyzed and explained, based on the mechanics of chip formation.Copyright