Ho Yeung

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.

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 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]

A comparative study of energy and material flow in modulation-assisted machining and conventional machining

A study has been made of deformation, forces and energy in modulation-assisted machining (MAM), wherein chip formation occurs in the presence of a controlled, low-frequency modulation superimposed on to the machining. A unique feature of the study is the use of high speed in situ imaging and image analysis to map material flow in the chip formation zone at high resolution; and simultaneous measurements of tool motions and forces, such that the instantaneous forces can be overlaid onto the chip formation process. The measurements show that the observed significant reductions in specific energy in MAM relative to conventional machining, when cutting ductile metals such as copper and Al 6061T6, are a consequence of chip formation with reduced strain levels in MAM. Additional insights into the chip formation are obtained by examining the effects of a chip aspect ratio parameter.Copyright

Surface flow in severe plastic deformation of metals by sliding

An in situ study of flow in severe plastic deformation (SPD) of surfaces by sliding is described. The model system – a hard wedge sliding against a metal surface – is representative of surface conditioning processes typical of manufacturing, and sliding wear. By combining high speed imaging and image analysis, important characteristics of unconstrained plastic flow inherent to this system are highlighted. These characteristics include development of large plastic strains on the surface and in the subsurface by laminar type flow, unusual fluid-like flow with vortex formation and surface folding, and defect and particle generation. Preferred conditions, as well as undesirable regimes, for surface SPD are demarcated. Implications for surface conditioning in manufacturing, modeling of surface deformation and wear are discussed.

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