Abstract:
Methods of machining a body to produce a chip are provided wherein the body is formed of a material and in a state such that the material exhibits sinuous flow during a machining operation. The methods include providing a layer located on a surface of the body, and machining the body by causing engagement between a cutting tool and the body in a contact region below an area of the surface having the coating layer thereon and moving the cutting tool relative to the body to produce the chip having the layer thereon. The layer reduces sinuous flow in the material of the body and the chip is formed primarily by laminar flow.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/194,082, filed Jul. 17, 2016, the contents of which are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support under CMMI1234961 and CMMI1363524 awarded by the National Science Foundation. The government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention generally relates to processes for machining materials. The invention particularly relates to systems and methods for improving the machinability of surfaces of relatively soft and/or ductile materials, such as but not limited to annealed metals and/or alloys. 
         [0004]    A typical machining process involves removal of material from a body with a cutting tool. The portion of material removed from the body is commonly referred to as a chip, and under appropriate conditions may be in the form of a continuous chip. For example,  FIG. 1  represents a cutting tool  12  in contact with and moving relative to a workpiece  14  to form and remove a portion  16  of the workpiece  14 , referred to as a bulk material, to yield a chip  18 . This example represents an ideal plane-strain machining process characterized by formation of the chip  18  by simple shear resulting in a smooth, laminar flow of the material. 
         [0005]    It is well known that machining a non-brittle metal in a soft state, for example, after being annealed, is significantly more difficult than machining the same metal in a hardened state, for example, after undergoing strain hardening. When the workpiece being cut is a metal in a soft state, the machining process generally requires a relatively large cutting force and results in an unusually thick chip. This difficulty in cutting, well known in industrial practice, has hitherto eluded fundamental explanation. Conventionally, at the mesoscale (for example, about 100 μm up to about 5 mm), the structure of the chip has been assumed to be homogeneous, resulting from laminar plastic flow as represented in  FIG. 1  (schematically represented by flow lines within the chip  18 ). Using such framework, augmented by ex situ observations, the high forces have generally been attributed to the thick chip developed in the process, without an explanation of the cause of such anomalous chip formation. 
         [0006]    In view of the above, it can be appreciated that it would be desirable if methods were available for machining relatively soft and/or ductile materials with reduced cutting forces and thinner resulting chips. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The present invention provides methods capable of producing chips by laminar flow in materials in which sinuous flow may otherwise occur during machining. 
         [0008]    According to one aspect of the invention, a method of machining a body to controllably produce a chip wherein the body is formed of a material and in a state such that the material exhibits sinuous flow during a machining operation is provided that includes providing a layer located on a surface of the body, and machining the body by causing engagement between a cutting tool and the body in a contact region below an area of the surface having the coating layer thereon and moving the cutting tool relative to the body to produce the chip having the layer thereon. The layer reduces sinuous flow in the material of the body and the chip is formed primarily by laminar flow. 
         [0009]    Other aspects of the invention include the chip produced/formed by the method described above. 
         [0010]    Technical effects of the method described above preferably include the capability of machining materials in a softened state to produce chips via laminar flow rather than sinuous flow inherent in certain materials, preferably resulting in reduced cutting forces and thinner chips. 
         [0011]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic representation of plane-strain cutting showing chip formation by smooth laminar flow with simple shear. 
           [0013]      FIG. 2  is an image showing a sinuous flow mode of deformation in an annealed copper workpiece during a machining operation. 
           [0014]      FIG. 3  is an image of an optical micrograph showing the chip of  FIG. 2 . 
           [0015]      FIG. 4  is an image representing strain distribution in the chip of  FIG. 2 , with mean &lt;ε&gt;=5.62. The highly inhomogeneous strain distribution inside the chip is reflective of multiple folds during deformation. 
           [0016]      FIG. 5  is a sequence of images (a, b, c, and d) with superimposed streaklines showing the development of folds during deformation in an annealed copper workpiece. 
           [0017]      FIG. 6  represents two adjacent streaklines demarcating a single fold formed during deformation of an annealed copper workpiece. P is the fold peak (curve maximum), M 1  and M 2  are fold troughs (curve minima), M is the midpoint of line M 1 M 2 , P′ is the maximum of the second streakline, and the axial line PP 0  and the line PM subtend angles φ and θ with M 1 -M 2 . The fold amplitude A and width W are the lengths of lines PM and M 1 M 2 , respectively. 
           [0018]      FIG. 7  is a scatter plot of φ and θ representing the first (circle), second (triangle), and third (square) streaklines from the free surface. The values fall around the 45° line, deviation from which implies non-uniform streakline spacing. Marker color indicates fold width (W), where darker color indicates a large fold and lighter color indicates a small fold. All of the wide folds underwent large shear and the smaller ones remained upright (φ and θ were about 90°). 
           [0019]      FIG. 8  is a histogram of fold widths. The mean width (Wm) is represented with a dashed line at about 50 μm and one standard deviation is represented with dotted lines. The mean wavelength of the folds was about 200 μm at the point of formation. 
           [0020]      FIG. 9  in an image representing a strain rate field with superimposed streaklines when machining a surface-hardened copper workpiece. A sharply defined narrow shear zone is shown, as assumed in conventional plasticity models. The flow was laminar with insignificant bump formation ahead of the tool-chip interface. 
           [0021]      FIG. 10  is a graph representing a comparison of cutting forces for various surface conditions on copper workpieces, including a hardened surface, an annealed surface, and a coated annealed surface. The insets schematically represent machining operations performed to obtain the represented data. 
           [0022]      FIG. 11  shows a plot of energies and cutting force versus cutting distance obtained during a machining operation performed on an annealed copper workpiece. The cutting energy was computed from force (E force ) measurements and PIV (E piv ) analysis. E piv =E sin +E sub  was obtained by integrating the stress along pathlines in the PIV flow field. E sin  and E sub  were the energies dissipated in the chip and the subsurface, respectively. The specific energy for sinuous flow (U sin ) was E sin  per unit volume. The cutting force F c  is also shown. 
           [0023]      FIG. 12  is a schematic representation of a machining process being performed on a workpiece comprising a hardened layer thereon having an induced strain and capable of reducing sinuous flow during the machining process. 
           [0024]      FIG. 13  is a schematic representation of a machining process being performed on a workpiece comprising a coating layer thereon capable of reducing sinuous flow during the machining process. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The following discussion is directed to processes and systems by which relatively soft and/or ductile metals and alloys, such as annealed metals, or other ductile materials can be machined with reduced cutting force and to yield a relatively thin chip, and the products made thereby. The machining process represented in  FIG. 1  represents a plane-strain machining process involving a cutting tool  12  in contact with and being moved relative to a workpiece  14  in order to cause plastic (irreversible) shape transformation in and remove an upper portion  16  of the workpiece  14  to form a chip  18 . The process is schematically represented in  FIG. 1  as characterized by formation of the chip  18  by simple shear resulting in a smooth, laminar flow of material in the upper portion  16  of the workpiece  14 . For convenience, processes, systems, and products disclosed below will be described in reference to the machining process and orientation represented in  FIG. 1 . However, it should be understood that the processes and systems of this disclosure are applicable to a variety of machining processes such as but not limited to cutting, turning, and boring processes, as well as other types of metal processes such as forming and surface conditioning processes. For convenience, consistent reference numbers are used throughout the drawings to identify the same or functionally equivalent elements. 
         [0026]    Referring to  FIG. 1 , a deformation zone is indicated between points A and O, h 0  is an initial thickness of the upper portion  16  of the workpiece (bulk material)  14  that is to be removed by the machining process, h c  is the thickness of the resulting chip  18 , and V 0  is the bulk material flow velocity. The arrow adjacent V 0  indicates the direction of relative travel between the cutting tool  12  and the workpiece  14 . As represented, h 0  is measured from an outermost surface  22  of the workpiece  14  (referred to herein as the free surface) to a material separation surface  24 , that is, the depth of the upper layer  16  to be removed to yield a final surface  20  of the resulting machined workpiece  14 . During the machining process, the workpiece  14  undergoes plastic shape transformation to form the chip  18  with the thickness h c . The face of the cutting tool  12  is in contact with the material in the deformation zone and is represented as being fixed at normal (90°) to the cutting direction. 
         [0027]    Though conventional wisdom has been that the structure of the chip  18  is homogeneous and results from laminar plastic flow (schematically represented by flow lines within the chip  18 ), investigations leading to the present invention determined that relatively soft and/or ductile metals, particularly annealed metals and/or alloys having a microstructure characterized by relatively large grains, do not exhibit laminar plastic flow during machining as conventionally assumed. Instead, these metals exhibit a mesoscopic deformation mode referred to herein as “sinuous flow,” a mode of deformation in the same genre as kinking and shear banding. 
         [0028]      FIGS. 2 through 4  contain images that depict sinuous flow in a workpiece  14  during a machining operation of a type carried out in various natural and industrial machining processes. The workpiece  14  was an annealed, oxygen-free (99.99%), high conductivity (OFHC) copper sample having an average grain size of about 500 μm. The workpiece  14  was annealed in air at 750° C. for four hours and oven cooled to room temperature. The machining process was performed as described above in relation to  FIG. 1 . The cutting tool  12  was a hard steel wedge that traveled at a velocity of V 0 =0.42 mm/s, and the cutting depth was maintained at h 0 =50 μm. During the machining operation, the flow of the material against the face of the cutting tool  12  was observed in situ and photographed using a high-speed camera. The images were post-processed using Particle Image Velocimetry (PIV) to obtain a comprehensive record of velocity, strain rate, and strain field histories. This enabled quantitative characterization of material flow past the edge of the cutting tool  12 . 
         [0029]      FIG. 2  is an image derived from a high speed image sequence that shows streaklines that represent a highly unsteady flow with significant vorticity. The streaklines are extensively folded over in the chip  18 , with peak-to-peak amplitudes in a single fold being as much as two-thirds of the chip thickness (h c  in  FIG. 1 ). Small surface protuberances or bumps, which formed in the compressive field just ahead of the face of the cutting tool  12 , were concluded to have triggered the folding. One such bump is represented as being bounded by two arrows ( 1  and  2 ) in  FIG. 2 . These arrows demarcate pinning points which were central to fold growth. The entire chip  18  thus formed by repeated folding of the incoming material, that is, sinuous flow, which bore little resemblance to any flow reported in classical plasticity. 
         [0030]    The occurrence of sinuous flow cannot easily be inferred purely from post-mortem structural observations in the chip  18  or force measurements. As an illustration, an optical micrograph of the removed chip  18  is shown in  FIG. 3 . The surface of the chip  18  (left side) shows repeated mushroom-like formations with gaps in between. This structure has previously been described in literature as resulting from homogeneous flow, supplemented by cracking on the chip free surface. In situ analysis in investigations leading to the present invention revealed that the strain field in the chip  18  was actually highly nonhomogeneous, as seen in  FIG. 4 .  FIG. 4  is an image representing strain distribution in the chip  18  of  FIG. 2 , with mean strain &lt;ε&gt;=5.62. The highly inhomogeneous strain distribution inside the chip  18  is reflective of multiple folds during deformation. As such,  FIG. 4  indicated that the previous belief that the structure seen in  FIG. 3  resulted from homogeneous flow was erroneous. 
         [0031]      FIG. 5  includes four frames (images a, b, c, and d) that represent the evolution of a bump into a fold over time. Two labeled points P 1  and P 2 , bounding the initial bump, moved along with the material during the machining process as apparent from inspection of subsequent frames. A white dotted line representative of an axis of the bump is shown as indicating the orientation of the impending fold. The shading of the streaklines depicts the underlying strain rate field obtained from PIV calculations. 
         [0032]    In image (a) of  FIG. 5 , P 1  and P 2 , corresponding to grain boundaries, delimited the initial bump and appeared to act as local pinning points, forcing the bump to deform plastically and resulting in a pronounced bulge on the free surface  22  in image (b). The underlying strain rate field reflected this deformation in the two local zones surrounding the initial bump (images a and b). The bump axis was nearly parallel in both frames. Simultaneous with surface bulging, the material of the workpiece  14  was also constantly forced against the vertical face of the cutting tool  12 . This constraint imparted a vertical velocity to each point in the material. The bulge in image (b) was hence sheared, causing the axis to rotate in a counter-clockwise direction as represented in image (c). The magnitude of shear increased as the material neared the face of the cutting tool  12 . The bulge was amplified while also reducing its original width (image d), with the material between P 1  and P 2  constituting a single impending fold. Folding was complete once the original bump axis was rotated by nearly 90°, at which time another bulge was initiated ahead of the face of the cutting tool  12  and the process repeated. 
         [0033]    It can be seen that the chip  18  comprised a series of folds, developed one after another in the manner described above. Corresponding folds in the streakline pattern provided quantitative geometric fold characteristics as well as variations along the chip thickness h c . The results of this analysis are summarized in  FIGS. 6 through 8 .  FIG. 6  represents two adjacent streaklines demarcating a single fold formed during deformation of an annealed copper workpiece  14 . P is the fold peak (curve maximum), M 1  and M 2  are fold troughs (curve minima), M is the midpoint of line M 1 M 2 , P′ is the maximum of the second streakline, and the axial line PP 0  and the line PM subtend angles φ and θ with M 1 M 2 . The fold amplitude A and width W are the lengths of lines PM and M 1 M 2 , respectively.  FIG. 7  is a plot of θ vs. φ representing inhomogeneous shear in the material. 
         [0034]    For symmetrically sheared folds, the maxima of adjacent streaklines were expected to lie on a line PM in  FIG. 6  which corresponds to a 45° dashed line in  FIG. 7 . However, local shear resulted in a varying distance between adjacent streaklines, as indicated in  FIG. 6 . Additionally, both φ and θ values were clustered near 0° and 180°, which indicate a large shear. Geometrically, this brought fold peaks P closer to the extrapolated minima line M 1 M 2 . Most wide folds underwent large shear, while a minor fraction (small folds) remained upright (φ and θ about 90°) and formed over existing larger folds. The distribution for width W, including the mean width Wm of about 50 μm and one standard deviation, is shown in  FIG. 8 . The wider folds (Wm≧150 μm) occurred near the beginning of the streaklines, getting progressively narrower as material flowed past the cutting tool. Subsequently, the small folds, constituting 10% of the total, were developed. The mean and maximum fold widths were smaller than the initial grain size in the material (about 500 μm). The average fold wavelength was about 200 μm at the point of formation. 
         [0035]    One consequence of this sinuous flow mechanism was that the resulting chip  18  was relatively thick, having a final thickness (h c ) of about fourteen times the initial thickness (h 0 ) as seen in the images of  FIG. 5 . However, this significant thickening was not a priori indicative of the actual unsteady folding phenomenon, for such a shape change can also be envisaged in the framework of ideal smooth laminar flow ( FIG. 1 ). Characteristically, however, the sinuous flow also produced a highly non-uniform strain field in the chip  18 , fluctuating between 4 and 8, that reflects the underlying fold pattern ( FIG. 4 ). The representative (volume-weighted) strain for sinuous flow was about 5.6 which was much lower than for an equivalent shape change by laminar flow, corresponding to a strain of about 8.1. 
         [0036]    Similar to strain, the specific energy U (energy per unit volume) for chip formation, that is, shape transformation, was also significantly smaller for the sinuous flow. By the usual integration of stress and strain along path lines in the sinuous flow field, the specific energy for sinuous flow U sin  was obtained as 2.9 J/mm 3  ( FIG. 11 ). In comparison, the corresponding value for an equivalent laminar flow (with ε≅8.1) was U lam =4.2 J/mm 3 , which was 45% greater than U sin . Based on the strain and specific energy, the shape transformation into a chip  18  was thus much more efficiently achieved by sinuous flow than by laminar flow. This is counterintuitive since, at first sight, the highly-folded, sinuous flow appears quite inefficient, involving extensive redundant deformation. But since selection of collective deformation modes is in general governed by their relative stability, the material&#39;s preference for sinuous flow is likely a result of a flow instability in smooth laminar flow. 
         [0037]    To confirm the above observations, additional investigations were performed on relatively soft and/or ductile materials other than copper, including a-brass and commercially pure aluminum. Chip formation for these materials was also by sinuous flow indicating that it is a truly mesoscopic mode, independent of the material&#39;s crystal structure. Those skilled in the art will appreciate that sinuous flow should not be confused with the transition between laminar and rotational dislocation motion, which occurs at a much smaller scale. 
         [0038]    The discovery of sinuous flow appears to explain the long-standing problem in machining relatively soft and/or ductile metals and/or alloys. In particular, the mechanism of fold formation during sinuous flow is strongly tied in with the large grain size and ductility common to annealed metals and is driven primarily by the ability of the material to undergo large plastic deformation. Microscopically, each grain roughly constitutes a single fold, consistent with both the formation mechanism ( FIG. 5 ) and the fold width distributions ( FIG. 8 ). 
         [0039]    Sinuous flow also provides an explanation of the Rehbinder effect, which is a documented phenomenon relating to a small reduction (less than 10%) of the cutting forces required to machine a workpiece upon application of a suitable volatile fluid (for example, CCl 4 ) on the free surface of the workpiece. This effect was described in the publication, “P. Rehbinder, “New physico-chemical phenomena in the deformation and mechanical treatment of solids,” Nature, 159:866-867 (1947), incorporated herein by reference in its entirety. The effect has traditionally been attributed to either microcracks on the free surface promoting a physico-chemical effect or a fundamental change in the dislocation structure near the surface of the workpiece. Besides the speculative nature of these explanations, the reports of the cutting force reductions have been inconsistent. However, sinuous flow provides an explanation for this effect, in that potentially any surface application, including volatile CCl 4 , may modify the surface mechanical state of a workpiece to some extent and inhibit initial bump formation ahead of a cutting tool. Consequently, folding may be diminished, resulting in lower necessary cutting forces. The inconsistent force reductions previously observed may be due to a large variability in the initial state (annealed, partially/fully-hardened) of the workpiece arising from the specific preparation procedures. 
         [0040]    In view of the above-noted investigations, it was concluded that the difficulty in machining annealed metals and/or alloys may be resolved if sinuous flow could be suppressed, reduced, or eliminated altogether, for example, by modifying the surface characteristics to overcome the effects of the large grain size and ductility of the annealed metals. Therefore, the following describes methods of machining workpieces formed of soft and/or ductile metals and/or alloys and other relatively soft and/or ductile materials, by which sinuous flow was shown to be suppressed or eliminated through the use of surface treatments and/or applications of coatings to their free surface prior to the machining operation. Such surface treatments and/or coatings are intended to modify the surface properties of the workpiece such that the resulting chips are formed by laminar flow, rather than sinuous flow. The inventors are not aware of any research into the Rehbinder effect that suggested modification of the free surface of the workpiece via pre-straining or adherence of coating layers. Rather, it is believed that the research was limited to volatile fluids that did not adhere to the free surface of the workpiece and instead tended to volatilize during the machining operation. 
         [0041]      FIG. 12  schematically represents a first nonlimiting embodiment of the invention, in which a workpiece  14  formed of a soft and/or ductile metal or alloy may undergo a surface treatment in order to yield a pre-strained hardened layer  30  on the workpiece  14  prior to performing a machining operation on the workpiece  14 . The hardened layer  30  is intended to modify the surface properties at the free surface  22  of the workpiece  14  such that the resulting chip  18  is formed by laminar flow, rather than sinuous flow. Preferably, the hardened layer  30  is treated to have a strain of equal to or greater than one. Such surface treatment causes refinement of the grain size and a reduction in the ductility of the metal or alloy at the free surface  22  thereof to remove triggers believed to effect sinuous flow, including bulge formation and the establishment of pinning points. The pre-straining may be accomplished by various surface deformation processes known in the art and which will not be discussed in any detail. Although the hardened layer  30  could have a thickness of greater than h 0 , it preferably has a thickness of equal to or, as represented in  FIG. 12 , less than h 0  in order for the final surface  20  that remains after the cut to have the original properties of the workpiece  14  (soft or ductile), or in order for portions of the chip  18  to have the original properties of the workpiece  14 , depending on which is the intended final product. Preferably, the hardened layer  30  has a minimum thickness sufficient to provide a surface hardness or ductility suitable for reducing or eliminating sinuous flow. 
         [0042]      FIG. 9  represents a machining operation that was performed on a hardened pre-strained copper workpiece  14 . The image represents a strain rate field with superimposed streaklines during the machining operation. The workpiece  14  was machined at a velocity of V 0 =0.42 mm/s, and the cutting depth was maintained at h 0 =50 μm. The hardened layer  30  had a depth of equal to or less than 50 μm. As represented, the material adjacent the cutting tool  12  exhibited a sharply defined, narrow shear zone that produced a relatively thin chip  18  formed by laminar flow. No significant bump formation ahead of the tool-chip interface was observed and folding was prevented. The cutting force and strain required to cut the pre-strained copper workpiece  14  were both about 70% below that required to cut the same material without pre-straining. These results indicated that performing surface treatments to form a hardened layer  30  in an annealed metal prior to a machining operation can significantly reduce sinuous flow during the machining operation. 
         [0043]      FIG. 13  schematically represents a second nonlimiting embodiment of the invention, in which a workpiece  14  formed of an annealed or otherwise relatively soft and/or ductile metal or alloy may undergo a coating process in order to provide a coating layer  40  on the workpiece  14  prior to performing a machining operation on the workpiece  14 . The coating layer  40  is intended to modify the surface properties at the original free surface  22  of the workpiece  14  on which the coating layer  40  was deposited, such that the resulting chip  18  is formed by laminar flow, rather than sinuous flow. Preferably, the coating layer is either an amorphous material, or a crystalline material having a reduced average grain size and a lower ductility than the metal or alloy of the workpiece  14  to remove triggers believed to effect sinuous flow, including bulge formation and the establishment of pinning points. The coating layer  40  may be deposited by various known deposition methods, and therefore particular coating techniques and parameters will not be discussed in any detail. The coating layer  40  can be quite thin, even much smaller than the undeformed chip thickness. Although the coating layer  40  could have a thickness of greater than h 0 , it preferably has a thickness of equal to or, as represented in  FIG. 13 , less than h 0  in order for the final surface  20  that remains after the cut to have the original properties of the workpiece  14  (soft or ductile), or in order for portions of the chip  18  to have the original properties of the workpiece  14 , depending on which is the intended final product. Preferably, the coating layer  40  has a minimum thickness sufficient to provide a surface hardness or ductility suitable for reducing or eliminating sinuous flow. As a nonlimiting example, the coating layer  40  may have a depth of about 50 μm or less, or depending on the application, 10 μm or less. As represented, the coating layer  40  is above the material separation surface  24  and away from the tool-chip contact region. 
         [0044]      FIG. 10  is a graph comparing cutting forces (force in the direction of V 0 ) for different surface conditions, including a hardened copper workpiece  14   a  processed to have a pre-strained hardened layer  40  (lower inset image), and an annealed copper workpiece  14   b  (upper inset image) having unmodified portions and portions with a coating layer  40  deposited thereon. Upper and lower insets schematically represent the machining operation performed on the annealed copper workpiece  14   b  and the hardened copper workpiece  14   a , respectively. As represented, the annealed copper workpiece  14   b  was surface-coated over half its length with a coating layer  40 , which was formed of a marking ink. The workpieces  14   a  and  14   b  were machined at a velocity of V 0 =0.42 mm/s, and the cutting depth was maintained at h 0 =50 μm. For the workpiece  14   a , the hardened layer  30  had a depth of about 50 μm, and for the workpiece  14   b , the coating layer  14   b  had a depth of about 10 μm. 
         [0045]    When machining the annealed copper workpiece  14   b , the cutting force in the unmodified region was very large and chip formation was via sinuous flow. Once the cutting tool  12  entered the coated region, a drastic reduction (greater than 50%) in the cutting force was observed and chip formation changed to laminar flow. This effect of the coating layer  40  was similar to that observed in the workpiece  14   a  due to the pre-strained hardened layer  30 . The subsequent application of the coating layer  40  on the free surface  22  of the hardened copper workpiece  14   a  yielded no measurable effect on required cutting forces. These results indicated that depositing the coating layer  40  on an annealed metal workpiece prior to a machining operation can significantly reduce sinuous flow during the machining operation. 
         [0046]    Various coating materials were tested on annealed workpieces, including inks, resins, paints, and lacquers which were found to suppress sinuous flow to various degrees. Such surface layer applications, however, did not have any noticeable influence on the forces and flow when cutting pre-strained workpieces, where the flow is intrinsically laminar. As such, suitable coating materials may include any material that is at least partly nonvolatile and capable of adequately adhering to the free surface of the workpiece, being applied as coating layer of desired thickness, and forming a coating layer having surface properties capable of suppressing the nucleation of unsteady flows, and occurrence of flow phenomena leading to folds in the chip, that is, sinuous flow. A particular example of a coating material is an ink commercially available under the brand name Dykem® owned by Illinois Tool Works which contains colored pigments in an alcohol (propanol+diacetone alcohol) medium. 
         [0047]    The above embodiments provide suitable surface treatments and coatings that can suppress sinuous flow, thereby enabling improved processing of ductile metals, alloys, or other materials that would otherwise exhibit sinuous flow during machining operations. The large reduction in forces translates directly into an equivalent energy reduction. As such, the surface treatments and coatings described herein may be used as effective and simple methods for improving the machinability of materials such as, but not limited to, stainless steels, copper, aluminum, tantalum, and titanium and nickel alloys widely used in automotive, aerospace, biomedical, and energy applications. For example, the reduced cutting forces and energy dissipation may provide benefits for industrial machining, such as but not limited to, avoiding or reducing chatter-vibration instability across a broader range of process conditions, improving component surface quality, and enhancing tool life. 
         [0048]    It is foreseeable that the surface treatments, coatings, and machining operations described above could be performed on separate machines and with the surface treatment or coating operation performed at any time prior to the machining operation, or in a single system in which the surface treatment or coating operation is performed immediately prior to the machining operation. For example, it is foreseeable that a system could, either continuously, in batches, or individually, provide a material (workpiece  14 ), induce a pre-strain in a surface of the material to provide a hardened layer  30  via a surface deformation process or apply and cure/dry a coating layer  40  on the surface of the material, and then perform a machining operation on the surface of the material, such as the machining process represented in  FIGS. 1, 12, and 13 . The scope of the invention includes the chips  18  formed by the embodiments described above. Such chips  18  may include the hardened layer  30  or the coating layer  40 , whichever is used, and at least a second layer comprising the material of the workpiece  14 . Depending on the application, it may be desirable to remove the hardened layer  30  (for example, by annealing) or the coating layer  40  after the machining operation has completed. 
         [0049]    It should be noted that while references have been made in this disclosure to improvements to the machinability of annealed metals and/or alloys, the concepts of this disclosure are also applicable to various non-annealed ductile metals and/or alloys, as well metals and alloys in a partially hardened state, that is, partially annealed. In addition, it is foreseeable and within the scope of the invention that various non-metal materials may be used with the above noted embodiments, including applying the coating layer  40  on certain polymers that would otherwise exhibit sinuous flow during a machining operation. As used herein, the term metal encompasses metals, alloys, and metallic materials. 
         [0050]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the workpiece  14  and cutting tool  12  could differ from that shown, and materials and processes/methods other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.