Controlling Gradation of Surface Strains and Nanostructuring by Large-Strain Machining R

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Scripta Materialia 60 (2009) 17–20 www.elsevier.com/locate/scriptamat

Controlling gradation of surface strains and nanostructuring by large-strain machining R. Calistes, a S. Swaminathan, b T.G. Murthy, a C. Huang, a C. Saldana, a M.R. Shankar c and S. Chandrasekar a, * aCenter for Materials Processing and Tribology, School of Industrial Engineering, Purdue University, 315 N Grant Street, West Lafayette, IN 47907-2023, USA bDepartment of Mechanical and Astronautical Engineering, Naval Postgraduate School, Monterey, CA 93943-5146, USA cDepartment of Industrial Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA Received 24 July 2008; revised 5 August 2008; accepted 14 August 2008 Available online 7 September 2008

Direct measurement of the deformation ﬁeld associated with the creation of a machined surface shows that the strains in the surface and chip are large and essentially equal. This is supported by microstructure and hardness data. By interpreting these results in the framework of an upper bound model for the deformation, control of severe plastic deformation parameters on the machined surface is seen to be feasible, suggesting interesting opportunities for nano- and microscale engineering of surface microstructures by machining. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Surfaces; Nanostructure; Severe plastic deformation; Machining

The formation of a chip during machining oc- Electron backscatter diffraction (EBSD) analysis of curs by severe plastic deformation (SPD) in a deforma- deformation in machined stainless steel indicates that tion zone ahead of the advancing tool cutting edge [1] . the subsurface microstructure is also ultrafine-grained Controlled, large plastic strains (of the order of 1–15) (UFG) and this UFG region extends tens of microme- can be imposed in the chip in a single pass of deforma- ters into the depth [12] . These UFG microstructures tion by design of the tool rake angle and deformation are a consequence of SPD of the surface occurring in zone geometry [2,3] . This SPD methodology has been a manner that is perhaps analogous to that on surfaces exploited as a means for producing bulk and particulate subjected to sliding, rolling, high-pressure torsion or nanostructured materials [3,4] . The deformation zone in mechanical attrition [13–17] . Nanoscale microstructures machining, while unitary in structure, is still of finite have also been reported on a copper surface produced geometry that permeates a zone ahead of the cutting by ‘‘mechanical grinding ” with a rotating spherical tool tool and into the surface of the bulk workpiece [5] . An [18] . While the occurrence of an UFG microstructure on inevitable consequence of this aspect is SPD in the mate- machined surfaces created under select conditions has rial layer near the machined surface. In general terms, been recognized, what appears not to have been consid- the strains in this layer are quite large (>5) at the surface ered is the possibility of systematically controlling the and decay with depth into the subsurface over a region SPD parameters (e.g. strain, strain rate and deformation that is tens of micrometers in extent [6–9] . The structure geometry) during the creation of this surface. Recent of the layer on copper surfaces created by abrasive studies of machining deformation using particle image machining has been found to consist of equiaxed grains velocimetry (PIV) [2,5] , although more focused on 30 nm in size with an orientation texture similar to understanding evolution and control of deformation in that of rolling [8,10] . A fine-scale microstructure of the chip, suggest the intriguing possibility of varying in 100 nm grains has been observed on hardened steel a systematic way the gradation of deformation strains surfaces under selected machining conditions [11] . on the surface. In this study, it is shown using direct measurement of strain rate and strain fields, and supported by microstructure and hardness data, that the * Corresponding author. E-mail: [email protected] deformation levels on the machined surface are very

18 R. Calistes et al. / Scripta Materialia 60 (2009) 17–20 similar to those in the chip. By interpreting these obser- displacement of ‘‘asperities ” specifically introduced onto vations in the framework of an upper bound model, the side face of the sample and applying the PIV tech- control of surface deformation using machining pro- nique, the displacement and velocity fields in the defor- cesses is discussed. Since deformation parameters deter- mation zone and adjoining machined surface were mine the microstructure, this opens up opportunities for obtained [2,5] . The strain rate field was estimated by realizing controlled nanostructuring and, more gener- spatial differentiation of the velocity field, and the strain ally, of controlling the microstructure of surfaces by field, by integration of the strain rate field along particle machining. trajectories. Commercial grade, oxygen-free high-conductivity The microstructure of the machined surface and chips (OFHC) copper (99.95%, Goodfellow) and brass (70 in copper was analyzed using transmission electron Cu-30 Zn) were machined at near-ambient temperature microscopy (TEM). Electron-transparent specimens (298 K) in a linear, plane strain machining set-up using representative of the surface and chip were prepared sharp, high-speed steel tools [2,19] . The samples, in the using a combination of mechanical polishing and elec- form of plates 3 mm in width, had initial grain sizes of trolytic jet-thinning. The specimens were then studied 35 ± 5 lm (Cu) and 200 lm (brass), and Vickers hard- in a JEOL 2000FX microscope operating at 200 kV. ness of 77 ± 3 kg mm –2 (Cu) and 80 kg mm –2 (brass). Hardness values for the surface and chip were obtained The tool was kept stationary and the sample was moved using Vickers indentation (100 g load) and nanoindenta- perpendicular to the tool cutting edge. An undeformed tion (Nanoindenter XP, MTS Nano Instruments). To chip thickness of 100–200 lm and a machining speed estimate the variation of hardness with depth into the of 10 mm s –1 were employed, this somewhat low speed subsurface, indentations were made on tapered sections being selected to ensure minimal temperature rise in of the samples (with the edge integrity protected by a the deformation zone, as confirmed by infrared ther- nickel layer) so as to obtain a high depth resolution of mography [19] . Tools of different rake angles ( À30 ° to the hardness. +20 °) were used to impose different levels of strain in Figure 1 a and b show the shear strain rate determined the deformation zone [1,2] . using PIV for copper (+20 °) and brass ( À30 °), respec- PIV, a correlation-based image analysis method, was tively. The region of high strain rate characterizes the used to directly map the strain rate and strain fields with deformation zone in these figures. A narrow region of particular attention being paid to evolution of deforma- severe deformation with strain rate of about 50–100 s –1 tion in the material constituting the machined surface extends across the chip width and, more importantly [2,5] . For mapping the deformation, one side of the plate for the present discussion, permeates some distance into sample was constrained along its entire length by a the subsurface underneath the tool as well, as shown by transparent glass block, 6 mm thick, to ensure minimal the arrows in Figure 1 . The extent of this permeation is side flow of the material during the machining while en- much greater in the copper even when using a positive abling direct observation of material flow through the rake angle tool than in the brass with the negative rake deformation zone and into the machined surface region. angle tool. As with the chip [2,5] , most of the strain A high-speed charge-coupled device (CCD) camera (Ko- resulting on the machined surface is imposed in this se- dak Motion Corder Analyzer Sr-Ultra, maximum frame vere deformation region when the material constituting rate 10 4 s–1 ), positioned stationary with respect to the the surface generated in the wake of the tool traverses cutting edge, was used to image the deformation zone it ( Fig. 1 ). The spatial contiguity of the chip and the ma- from the side of the sample constrained by the glass chined surface suggests that the level of strain on the block. A sequence of images, in time steps ( Dt) corre- machined surface is probably the same as that prevailing sponding to the framing rate of the camera (250 s –1 ) in the chip. The permeation of the severe deformation was collected to characterize the deformation field region into the subsurface for a given material increases parameters. By following the motion and relative with decreasing rake angle such that the more negative

Figure 1. Shear strain rate ﬁelds obtained using the PIV for (a) copper, +20 ° rake and (b) brass, À30 ° rake. AB and CD are two particle trajectories along which the strain rate ﬁeld was integrated to obtain subsurface strains. Particle trajectories into the chip were used to estimate chip strains. Author's personal copy

R. Calistes et al. / Scripta Materialia 60 (2009) 17–20 19 the rake angle, the more intense and more extensive the chip thickness, the latter being a consequence of the subsurface straining on the machined surface. Impor- self-similar nature of the deformation in machining. tantly, the analysis of material flow using the PIV also These observations have also been noted in finite ele- reveals a bifurcation of this flow ahead of the cutting ment analysis of machining [21] . Thus, for example, by tool, with one part of the material becoming a part of machining with negative rake angles and a large unde- the nanostructured chip and the other being subducted formed chip thickness, the extent of the severely de- to become the machined surface. The point of this bifur- formed layer on a machined surface can be enhanced. cation shows a strong variation with the deformation Consistent with the strain levels, the Vickers hardness geometry wherein, as the rake angle becomes more neg- on the machined surface of copper is high, 155 kg ative, the bifurcation point shifts upward, and hence the mm –2 , and decreases with increasing depth into the sub- amount of the subducted material and extent of the sur- surface, eventually reaching that of the bulk material at face deformation strain increase at the smaller (more a depth of 100 lm ( Fig. 2 ). The surface hardness of negative) rake angles ( Fig. 1 b). 155 kg mm –2 is practically indistinguishable from that The extent of deformation on the machined surface of the copper chip created at this deformation condition. and its correspondence to the level of SPD in the chip Furthermore, when the copper was machined at liquid was confirmed by direct estimates of the shear strain de- nitrogen temperature, both the surface and chip showed rived from the PIV analysis. The variation of strain with a higher hardness value of 190 kg mm –2 , again indicat- depth into the surface is shown in Figure 2 for brass and ing an equivalency of the deformation condition in these copper. The strains were estimated by integrating the two regions. Nanoindentation data were consistent with strain rate field along particle trajectories (e.g. dotted the Vickers hardness values. lines AB and CD in Fig. 1 ) through the deformation Figure 3 shows a TEM micrograph and selected-area zone [5] . The plastic strain levels are seen to be quite diffraction pattern (inset) representative of the fine-scale large near the surface and approaching those of the lev- microstructure on the machined surface of copper els in the chip. Furthermore, the strains are greater for (strain 8). The microstructure is seen to consist of the surface created with the negative rake angle tool cor- nanoscale structures with an average size of 175 ± 30 responding to the greater levels of strain in the chips at nm. This microstructure, as well as those recorded with the negative rake angles. The surface strains obtained by the other rake angle tools, were found to mirror those of extrapolation trends of the subsurface strain distribu- chips produced under the same deformation conditions tions are about 3 (+10 ° rake) and 10 ( À30 ° rake) in [3] and of copper created by multipass equal channel the brass, and 5 (+20 ° rake) in the copper. These strain angular processing at similar levels of strain [22] . Obser- values are essentially indistinguishable from those esti- vations of evolution of deformation/microstructure in mated in the chip using the PIV data; for reference, large-strain machining of materials such as Pb, Ti, Al the corresponding chip strains are also given in Figure 6061-T6 and stainless steel also support this correspon- 2. Furthermore, the strain values are consistent with dence [2,5,12,19] . those inferred from analysis of curvature of flow lines The observations suggest interesting opportunities near the surface [20] . This close correspondence between for controlling the strain and strain rate in generation the strain on the surface and in the chip shows that the of surfaces by machining, and the microstructure on ma- chip strain, which can be approximately (but analyti- chined surfaces. An approximate estimate of the shear cally) estimated using the upper bound model, is a good strain ( c) imposed by the SPD can be obtained from indicator of the deformation on the machined surface. the upper bound model as [1,23] : While the strains decay with depth into the subsurface k 1 (Fig. 2 ), there is nevertheless a region, 100 lm thick, c ¼ þ À 2 tan a; ð1Þ on the machined workpiece that is intensely strained. cos a k cos a The thickness of this deformed layer increases with where k = t c/t is the chip compression ratio (=1/ r, where decreasing rake angle and with increasing undeformed r is the cutting ratio in machining), tc and t are the deformed and undeformed chip thickness, respectively, and a is the tool rake angle. This strain value can be taken as indicative of that prevailing on the machined

Figure 2. Variation of strain ( c) with depth from the machined surface in copper and brass. The dotted curves are extrapolations used to estimate the strains at the surface. The strain values in the chip are marked as A (brass, À30 ° rake), B (copper, +20 ° rake) and C (brass, +10 ° rake). Also shown is the variation of hardness with depth for Figure 3. TEM image of copper surface showing nanoscale micro- copper. structure, c 8.0 ( À10 ° rake). Author's personal copy

20 R. Calistes et al. / Scripta Materialia 60 (2009) 17–20 surface because of the correspondence established be- Since these determine the resulting microstructure and tween the surface and chip strains. While the model texture on the machined surface/subsurface, unprece- tends to overestimate the strain at negative a [2] , it nev- dented opportunities exist for controlled engineering of ertheless provides a guiding framework for analyzing the surfaces using relatively simple machining processes. deformation. In conventional machining, the strain is determined This work was supported in part by the NSF via primarily by a since k (i.e. tc) is not controllable but is grants CMMI 0626047 and CMMI 0654250. an outcome of the deformation [1] . Eq. (1) indicates that the strain is small at large a, where k is typically close to 1 [1] , but increases quite steeply with decreasing a espe- [1] M.C. Shaw, Metal Cutting Principles, second ed., Clar- cially for negative a. Thus, by employing tools with large endon Press, Oxford, 1984. negative a, a larger strain can be imposed on the ma- [2] S. Lee, J. Hwang, M.R. Shankar, S. Chandrasekar, W.D. chined surface in a given material consistent with the re- Compton, Metallurgical and Materials Transactions A 37 sults of the present study. Furthermore, since the (2006) 1633. machining process is self-similar as long as t is much [3] W. Moscoso, M.R. Shankar, J.B. Mann, W.D. Compton, greater than the tool edge radius, the depths of the S. Chandrasekar, Journal of Materials Research 22 (2007) 201. strained layer and of the surface layer with a fine-scale [4] J.B. Mann, C. Saldana, S. Chandrasekar, W.D. Compton, microstructure may be increased significantly by using K.P. Trumble, Scripta Materialia 57 (2007) 909. larger t and a more negative a. There is a critical value [5] M.R. Shankar, B.C. Rao, S. Lee, S. Chandrasekar, A.H. of a below which chip-formation ceases to occur as King, W.D. Compton, Acta Materialia 54 (2006) 3691. per the upper bound model. Below this a, material flow [6] L.E. Samuels, Metallographic Polishing by Mechanical around the cutting edge resembles a process of ‘‘large- Methods, fourth ed., American Society for Materials strain extrusion ”; in this variant of machining, large- International, Materials Park, OH, 2003. strains can be imposed over thick layers with attendant [7] K. Zum Gahr, Microstructure and Wear of Materials, nanostructuring of the surface. This deformation case is Elsevier, Amsterdam, 1987. probably similar to that occurring in some types of roll [8] H. Wilman, Wear 14 (1969) 249. [9] P.L.B. Oxley, The Mechanics of Machining, Ellis Hor- processing of the surface. In contrast, the surface defor- wood, Chichester, 1989. mation condition corresponding to a small t, large neg- [10] D.M. Turley, L.E. Samuels, Metallography 14 (1981) 275. ative a case, resembles that in abrasive machining, [11] S. Akcan, S. Shah, S.P. Moylan, P.N. Chhabra, S. sliding or surface roll processing with imposition of Chandrasekar, H.T.Y. Yang, Metallurgical and Materials large plastic strains [1,6,9] . But systematic control of Transactions A 33 (2002) 1245. deformation parameters in this regime is difficult, if [12] R. M’Saoubi, L. Ryde, Materials Science and Engineering not, impossible, since these will be determined by meso- A 405 (2005) 339. scale characteristics of the tool cutting edge, including [13] D.A. Rigney, Wear 245 (2000) 1. edge radius and topography. At the other extreme, the [14] G. Baumann, H.J. Fecht, S. Liebelt, Wear 191 (1996) 133. deformation imposed on the machined surface can be [15] D.A. Hughes, D.C. Chrzan, Q. Liu, N. Hansen, Physical Review Letters 81 (1998) 4664. significantly reduced by employing tools with large posi- [16] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progress tive a. This is the preferred condition for ensuring a sur- in Materials Science 45 (2000) 103. face microstructure that shows the least deviation from [17] K.Y. Zhu, A. Vassel, F. Brisset, K. Lu, J. Lu, Acta that of the bulk. Large-strain extrusion machining, a Materialia 52 (2004) 4101. constrained chip-formation variant of machining, pro- [18] W.L. Li, N.R. Tao, K. Lu, Scripta Materialia 59 (2008) vides for independent control of both k and a through 546. the application of a constraint at the point of chip-for- [19] C. Huang, T.G. Murthy, M.R. Shankar, R. M’Saoubi, S. Chandrasekar, Scripta Materialia 58 (2008) 663. mation fixing tc a priori [3,23] . This two-parameter control also enables extraordinary control of deformation [20] J.H. Dautzenberg, J.H. Zaat, Wear 23 (1973) 9. levels. [21] M. Sevier, H.T.Y. Yang, S. Lee, S. Chandrasekar, Metallurgical and Materials Transactions B 38 (2007) 927. In summary, by direct measurement of the deforma- [22] F. Dalla Torre, R. Lapovok, J. 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