FACTA UNIVERSITATIS Series: Mechanical Engineering Vol. 17, N o 2, 2019, pp. 161 - 168 https://doi.org/10.22190/FUME190330023K © 2019 by University of Niš, Serbia | Creative Commons License: CC BY-NC-ND Original scientific paper  IMPACT OF INDENTOR SLIDING VELOCITY AND LOADING REPETITION FACTOR ON SHEAR STRAIN AND STRUCTURE DISPERSION IN NANOSTRUCTURING BURNISHING Viktor P. Kuznetsov 1 , Andrey S. Skorobogatov 1 , Vladimir G. Gorgots 2 1 Ural Federal University, Russia 2 Kurgan State University, Russia Abstract. The article probes into a relationship of the shear strain intensity and the shear strain rate in the surface layer and the sliding velocity of a spherical indentor and its loading repetition factor. It brings forward an experimental procedure to evaluate the shear strain intensity and rate by analyzing the geometrical parameters of the bulge of plastically edged metal and the thickness of the shifted layer relative to different sliding velocities and feed rates. Key Words: Surface Layer, Nanostructuring Burnishing, Spherical Indentor, Severe Plastic Shear Deformation, Sliding Velocity, Loading Repetition Factor 1. INTRODUCTION Burnishing implementation as a surface plastic deforming technique has been widely applied for finishing surface layers of precision parts on metal-cutting machines. Application of burnishing brings about adequate dimensional accuracy; it also succeeds in decreasing the height of microprofile irregularities by several times, increasing the surface layer microhardness and creating a substantial extent of compressive residual stresses. A number of authors [1-7] have studied finishing and hardening burnishing without taking into consideration the possibility of forming nanocrystalline structures. To promote burnishing the papers [8-16] bring into the foreground a finishing technique for forming a nanocrystalline structure in the surface layer, which puts to use the method of severe plastic shear deformation (SPD), named nanostructuring burnishing (NSB). The NSB technique makes it possible to impart unique performance attributes to the surface layer, Received March 30, 2019 / Accepted June 02, 2019 Corresponding author: Viktor P. Kuznetsov Ural Federal University, Institute of New Materials and Technologies, 19 Mira Street, Ekaterinburg 620002, Russia E-mail: wpkuzn@mail.ru 162 V.P. KUZNETSOV, A.S. SKOROBOGATOV, V.G. GORGOTS and it can be implemented in the process of machining on modern lathes and machining centers. However, the influence of the indentor sliding velocity and the loading repetition factor on the evolution of SPD as well as the formation of the nanocrystalline structure in the thin surface layer have not been studied in depth. Zhao et al. [17] study the formation of a gradient nano/microstructured layer in a sample of pure copper; the layer is produced by severe plastic deformation with roller burnishing (SPRB). The experimental results showed that the gradient layer had a thickness over 100 µm. The significantly increased hardness close to the surface resulted predominantly from decreasing the grain sizes. The nano-dimension grains were randomly oriented; most boundaries turned out to be low angle ones. It was in the nanostructure that dislocation activities prevailed, accompanied by rotating grains in the local area. Roland et al. [18] describe a research study of the influence that a nanocrystalline surface layer exerts on fatigue properties of a 3161 stainless steel; this layer is obtained by applying mechanical attrition treatment to the surface (SMAT). Qualitative surface layers containing ultra-fine grains or nano-grains can be formed in the process of cryogenic burnishing of an AI 7050-T7451 alloy with a roller tool [19]; it can be achieved by means of applied severe plastic deformation and dynamic recrystallization (DRX) inherent in it. The article compares dry and cryogenic burnishing. It shows that refined layers containing nano-grains come into being in the surface that undergoes cryogenic treatment. The average hardness at a depth of 200 µm is higher after cryogenic burnishing than after dry burnishing by 9.5%, 17.5 % and 24.8 % at a rate of 25, 50 and 100 m/min, respectively. A number of authors [20-26] describe the research that has been done into effects of severe plastic deformation parameters on the dispersion of grain structures in structural materials. They show that for the formation of a nanocrystalline structure it is necessary that the shear strain intensity exceeding 2 and the shear strain rate being above 10 2 s -1 should be ensured. The present research aims at establishing a relationship of the indentor sliding velocity and an integral parameter of loading repetition factor and the shear strain intensity and shear strain rate in the surface layer of X20Cr13 steel and the dispersion degree of the formed nanocrystalline structure. 2. MATERIALS AND METHODS An experiment-calculated procedure is brought forward to determine the plastic shear strain intensity and rate in the surface layer. The procedure is based on measuring the length of the bulge, which is edged by the tool, applying 3D profilometry and the depth, to which shear strain propagates, by examining the deformed surface layer with scanning microscopy (Fig.1). Material shear strain  can be determined as a tangent of the shear angle or as a ratio of length lr of the edged metal bulge to thickness hs of the sheared layer: r s tg l h    (1) The true plastic shear strain is calculated with the following relation: ln(1 ) ln(1 ) r s l h      (2) Impact of Indentor Sliding Velocity and Loading Repetition Factor on Shear Strain and Structure... 163 Fig. 1 Location of relative deformation (a) and scanning microscopy of the deformed layer (b) The value of the true shear strain makes it possible to determine a relation of the mean strain rate to indentor sliding velocity vb and loading repetition factor nc relative to the elementary volume in the material. The duration of a single-time action of the indentor on the elementary volume is determined with a relation of contact spot length ac to sliding velocity vb: 1 c b t a v (3) The contact spot length value was determined by measuring width of single path of indenter made with a giving burnishing force as shown in [14]. The resulting spot length value is 370±6 μm. As the tool is fed, generally the indentor acts upon the elementary volume several times. To take the number of these actions into consideration V.P. Kuznetsov’s paper [14] introduces an integral parameter of loading repetition factor nc, which is determined as follows: c c b n a f (4) The total deformation time for nc of the indentor’s actions is found as follows: 1 c c t c b a n t t n v   (5) Thus, the mean shear strain rate in nanostructuring burnishing of the surface layer can be calculated in the following way: b t c c v t a n     (6) Having found the value of accumulated edged material bulge lra and shifted layer thickness hs, it is now possible to evaluate accumulated shear strain intensity . To perform the present experiment-calculated procedure a scheme for burnishing circular paths on a flat sample with a high-precision stopping of the tool (Fig. 2, a) is proposed. These kinematics of the tool lead to forming an accumulated bulge of plastically shifted material at the end of the path (Fig. 2, b). 164 V.P. KUZNETSOV, A.S. SKOROBOGATOV, V.G. GORGOTS Fig. 2 Burnishing a path with a high-precision stopping (a) for forming an accumulated bulge of plastically edged material (b) The experimental study was done by machining X20Cr13 steel samples with a 2mm- radius ASPM-3 polycrystalline diamond (PCD) indentor, the burnishing force being Fb = 340 N. The paths were 10mm wide and 70mm long; three different feed rates were applied fb=0.06, 0.04 and 0.02 mm/rev at sliding velocities vb=3, 6, 8, 11, 14, 20 and 26 m/min. At these feed rates the loading repetition factor was, respectively, nc=4.2, 6.3 and 12.6. In this manner, within the framework of a full factorial experiment 21 paths were machined with a unique combination of feed and velocity parameters fb and vb. Machining was done on an OKUMA MA600BII machining center. Geometrical parameters of the bulges produced at each path were studied by optical 3D profilometry on a WYCO NT1100 profilometer. After it, a cross-section metallographic sample was prepared from the middle of each path; these samples were examined by scanning electron microscopy with a Zeiss AURIGA CrossBeam to estimate the thickness of the sheared layer. The structural analysis was done by transmission electron microscopy with a JEOL JEM 2100. The foils were taken from the surface in the middle of each path. 3. RESULT AND DISCUSSIONS On the basis of the results obtained by optical 3D profilometry and scanning electron microscopy (Fig. 3) the sought values of the plastically edged metal bulge length and the sheared layer thickness (Table 1) were found. The results thus acquired show that as the sliding velocity rises to 11 m/min, an increase of the accumulated bulge length and the sheared layer thickness regardless of the loading repetition factor can be observed. At the same time, a further increase of the velocity leads to a reverse effect. What is more, the maximum thickness of the sheared layer practically does not correlate with the loading repetition factor and remains within the range from 6.29 to 6.63 µm. Meanwhile, the length of the accumulated bulge of plastically edged material grows significantly when the loading repetition factor is increased from 272 µm at nc=4.2 to 348 µm at nc=12.5. Inserting the results of the measurements in (2), (3) and (6), the relations of the plastic shear strain intensity and the plastic shear strain rate in the surface layer of X20Cr13 steel were found (Fig. 4). Impact of Indentor Sliding Velocity and Loading Repetition Factor on Shear Strain and Structure... 165 Fig. 3 Scanning electron microscopy of the surface layer in the paths produced at loading repetition factors of 4.2 (a, b); 6.3 (c, d) and 12.5 (e, f). Table 1 Results of measurements Sliding velocity vb, m/min nc=4.2 nc=6.3 nc=12.5 Bulge length lr, μm Layer thickness hs, μm Bulge length lr, μm Layer thickness hs, μm Bulge length lr, μm Layer thickness hs, μm 3 40 5.1 48 3.24 81 3.66 6 78 5.1 42 5.54 59 5.22 8 229 5.75 183 5.58 174 5.65 11 272 6.29 277 6.47 348 6.63 14 204 6.35 202 6.42 227 5.87 20 179 5.99 165 5.76 205 5.7 26 175 6.04 169 6.08 201 6.01 Fig. 4 Relation of the plastic shear strain intensity (a) and the plastic shear strain rate (b) to the sliding velocity when burnishing at feed rates fb=0.02 mm/rev (nc=12.5), 0.04 mm/rev (nc=6.3) and 0.06 mm/rev (nc=4.2) 166 V.P. KUZNETSOV, A.S. SKOROBOGATOV, V.G. GORGOTS The maximum shear strain intensity =3.75…4 is obtained when burnishing is done at an indentor sliding velocity of 11 m/min. At lower velocities the shear strain intensity decreases considerably. At the same time, raising the velocity leads to stabilizing the shear strain intensity at 3.4…3.6. These results turn out to agree well with the experimental data acquired in the papers [14] which describe a similar correlation of the sliding velocity to the microhardness in the surface layer. Besides, the values of the shear strain intensity are confirmed by mathematic simulation presented in the paper [12]. Another conclusion can be drawn that at higher sliding velocities a surface layer shear strain comparable in regard to its intensity may be obtained at substantially higher shear strain rates. Fig. 5 Transmission electron microscopy of the surface layer in the paths obtained at loading repetition factors of 4.2 (a, b, c); 6.3 (d, e, f) и 12.5 (g, h, i). Impact of Indentor Sliding Velocity and Loading Repetition Factor on Shear Strain and Structure... 167 The microstructural analysis of the foils showed that the nanocryslalline structure evolves with various dispersion degrees depending on different combinations of the parameters of the sliding velocity and loading repetition factor that are under study. In addition to it, a relation can be traced between the dispersion of the structure being formed and the shear strain intensity in the material of the surface layer. For instance, a composite UFG- and nanocrystalline structure develops at a sliding velocity of 6 m/min and shear strain intensities below 2.8 regardless of loading repetition factor values (Fig. 5, a, d, g). In the cases when the velocity is 11 m/min and the shear strain intensity reaches 3.75…4. A homogeneous nanocrystalline structure is observed in the surface layer; it is proved by a circular shape of the interferogrammas (Fig. 5, b, e, h). If the velocity is increased to 20 m/min, the dispersion of the structure significantly decreases as a result of the shear strain intensity going down (Fig. 5, c, f, i). 4. CONCLUSIONS Notwithstanding considerable differences in the shear strain intensity and the shear strain rate, each combination of the parameters of the loading repetition factor and indentor sliding velocity within the framework of the research meets the conditions necessary for a nanocrystalline structure to form (≥2,  ≥102 s-1). The research proves that higher values of the shear strain intensity and rate are achieved in nanostructuring burnishing. This appears to be stimulating for obtaining higher dispersion degrees of the grain structure and ensures formation of a homogeneous surface layer. The results thus obtained explain the differences in the microhardness of nanostructured surfaces occurring in the cases when the sliding velocity is altered; these differences were found earlier in the papers [13, 15]. 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