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Cutting edge preparation of cutting tools using plasma discharges in electrolyte
Journal of Manufacturing Processes 46 (2019) 234–240
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Cutting edge preparation of cutting tools using plasma discharges in electrolyte
T
Tomáš Vopát, Štefan Podhorský, Martin Sahul, Marián Haršáni Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava, Institute of Production Technologies, Jána Bottu 25, Trnava, 91724, Slovakia
A R T I C LE I N FO
A B S T R A C T
Keywords: Cutting edge microgeometry Edge preparation Cutting tool geometry Machining Plasma Discharges in electrolyte Cemented carbide
The paper deals with the issue of cutting edge preparation and tool treatment before coating. In this article, the novel method called edge preparation by plasma discharges in electrolyte is presented. The mechanism of material removal is based on melting and vaporisation instead of abrasion and it is not influenced or limited by the hardness of the tool material. The shape of the vapour-plasma envelope leads to intensive material removal from the edges of cutting tools. This phenomenon was applied to round the sharp edges of electrically conducting cutting tools. The tested cutting tool material of samples was cemented carbide. The cutting edge radii were formed by immersing the cutting tool into an electrolyte. The increasing the processing time results in a larger cutting edge radius. The values of cutting edge radii increased from 10 μm to 45 μm in 50 s of treatment by plasma discharges in electrolyte. Moreover, the grinding marks on the surfaces of the cemented carbide turning insert prepared by plasma discharges in electrolyte were smoothed. The quality of surface roughness of cemented carbide samples was increased. If cemented carbide turning inserts are prepared using plasma discharges in electrolyte after grinding then the values of surface roughness parameter Ra will be lower from 0.223 μm to 0.186 μm in the cross direction to the grinding marks and from 0.183 μm to 0.117 μm in the longitudinal direction.
1. Introduction The theoretical analysis was focused on the issue of cutting edge microgeometry, discussing the classification and importance of cutting edge microgeometry and edge preparation methods. Publications in [1,2] were focused on the optimum shape of cutting edge and surface of cutting tool that was prepared by various preparation methods. Publication [3] summarized the issue of cutting edge microgeometry. Authors determined to what extent does cutting edge microgeometry affect the wear behavior, the chip formation and forces and the surface integrity. In the experiment, Yue et al. [4] showed the good quality of surface roughness in machining brittle and hard material by diamond cutting tools with sharp cutting edges. The new conception of process of a 5-axis brushing for the forming asymmetrical cutting edge microgeometry was demonstrated in [5]. Publication [6] deals with the designing and cutting edge microgeometry. For the edge preparation was used the method of wet abrasive jet machining with a robot guided system that allows forming a specific design of the cutting edge. Yussefian et al. [7] used electrical discharge machining as a novel
method for the cutting edge preparation. The cutting edge was immersed into an appropriate counterface. This new method for edge preparation is not influenced or limited by the hardness of the tool material. The new conception of process for the surface treatment and edge preparation using magneto-abrasive is discussed in [8]. The using of laser ablation for manufacturing the complex-shaped cutting edge microgeometry on the cemented carbide cutting tools could have an advantageous effect on the tool life [9]. The laser beam ablation and the abrasive flow machining were used for cutting edge preparation of cemented carbide cutting tools with respect to the manufacturing of complex-shaped microgeometries [10]. The appropriate set conditions of laser machining eliminate the negative effects of thermal affected zones associated with its thickness, material properties and coating adhesion to the substrate [11]. Denkena et al. [12] found that mill wear behaviour depends on the design of the location and the undercut at the cutting edge. The cutting edge radius is very important factor in consideration of chip formation process especially in micro machining [13]. Authors in [14] confirmed that cutting forces are negatively affected by small feed rate near the cutting edge radius due to a disturbed chip formation. The issue of the symmetric microgeometry of the cutting edge on
E-mail address: [email protected] (T. Vopát). https://doi.org/10.1016/j.jmapro.2019.08.033 Received 1 July 2019; Received in revised form 22 August 2019; Accepted 28 August 2019 Available online 24 September 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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the mechanics of chip formation was first reported by Albrecht [15]. Ventura et al. [16] documented that asymmetric microgeometry increases tool life. The applicability of the prepared cutting inserts was demonstrated and the use of asymmetric microgeometries in hard turning proved to be adequate with respect to compressive residual stresses, though higher cutting forces were obtained. Denkena et al. [17] proposed that edge microgeometry can be characterised by four parameters as Sα (cutting edge start point on flank from virtual sharp edge), Sγ (cutting edge start point on rake face from virtual sharp edge) and Δr (distance of the edge apex to the virtual sharp edge) and parameter φ locates the tool apex relative to the tool faces. Rodriguez [18] and Wyen [19] et al. attempted to address the issue above by defining the different approaches as Denkena et al. described. Ventura et al. [20] investigated the influence of the cutting edge geometry on the performance of cutting inserts made of cubic nitride boron. The cutting edge rounding were modified by several chamfers. The tests of CBN cutting inserts were performed during the interrupted hard turning. The tool wear and mechanisms were analysed. The best cutting edge microgeometry was evaluated. Rech et al. [21] showed that the behaviour of coated milling inserts depends strongly on the surface treatment before the coating. The modifying cutting edge radius and tool surface are the main criteria that affect wear resistance. Puneet et al. [22] determined the influence of particular surface treatments on the tool life of PVD coated drills. In the experiments, authors investigated that drills prepared using a lower particle size of Al2O3 medium increased tool life and adhesion strength. It was related to the surface roughness of the substrate. The process of plasma discharges in electrolyte, also known as plasma treatment in electrolyte, plasma-electrolytic process, electrolytic-plasma process or plasma electrolysis is utilised for the surface treatment of metallic parts, for example for polishing [23,24], surface cleaning [25], heat treatment, thermochemical treatment and anodic oxidation [26]. It is a relatively nonstandard method of surface treatment and it is an environmentally friendlier alternative [23] to today’s conventional electrochemical process. In this article, authors presented a novel method for the edge preparation of cutting tools using plasma discharges in electrolyte (PDE). The use of the method of the cutting edge preparation of a tool and a tool machined in this manner has been patented due to its originality [27].
decreasing the area open for the electric current, and in such a way, further increasing the local density of the electric current in the places free from bubbles. It means a further increase in the electrolyte overheating above the surface of the active electrode, bubble boiling increases, the local density of the electric current continues to grow and it results in an avalanche effect. Vapour bubbles coalesce into a film and the bubble boiling is replaced by stable film boiling [29,30]. The film is formed mostly from evaporated water as was explained above, but there is some gas (oxygen) due to the process of electrolysis which takes part concurrently. The phenomena described takes place on the active electrode only. The counter electrode must have a much larger surface area [23,24] so there is much less density of the electric current, much less concentrated heat released and therefore no gaseous film forms on the passive electrode. The thin gaseous film on the active electrode is electrically nonconductive under normal circumstances and separates the metal surface from the electrolyte. In this way, the electric current is broken so the electric circuit gets disconnected. However, when the voltage between the electrodes is high enough - a few hundred volts, the gaseous film becomes ionised due to the high value of electric field intensity inside the volume of the thin film [24]. It becomes electrically conductive in such cases and the electric current flows through this film in the form of glow discharge. The ionised gaseous layer over the surface of the active electrode is called a vapour-plasma envelope envelope [25,30,31], shell [30] or skin [23]. The glow discharge formed on the active electrode consists of many ionised channels, narrow columns (pinch effect). These discharge columns always run toward the summits or protuberances of the surface profile since the distance between the metal surface and the wall of electrolyte is shortest here. In this way the surface summits are quickly removed [28]. When the material of a summit is removed by the discharge, the column of discharge moves to the next surface summit where the distance to the opposite wall of the electrolyte is less. Moreover, each summit of the surface acts as a concentrator of the electric field strength so the discharge columns are “’attracted” here. Thus the surface texture is continually smoothed and the value of the surface roughness decreases. The process is of a dynamic nature. This phenomenon is just utilised in the case of PDE treatment. There are several theories about the mechanism of material removal from the metal surface [32]. The process of the plasma discharge is explained in more detail according to streamer theory [24,32,33]. 2.2. Utilisation of the plasma discharges in electrolyte for edge rounding
2. Process concept of a novel method for cutting edge preparation The process of PDE treatment (like the common electro-polish) is often used, not as a method of surface treatment, but for deburring [23] and edge rounding purposes of machined pieces. The essential feature of this technology is the formation of the vapour-plasma envelope around the active electrode, i.e. around the treated object. The gaseous volume of the vapour-plasma envelope displaces the electrolyte from the metal surface and a wall of the electrolyte is formed in such a way. A system consisting of two parallel electrodes is then created; the first electrode is the metallic surface of the treated object and the second one is the wall of the displaced electrolyte. The gaseous film as well as the wall of the electrolyte copies the surface of the metal and the film has some uniform thickness which depends on the process parameters. But the uniformity of the thickness of the vapour-plasma envelope is broken on the sharp edges of the treated object. The wall of electrolyte cannot copy the exact shape of the edge; it bends at a certain radius. The radius depends first of all on the physical properties of the liquid, but the thickness of the vapourplasma envelope is definitely less here than the overall thickness (Fig. 1). The thickness of the vapour-plasma envelope is a key feature of the process since it determines the density of the electric current passing through it. The less thick it is, the higher the intensity of the electric
2.1. Basic principle of the plasma-electrolytic process The principle of the process of plasma-electrolytic process was discussed in [28] by Podhorsky. This principle was also utilised for the preparation of cutting tools using PDE. The schematics of the PDE can resemble common electro-polishing at first glance. The metal part to be treated is immersed into electrolyte and is anodically polarised, i.e. connected to the positive pole of a DC electric current supply. The next electrode is connected to the negative pole of the power supply. The main differences between the plasma-electrolytic treatment and common electro-polishing are in the value of the voltage and in the chemical composition of the electrolyte. The high concentrated mixture of acids used for standard electro-polishing is usually displaced by a low-concentrated aqueous solution of chemically neutral salts. A thin gaseous film is formed on the entire treated surface due to a high value of current density on the interface between the electrolyte and the metal surface. The high value of the current density means a high amount of heat is released so local boiling of the electrolyte takes place here [26]. The boiling of electrolyte is accompanied by the generation of bubbles which grow and detach the surface of the active electrode. Vapour bubbles cover a part of the active electrode, thereby 235
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The electrolyte used during the experiment was made up from 10% water solution of ammonium sulphate in a volume of 25 litres. The temperature of the electrolyte was kept at 70 °C by a liquid cooling/ heating system. The voltage between the electrodes was set to 300 V. The tested cutting tool material of samples was cemented carbide. The specific grade of the cemented carbide was K20 - K30 by ISO (89.6% of WC, 10% of Co and 0.4% of other carbides). The sizes of samples and their shape are labelled according to standardized designation of turning inserts. The surface area of the one cemented carbide turning insert was approximately 522 mm2. It is a surface area that is in contact with the electrolyte.
Fig. 1. The principle of the cutting edge rounding.
field is so the current density is higher as well [29]. Moreover, each edge acts as a concentrator of the electric field strength, which concurrently increases the local density of the electric current [31]. The higher the current density is, the higher the material removal rate from the surface is. This fact leads to intensive material removal from the edges of the treated object. The edges gradually become more and more rounded. When visually observing the PDE process, the vapour-plasma envelope illuminates and the luminescence is by far the most intense just on the edges of the treated object. This phenomenon can be exploited to generate controlled cutting edge radii on cutting tools by immersing the cutting tools with sharp cutting edges into an appropriate electrolyte. It represents an innovative perspective that amplifies the usage of PDE in the area of cutting tool production. Furthermore, as the mechanism of material removal is based on melting and vaporisation instead of abrasion, material removal is not influenced or limited by the hardness of the tool material. It just needs to possess the required electrical conductivity. Prepared cutting tools may have a simple or complex shape and can be a cutting insert, a milling tool, a drill or other.
3.2. Cutting edges rounding and surface roughness The aim of this experiment was to round the cutting edges and observe the surface roughness. In this experiment, the CNHA 120408 cemented carbide turning inserts were selected. The cemented carbide turning inserts had not had any edge preparation, however every surfaces were grinded. A JEOL JSM 7600 F scanning electron microscope (SEM) was used to observe the cutting edge and surface after preparation using PDE and before preparation. The surface roughness parameter Ra was measured by Surtronic 3+ measuring device for comparison of the changes in surface roughness. The arithmetical mean deviation of the assessed profile (Ra) was measured on the rake face of the cemented carbide turning insert in the cross direction to the grinding marks (black line) and the longitudinal direction (white line), as seen from the SEM image (Fig. 3). In the experiment, the following process parameters were chosen:
• C e – the concentration of the electrolyte 10%, • T e – the temperature of the electrolyte of 70 °C, • t – the processing time of 30 s.
3. Materials and methods
3.3. Formation of cutting edge radius controlled by PDE process 3.1. Experimental details The aim of the experimental analysis was to form the cutting edge radius of varied size. The CNMG 120408E-SM cemented carbide turning inserts were selected for the research. The following process parameters were set in the experiment:
The technology equipment for PDE treatment in electrolyte consists of two separate units: a power supply unit supplying direct current to the electrodes of the electrolytic circuit and a bath unit with the electrolyte, where the actual PDE process takes place. The supply unit comprises a control unit. The issue of stable power supply was crucial in the development of related technology since the electrolytic circuit in the working area has a negative dynamic resistance [31]. The power supply output limits the maximum size of the area to be treated, it is about 5–6 dm2 for the power supply used (35 kVA). It is impossible to directly control the value of the current density in PDE treatment. The current density is determined by the process parameters; its value can be reduced by increasing the temperature of the electrolyte, which, however, has a negative impact on the final quality of the treated surface. A bath unit consists of a working vessel with technological accessories such as pumps, a heat exchanger, a ventilation device with a condenser, a hanging unit, a protective cover of the vessel, a bath with rinse water, heating elements and sensors. From a construction point of view, a bath unit represents a rather complex system when compared to the device for electrochemical operations. From an electrical point of view, a bath unit represents a simple circuit: it consists of a vessel with electrolyte and a hanging rod used for hanging the holders with treated objects in the vessel. The vessel also serves as the counting electrode – the cathode. The vessel of dimension 300 x 150 mm (top view) and of height 300 mm, made from AISI 304 grade stainless steel, has been used. Experimental samples were fixed to a holder with nuts, both made of stainless steel (AISI 304). The holder had no surface insulation, since no electric current measurement had been planned. The holder with the samples was immersed into the centre of the vessel, in the depth of 150 mm under the level of the electrolyte, as seen from the Fig. 2.
• C e – the concentration of the electrolyte 10%, • T e - temperature of the electrolyte of 70 °C The process parameters as the concentration and the temperature of the electrolyte were kept constant while the processing time was varied (t 1 = 10 s, t 2 = 30 s, t 3 = 50 s, t4 = 120 s, t5 = 180 s, t6 = 240 s, t7 = 300 s) during the edge preparation of PDE. The cemented carbide turning inserts were cleaned in an ultrasonic cleaning device and dried after the edge preparation process. To observe the cutting edges after the PDE, a JEOL JSM 7600 F scanning electron microscope (SEM) was used to provide various views as in the previous preliminary experiment. The shape of the cutting edge microgeometry was observed in the cutting edge plane Ps as shown in Fig. 4b. For this reason, Fig. 4a has been illustrated for better explanation. The reason why the cutting edge radius was observed in this cutting edge plane Ps is because the authors needed to identify potential defects which would be located on the cutting edge. In this area of the nose radius, the cutting tool is in contact with the workpiece when machining. 4. Results 4.1. Cutting edges rounding and surface roughness Fig. 5 shows the SE micrographs of cutting edges after grinding 236
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Fig. 2. Fixing the cutting inserts in the electrolyte bath during the experiments.
The sharp cutting edge was rounded after edge preparation (Fig. 5b). Moreover, the burrs and chipping were not observed on the cutting edge. The results of the experiment confirmed the assumption that it is possible to utilise PDE for cutting edge preparation. As seen from the SEM images, the grinding marks on the rake face of the cemented carbide turning insert prepared by PDE were smoothed (Fig. 5b) in contrast to Fig. 5a. Moreover, the results of measurement of surface roughness parameter Ra (arithmetical mean deviation of the assessed profile) of cemented carbide turning inserts prepared by PDE and unprepared ones are shown in Fig. 6. It is clear from the graph, if cemented carbide turning inserts are prepared using plasma discharges in electrolyte after grinding then the values of surface roughness parameter Ra will be lower from 0.223 μm to 0.186 μm in the cross direction to the grinding marks and from 0.183 μm to 0.117 μm in the longitudinal direction. 4.2. Formation of cutting edge radius controlled by PDE process Fig. 3. Surface roughness parameter Ra measurement on the rake face of the cemented carbide turning insert.
This section discusses in greater detail the formation of cutting edge radius size controlled by PDE process. The magnified micrographs of the cemented carbide turning insert prepared by PDE treatment and before are shown in Fig. 7. Fig. 7a shows the SE micrographs of cutting edges after sintering and microblasting before they were prepared by PDE. This cemented carbide turning insert (Fig. 7a) was manufactured with rn =10.36 μm, where dry microblasting was only used for deburring but cutting edges were not further prepared. After observation with a SEM, the cutting edge was rounded after edge preparation and no defects were observed on the cutting edge (Fig. 7b). The forming of the cutting edge radius runs along the whole cemented carbide turning insert on both sides at once.
before they were prepared (Fig. 5a) and after the preparation by PDE (Fig. 5b). Cemented carbide turning insert after grinding with no preparation did not have perfect sharp cutting edges as seen from the SE micrographs. There are observed burrs and edge chipping which are typical defects of cutting edges after grinding (Fig. 5a). However, the sizes of these defects have to be smaller than the planned cutting edge radius size of the cemented carbide turning insert after preparation. If the sizes of these defects are larger than the planned cutting edge radius size, cutting edge microgeometry will be affected due to other shapes which remain after grinding.
Fig. 4. SEM image of cutting edge observation (U = 300 V, Te =70 °C, Ce = 10%, t = 30 s). 237
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Fig. 5. SEM images of cutting edge formation and tool surfaces.
cutting edge radii increased from 10 μm to 45 μm in 50 s of PDE treatment. As seen from the graph, the dependence of the cutting edge radius size on the processing time is nonlinear, its speed progressively decreases. This fact is in full compliance with the theory of edge rounding mechanism, described above. By regression analysis for the experimental data using the least squares method, the dependence can be defined by the next formula Eqn (1):
rn = 4.89 + 0.957 * t − 0.00124 * t 2
(1)
However, it is not needed to form a cutting edge radius that has large values when cutting tools are primarily used for general machining. The processing time of edge preparation takes approximately up to 70 s with respect to the specific application of the cutting tools. These values of cutting edge radii present a wide range of cutting with respect to the machining conditions, machined material and other aspects. By limiting the measured data to the interval from 0 s to 120 s for the regression analysis, a linear expression Eqn (2) can be used instead of Formula (1) within this interval:
Fig. 6. Measured values of surface roughness parameter Ra of cemented carbide turning inserts (U = 300 V, Te =70 °C, Ce = 10%, t = 30 s).
rn = 5.79 + 0.838 * t
Fig. 8 shows the plotted 3D and 2D shapes of cutting edge microgeometry. The cemented carbide turning inserts were measured with respect to the shape of the cutting edge. Then, the influence of edge preparation time on the cutting edge formation was evaluated from the measurement of the cutting edge radius on a Zeiss LSM 700 laser confocal microscope. Another measurement of cutting edge radius size was evaluated from the SEM micrographs from the JEOL JSM 7600 F scanning electron microscope. Every measurement of a cutting edge radius size was repeated five times. After that, the average value was calculated and inserted into the graph (Fig. 9). The plotted graph in Fig. 9 expresses the dependence of cutting edge radius size on the processing time. The results of the experiment confirmed that cutting edge radii of varied sizes are possible to be formed using PDE. From the graph (Fig. 9) it is clear that increasing the processing time t results in a larger cutting edge radius rn. The values of
(2)
From the point of view of such quick processing time, this novel method of edge preparation of cutting tools by PDE can be one of the most productive methods. 5. Conclusion This paper presents the novel process of using plasma discharges in electrolyte for the edge preparation of cutting tools. The principle of the process of plasma discharges in electrolyte was demonstrated. When the voltage between the electrodes (cutting tool) is a few hundred volts, the gaseous film becomes ionised due to a high value of the electric field intensity inside the volume of the thin film. It becomes electrically conductive in such cases and the electric current flows through this film in the form of glow discharge. The ionised gaseous layer (or film) over 238
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Fig. 7. SEM images of cutting edge microgeometry observation.
Fig. 8. Example of the measurement of a cutting edge radius on a Zeiss LSM 700 laser confocal microscope after an edge preparation time t1 of 10 s (U = 300 V, Te =70 °C, Ce = 10%).
arithmetical mean deviation of the assessed profile (Ra) was measured on the rake face of the cemented carbide turning insert. Since, each summit of the surface acts as a concentrator of the electric field strength so the discharge columns are “’attracted” here. Thus the surface texture
the surface of the cutting tool is called a vapour-plasma envelope where material removal from the surface occurs. The surface roughness parameter Ra was measured for comparison of the changes in surface roughness after PDE treatment and before. The 239
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Fig. 9. The dependence of the cutting edge radius size on the processing time (U = 300 V, Te =70 °C, Ce = 10%).
was continually smoothed and the value of the surface roughness decreased after PDE treatment. The principle of the formation of the cutting edge radius on the cemented carbide turning insert was discussed. The gaseous film, as well as the wall of the electrolyte, copies the surface of the cutting tool and the film has some uniform thickness which depends on the process parameters. The wall of electrolyte cannot copy the exact shape of the edge; it bends at a certain radius. The thickness of the vapour-plasma envelope in the area of the cutting edge is definitely less than the overall thickness; therefore the material removal from the surface is the most intensive here. The cutting edges gradually become rounded. In the experiment, the formation of a cutting edge radius on the cemented carbide cutting insert using PDE was proven. The increasing the processing time of edge preparation by PDE results in a larger cutting edge radius. The dependence of the cutting edge radius size on the processing time is nonlinear, its speed progressively decreases. Further work will focus on the influence of the other process parameters of PDE on cutting edge radius sizes and the surface roughness of cutting tools. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Scientific Grant Agency of the Slovak Republic under the grant no. 1/0097/17 and the Slovak Research and Development Agency of the Slovak Republic under the Contract no. APVV-16-0057. References [1] Bouzakis KD, Bouzakis E, Kombogiannis S, Makrimallakis S, Skordaris G, Michailidis N, et al. Effect of cutting edge preparation of coated tools on their performance in milling various materials. CIRP J Manuf Sci Technol 2014;7:264–73. https://doi.org/10.1016/J.CIRPJ.2014.05.003. [2] Uhlmann E, Oberschmidt D, Löwenstein A, Kuche Y. Influence of cutting edge preparation on the performance of Micro milling tools. Procedia CIRP 2016;46:214–7. https://doi.org/10.1016/J.PROCIR.2016.03.204. [3] Denkena B, Biermann D. Cutting edge geometries. CIRP Ann 2014;63:631–53. https://doi.org/10.1016/J.CIRP.2014.05.009. [4] Yue X, Xu M, Du W, Chu C. Effect of cutting edge radius on surface roughness in diamond tool turning of transparent MgAl2O4 spinel ceramic. Opt Mater (Amst) 2017;71:129–35. https://doi.org/10.1016/J.OPTMAT.2016.04.017. [5] B. Denkena, L. de Leon, E. Bassett Five-Axis brushing for cutting edge preparation. ATZ produktion Worldw n.d.;2:18–21. doi:10.1007/bf03224185. [6] Biermann D, Aßmuth R, Schumann S, Rieger M, Kuhlenkötter B. Wet abrasive jet machining to prepare and design the cutting edge Micro Shape. Procedia CIRP
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Cutting edge preparation of cutting tools using plasma discharges in electrolyte
T
Tomáš Vopát, Štefan Podhorský, Martin Sahul, Marián Haršáni Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava, Institute of Production Technologies, Jána Bottu 25, Trnava, 91724, Slovakia
A R T I C LE I N FO
A B S T R A C T
Keywords: Cutting edge microgeometry Edge preparation Cutting tool geometry Machining Plasma Discharges in electrolyte Cemented carbide
The paper deals with the issue of cutting edge preparation and tool treatment before coating. In this article, the novel method called edge preparation by plasma discharges in electrolyte is presented. The mechanism of material removal is based on melting and vaporisation instead of abrasion and it is not influenced or limited by the hardness of the tool material. The shape of the vapour-plasma envelope leads to intensive material removal from the edges of cutting tools. This phenomenon was applied to round the sharp edges of electrically conducting cutting tools. The tested cutting tool material of samples was cemented carbide. The cutting edge radii were formed by immersing the cutting tool into an electrolyte. The increasing the processing time results in a larger cutting edge radius. The values of cutting edge radii increased from 10 μm to 45 μm in 50 s of treatment by plasma discharges in electrolyte. Moreover, the grinding marks on the surfaces of the cemented carbide turning insert prepared by plasma discharges in electrolyte were smoothed. The quality of surface roughness of cemented carbide samples was increased. If cemented carbide turning inserts are prepared using plasma discharges in electrolyte after grinding then the values of surface roughness parameter Ra will be lower from 0.223 μm to 0.186 μm in the cross direction to the grinding marks and from 0.183 μm to 0.117 μm in the longitudinal direction.
1. Introduction The theoretical analysis was focused on the issue of cutting edge microgeometry, discussing the classification and importance of cutting edge microgeometry and edge preparation methods. Publications in [1,2] were focused on the optimum shape of cutting edge and surface of cutting tool that was prepared by various preparation methods. Publication [3] summarized the issue of cutting edge microgeometry. Authors determined to what extent does cutting edge microgeometry affect the wear behavior, the chip formation and forces and the surface integrity. In the experiment, Yue et al. [4] showed the good quality of surface roughness in machining brittle and hard material by diamond cutting tools with sharp cutting edges. The new conception of process of a 5-axis brushing for the forming asymmetrical cutting edge microgeometry was demonstrated in [5]. Publication [6] deals with the designing and cutting edge microgeometry. For the edge preparation was used the method of wet abrasive jet machining with a robot guided system that allows forming a specific design of the cutting edge. Yussefian et al. [7] used electrical discharge machining as a novel
method for the cutting edge preparation. The cutting edge was immersed into an appropriate counterface. This new method for edge preparation is not influenced or limited by the hardness of the tool material. The new conception of process for the surface treatment and edge preparation using magneto-abrasive is discussed in [8]. The using of laser ablation for manufacturing the complex-shaped cutting edge microgeometry on the cemented carbide cutting tools could have an advantageous effect on the tool life [9]. The laser beam ablation and the abrasive flow machining were used for cutting edge preparation of cemented carbide cutting tools with respect to the manufacturing of complex-shaped microgeometries [10]. The appropriate set conditions of laser machining eliminate the negative effects of thermal affected zones associated with its thickness, material properties and coating adhesion to the substrate [11]. Denkena et al. [12] found that mill wear behaviour depends on the design of the location and the undercut at the cutting edge. The cutting edge radius is very important factor in consideration of chip formation process especially in micro machining [13]. Authors in [14] confirmed that cutting forces are negatively affected by small feed rate near the cutting edge radius due to a disturbed chip formation. The issue of the symmetric microgeometry of the cutting edge on
E-mail address: [email protected] (T. Vopát). https://doi.org/10.1016/j.jmapro.2019.08.033 Received 1 July 2019; Received in revised form 22 August 2019; Accepted 28 August 2019 Available online 24 September 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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the mechanics of chip formation was first reported by Albrecht [15]. Ventura et al. [16] documented that asymmetric microgeometry increases tool life. The applicability of the prepared cutting inserts was demonstrated and the use of asymmetric microgeometries in hard turning proved to be adequate with respect to compressive residual stresses, though higher cutting forces were obtained. Denkena et al. [17] proposed that edge microgeometry can be characterised by four parameters as Sα (cutting edge start point on flank from virtual sharp edge), Sγ (cutting edge start point on rake face from virtual sharp edge) and Δr (distance of the edge apex to the virtual sharp edge) and parameter φ locates the tool apex relative to the tool faces. Rodriguez [18] and Wyen [19] et al. attempted to address the issue above by defining the different approaches as Denkena et al. described. Ventura et al. [20] investigated the influence of the cutting edge geometry on the performance of cutting inserts made of cubic nitride boron. The cutting edge rounding were modified by several chamfers. The tests of CBN cutting inserts were performed during the interrupted hard turning. The tool wear and mechanisms were analysed. The best cutting edge microgeometry was evaluated. Rech et al. [21] showed that the behaviour of coated milling inserts depends strongly on the surface treatment before the coating. The modifying cutting edge radius and tool surface are the main criteria that affect wear resistance. Puneet et al. [22] determined the influence of particular surface treatments on the tool life of PVD coated drills. In the experiments, authors investigated that drills prepared using a lower particle size of Al2O3 medium increased tool life and adhesion strength. It was related to the surface roughness of the substrate. The process of plasma discharges in electrolyte, also known as plasma treatment in electrolyte, plasma-electrolytic process, electrolytic-plasma process or plasma electrolysis is utilised for the surface treatment of metallic parts, for example for polishing [23,24], surface cleaning [25], heat treatment, thermochemical treatment and anodic oxidation [26]. It is a relatively nonstandard method of surface treatment and it is an environmentally friendlier alternative [23] to today’s conventional electrochemical process. In this article, authors presented a novel method for the edge preparation of cutting tools using plasma discharges in electrolyte (PDE). The use of the method of the cutting edge preparation of a tool and a tool machined in this manner has been patented due to its originality [27].
decreasing the area open for the electric current, and in such a way, further increasing the local density of the electric current in the places free from bubbles. It means a further increase in the electrolyte overheating above the surface of the active electrode, bubble boiling increases, the local density of the electric current continues to grow and it results in an avalanche effect. Vapour bubbles coalesce into a film and the bubble boiling is replaced by stable film boiling [29,30]. The film is formed mostly from evaporated water as was explained above, but there is some gas (oxygen) due to the process of electrolysis which takes part concurrently. The phenomena described takes place on the active electrode only. The counter electrode must have a much larger surface area [23,24] so there is much less density of the electric current, much less concentrated heat released and therefore no gaseous film forms on the passive electrode. The thin gaseous film on the active electrode is electrically nonconductive under normal circumstances and separates the metal surface from the electrolyte. In this way, the electric current is broken so the electric circuit gets disconnected. However, when the voltage between the electrodes is high enough - a few hundred volts, the gaseous film becomes ionised due to the high value of electric field intensity inside the volume of the thin film [24]. It becomes electrically conductive in such cases and the electric current flows through this film in the form of glow discharge. The ionised gaseous layer over the surface of the active electrode is called a vapour-plasma envelope envelope [25,30,31], shell [30] or skin [23]. The glow discharge formed on the active electrode consists of many ionised channels, narrow columns (pinch effect). These discharge columns always run toward the summits or protuberances of the surface profile since the distance between the metal surface and the wall of electrolyte is shortest here. In this way the surface summits are quickly removed [28]. When the material of a summit is removed by the discharge, the column of discharge moves to the next surface summit where the distance to the opposite wall of the electrolyte is less. Moreover, each summit of the surface acts as a concentrator of the electric field strength so the discharge columns are “’attracted” here. Thus the surface texture is continually smoothed and the value of the surface roughness decreases. The process is of a dynamic nature. This phenomenon is just utilised in the case of PDE treatment. There are several theories about the mechanism of material removal from the metal surface [32]. The process of the plasma discharge is explained in more detail according to streamer theory [24,32,33]. 2.2. Utilisation of the plasma discharges in electrolyte for edge rounding
2. Process concept of a novel method for cutting edge preparation The process of PDE treatment (like the common electro-polish) is often used, not as a method of surface treatment, but for deburring [23] and edge rounding purposes of machined pieces. The essential feature of this technology is the formation of the vapour-plasma envelope around the active electrode, i.e. around the treated object. The gaseous volume of the vapour-plasma envelope displaces the electrolyte from the metal surface and a wall of the electrolyte is formed in such a way. A system consisting of two parallel electrodes is then created; the first electrode is the metallic surface of the treated object and the second one is the wall of the displaced electrolyte. The gaseous film as well as the wall of the electrolyte copies the surface of the metal and the film has some uniform thickness which depends on the process parameters. But the uniformity of the thickness of the vapour-plasma envelope is broken on the sharp edges of the treated object. The wall of electrolyte cannot copy the exact shape of the edge; it bends at a certain radius. The radius depends first of all on the physical properties of the liquid, but the thickness of the vapourplasma envelope is definitely less here than the overall thickness (Fig. 1). The thickness of the vapour-plasma envelope is a key feature of the process since it determines the density of the electric current passing through it. The less thick it is, the higher the intensity of the electric
2.1. Basic principle of the plasma-electrolytic process The principle of the process of plasma-electrolytic process was discussed in [28] by Podhorsky. This principle was also utilised for the preparation of cutting tools using PDE. The schematics of the PDE can resemble common electro-polishing at first glance. The metal part to be treated is immersed into electrolyte and is anodically polarised, i.e. connected to the positive pole of a DC electric current supply. The next electrode is connected to the negative pole of the power supply. The main differences between the plasma-electrolytic treatment and common electro-polishing are in the value of the voltage and in the chemical composition of the electrolyte. The high concentrated mixture of acids used for standard electro-polishing is usually displaced by a low-concentrated aqueous solution of chemically neutral salts. A thin gaseous film is formed on the entire treated surface due to a high value of current density on the interface between the electrolyte and the metal surface. The high value of the current density means a high amount of heat is released so local boiling of the electrolyte takes place here [26]. The boiling of electrolyte is accompanied by the generation of bubbles which grow and detach the surface of the active electrode. Vapour bubbles cover a part of the active electrode, thereby 235
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The electrolyte used during the experiment was made up from 10% water solution of ammonium sulphate in a volume of 25 litres. The temperature of the electrolyte was kept at 70 °C by a liquid cooling/ heating system. The voltage between the electrodes was set to 300 V. The tested cutting tool material of samples was cemented carbide. The specific grade of the cemented carbide was K20 - K30 by ISO (89.6% of WC, 10% of Co and 0.4% of other carbides). The sizes of samples and their shape are labelled according to standardized designation of turning inserts. The surface area of the one cemented carbide turning insert was approximately 522 mm2. It is a surface area that is in contact with the electrolyte.
Fig. 1. The principle of the cutting edge rounding.
field is so the current density is higher as well [29]. Moreover, each edge acts as a concentrator of the electric field strength, which concurrently increases the local density of the electric current [31]. The higher the current density is, the higher the material removal rate from the surface is. This fact leads to intensive material removal from the edges of the treated object. The edges gradually become more and more rounded. When visually observing the PDE process, the vapour-plasma envelope illuminates and the luminescence is by far the most intense just on the edges of the treated object. This phenomenon can be exploited to generate controlled cutting edge radii on cutting tools by immersing the cutting tools with sharp cutting edges into an appropriate electrolyte. It represents an innovative perspective that amplifies the usage of PDE in the area of cutting tool production. Furthermore, as the mechanism of material removal is based on melting and vaporisation instead of abrasion, material removal is not influenced or limited by the hardness of the tool material. It just needs to possess the required electrical conductivity. Prepared cutting tools may have a simple or complex shape and can be a cutting insert, a milling tool, a drill or other.
3.2. Cutting edges rounding and surface roughness The aim of this experiment was to round the cutting edges and observe the surface roughness. In this experiment, the CNHA 120408 cemented carbide turning inserts were selected. The cemented carbide turning inserts had not had any edge preparation, however every surfaces were grinded. A JEOL JSM 7600 F scanning electron microscope (SEM) was used to observe the cutting edge and surface after preparation using PDE and before preparation. The surface roughness parameter Ra was measured by Surtronic 3+ measuring device for comparison of the changes in surface roughness. The arithmetical mean deviation of the assessed profile (Ra) was measured on the rake face of the cemented carbide turning insert in the cross direction to the grinding marks (black line) and the longitudinal direction (white line), as seen from the SEM image (Fig. 3). In the experiment, the following process parameters were chosen:
• C e – the concentration of the electrolyte 10%, • T e – the temperature of the electrolyte of 70 °C, • t – the processing time of 30 s.
3. Materials and methods
3.3. Formation of cutting edge radius controlled by PDE process 3.1. Experimental details The aim of the experimental analysis was to form the cutting edge radius of varied size. The CNMG 120408E-SM cemented carbide turning inserts were selected for the research. The following process parameters were set in the experiment:
The technology equipment for PDE treatment in electrolyte consists of two separate units: a power supply unit supplying direct current to the electrodes of the electrolytic circuit and a bath unit with the electrolyte, where the actual PDE process takes place. The supply unit comprises a control unit. The issue of stable power supply was crucial in the development of related technology since the electrolytic circuit in the working area has a negative dynamic resistance [31]. The power supply output limits the maximum size of the area to be treated, it is about 5–6 dm2 for the power supply used (35 kVA). It is impossible to directly control the value of the current density in PDE treatment. The current density is determined by the process parameters; its value can be reduced by increasing the temperature of the electrolyte, which, however, has a negative impact on the final quality of the treated surface. A bath unit consists of a working vessel with technological accessories such as pumps, a heat exchanger, a ventilation device with a condenser, a hanging unit, a protective cover of the vessel, a bath with rinse water, heating elements and sensors. From a construction point of view, a bath unit represents a rather complex system when compared to the device for electrochemical operations. From an electrical point of view, a bath unit represents a simple circuit: it consists of a vessel with electrolyte and a hanging rod used for hanging the holders with treated objects in the vessel. The vessel also serves as the counting electrode – the cathode. The vessel of dimension 300 x 150 mm (top view) and of height 300 mm, made from AISI 304 grade stainless steel, has been used. Experimental samples were fixed to a holder with nuts, both made of stainless steel (AISI 304). The holder had no surface insulation, since no electric current measurement had been planned. The holder with the samples was immersed into the centre of the vessel, in the depth of 150 mm under the level of the electrolyte, as seen from the Fig. 2.
• C e – the concentration of the electrolyte 10%, • T e - temperature of the electrolyte of 70 °C The process parameters as the concentration and the temperature of the electrolyte were kept constant while the processing time was varied (t 1 = 10 s, t 2 = 30 s, t 3 = 50 s, t4 = 120 s, t5 = 180 s, t6 = 240 s, t7 = 300 s) during the edge preparation of PDE. The cemented carbide turning inserts were cleaned in an ultrasonic cleaning device and dried after the edge preparation process. To observe the cutting edges after the PDE, a JEOL JSM 7600 F scanning electron microscope (SEM) was used to provide various views as in the previous preliminary experiment. The shape of the cutting edge microgeometry was observed in the cutting edge plane Ps as shown in Fig. 4b. For this reason, Fig. 4a has been illustrated for better explanation. The reason why the cutting edge radius was observed in this cutting edge plane Ps is because the authors needed to identify potential defects which would be located on the cutting edge. In this area of the nose radius, the cutting tool is in contact with the workpiece when machining. 4. Results 4.1. Cutting edges rounding and surface roughness Fig. 5 shows the SE micrographs of cutting edges after grinding 236
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Fig. 2. Fixing the cutting inserts in the electrolyte bath during the experiments.
The sharp cutting edge was rounded after edge preparation (Fig. 5b). Moreover, the burrs and chipping were not observed on the cutting edge. The results of the experiment confirmed the assumption that it is possible to utilise PDE for cutting edge preparation. As seen from the SEM images, the grinding marks on the rake face of the cemented carbide turning insert prepared by PDE were smoothed (Fig. 5b) in contrast to Fig. 5a. Moreover, the results of measurement of surface roughness parameter Ra (arithmetical mean deviation of the assessed profile) of cemented carbide turning inserts prepared by PDE and unprepared ones are shown in Fig. 6. It is clear from the graph, if cemented carbide turning inserts are prepared using plasma discharges in electrolyte after grinding then the values of surface roughness parameter Ra will be lower from 0.223 μm to 0.186 μm in the cross direction to the grinding marks and from 0.183 μm to 0.117 μm in the longitudinal direction. 4.2. Formation of cutting edge radius controlled by PDE process Fig. 3. Surface roughness parameter Ra measurement on the rake face of the cemented carbide turning insert.
This section discusses in greater detail the formation of cutting edge radius size controlled by PDE process. The magnified micrographs of the cemented carbide turning insert prepared by PDE treatment and before are shown in Fig. 7. Fig. 7a shows the SE micrographs of cutting edges after sintering and microblasting before they were prepared by PDE. This cemented carbide turning insert (Fig. 7a) was manufactured with rn =10.36 μm, where dry microblasting was only used for deburring but cutting edges were not further prepared. After observation with a SEM, the cutting edge was rounded after edge preparation and no defects were observed on the cutting edge (Fig. 7b). The forming of the cutting edge radius runs along the whole cemented carbide turning insert on both sides at once.
before they were prepared (Fig. 5a) and after the preparation by PDE (Fig. 5b). Cemented carbide turning insert after grinding with no preparation did not have perfect sharp cutting edges as seen from the SE micrographs. There are observed burrs and edge chipping which are typical defects of cutting edges after grinding (Fig. 5a). However, the sizes of these defects have to be smaller than the planned cutting edge radius size of the cemented carbide turning insert after preparation. If the sizes of these defects are larger than the planned cutting edge radius size, cutting edge microgeometry will be affected due to other shapes which remain after grinding.
Fig. 4. SEM image of cutting edge observation (U = 300 V, Te =70 °C, Ce = 10%, t = 30 s). 237
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Fig. 5. SEM images of cutting edge formation and tool surfaces.
cutting edge radii increased from 10 μm to 45 μm in 50 s of PDE treatment. As seen from the graph, the dependence of the cutting edge radius size on the processing time is nonlinear, its speed progressively decreases. This fact is in full compliance with the theory of edge rounding mechanism, described above. By regression analysis for the experimental data using the least squares method, the dependence can be defined by the next formula Eqn (1):
rn = 4.89 + 0.957 * t − 0.00124 * t 2
(1)
However, it is not needed to form a cutting edge radius that has large values when cutting tools are primarily used for general machining. The processing time of edge preparation takes approximately up to 70 s with respect to the specific application of the cutting tools. These values of cutting edge radii present a wide range of cutting with respect to the machining conditions, machined material and other aspects. By limiting the measured data to the interval from 0 s to 120 s for the regression analysis, a linear expression Eqn (2) can be used instead of Formula (1) within this interval:
Fig. 6. Measured values of surface roughness parameter Ra of cemented carbide turning inserts (U = 300 V, Te =70 °C, Ce = 10%, t = 30 s).
rn = 5.79 + 0.838 * t
Fig. 8 shows the plotted 3D and 2D shapes of cutting edge microgeometry. The cemented carbide turning inserts were measured with respect to the shape of the cutting edge. Then, the influence of edge preparation time on the cutting edge formation was evaluated from the measurement of the cutting edge radius on a Zeiss LSM 700 laser confocal microscope. Another measurement of cutting edge radius size was evaluated from the SEM micrographs from the JEOL JSM 7600 F scanning electron microscope. Every measurement of a cutting edge radius size was repeated five times. After that, the average value was calculated and inserted into the graph (Fig. 9). The plotted graph in Fig. 9 expresses the dependence of cutting edge radius size on the processing time. The results of the experiment confirmed that cutting edge radii of varied sizes are possible to be formed using PDE. From the graph (Fig. 9) it is clear that increasing the processing time t results in a larger cutting edge radius rn. The values of
(2)
From the point of view of such quick processing time, this novel method of edge preparation of cutting tools by PDE can be one of the most productive methods. 5. Conclusion This paper presents the novel process of using plasma discharges in electrolyte for the edge preparation of cutting tools. The principle of the process of plasma discharges in electrolyte was demonstrated. When the voltage between the electrodes (cutting tool) is a few hundred volts, the gaseous film becomes ionised due to a high value of the electric field intensity inside the volume of the thin film. It becomes electrically conductive in such cases and the electric current flows through this film in the form of glow discharge. The ionised gaseous layer (or film) over 238
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Fig. 7. SEM images of cutting edge microgeometry observation.
Fig. 8. Example of the measurement of a cutting edge radius on a Zeiss LSM 700 laser confocal microscope after an edge preparation time t1 of 10 s (U = 300 V, Te =70 °C, Ce = 10%).
arithmetical mean deviation of the assessed profile (Ra) was measured on the rake face of the cemented carbide turning insert. Since, each summit of the surface acts as a concentrator of the electric field strength so the discharge columns are “’attracted” here. Thus the surface texture
the surface of the cutting tool is called a vapour-plasma envelope where material removal from the surface occurs. The surface roughness parameter Ra was measured for comparison of the changes in surface roughness after PDE treatment and before. The 239
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Fig. 9. The dependence of the cutting edge radius size on the processing time (U = 300 V, Te =70 °C, Ce = 10%).
was continually smoothed and the value of the surface roughness decreased after PDE treatment. The principle of the formation of the cutting edge radius on the cemented carbide turning insert was discussed. The gaseous film, as well as the wall of the electrolyte, copies the surface of the cutting tool and the film has some uniform thickness which depends on the process parameters. The wall of electrolyte cannot copy the exact shape of the edge; it bends at a certain radius. The thickness of the vapour-plasma envelope in the area of the cutting edge is definitely less than the overall thickness; therefore the material removal from the surface is the most intensive here. The cutting edges gradually become rounded. In the experiment, the formation of a cutting edge radius on the cemented carbide cutting insert using PDE was proven. The increasing the processing time of edge preparation by PDE results in a larger cutting edge radius. The dependence of the cutting edge radius size on the processing time is nonlinear, its speed progressively decreases. Further work will focus on the influence of the other process parameters of PDE on cutting edge radius sizes and the surface roughness of cutting tools. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Scientific Grant Agency of the Slovak Republic under the grant no. 1/0097/17 and the Slovak Research and Development Agency of the Slovak Republic under the Contract no. APVV-16-0057. References [1] Bouzakis KD, Bouzakis E, Kombogiannis S, Makrimallakis S, Skordaris G, Michailidis N, et al. Effect of cutting edge preparation of coated tools on their performance in milling various materials. CIRP J Manuf Sci Technol 2014;7:264–73. https://doi.org/10.1016/J.CIRPJ.2014.05.003. [2] Uhlmann E, Oberschmidt D, Löwenstein A, Kuche Y. Influence of cutting edge preparation on the performance of Micro milling tools. Procedia CIRP 2016;46:214–7. https://doi.org/10.1016/J.PROCIR.2016.03.204. [3] Denkena B, Biermann D. Cutting edge geometries. CIRP Ann 2014;63:631–53. https://doi.org/10.1016/J.CIRP.2014.05.009. [4] Yue X, Xu M, Du W, Chu C. Effect of cutting edge radius on surface roughness in diamond tool turning of transparent MgAl2O4 spinel ceramic. Opt Mater (Amst) 2017;71:129–35. https://doi.org/10.1016/J.OPTMAT.2016.04.017. [5] B. Denkena, L. de Leon, E. Bassett Five-Axis brushing for cutting edge preparation. ATZ produktion Worldw n.d.;2:18–21. doi:10.1007/bf03224185. [6] Biermann D, Aßmuth R, Schumann S, Rieger M, Kuhlenkötter B. Wet abrasive jet machining to prepare and design the cutting edge Micro Shape. Procedia CIRP
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