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Research on Electrical Discharge Grinding Technics and Tool's Life of Polycrystalline Cubic Boron Nitride Cutting Tool
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 637 – 642
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, Bilbao, Spain
Research on Electrical Discharge Grinding Technics and tool's life of Polycrystalline cubic Boron Nitride Cutting Tool Yunhai JIAa,b*, Linxin ZHUb a
Beijing Key Laboratory of Electrical Discharge Machining Technology, Beijing 100191, China b Beijing Institute of Electro-Machining, Beijing 100191, China
* Corresponding author. Tel.: +86-010-5153-0520; fax: +86-010-5153-0520. E-mail address: [email protected]
Abstract Electrical discharge grinding is part of the most widely used methods to machine polycrystalline cubic boron nitride cutting tools. Polycrystalline cubic boron nitride compact samples at different granularity size are the research object. Electrode running speed, peak current and pulse duration is selected as the main process parameters. Material removal volume and the electrode wear set as the evaluation index of productive efficiency. Workpiece surface roughness value sets as an evaluation standard of processing quality. Through electrical discharge grinding experiments, combined with scanning electron microscopy observation, energy spectrum analyzer and roughness tester, the influences of the main process parameters on electrical discharge grinding are analyzed, results show that polycrystalline cubic boron nitride crystal granularity size has some effect on the electrical discharge grinding. With the increase of the electrode rotation speed, the workpiece material removal volume is increased, but the increase rate becomes slow. Electrode wear is further increased with the increase of the electrode rotation speed. With the peak current increase, material removal volume and the surface roughness value of the workpiece material increase, electrode wear reduces. Through the turning test, the relationship between service life of polycrystalline cubic boron nitride cutting tools that are machined by electrical discharge grinding and cutting tool flank wide, workpiece surface roughness are discussed. The results showed that to adjust electrical discharge grinding parameter, such as pulse duration and machining electric currents, can reduce the depth of the effect layer and extend the service life of polycrystalline cubic boron nitride cutting tool. The research provides a valuable experimental reference for drawing up electrical discharge grinding technics of polycrystalline cubic boron nitride cutting tool. © 2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Polycrystalline cubic boron nitride (PcBN), electrical discharge grinding (EDG), affected layer, cutting tool's life, machine technics
1. Introduction The improvement in machining performance, which leads to high production rates, high tolerance, good surface integrity and the ability to machine advanced materials, depends not only on improved cutting tool materials but also depends on other enabling technologies. Increasingly high surface integrity and close dimensional tolerances are required, which requires tools to have lower wear rates and much tighter specification of tool dimensions. Polycrystalline cubic boron nitride (PcBN) tool that is one of substitution products of traditional cutting tool has been widely applied in the manufacturing industry of developed countries in recent 20 years. The use of PcBN tool, compared to solid tungsten
carbide, has enabled machining speeds to be completed. Thread quality to be improved and the burr to be virtually eliminated [1]. To utilize Polycrystalline cubic boron nitride (PcBN) cutting tool makes precision hard turning possible, in which the hardness of the workpiece is up to 58-62 HRC [2]. So the carbide cutting tool has been massively substituted by the PcBN cutting tool. Electrical discharge grinding (EDG) belongs to a noncontact machining, which is widely used in rigid metal processing and also utilized in the workpieces that cannot be accomplished by the traditional machining process. It is a form of discharge machining process that based on the pulse discharging electric corrosion principle. It uses thermal energy to machine electrically conductive hard material parts
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.146
638
Yunhai Jia and Linxin Zhu / Procedia CIRP 68 (2018) 637 – 642
regardless of their geometry [3]. Electrical discharge grinding is the principal method of machining PcBN cutting tool because of the tool electrode simply forming and small loss. It has actually been used in machining complex shape cutting edge of PcBN cutting tool. It is a biological phenomenon that workpiece surface affected layer is produced in electrical discharge machining [4]. The existence of surface affected layer has affected the service life of the tool, the workpiece material and technological parameter of electrical discharge machining impact the affected layer thickness and its physical property [4]. There are a lot valuable research results on PcBN cutting tools and EDM technics. M.Boujelbene et al [5] used X200Cr15 and 50CrV4 steel for a study of the influence of machining parameters on the surface integrity of electrical discharge machining. The results of the study showed that increasing energy discharges increase instability and therefore. The quality of the workpiece surface became rougher and the white layer thickness increased. In Jiannan Zhao paper, a novel laser/water jet heat treatment (LWH) process is investigated to enhance the surface hardness of a dual phase boron nitride (BN) material composed of 50% cubic and 50% wurtzite phases. Results indicate that experimentally measured hardness increase is dependent on the processing parameter such as laser fluence and overlap between heat treatment passes [6]. Ko-Ta Chiang in his paper indicated that the main two significant factors on the value of the material removal rate (MRR) were the discharge current and the duty factor. The discharge current and the pulse on time plus have statistical significance on both the value of the electrode wear ratio (EWR) and the surface roughness (SR) [7]. ST Chen and YC Lai present a compound process combining composite electroforming with wire Electrical Discharge Machining (wEDM) for developing a thin cubic boron nitride (CBN) grinding-tool. Experimental results indicate that the electroforming process can create a Ni-based CBN layer of high-integrity under a current density of 2 ASD and concentration of 8.6 g/l CBN grits. Moreover, the CBN grinding-edge of 10 ȝm in thickness can be achieved [8]. J Kozak studied the abrasive electro-discharge grinding (AEDG) process performance while machining such advanced materials as polycrystalline diamond (PCD), metal matrix composite AL-SiC, polycrystalline cubic boron nitride (PCBN) and electric conductive ceramics [9]. The purpose of this paper is to analyze microstructure and composition of the affected layer, the relations between electrical discharge grinding parameters and microstructure of PcBN affected layer. Further, the influence of the existence of the affected layer on cutting tool life is studied. These studies provide experimental support for establishing the reasonable technics of electrical discharge grinding PcBN cutting tool. Nomenclature v ip ton
the electrode speed the peak current the pulse duration
toff VW VE Raw
the pulse interval workpiece material removal volume electrode wear workpiece surface roughness
2. Experiments In general, rotation disk copper electrode is used as a tool electrode in electrical discharge grinding. Different from the traditional EDM, PcBN electrical discharge grinding belongs to precision machining, so the machining parameters value is small. In addition, working medium of electrical discharge grinding generally utilizes the injection processing. There are many process parameters of EDG, a few of the most important discharge parameters can only be chosen to study, such as: the electrode speed v, the peak current ip and the pulse duration ton. While electrode wears VE, workpiece material removal volume VW and workpiece surface roughness Raw that are involved in the processing efficiency and processing quality is selected as evaluation index. PcBN compacts (10μm and 25μm crystal granularity sizes) produced by SF DIAMOND Corporation are selected as the experimental sample. Disk shaped copper that has a diameter of 150 millimeters and a thickness of 40 millimeters is used as a tool electrode. The electrical discharge machine named BDM-903 (as showed in Fig. 1) produced by Beijing Institute of Electro-machining is selected as experimental machining equipment. The SB001 X-ray diffractometer (XRD), S-4800 scanning electron microscope (SEM) and TR240 surface roughness tester are chosen as experimental analyzer.
Fig. 1. Experimental machining equipment.
Experimental process is as follow. A piece of PcBN compact is incised in several pieces, and then welded in turning tools. After that, those samples are cleaned by ultrasonic wave machine and cleaned in acetone liquid, dried in the air, and then machined by EDG in different machining parameters (the discharge machining parameters are shown in table 1). After the machine, those cutting tool samples are cleaned by ultrasonic wave machine and cleaned in acetone liquid, dried in the air, and then analyzed by XRD and SEM. The last, using those cutting tool samples to machine experiment workpiece in dry turning in a lathe (CAK6140). ĭ60×150mm bearing steel rods GCr15 are selected as the cutting experiment workpieces. The chemical composition of
Yunhai Jia and Linxin Zhu / Procedia CIRP 68 (2018) 637 – 642
639
GCr15 is shown in Table 2. The cutting tool samples geometry parameters are presented in Table 3. Table 1. The discharge machining parameters. EDG machining parameters
Rough machining
Semiprecision machining
Precision machining
Electrode speed v (m/min)
100
60
50
Open circuit voltage U (v)
120
120
90
Peak current ip (A)
6
3
1
Pulse duration ton (ȝV)
35
20
5
Pulse interval toff (ȝV)
15
15
10
Table 2. &RPSRVLWLRQRIZRUNSLHFHPDWHULDO ĭ60×150mm). Workpiece material GCr15 (%)
C
Mn
Cr
Si
P
Hardness
0.95 ~1.05
0.25 ~0.45
1.40 ~1.65
0.15~0.35
0.025
59~62 HRC
Table 3. Cutting tool geometry parameters. Geometry parameters
Value
Rake angle Ȗ0(°)
0
Flank angle Į0 (º)
8
Blade angle ȜS(º)
5
Edge angle kr (º)
75
Nose radial rİ (mm)
0.3
Vice chamfering width br1 (mm)
0.2
Vice chamfering angle Ȗ01 (°)
-10
Fig. 3. XRD analysis of EDG PcBN cutting tool surface (a) Not machining; (b) rough machining; (c) semi-precision machining; (d) precision machining.
3. Experimental analysis
3.2. SEM analysis
3.1. XRD analysis
A SEM photograph of unprocessed 25μm crystal granularity PcBN is illustrated in Fig. 4 (a). Fig. 4 (b) is part of 25μm crystal granularity PcBN surface SEM comparison charts of electrical discharge grinding. It can be seen from these graphs that PcBN compact sample surface is flat and smooth before electrical discharge grinding, but after machining, a number of loose and hole, high temperature melting traces, local squamous lines and molten material are shown in the sample surface. Fig. 4 (c) is the SEM photograph of 10μm crystal granularity PcBN surface after electrical discharge grinding. From this graph, it can be observed that the surfaces of EDG machined PcBN cutting tools present white affected layer. The affected layer thickness of EDG rough machined PcBN cutting device is about 65 WR ȝP and the affected layer thickness of EDG precision machined PcBN cutting device is about 10 to 17ȝP
Several different machines cutting tool samples are analysed by the X-ray diffraction, the result as showed in Fig. 3. It can be seen in Fig. 3 that the main phase of no machining sample includes the BN, Ti2C, B2Ti, W3N4, Co. The WC, WN and TiO appear in the EDG machining sample besides BN, Ti2C, B2Ti, W3N4, Co. It indicated that the PcBN samples surface present an affected layer containing the WC, WN and TiO after electrical discharge grinding.
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Fig. 5. Energy spectrum of PcBN compact samples (a) Not machining; (b) EDG rough machining.
Through the above analysis, It can generally be accepted that PcBN machining in EDG process is hot melt adhesive, which results in the overall loss of PcBN particles and realizes the grinding removal; in this process, accompanied by replacement and oxidation between elements. 4. Machining parameters analysis 4.1. Electrode speed Fig. 4. SEM of PcBN compact samples surface (a) Not machining; (b) EDG machining (c) EDG rough machining. (d) EDG precision machining.
3.3. Energy Spectrum analysis An energy spectrum photograph of unprocessed PcBN sample is illustrated in Fig. 5 (a). Fig. 5 (b) is the energy spectrum photograph of PcBN sample surface after electrical discharge grinding. The energy spectrum can be intuitively found boron, aluminium element content decreased, nitrogen, tungsten element content increased, while the emergence of copper. Aluminium element is the primary element of the binder, which also guarantees the existence of a conductive adhesive. Boron mainly composed of boron nitride, which is removed, thus reducing the content of the electrical discharge grinding process. the reason of increasing nitrogen and tungsten element content, is the chemical reaction between nitrogen and tungsten occurs to produce the WN, WC, TiO in the discharge grinding process at high temperature, and other substances deposited on the surface of PcBN those made of the affected layer, so the nitrogen and tungsten element content in the test increased. The emergence of copper is used mainly copper electrode processing, combining elements copper and oxygen under high temperature metal oxides deposited on PcBN surface.
In order to analyze the effects of electrode speed on machining efficiency and machining quality, the discharge parameters have been set as follow, open circuit voltage U = 120V, peak current ip = 4A, pulse width ton ȝV , pulse interval toff ȝV Discharge machining time is about 90 seconds. The test results are shown in Fig. 6.
Fig. 6. The effect of tool electrode speed on machining efficiency and quality (a) workpiece material removal volume & electrode wear; (b) workpiece surface roughness.
From Fig.6, it can be seen that the workpiece material removal rate is the lowest and the workpiece surface roughness value is larger when the electrode does not rotate. However, as the electrode rotating, the machining speed and machining quality is significantly increased. These results
Yunhai Jia and Linxin Zhu / Procedia CIRP 68 (2018) 637 – 642
from the fact that the electrode rotation improved discharge machining conditions accelerated the workpiece material erosion rate. With the increase of the electrode rotation speed, one is that the workpiece material removal is increased, but the increase rate becomes slow, the other is that the electrode wear also increases. This is caused by electrode wear mainly occurred in electrical discharge machining early stage, along with the increase of electrode rotation speed, the discharge point move quickly, discharge frequency increased sharply, so the electrode wear increase finally. With the increase of the electrode rotation speed, the workpiece surface roughness value reduced first, and then increased, but the electrode rotation speed of 50 m/min to 100 m/min, the workpiece surface roughness value changed little. It can be seen from the Fig 6, the electrode rotation speed impact on workpiece surface roughness has nothing to do with the granularity of the PcBN compact. Thought from the two aspects influencing of the electrode rotation speed on the machining speed and machining quality, it can be found that processing integrated works best when the electrode rotation speed is 50~80 m/min.
641
roughness are slowly increased, and the electrode wear is decreased dramatically; while the peak current greater than 4 amperes, the workpiece material removal volume and workpiece surface roughness values are rapidly increasing, slowly reduce electrode wear. Meanwhile, it can be seen from Fig.7, when the peak current is below 2 amperes, PcBN granularity size has little effect on machining efficiency and machining quality; but when the peak current is greater than 6 amperes, PcBN granularity on the machining efficiency and machining quality of workpiece has great influence. 4.3. Pulse duration In order to analyze the effects of pulse duration on machining efficiency and machining quality, the discharge parameters have been set as follows, open circuit voltage U = 120V, peak current ip =4A, pulse interval toff =ȝV, electrode speed v = 60 m/min. The test results are shown in Fig. 8.
4.2. Peak current In order to analyze the effects of peak current on machining efficiency and machining quality, the discharge parameters have been set as follows, open circuit voltage U = 120V, pulse duration ton = 20ȝs , pulse interval toff = 15ȝs, electrode speed v = 60 m/min. The test results are presented in Fig. 7.
Fig. 8. The effect of pulse duration on machining efficiency (a) workpiece material removal volume & electrode wear; (b) workpiece surface roughness.
Fig. 7. The effect of peak current on machining efficiency (a) workpiece material removal volume & electrode wear; (b) workpiece surface roughness.
With the increase of the peak current, material removal volume and workpiece surface roughness value are increasing, but electrode wear is decreasing. When the peak current is less than 4 amperes, workpiece material removal and surface
It can be found from Fig 8, with the increase in pulse duration, the PcBN material removal volume is increasing rapidly. The reason is that the longer the pulse duration, the longer the discharge time, the more energy is generated between the electrode and workpiece, which accelerated the PcBN material removal volume. Meanwhile, as the pulse duration increases, electrode wear is reduced. Electrode wear occurred mainly in the front edge of the pulse, when the pulse interval constant, the larger the pulse duration, the fewer the number of pulses delivered by the same time, so the smaller the electrode wear. The PcBN surface roughness value is increased with the increase of pulse duration. The PcBN surface roughness value changes more smoothly when the pulse duration is below 25 ȝs, and then increases rapidly.
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Yunhai Jia and Linxin Zhu / Procedia CIRP 68 (2018) 637 – 642
5. Research on cutting tool's life In order to study the relationship between PcBN cutting tool blade quality and tool life, several PcBN cutting tool samples that machined by EDG (the discharge machining parameters are shown in table 1) are used to turning experiment workpieces in the cutting speed of 160 m/min, depth of cut 0.1 mm and feed input of 0.1 mm/r. The results are shown in Fig.9.
(1) The affected layer is inevitably presented after electrical discharge grinding. The WC, WN and TiO are shown in the affected layer. (2) PcBN crystal granularity size has some effect on the electrical discharge grinding. (3) With the increase of the electrode rotation speed, the PcBN material removal volume and electrode wear are raised, but the surface roughness and the affected layer thickness quickly decreases, then increases slowly. For machining quality and machining speed, the influence of the electrode change is weaker than that of peak current and pulse duration. (4) With the increase of the peak current, PcBN removal volume, the surface roughness value and affect layer thickness increase, electrode wear decreases. Therefore, the peak current is generally set little than 6 amperes. (5) With the increase of pulse duration, the PcBN material removal volume, the PcBN cutting tool's surfaces roughness value and the affected layer thickness are increasing, but electrode wear is reduced. The PcBN surface roughness value and the affected layer thickness changes more smoothly when the pulse duration LVEHORZȝVDQGWKHQLQFUHDVHVUDSLGO\ (6) PcBN surface affected layer thickness and surface roughness have a notable impact on cutting tool's life. The reduction of the affected layer thickness or the enhancement of cutting tool surface roughness can improve workpiece surface quality and extend PcBN cutting tool's life. Acknowledgements
Fig. 9. Relation curve between cutting tool flank wide and cutting distance (a) cutting tool flank wide; (b) workpiece surface roughness.
This research is supported by the Beijing Natural Science Foundation the Grant No. 3162013. References
From these two graphs, it can be undoubtedly found that PcBN surface affected layer thickness and surface roughness have significant impact on the workpiece surface quality and cutting tool's life. If setting 0.15mm as cutting tool flank wear width failure point, from Fig. 9 (a), it can be found that the life of rough machined cutting tool is only 0.065km, the life of semi- precision machined cutting tool is about 0.3km and that of precision machined cutting tool is about 0.4km, but for mechanical machining cutting tool, the life is about 0.46km. If VHWWLQJȝPDVGCr15 rods surface roughness failure point, from Fig.9 (b), it distinctly indicates that the life of EDG rough machined cutting tool is only 0.05km, the life of EDG semi-precision machined cutting tool is about 0.28km and that of EDG precision machined cutting tool is about 0.36km. Nonetheless, for mechanical machining cutting tool, the life is about 0.5km. So the conclusion can be obtained from that to reduce the thickness of the affected layer and the cutting tool roughness value, the cutting tool’s life can be extended. 6. Summary From the above analysis, the following conclusions can be obtained:
[1] M.W.Cook, P.K.Bossom. Trends and recent developments in the material manufacture and cutting tool application of polycrystalline diamond and polycrystalline cubic boron nitride. International Journal of Refractory Metals & Hard Materials 2000;18:147-151. [2] Adilson José de Oliveira, Anselmo Eduardo Diniz and Davi Janini Ursolino. Hard turning in continuous and interrupted cuts with PCBN and whisker-reinforced cutting tools. Journal of Materials Processing Technology 2009;209:52-62. [3] Yu Jiashan. Theory of electrical discharge machining. Bei Jing: National Defence Industry Press; 2011. p. 55-71. [4] Yunhai Jia. Electrode Material Effect on Electrical Discharge Machining PcBN Cutting Tool Processing 2012;399:1667-1671. [5] M.Boujelbene, E.Bayraktar, W.Tebni. Influence of machining parameters on the surface integrity in electrical discharge machining. Archives of Materials Science and Engineering 2009;37:110-116. [6] Jiannan Zhao, Pranav Shrotriya. Ultrahard Polycrystalline Cubic Boron Nitride Composite through Hybrid Laser/Waterjet Heat (LWH) Treatment. Procedia Manufacturing 2016;5:747-760. [7] Ko-Ta Chiang. Modeling and analysis of the effects of machining parameters on the performance characteristics in the EDM process of AL2O3+TiC mixed ceramic. Int J Adv Manuf Technology 2008;37:523528. [8] ST Chen, YC Lai. Development of a Thin CBN Grinding-Tool by Compound Process. Applied Mechanics & Materials 2013;421:359-363. [9] J Kozak. Abrasive electrodischarge grinding (AEDG) of advanced materials. Archives of Civil & Mechanical Engineering 2002;2:83-101.
ScienceDirect Procedia CIRP 68 (2018) 637 – 642
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, Bilbao, Spain
Research on Electrical Discharge Grinding Technics and tool's life of Polycrystalline cubic Boron Nitride Cutting Tool Yunhai JIAa,b*, Linxin ZHUb a
Beijing Key Laboratory of Electrical Discharge Machining Technology, Beijing 100191, China b Beijing Institute of Electro-Machining, Beijing 100191, China
* Corresponding author. Tel.: +86-010-5153-0520; fax: +86-010-5153-0520. E-mail address: [email protected]
Abstract Electrical discharge grinding is part of the most widely used methods to machine polycrystalline cubic boron nitride cutting tools. Polycrystalline cubic boron nitride compact samples at different granularity size are the research object. Electrode running speed, peak current and pulse duration is selected as the main process parameters. Material removal volume and the electrode wear set as the evaluation index of productive efficiency. Workpiece surface roughness value sets as an evaluation standard of processing quality. Through electrical discharge grinding experiments, combined with scanning electron microscopy observation, energy spectrum analyzer and roughness tester, the influences of the main process parameters on electrical discharge grinding are analyzed, results show that polycrystalline cubic boron nitride crystal granularity size has some effect on the electrical discharge grinding. With the increase of the electrode rotation speed, the workpiece material removal volume is increased, but the increase rate becomes slow. Electrode wear is further increased with the increase of the electrode rotation speed. With the peak current increase, material removal volume and the surface roughness value of the workpiece material increase, electrode wear reduces. Through the turning test, the relationship between service life of polycrystalline cubic boron nitride cutting tools that are machined by electrical discharge grinding and cutting tool flank wide, workpiece surface roughness are discussed. The results showed that to adjust electrical discharge grinding parameter, such as pulse duration and machining electric currents, can reduce the depth of the effect layer and extend the service life of polycrystalline cubic boron nitride cutting tool. The research provides a valuable experimental reference for drawing up electrical discharge grinding technics of polycrystalline cubic boron nitride cutting tool. © 2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Polycrystalline cubic boron nitride (PcBN), electrical discharge grinding (EDG), affected layer, cutting tool's life, machine technics
1. Introduction The improvement in machining performance, which leads to high production rates, high tolerance, good surface integrity and the ability to machine advanced materials, depends not only on improved cutting tool materials but also depends on other enabling technologies. Increasingly high surface integrity and close dimensional tolerances are required, which requires tools to have lower wear rates and much tighter specification of tool dimensions. Polycrystalline cubic boron nitride (PcBN) tool that is one of substitution products of traditional cutting tool has been widely applied in the manufacturing industry of developed countries in recent 20 years. The use of PcBN tool, compared to solid tungsten
carbide, has enabled machining speeds to be completed. Thread quality to be improved and the burr to be virtually eliminated [1]. To utilize Polycrystalline cubic boron nitride (PcBN) cutting tool makes precision hard turning possible, in which the hardness of the workpiece is up to 58-62 HRC [2]. So the carbide cutting tool has been massively substituted by the PcBN cutting tool. Electrical discharge grinding (EDG) belongs to a noncontact machining, which is widely used in rigid metal processing and also utilized in the workpieces that cannot be accomplished by the traditional machining process. It is a form of discharge machining process that based on the pulse discharging electric corrosion principle. It uses thermal energy to machine electrically conductive hard material parts
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.146
638
Yunhai Jia and Linxin Zhu / Procedia CIRP 68 (2018) 637 – 642
regardless of their geometry [3]. Electrical discharge grinding is the principal method of machining PcBN cutting tool because of the tool electrode simply forming and small loss. It has actually been used in machining complex shape cutting edge of PcBN cutting tool. It is a biological phenomenon that workpiece surface affected layer is produced in electrical discharge machining [4]. The existence of surface affected layer has affected the service life of the tool, the workpiece material and technological parameter of electrical discharge machining impact the affected layer thickness and its physical property [4]. There are a lot valuable research results on PcBN cutting tools and EDM technics. M.Boujelbene et al [5] used X200Cr15 and 50CrV4 steel for a study of the influence of machining parameters on the surface integrity of electrical discharge machining. The results of the study showed that increasing energy discharges increase instability and therefore. The quality of the workpiece surface became rougher and the white layer thickness increased. In Jiannan Zhao paper, a novel laser/water jet heat treatment (LWH) process is investigated to enhance the surface hardness of a dual phase boron nitride (BN) material composed of 50% cubic and 50% wurtzite phases. Results indicate that experimentally measured hardness increase is dependent on the processing parameter such as laser fluence and overlap between heat treatment passes [6]. Ko-Ta Chiang in his paper indicated that the main two significant factors on the value of the material removal rate (MRR) were the discharge current and the duty factor. The discharge current and the pulse on time plus have statistical significance on both the value of the electrode wear ratio (EWR) and the surface roughness (SR) [7]. ST Chen and YC Lai present a compound process combining composite electroforming with wire Electrical Discharge Machining (wEDM) for developing a thin cubic boron nitride (CBN) grinding-tool. Experimental results indicate that the electroforming process can create a Ni-based CBN layer of high-integrity under a current density of 2 ASD and concentration of 8.6 g/l CBN grits. Moreover, the CBN grinding-edge of 10 ȝm in thickness can be achieved [8]. J Kozak studied the abrasive electro-discharge grinding (AEDG) process performance while machining such advanced materials as polycrystalline diamond (PCD), metal matrix composite AL-SiC, polycrystalline cubic boron nitride (PCBN) and electric conductive ceramics [9]. The purpose of this paper is to analyze microstructure and composition of the affected layer, the relations between electrical discharge grinding parameters and microstructure of PcBN affected layer. Further, the influence of the existence of the affected layer on cutting tool life is studied. These studies provide experimental support for establishing the reasonable technics of electrical discharge grinding PcBN cutting tool. Nomenclature v ip ton
the electrode speed the peak current the pulse duration
toff VW VE Raw
the pulse interval workpiece material removal volume electrode wear workpiece surface roughness
2. Experiments In general, rotation disk copper electrode is used as a tool electrode in electrical discharge grinding. Different from the traditional EDM, PcBN electrical discharge grinding belongs to precision machining, so the machining parameters value is small. In addition, working medium of electrical discharge grinding generally utilizes the injection processing. There are many process parameters of EDG, a few of the most important discharge parameters can only be chosen to study, such as: the electrode speed v, the peak current ip and the pulse duration ton. While electrode wears VE, workpiece material removal volume VW and workpiece surface roughness Raw that are involved in the processing efficiency and processing quality is selected as evaluation index. PcBN compacts (10μm and 25μm crystal granularity sizes) produced by SF DIAMOND Corporation are selected as the experimental sample. Disk shaped copper that has a diameter of 150 millimeters and a thickness of 40 millimeters is used as a tool electrode. The electrical discharge machine named BDM-903 (as showed in Fig. 1) produced by Beijing Institute of Electro-machining is selected as experimental machining equipment. The SB001 X-ray diffractometer (XRD), S-4800 scanning electron microscope (SEM) and TR240 surface roughness tester are chosen as experimental analyzer.
Fig. 1. Experimental machining equipment.
Experimental process is as follow. A piece of PcBN compact is incised in several pieces, and then welded in turning tools. After that, those samples are cleaned by ultrasonic wave machine and cleaned in acetone liquid, dried in the air, and then machined by EDG in different machining parameters (the discharge machining parameters are shown in table 1). After the machine, those cutting tool samples are cleaned by ultrasonic wave machine and cleaned in acetone liquid, dried in the air, and then analyzed by XRD and SEM. The last, using those cutting tool samples to machine experiment workpiece in dry turning in a lathe (CAK6140). ĭ60×150mm bearing steel rods GCr15 are selected as the cutting experiment workpieces. The chemical composition of
Yunhai Jia and Linxin Zhu / Procedia CIRP 68 (2018) 637 – 642
639
GCr15 is shown in Table 2. The cutting tool samples geometry parameters are presented in Table 3. Table 1. The discharge machining parameters. EDG machining parameters
Rough machining
Semiprecision machining
Precision machining
Electrode speed v (m/min)
100
60
50
Open circuit voltage U (v)
120
120
90
Peak current ip (A)
6
3
1
Pulse duration ton (ȝV)
35
20
5
Pulse interval toff (ȝV)
15
15
10
Table 2. &RPSRVLWLRQRIZRUNSLHFHPDWHULDO ĭ60×150mm). Workpiece material GCr15 (%)
C
Mn
Cr
Si
P
Hardness
0.95 ~1.05
0.25 ~0.45
1.40 ~1.65
0.15~0.35
0.025
59~62 HRC
Table 3. Cutting tool geometry parameters. Geometry parameters
Value
Rake angle Ȗ0(°)
0
Flank angle Į0 (º)
8
Blade angle ȜS(º)
5
Edge angle kr (º)
75
Nose radial rİ (mm)
0.3
Vice chamfering width br1 (mm)
0.2
Vice chamfering angle Ȗ01 (°)
-10
Fig. 3. XRD analysis of EDG PcBN cutting tool surface (a) Not machining; (b) rough machining; (c) semi-precision machining; (d) precision machining.
3. Experimental analysis
3.2. SEM analysis
3.1. XRD analysis
A SEM photograph of unprocessed 25μm crystal granularity PcBN is illustrated in Fig. 4 (a). Fig. 4 (b) is part of 25μm crystal granularity PcBN surface SEM comparison charts of electrical discharge grinding. It can be seen from these graphs that PcBN compact sample surface is flat and smooth before electrical discharge grinding, but after machining, a number of loose and hole, high temperature melting traces, local squamous lines and molten material are shown in the sample surface. Fig. 4 (c) is the SEM photograph of 10μm crystal granularity PcBN surface after electrical discharge grinding. From this graph, it can be observed that the surfaces of EDG machined PcBN cutting tools present white affected layer. The affected layer thickness of EDG rough machined PcBN cutting device is about 65 WR ȝP and the affected layer thickness of EDG precision machined PcBN cutting device is about 10 to 17ȝP
Several different machines cutting tool samples are analysed by the X-ray diffraction, the result as showed in Fig. 3. It can be seen in Fig. 3 that the main phase of no machining sample includes the BN, Ti2C, B2Ti, W3N4, Co. The WC, WN and TiO appear in the EDG machining sample besides BN, Ti2C, B2Ti, W3N4, Co. It indicated that the PcBN samples surface present an affected layer containing the WC, WN and TiO after electrical discharge grinding.
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Fig. 5. Energy spectrum of PcBN compact samples (a) Not machining; (b) EDG rough machining.
Through the above analysis, It can generally be accepted that PcBN machining in EDG process is hot melt adhesive, which results in the overall loss of PcBN particles and realizes the grinding removal; in this process, accompanied by replacement and oxidation between elements. 4. Machining parameters analysis 4.1. Electrode speed Fig. 4. SEM of PcBN compact samples surface (a) Not machining; (b) EDG machining (c) EDG rough machining. (d) EDG precision machining.
3.3. Energy Spectrum analysis An energy spectrum photograph of unprocessed PcBN sample is illustrated in Fig. 5 (a). Fig. 5 (b) is the energy spectrum photograph of PcBN sample surface after electrical discharge grinding. The energy spectrum can be intuitively found boron, aluminium element content decreased, nitrogen, tungsten element content increased, while the emergence of copper. Aluminium element is the primary element of the binder, which also guarantees the existence of a conductive adhesive. Boron mainly composed of boron nitride, which is removed, thus reducing the content of the electrical discharge grinding process. the reason of increasing nitrogen and tungsten element content, is the chemical reaction between nitrogen and tungsten occurs to produce the WN, WC, TiO in the discharge grinding process at high temperature, and other substances deposited on the surface of PcBN those made of the affected layer, so the nitrogen and tungsten element content in the test increased. The emergence of copper is used mainly copper electrode processing, combining elements copper and oxygen under high temperature metal oxides deposited on PcBN surface.
In order to analyze the effects of electrode speed on machining efficiency and machining quality, the discharge parameters have been set as follow, open circuit voltage U = 120V, peak current ip = 4A, pulse width ton ȝV , pulse interval toff ȝV Discharge machining time is about 90 seconds. The test results are shown in Fig. 6.
Fig. 6. The effect of tool electrode speed on machining efficiency and quality (a) workpiece material removal volume & electrode wear; (b) workpiece surface roughness.
From Fig.6, it can be seen that the workpiece material removal rate is the lowest and the workpiece surface roughness value is larger when the electrode does not rotate. However, as the electrode rotating, the machining speed and machining quality is significantly increased. These results
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from the fact that the electrode rotation improved discharge machining conditions accelerated the workpiece material erosion rate. With the increase of the electrode rotation speed, one is that the workpiece material removal is increased, but the increase rate becomes slow, the other is that the electrode wear also increases. This is caused by electrode wear mainly occurred in electrical discharge machining early stage, along with the increase of electrode rotation speed, the discharge point move quickly, discharge frequency increased sharply, so the electrode wear increase finally. With the increase of the electrode rotation speed, the workpiece surface roughness value reduced first, and then increased, but the electrode rotation speed of 50 m/min to 100 m/min, the workpiece surface roughness value changed little. It can be seen from the Fig 6, the electrode rotation speed impact on workpiece surface roughness has nothing to do with the granularity of the PcBN compact. Thought from the two aspects influencing of the electrode rotation speed on the machining speed and machining quality, it can be found that processing integrated works best when the electrode rotation speed is 50~80 m/min.
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roughness are slowly increased, and the electrode wear is decreased dramatically; while the peak current greater than 4 amperes, the workpiece material removal volume and workpiece surface roughness values are rapidly increasing, slowly reduce electrode wear. Meanwhile, it can be seen from Fig.7, when the peak current is below 2 amperes, PcBN granularity size has little effect on machining efficiency and machining quality; but when the peak current is greater than 6 amperes, PcBN granularity on the machining efficiency and machining quality of workpiece has great influence. 4.3. Pulse duration In order to analyze the effects of pulse duration on machining efficiency and machining quality, the discharge parameters have been set as follows, open circuit voltage U = 120V, peak current ip =4A, pulse interval toff =ȝV, electrode speed v = 60 m/min. The test results are shown in Fig. 8.
4.2. Peak current In order to analyze the effects of peak current on machining efficiency and machining quality, the discharge parameters have been set as follows, open circuit voltage U = 120V, pulse duration ton = 20ȝs , pulse interval toff = 15ȝs, electrode speed v = 60 m/min. The test results are presented in Fig. 7.
Fig. 8. The effect of pulse duration on machining efficiency (a) workpiece material removal volume & electrode wear; (b) workpiece surface roughness.
Fig. 7. The effect of peak current on machining efficiency (a) workpiece material removal volume & electrode wear; (b) workpiece surface roughness.
With the increase of the peak current, material removal volume and workpiece surface roughness value are increasing, but electrode wear is decreasing. When the peak current is less than 4 amperes, workpiece material removal and surface
It can be found from Fig 8, with the increase in pulse duration, the PcBN material removal volume is increasing rapidly. The reason is that the longer the pulse duration, the longer the discharge time, the more energy is generated between the electrode and workpiece, which accelerated the PcBN material removal volume. Meanwhile, as the pulse duration increases, electrode wear is reduced. Electrode wear occurred mainly in the front edge of the pulse, when the pulse interval constant, the larger the pulse duration, the fewer the number of pulses delivered by the same time, so the smaller the electrode wear. The PcBN surface roughness value is increased with the increase of pulse duration. The PcBN surface roughness value changes more smoothly when the pulse duration is below 25 ȝs, and then increases rapidly.
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5. Research on cutting tool's life In order to study the relationship between PcBN cutting tool blade quality and tool life, several PcBN cutting tool samples that machined by EDG (the discharge machining parameters are shown in table 1) are used to turning experiment workpieces in the cutting speed of 160 m/min, depth of cut 0.1 mm and feed input of 0.1 mm/r. The results are shown in Fig.9.
(1) The affected layer is inevitably presented after electrical discharge grinding. The WC, WN and TiO are shown in the affected layer. (2) PcBN crystal granularity size has some effect on the electrical discharge grinding. (3) With the increase of the electrode rotation speed, the PcBN material removal volume and electrode wear are raised, but the surface roughness and the affected layer thickness quickly decreases, then increases slowly. For machining quality and machining speed, the influence of the electrode change is weaker than that of peak current and pulse duration. (4) With the increase of the peak current, PcBN removal volume, the surface roughness value and affect layer thickness increase, electrode wear decreases. Therefore, the peak current is generally set little than 6 amperes. (5) With the increase of pulse duration, the PcBN material removal volume, the PcBN cutting tool's surfaces roughness value and the affected layer thickness are increasing, but electrode wear is reduced. The PcBN surface roughness value and the affected layer thickness changes more smoothly when the pulse duration LVEHORZȝVDQGWKHQLQFUHDVHVUDSLGO\ (6) PcBN surface affected layer thickness and surface roughness have a notable impact on cutting tool's life. The reduction of the affected layer thickness or the enhancement of cutting tool surface roughness can improve workpiece surface quality and extend PcBN cutting tool's life. Acknowledgements
Fig. 9. Relation curve between cutting tool flank wide and cutting distance (a) cutting tool flank wide; (b) workpiece surface roughness.
This research is supported by the Beijing Natural Science Foundation the Grant No. 3162013. References
From these two graphs, it can be undoubtedly found that PcBN surface affected layer thickness and surface roughness have significant impact on the workpiece surface quality and cutting tool's life. If setting 0.15mm as cutting tool flank wear width failure point, from Fig. 9 (a), it can be found that the life of rough machined cutting tool is only 0.065km, the life of semi- precision machined cutting tool is about 0.3km and that of precision machined cutting tool is about 0.4km, but for mechanical machining cutting tool, the life is about 0.46km. If VHWWLQJȝPDVGCr15 rods surface roughness failure point, from Fig.9 (b), it distinctly indicates that the life of EDG rough machined cutting tool is only 0.05km, the life of EDG semi-precision machined cutting tool is about 0.28km and that of EDG precision machined cutting tool is about 0.36km. Nonetheless, for mechanical machining cutting tool, the life is about 0.5km. So the conclusion can be obtained from that to reduce the thickness of the affected layer and the cutting tool roughness value, the cutting tool’s life can be extended. 6. Summary From the above analysis, the following conclusions can be obtained:
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