Ni Cermet Machining

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ScienceDirect Procedia CIRP 42 (2016) 18 – 22

18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)

Experimental Investigation of EDM Parameters for TiC/Ni Cermet Machining Feng Yeruia, GuoYongfenga, Li Zongfenga a Harbin Institute of Techenology,No.92,West Dazhi Street, Harbin,150001, China * Corresponding author. Tel.: +86-0451-86418084; fax: +86-0451-86418084. E-mail address: [email protected]

Abstract Electrical discharge machining (EDM) is an effective method for TiC/Ni metal ceramic processing. In this paperˈthe influence of peak current, pulse duration on the surface roughness, material removal rate and material removal mode (MRD) is studied. Experimental results indicate that with the increase of peak current, the surface roughness and material removal rate increases gradually; with the increase of the pulse duration the surface roughness of the workpiece increases gradually, the material removal rate increases first and then decrease. The machined surface are measured by a scanning electron microscope (SEM), when the peak current is small, the main MRD is melting, gasification of nickel and TiC and integrate grains removal of TiC, it turns to be more obvious of melting, gasification of nickel with the current increase. The change of pulse duration has little effect on the MRD, which is mainly melting, gasification of nickel and TiC and thermal explosion spalling of TiC. © 2016 2016 The The Authors. Authors.Published Publishedby byElsevier ElsevierB.V. B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM (http://creativecommons.org/licenses/by-nc-nd/4.0/). XVIII). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) Keywords: EDM; TiC/Ni cermet; material removal mode; surface roughness; material removal rate

1. Introduction TiC/Ni, as a typical cermet material, with high hardness, high strength, high wear resistance, high corrosion resistance, high temperature resistance and other excellent properties, has extensive application in aerospace, manufacturing and petrochemical industry etc. [1, 2]. TiC/Ni is difficult to machine by traditional cutting machining method due to the brittleness and low impact toughness, as well as low precision and high manufacturing cost limit the application and dissemination of TiC/Ni. Electrical discharge machining (EDM), using the electro-thermal action during discharge to remove workpiece material without any contact, can machine any conducting materials, so it has great superiority in process of EDM for TiC/Ni cermet. It will be more widely used if we can improve machining quality and efficiency , even more the remove mode of the material. Research and development on EDM of cermet has been studied by a number of researchers. Lauwers et al researched the influence of composition and grain magnitude of WC based cermets on WEDM, the results shows machining speed decrease with increase of grain size [3]. Kumar et al

researched the machining process of TiCN-20wt.% cermet electrode machining high speed steel, the results indicates the material removal rate ( MRR) is high, while the electrode wear rate (EWR) is low when electrode material is TiCN– 20Ni–10TaC cermet [4]. Landfried et al researched the machining effect of conducting phase TiC,TiN,TiCN,TiB2 and WC in EDM process for zirconia-toughened alumina (ZTA), it is found that the machining process will be more stable when the cermet conducting phase added as electrode composite [5]. Lin et al took experimental research of EDM process for Al2O3-TiC, studied the influence of polarity, peak current, pulse duration and open circuit voltage on MRR, EWR and surface roughness [6]. All of the reported researches are focused on EDM of cermets except TiC/Ni. So in this paper, the EDM machining process of TiC/Ni (TiC (70wt%) and Ni (30wt%)) is investigated, peak current and pulse duration are chosen for the parameters to study the influence on MMR and surface roughness. The material removal mode (MRD) of the EDM machining process is also researched using scanning electron microscope (SEM), and the effects of peak current and pulse duration are analyzed.

2212-8271 © 2016 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 organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.177

Feng Yerui et al. / Procedia CIRP 42 (2016) 18 – 22

2. Materials and methods This paper uses the HCD400K precision EDM machine to complete all the experiments, Kerosene is used as insulation medium, diameter of the copper electrode is 2 mm, connecting to the power supply anode, each experiment was repeated three times, and the results shown in this paper are average value of three experimental results. The work piece is TiC/Ni cermet with the thickness of 3 mm, which is manufactured by Harbin Institute of Technology through self-propagating high temperature synthesis and pseudo hot isostatic pressing (SHS/PHIP), its composition includes TiC (70wt%) and Ni (30wt%). The thermal physical properties are as follows: the melting point is 2,856 K, the density is 5.67 g/cm3, the thermal expansion coefficient is 8.4×10-6 K-1. Peak current and pulse duration parameters are shown in Table 1 and Table 2, respectively. Other experimental parameters and their level are as follows: the pulse-off time is 50 μs, the open circuit voltage is 280 V, the servo voltage is 40 V. In this paper, the workpiece is connected to pulse generator cathode to avoid the central defects induced by polarity effect. The specific technical objectives are as follows: ķ The mathematical equation of material removal rate:

9 W

055

Table 1 Peak current and pulse duration settings

Parameter pulse duration Ton ˄µs˅

Settings 0.64 2 4.6 8.6 Table 2 pulse duration settings Settings 5

Fig. 1 Effect of peak current on MRR

(1)

where MRR is the material removal rate. V is the volume material removed, which is the product of the processing area and the actual processing depth, the actual processing depth equal to the target depth minus the length of the electrode. t is time of electrical discharge machining, which can be obtained by the recorder of machine tool. ĸ The surface roughness (SR), measured by OLS3000 laser scanning confocal microscope in the research. Parameter Peak current Ip˄A˅

increase of SR is associate to the increase of single discharge pulse energy. As the pulse duration and pulse-off time is a constant value, single discharge pulse energy is a value varying with peak current. More material will be removed with the higher single discharge pulse energy, and the surface discharge cavity will be larger and deeper, these make the SR increase with the increase of peak current.

10

20

30

40

12

50

16

60

19.2

80

3. Results and discussion 3.1. Effect of peak current on MRR and SR As the polarity, pulse-off time, open circuit voltage and servo voltage are constant, the pulse duration is set 15µs to learn the effect of peak current on MRR and SR. Figure 1and Figure 2 present the effect of peak current on MRR and SR, respectively. As can be seen in Fig. 1, with the peak current increase from 0.64 A to 19.2 A, the MRR increase from 0.395 mm3/min to 3.244 mm3/min, and the increase trend is obvious. This is because the increase of peak current increases the energy of single pulse, induced the material removal volume increase during a single discharge pulse, so make the MRR increase. In Fig. 2, the SR increase from Ra 4.54 µm to Ra 12.06 µm with the peak current increase from 0.64 A to 19.2 A. the

Fig. 2 Effect of peak current on SR

3.2. Effect of pulse duration on MRR and SR The polarity, pulse-off time, open circuit voltage and servo voltage are constant values, the peak current is set 4.6 A to learn the effect of pulse duration on MRR and SR. Figure 3 and Figure 4 present the effect of pulse duration on MRR and SR, respectively. Fig. 3 illustrates that MRR increase initially and then decrease with the increase of pulse duration, and the inflection point is found at the pulse duration of 30 µs, has the highest MRR of 1.082 mm3/min. When pulse duration increase from 5 µs to 30 µs, the single discharge pulse energy increase because other constant value parameter, and the material removal volume caused by a single pulse duration increase, and the result is MRR increase with the increase of pulse duration when it is set no more than 30 µs. The machining process is stable, and discharge is the main gap status. While the pulse duration increase from 30 µs to 80 µs, with the increase of material removal volume, the debris generated during machining process grows, and it is difficult to flow away the debris from the gap between electrode and workpiece. This induces arc and short gap status between electrode and workpiece, the result is electrode ( Z axis ) retracting frequently, and decrease the actual machining time,

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and the result is MRR decrease with the increase of pulse duration when it is set more than 30 µs. Fig. 4 shows that SR increase at high speed initially and then increase slowly with the increase of pulse duration, the value of SR increases from 4.42 µm to 9.63 µm. The single discharge pulse energy increases with the increase of pulse duration, so the surface discharge cavity will increase at high speed initially. When the pulse duration increase to a certain scale (more than 20 µs), discharge occurs between debris generated during machining process and electrode or workpiece, distribute some single discharge pulse energy, make the SR increase slowly.

partly flow away from the gap in terms of debris, partly bond to the machined surface.

Element Wt% CK 44.99 OK 04.18 TiK 31.13 NiK 16.49 CuK 03.21 Matrix Correction

At% 75.09 05.23 13.03 05.63 01.01 ZAF

Fig. 5 Surface analysis of machined TiC/Ni cermet by EDS

3.4. Effect of peak current on MRD

Fig. 3 Effect of pulse duration on MRR

Fig. 4 Effect of pulse duration on SR

3.3. Analysis of machined surface of TiC/Ni Cermet Fig. 5 shows the machined surface composites of TiC/Ni cermet measured by Energy Dispersive Spectrometer (EDS), the nickel, carbon and titanium are mainly composites, as much as 92.61wt%, besides there are little oxide and copper. During the machining process, the temperature of discharge path can be more than 5000K [7], TiC, Ni and Cu, composites of workpiece and electrode, are gasified by transient high temperature caused by electrical discharge machining. Part of the gasified composites break down into Ti4+, C4-, Ni2+ and Cu2+, others stay the initial status of TiC molecule, Ni and Cu atoms. Environment fluid kerosene break down to C and H, part of the C shows ionic status: C 4and C4+. Considering the O2 dissolving in kerosene, then some chemical interaction will occur. TiO2, NiO, CuO, CO2 and H2 can be synthetized during the discharge, then the solid composites include the electrode and workpiece materials

Fig. 6 shows scanning electron microscope (SEM) images of EDM surface machined at different peak current, the square frame in the smaller pictures on upper right corner of the pictures is the measured place of 5000 X pictures. When machining the specimen, the pulse duration is set 15 µs. When peak current is below 4.6 A, the phase of melting and resolidification, spherical gasified resolidification grain, spherical melting and resolidification grain (distinguished by shape and linkage style with matrix material), micro cracks, residual cavity of removed TiC grains were obviously seen on the machined surface. When peak current is small, the current density decrease, the discharge energy is low, and the discharge explosive impact is not large enough to destroy the grains. The MRD is mainly melting, gasification of nickel and TiC and integrate grains removal of TiC, with little influence of thermal stress. During setting scale of peak current, discharge is main gap status, machining process is stable, surface cavity is small, the machined woekpiece has fine SR. As peak current increase from 4.6 A to 16 A, the phenomenon of melting and gasification of nickel is more obvious, and brittle rupture of TiC grains can be seen in the picture. With the increase of peak current, the single discharge energy increases, the quantity of heat released in the gap increases, influence caused by thermal melting and gasification increases, then the explosive power increase large enough to destroy the TiC grains. It can be deduced that materials removed by thermal stress increased, caused big grains resolidificated bonded to machined surface. During machining process, the status of short and arc carried out frequently, caused bad SR. The main MRD is melting, gasification of nickel and TiC and integrate grains removal , brittle rupture induced by thermal stress of TiC.

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energy increases, the phenomenon of bigger melting and resolidification ball, micro cracks, porosities and residual cavities of removed TiC grains can be seen easily. The material resolidficsation on the machined surface grows, which make SR worse. The main MRD is melting, gasification of nickel and TiC and thermal explosion spalling of of TiC.

Fig. 6 SEM images of EDM surface using different peak current: (a) Ip =0.64 A (b) Ip=4.6 A (c) Ip=16 A 3.5. Effect of pulse duration on MRD Fig. 7 shows scanning electron microscope (SEM) images of EDM surface using different pulse duration, the square frame in the smaller pictures on upper right corner of the pictures is the measure place of the 5000 X pictures. When machining the specimen, the peak current is set 4.6 A. It can be seen melting and resolidification spherical shaped materials, can be called ball, melting and resolidification grain, micro cracks, porosities and residual cavity of removed TiC grains were obviously seen in all pictures. Changes of pulse duration have little effect on MRD of TiC/Ni. With increase of pulse duration, the discharge

Fig. 7 SEM images of EDM surface using different pulse duration: (a) Ton =5 µs (b) Ton =40 µs (c) Ton =80 µs

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4. Conclusion In this paper, the process of EDM for TiC/Ni cermet is investigated, influences of peak current, pulse duration on SR, MRR and MRD is studied. The key findings are summarized as below. 1) MRR and SR increase with the increase of peak current, this is because the single pulse energy increase. 2) MRR increases with the increase of pulse duration when pulse duration below 30 µs. MRR decreases when pulse duration above 30 µs, and SR increase slowly. The reason is debris generated during the process destroy discharge status. 3) When the peak current is small, the main MRD is melting, gasification of nickel and TiC, integrate grains removal of TiC, little influence of thermal stress. When peak current increase, main MRD turns to be melting, gasification of nickel and TiC and integrate grains removal, brittle rupture induced by thermal stress of TiC. 4) The relation between pulse duration changes and MRD is faint. When peak current is set, the main MRD is melting, gasification of nickel and TiC and thermal explosion spalling of TiC.

Acknowledgements The authors gratefully acknowledge the help of Mr Zhang, Mr Ling and Mr Ma. This research is supported by the college

and universities Doctoral Program Special Research Foundation (20112302110007) of Chinese Ministry of Education, Chinese National Nature Science Foundation (51275110) and the Harbin Innovation Fund (2015RAXXJ026). References [1] Xu Q, Zhang X H, Han J C. PROGRESS IN RESEARCH ON CERMETS. Cemented Carbide, 2002, 4: 007. [2] Chen Y Y, Zou Z G, Long F. Synthesis of TiC-based Cermet and the Status-quo of Its Research and Application. Titanium Industry Progress, 2007, 3: 1. [3] Bert Lauwers, Weidong Liu, Wesley Eeraerts. Influence of the Composition of WC-Based Cermets on Manufacturability by Wire-EDM. Journal of Manufacturing Processes, 2006, 8(2):83-89. [4] B.V. Manoj Kumar, J. Ramkumar, Bikramjit Basu, S. Kang. Electrodischarge Machining Performance of TiCN-based Cermets. International Journal of Refractory Metals and Hard Materials, 2007, 25(4): 293-299. [5] Richard Landfried, Frank Kern, Rainer Gadow, et al. Development of Electrical Discharge Machinable ZTA Ceramics with 24 vol% of TiC, TiN, TiCN, TiB2 and WC as Electrically Conductive Phase. International Journal of Applied Ceramic Technology, 2013, 10(3): 509-518. [6] Lin Y C, Wang A C, Wang D A, Chen C C. Machining performance and optimizing machining parameters of Al2O3–TiC ceramics using EDM based on the Taguchi method[J]. Materials and Manufacturing Processes, 2009, 24(6): 667-674. [7] A. Kojima, W. Natsu, M. Kunieda. Spectroscopic measurement of arc plasma diameter in EDM. CIRP Annals-Manufacturing Technology, 2008, 57: 203-207..