Study of Micro Groove Machining by Micro ECM

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ScienceDirect Procedia CIRP 42 (2016) 418 – 422

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

Study of micro groove machining by micro ECM Chuangchuang Chen, Jianzhong Li, Shicheng Zhan, Zuyuan Yu*, Wenji Xu Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, China * Corresponding author. Tel.: +86-0411-84707231; E-mail address: [email protected]

Abstract Micro ECM has the ability to generate the micro features such as micro holes, 3D micro cavities in electrically conductive materials. In this study, the influence of parameters (voltage, on-time and frequency of pulse generator, electrolyte flow, etc.) on the machining performance such as material removal rate (MRR) and accuracy on micro groove machining by micro ECM are investigated. Total of 19 grooves were machined on a self-made micro ECM equipment using a nanosecond pulse generator. Experimental results were recorded and analyzed. It was found that the corner radius and the taper between the sidewall and the bottom surface of a groove vary with the voltage and pulse on-time, the frequency of pulse, electrolyte system besides of the MRR and surface roughness. The electrolyte system with double nozzle is proposed to shorten the geometrical deviation such as the corner and the taper of the right and left sidewall of a groove. The results are analyzed. © 2016 2016 The B.V. © The Authors. Authors.Published PublishedbybyElsevier Elsevier 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: ECM; Micromachining; Groove; Angle; Radius

1. Introduction A micro groove is a basic geometric feature of a micro part. There are various techniques available to fabricate micro grooves such as micro electrical discharge machining (EDM), laser beam machining (LBM), micro ultrasonic machining (USM) and micro electrical chemical machining (ECM), etc. [1]. However, when compared with micro EDM, LBM and USM, micro ECM is a promising solution for the fabrication of micro 3D structures due to its special characteristics, such as no tool wear and no machining force, regardless of the material hardness, high surface quality and absence of machining stress, recast layer, heat affected zone and the micro-cracking [2-4]. In order to improve the localized corrosion of micro ECM, the ultrashort voltage pulses was applied between a tool electrode and a workpiece, allowing the 3D machining of conducting materials with micrometer precision proposed by Schuster [5]. B.H.Kim put forward a disk-type electrode to apply to prevent the taper of side wall in a micro feature during micro ECM[6]. This paper is to study the influence of machining parameters on the machining performance in micro groove machining by ECM. After the introduction of experimental equipment and

parameters, the experimental results are analyzed. To reduce the corner radius and taper of side wall of the grooves, an electrolyte system with double nozzle is proposed. The results are compared with those using single nozzle. This paper is summarized in the final section. 2. Experimental equipment and parameters 2.1. Electrochemical reaction In micro ECM, metal is removed by electrochemical dissolution. In this study, dilute sulphuric acid was used as electrolyte, the workpiece was connected to the anode and the electrode was connected to the cathode of an ultra-short pulse power supply. The electrochemical reaction that takes place in the workpiece is:

Fe o Fe2   2e

(1)

Fe  2OH  o Fe(OH ) 2 p 2e 

(2)

and around the electrode,

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.2016.02.224

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(3)

2.2. Experimental equipment Fig. 1 shows the structure of the experimental equipment for micro ECM. The experimental equipment includes XYZ axes moving stages with 1ȝm repeatable positioning accuracy and 0.1ȝm resolution, an ultra-short pulse power supply, a electrolyte flow system, a wire electrical discharge grinding unit (WEDG) for dressing micro electrode tools on machine, a computer control system, a signal acquisition system, etc. Z axis WEDG Rotation axis Electrode

Oscilloscope Ultra short pulses

Pump Workpiece

Current DAQ card probe

Electrolyte

Motion control card CCD

Workbench Y axis

Fig.2. Ɏ80 ȝm micro electrode fabricated by WEDG.

3. Experimental results and discussion To achieve micro groove machining with high quality and accuracy in this study, the micro groove machining is carried out by means of electrochemical milling process with a rotational electrode. The groove is machined layer by layer. In each layer, the electrode is controlled to move along the tool path shown in Fig. 3.

X axis Shake proof mounting (a)

Structure of experimental equipment.

Electrode Electrolyte Distribution Right Sidewall

Left Sidewall

Workpiece

Fig.3. Scanning tool path (b)

Photo of experimental equipment.

Fig.1. Micro ECM processing machine tool

3.1. Effect of pulse voltage and on-time on geometric accuracy

2.3. Machining Parameters setting To study the effect of processing parameters in micro ECM processing of micro grooves, experiments were conducted under various machining conditions. Table 1 lists the experimental conditions of micro ECM. The experiments were performed under various pulse on-time, frequency and voltage. To prevent electrode deformation during machining, tungsten is used as the electrode material because of its high electrical and thermal conductivities as well as high degree of stiffness in micro ECM. Fig. 2 shows the prepared electrode.

In micro ECM, the pulsed power supply condition is one of the key factors that determine the machining quality. In this paper, RA is defined as the corner radius between the side wall and the bottom surface, ș as the taper of the side wall, RAR is defined as the right RA and RAL is defined as the left RA, and the same as șR and șL. These key factors are described in Fig.4.

Table 1. Machining conditions Items

șL Parameters

Electrode

tungsten, ĭ 80μm

Workpiece Electrolyte

304 stainless steel, 25*25*0.5mm3 0.2mol/L H2SO4 solution

Tool rotation rate

1000 rpm

Feed velocity

XY 2μm/s, Z 1μm/s

șR

RAL

Z

X

RAR

Micro groove

Fig.4. Key factors of the micro groove

Chuangchuang Chen et al. / Procedia CIRP 42 (2016) 418 – 422

Fig. 5 shows the micro groove-array machined under different pulse voltages and pulse on-times. The RAs and șs were measured using an optical microscope. Table 2 shows the machining conditions of the micro groove-array. As shown in Fig. 6 and 7, it can be seen clearly that both of RA and ș increase with an increase of either the applied pulse voltage or the pulseon time. To achieve high geometrical accuracy, a pulse with low voltage and short pulse on-time is preferred.

(b)

3.2. Effect of pulse frequency on MRR

Fig.5. Micro groove-array by micro ECM (ĭ80ȝm micro electrode, 304 SS, 150KHz pulse frequency, 0.2mol/L H2SO4) Table 2ˊ Machining conditions of micro groove-array Groove No.

Voltage (V)

Pulse ontime (ns)

Groove No.

Voltage (V)

Pulse ontime (ns)

1

9

100

5

6

150

2

8

100

6

6

125

3

7

100

7

6

100

4

6

100

8

6

75

100

RAR șR

115 110 105

60

100 40

ș (°)

RA (ȝm)

80

120

RAL șL

80 7 8 Pulse amplitude (V)

5

13

85 6

5.5

4.5

11

9

Fig.6. The angle between the side wall and the bottom surface (RA) and the taper of the side wall (ș) of the micro groove according to the pulse voltage; (150 KHz, 100ns pulse on-time)

4

9

3.5

7 5

3 50

10

RAR

RAL

șR

șL

110 108 106 104 102 100 98 96 94 92 90

RA (ȝm)

8 6 4 2 0 75

100 125 Pulse on-time (ns)

100 150 Pulse frequency (KHz)

200

Fig.8. MRR vs. pulse frequency (6V, 100ns pulse on-time)

ș (e)

12

Processing time

15

90

0

17 MRR

95

20

Fig. 8 shows the relationship between the MRR and the pulse frequency in the process of micro groove machining by micro ECM. The sum of pulse on-time increases per unit time as pulse frequency increases. This indicates that more electrical current is used in material removal, leading to a higher MRR. Therefore, the higher the pulse frequency is, the higher the MRR and the shorter processing time are. As shown in Fig.9 (a), a micro groove was machined with very small RA (within 7ȝm) and ș (”93°). The MRR value increased to 14601 ȝm3/min, under the follow conditions: ĭ80ȝm electrode, pulse voltage of 6V, pulse on-time of 100ns, pulse frequency of 200 KHz, 0.2mol/L H2SO4 electrolyte and feed rate of 2ȝm/s. Fig.9 (b) shows the profile of the bottom surface with the average surface roughness Ra value of 70nm.

Processing time (h)

(a)

In Fig. 5 (a), it is observed that the shape accuracy (RA, ș) of groove 4 is much better than that of grooves from 1 to 3. It proves that the high machining accuracy is obtained under a low pulse voltage. From grooves of 5 to 8, it can be seen that the machining accuracy is improved compared with those of grooves 1 to 4. The pulse on-time varied from 125ns to 75ns. When the pulse on-time is down to 75ns, however, there is not enough time for the double-layer charging, decreasing material removal ability. The electrode traveled at a constant speed, leading to the occurrence of short circuits, or the contact of the electrode and the side wall of groove. The side wall of groove 8 (Voltage 6V, pulse on-time 75ns) in Fig.5 (b) shows the contact result of the electrode and the workpiece. Based on the experimental results of micro groove machining process, when 6V pulse voltage with 100ns pulse on-time was applied, the RA can be limited to 6μm and the ș can be controlled within 94° perpendicularly.

MRR (1000ȝm3/min)

420

150

Fig.7. RA and ș of the micro groove according to the pulse on-time (150 KHz, 6V pulse amplitude)

(a)

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Chuangchuang Chen et al. / Procedia CIRP 42 (2016) 418 – 422

Fig.9. (a) Micro groove machined by micro ECM (125x165x80Ɋm3); (b) The bottom surface morphology of the machined micro groove.

60

4. Optimization of electrolyte flow system

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50

RAL ɽL

100 95

30

90 85

20

80 10

75

0

70 1

2

3

4

5

6 7 8 Group

9 10 11 12 13

Fig.10. Various RA and ș

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105

40 RA (ʅm)

In the process of micro groove machining by micro ECM, the electrolyte flow pattern has a great influence on machining accuracy. Fig.10 shows that the value of RAR is larger than that of RAL, and șR is larger than șL. This was caused by the electrolyte flow pattern shown in Fig. 11 (a). The electrolyte flow is provided using a single nozzle. In the working areas A and B, the electrolyte property is affected by the nozzle flow direction and the rotation of electrode. In Area A of Fig. 11 (a), the direction of electrode rotation is the same as the electrolyte flow direction. In Area B of Fig. 11 (b), the direction of electrode rotation is opposite to the electrolyte flow direction. Therefore, the electrode flow rate in Area A is higher than that of Area B, which results in the difference of corner radius and the taper of the side wall of the groove.

110 RAR ɽR

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%

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(a)

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(b)

Fig.11. (a) Electrolyte system with single nozzle; (b) Electrolyte system with double nozzles jetting opposite to each other

ɽ(°)

(b)

To reduce the difference, double nozzle electrolyte flow is proposed in this paper, shown in Fig. 11 (b). The symmetrical distribution of electrolyte flow with the usage of double nozzles jetting system leads to the even material removal on both side walls. Consequently, the machining accuracy is improved. To verify the proposed method, two groups of experiments are performed under the machining conditions listed Table 1. The experimental results are shown in Fig. 12. It can be seen that the side walls of grooves are almost vertical to the bottom surfaces. The taper of side wall and the corner radius are measured and compared with the corresponding single nozzle flow pattern, shown in Fig. 13. It is obvious that the difference between RAR and RAL, and șR and șL was reduced significantly when the double nozzle electrolyte flow was applied.

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Chuangchuang Chen et al. / Procedia CIRP 42 (2016) 418 – 422

large. To solve this problem, a double nozzle electrolyte flow is proposed, significantly reducing the difference. Acknowledgements Authors are thankful for the support from National Science and Technology Major Project of the Ministry of Science and Technology of China (No. 2013ZX04001-091) and National Science Foundation of China (NSFC) (No. 51475075 and 51075053). References Fig.12 Micro groove-array (6V, 100ns pulse on-time, 0.2mol/L H2SO4; groove 1, 2: 200 KHz pulse frequency; groove 3, 4: 100 KHz pulse frequency) 100 99 98

Single nozzle jetting electrolyte system Double nozzles jetting electrolyte system

97 ș (°)

96 95 94 93 92 91 90 șR

șL

Single nozzle jetting electrolyte system 15 Double nozzles jetting electrolyte system

RA(ȝm)

13

11

9

7

5 RAR

RAL

Fig.13 Effect of electrolyte system on geometric accuracy

5. Summary This paper is a study of the influence of machining parameters on machining performance of micro ECM in micro groove processing. It was found that the taper of groove side walls and the corner radius increase with an increase of pulse voltage and on-time. When the pulse voltage of 6V and pulse on-time of 100ns were set, the optimal results were obtained. It was also found that the electrolyte flow pattern has a significant influence on the machining accuracy. When a single nozzle was used, the difference of side wall tapers and the corner radius is

[1] T. Masuzawa. State of the Art of Micromachining. CIRP Annals 2000; 49(2):473-488. [2] Z.Y.Yu, J.Kozak, K.P.Rajurkar. Modeling and Simulation of Micro EDM Process. CIRP Annals 2003; 52(1):143-146. [3] D.Zhu, K.Wang, N.S.Qu. Micro Wire Electrochemical Cutting by Using in Situ Fabricated Wire Electrode. CIRP Annals 2007; 56(1):241-244. [4] K.P.Rajurkar, G.Levy, A.Malshe. Micro and Nano Machining by ElectroPhysical and Chemical Processes. CIRP Annals 2006; 55(2): 643-666. [5] R.Schuster, V.Kirchner, P.Allongue. Electrochemical Micromachining. Science 2000; 289:98-101. [6] B.H.Kim, S.H.Ryu, D.K.Choi. Micro Electrochemical Milling. Journal of micromechanics and microengineering 2005; 15:124-12.