Selective Surface Texturing Using Electrolyte Jet Machining

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ScienceDirect Procedia CIRP 13 (2014) 345 – 349

2nd CIRP Conference on Surface Integrity (CSI)

Selective surface texturing using electrolyte jet machining Takuma Kawanakaa*, Shigeki Katoa, Masanori Kuniedaa, James W. Murrayb, Adam T. Clareb a

Department of Precision Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-8685, Japan Institute for Advanced Manufacturing, Department of M3, University of Nottingham, Nottingham NG7 2RD, UK * Corresponding author. Tel.: +81-3-5841-6463; fax: +81-3-5841-1952. E-mail address: [email protected]. b

Abstract

This paper describes development of a method for selective surface texturing using electrolyte jet machining. Electrolyte jet machining is an electrochemical machining method in which dissolution occurs selectively where the electrolyte jet hits the surface of the anode. This process is characterized by the ability to control the surface finish of the removed or added micro patterns by the current density in the electrolyte jet. Higher current density results in a mirror-like surface, while lower current density realizes significantly rough and complicated structures which are difficult to obtain with other machining processes. © Published by Elsevier B.V. This © 2014 2014The TheAuthors. Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of The International Scientific Committee of the “2nd Conference on Selection and peer-review under responsibility of The International Scientific Committee of the “2nd Conference on Surface Integrity” Surface Integrity” in the person theDragos Conference Prof Dragos Axinte [email protected] in the person of the Conference ChairofProf Axinte Chair [email protected] Keywords: Texture; Electro chemical machining (ECM); Stainless steel; Electrolyte jet; Micro machining; Electrolyte jet machining (EJM).

1. Introduction Electrolyte jet machining (EJM) [1, 2] is an adaptation of electrochemical machining. In EJM, a workpiece is machined only in the area hit by the electrolyte jet which is ejected from a nozzle. By translating the jet over the workpiece, intricate patterns can be fabricated without the use of special mask [3]. Even three-dimensional shapes can be machined by controlling the current and dwelling time of the jet over the workpiece [4]. Since EJM is an electrochemical process, there are no burrs, cracks, or heat affected zones generated by the process. This process can be used not only for removing processes by anodic dissolution, but also for coloring process by anodic oxidation [5]. Furthermore, by reversing the polarity, 3D additive manufacturing can be performed [6]. In the previous research [7], it was found that both glossy surface and considerably rough surface can be obtained by controlling the current density. Locally textured surfaces with a large variety in surface topography have the

potential to be used for micro fluidic systems, tribologically functional surfaces, and biomedical applications. Hence, this paper investigates the influence of current density and electrolyte on the surface morphologies of stainless steel machined by EJM. 2. Principle of electrolyte jet machining Electrolyte jet machining is carried out by jetting electrolytic aqueous solution from the nozzle toward the workpiece while applying voltage to the gap as shown in Fig. Electrolyte jet 1. Nozzle (-)

Current density

Workpiece (+)

Hydraulic jump

Fig. 1 Principle of electrolyte jet machining

2212-8271 © 2014 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/3.0/). Selection and peer-review under responsibility of The International Scientific Committee of the “2nd Conference on Surface Integrity” in the person of the Conference Chair Prof Dragos Axinte [email protected] doi:10.1016/j.procir.2014.04.058

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Takuma Kawanaka et al. / Procedia CIRP 13 (2014) 345 – 349

Fig. 2 shows the electric potential distribution in the electrolyte flow ejected from a cylindrical nozzle and the resultant current density distribution over the workpiece surface, calculated by Yoneda et al [8]. When the electrolyte jet hits the workpiece at a sufficiently high flow rate, the electrolyte flows rapidly outward in a fast thin layer, and suddenly changes in its thickness in the area far away from the nozzle due to the hydraulic jump phenomenon. Only when this fast thin layer is formed, distribution of the current density can be concentrated under the nozzle as shown in Fig. 2 (b). As a result, the material under the jet is selectively removed because of electrolytic dissolution.

Z Y

X

Cylindrical nozzle

᧩ ᧧ Electrolyte tank

Constant current power supply

Workpiece

Fig. 3 Experimental equipment Table 1. Machining conditions Workpiece material

Stainless steel (SUS304)

3. Experiment Machining current [A]

A schematic view of the experimental equipment is shown in Fig. 3. The workpiece was set on a table which was placed in a work sink to drain the electrolyte. The work sink and nozzle were installed on an orthogonaltype robot whose XYZ axes were numerically controlled. Since the electrolyte was supplied from a gear pump, the flow rate was controlled by varying the pump revolution speed. The polarity of the workpiece was set positive with respect to the nozzle in the removal process. SEM imaging was performed using a JEOL JSM-6010LV. Surface roughness and profiles were taken using a scanning white light interferometer (Zygo Newview700). Surface roughness Ra was measured at four different parts on the bottom surface of machined dimples. Machining conditions are shown in Table 1. Current density is defined as the machining current divided by the nozzle inner area because current density is not uniform over the workpiece.

2

3.13, 24.9 - 186.9

Machining time [s]

2.67 - 13.3, 200

Gap width [mm]

0.5

Flow rate [ml/s]

5.4

Nozzle inner diameter [mm]

1.43

Electrolyte

NaNO3aq 20wt% NaClaq 20wt%

4. Experimental results and discussion 4.1. Influence of current density In order to investigate the influence of current density on surface roughness and morphology, stainless steel (SUS304) was machined with increasing current densities from 25A/cm2 to 187A/cm2. In this experiment, a sodium nitrate aqueous solution with 20 weight % was

Nozzle inner diameter 2a㻌

3a㻌 2a㻌 a㻌

0㻌

0.3V0㻌 0.5V0㻌 0.7V0㻌 0.9V0㻌

a/2㻌

3a㻌 4a㻌 a㻌 2a㻌 Position in radial direction㻌

(a) Electric potential distribution㻌

Normalized current density [a.u.]㻌

Nozzle

Distance from anode surface㻌

㻌

6a㻌

4a㻌

0.05, 0.4 - 3.0

Current density [A/cm ]

ȭ2a㻌

5a㻌

Gear pump

3㻌

2㻌

1㻌

0㻌

a㻌 2a㻌 3a㻌 Position in radial direction㻌 (b) Current density distribution㻌

Fig. 2 Electric potential and current density distributions in impinging cylindrical jet㻌

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At current density of 87A/cm2, significantly smooth surface was obtained. At 37A/cm2, the morphology was anisotropic influenced by the surface finish of the material prior to EJM texturing (Fig. 7). In contrast, the morphology at 137A/cm2 was isotropic.

used. By decreasing the machining time inversely proportional to the current density, the total electric charge was maintained the same, thereby approximately 50μm deep dimples were obtained for every condition. Fig. 4 shows photographs of the dimples machined with increasing the current density. The machined surfaces were not glossy when the current density was lower than 50A/cm2, while mirror like surfaces were obtained with higher current densities. Fig. 5 shows the relationship between surface roughness Ra and current density. With increasing the current density, surface roughness rapidly decreased, thereafter it gradually increased. The finest surface roughness was Ra = 14.1nm under the current density of 87A/cm2.

i = 37

i = 50

nm㻌

-500㻌

Fig. 7 Surface finish of the material prior to EJM texturing (Ra: 176nm)㻌

5mm㻌

2

Current density, i (A/cm )㻌 i = 25

500㻌

i = 62

i = 75

i = 112 i = 125 i = 137 i = 150 i = 162

i = 87

i = 100 㻌

4.3. Porous structure surface texturing Surfaces machined under lower current densities showed complex porous features. In order to reveal the influence of metallographic structure on the surface morphology, stainless steel was textured with machining time of ten seconds. Fig. 8 shows SEM images of the metallographic structure of a stainless steel surface before and after the texturing process. To observe the metallographic structure before texturing, the surface was etched using a nitrohydrochloric acid. Pitting erosion was observed over the textured surface. However, there are no relationship between the locations of grain boundaries and generated pits. Change in the surface morphology of stainless steel was investigated using different electrolytes. Fig. 9 shows SEM images of surface morphologies at the center of the dimples machined under current density of 3.13A/cm2 using sodium nitrate and sodium chloride aqueous solutions. Machining time was 200 seconds. A large difference in average pore size indicates that the microstructure does not influence the surface morphology obtained by EJM under the conditions used in the present work.

i = 174 i = 187㻌

Fig. 4 Dimples machined with increasing current densities㻌

Surface roughness Ra [nm]

200

100

0 0

50

100 150 200 Current density [A/cm2] Fig. 5 Roughness of stainless steel machined with increasing

4.2. Mirror like surface texturing Fig. 6 shows surface morphologies at the center of the dimples machined under different current densities measured using the scanning white light interferometer.

200㻌 nm㻌 50 μm㻌 2

37A/cm 㻌 Ra: 74.3nm㻌

50 μm㻌

50 μm㻌 2


Fig. 6 Surface morphologies under different current densities on stainless steel 㻌

2


-200㻌

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20 μm㻌

20 μm㻌 (b) Material surface after 10s of texturing㻌

(a) Raw material surface before texturing㻌

Fig. 8 Location of pits generated by electrochemical erosion relative to grain boundaries㻌

100 μm㻌

100 μm㻌

(a) Sodium nitrate aqueous solution㻌

(b) Sodium chloride aqueous solution㻌 Fig. 9 Influence of electrolyte on surface morphology of stainless steel 㻌

5. Conclusions

References

In this study, a method for selective surface texturing by EJM was developed and the influences of current density and electrolyte on surface morphology were investigated. The following conclusions were obtained. z Surface textures can be controlled by changing current density using EJM. z With increasing current density, surface roughness rapidly decreased, thereafter it gradually increased. The finest surface roughness obtained was Ra = 14.1nm under the current density of 87A/cm2. z Complicate porous structures were obtained under lower current densities. z Surface morphology was dependent on electrolyte. Larger pores were generated with NaCl aqueous solution than NaNO3. z This method is useful to manufacture micro fluidic systems, tribologically functional surfaces, and biologically compatible surfaces.

[1] Ippolito R, Tornincasa S, Capello G (1981) Electron-Jet Drilling. Annals of the CIRP 30(1):87– 89. [2] Kozak J (1989) Some Aspects of Electro Jet Drilling. 4th International Conference on Developments in Production Engineering Design & Control, 363–369. [3] Kunieda M, Yoshida M, Yoshida H, Akamatsu Y (1993) Influence of Micro Indents Formed by Electro-chemical Jet Machining on Rolling Bearing Fatigue Life. ASME PED 64:693–699. [4] Natsu W, Ooshiro S, Kunieda M (2008) Research on Generation of Three dimensional Surface with Micro-electrolyte Jet Machining. CIRP Journal of Manufacturing Science and Technology 1:27–34. [5] Mori Y, Kunieda M (1997) Maskless Coloring of Titanium Alloy using Electrolyte Jet. Proceedings of JSEME Annual Meeting in 1997, 13–16. (in Japanese). [6] Kunieda M, Katoh R, Mori Y (1998) Rapid Prototyping by Selective Electro Deposition using Electrolyte Jet. Annals of the CIRP 47(1):161–164 [7] T. Ikeda, W. Natsu, M.Kunieda, Electrolyte Jet Machining Using Multiple Nozzles, IJEM, 2006, No.11, pp.25-32. [8] Yoneda K, Kunieda M (1996) Numerical Analysis of Cross Section Shape of Micro-Indents Formed

Acknowledgements This work was supported by the Grants-in-Aid for Scientific Research (Challenging Exploratory Research 23656096), 2011.


by the Electrochemical Jet Machining. Journal of JSEME 29(63):1–8. (in Japanese).

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