Fabrication of surface microstructures by mask electrolyte jet machining

International Journal of Machine Tools & Manufacture 148 (2020) 103471

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International Journal of Machine Tools and Manufacture journal homepage: http://www.elsevier.com/locate/ijmactool

Fabrication of surface microstructures by mask electrolyte jet machining Ming Wu a, b, Jiangwen Liu a, Junfeng He a, Xiaolei Chen a, Zhongning Guo a, * a b

College of Mechanical and Electrical Engineering, Guangdong University of Technology, Guangzhoug, 510000, China Guangzhou Key Laboratory of Nontraditional Machining and Equipment, Guangzhou, 510006, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrochemical micromachining Electrolyte jet machining Microfabrication Surface microstructures

This paper presents a novel electrochemical processing technique, mask electrolyte jet machining (MEJM), for the fabrication of surface microstructures. MEJM combines jet electrochemical micromachining (Jet-ECM) and through-mask electrochemical micromachining (TMEMM), combining the advantages of TMEMM, which is a high-throughput process, and of Jet-ECM, with its adjustable flow field. The effects of a mobile nozzle on electrolyte flow are investigated, and a new modeling approach for large translational movements is proposed. An analysis of the accuracy and reliability of the proposed method is presented. Microprotrusions and micro­ dimples are produced to high precision and with excellent consistency of dimensional variation (maximum standard deviation 2.171 μm). The results suggest the possibility of an affordable technique for batch fabrication of surface microstructures with high efficiency and precision.

1. Introduction In the last few decades, there has been increasing interest in the application of surface structures, particularly on the micro- and nano­ scale, owing, for example, to their tribological and wetting properties [1, 2]. A variety of approaches to the fabrication of micro/nanoscale surface structures have been investigated. Electrochemical micromachining (ECM) is widely employed [3–7]. This is a noncontact and nonthermal process and thus does not lead to tool wear or to mechanical and thermal residual stresses that might initiate changes in material properties [8,9]. The material removal rate can be adjusted by controlling the applied current. A wide range of metallic materials, regardless of their hardness and thermal resistance, can be easily machined by ECM. For the fabrication of surface structures, a number of techniques based on ECM are available, including jet electrochemical machining and through-mask electrochemical micromachining. Jet electro­ chemical machining (Jet-ECM) employs a jet of electrolyte: a stream of electrolyte flows through a nozzle and impinges directly on the work­ piece, so that the electrochemical reaction is localized to a small region. Sen and Shan [10] investigated the effects of process parameters such as applied voltage, nozzle diameter, and electrolyte pressure on the quality of small holes produced by Jet-ECM. This technique has demonstrated its capacity to obtain various shapes down to the micrometer range [11], even its capability of fabricating microstructures on metal carbides [12]. However, Jet-ECM suffers from a relatively slow speed because of the

need for sequential processing. Costa and Hutchings [13] employed textured tools consisting of arrays of holes to fabricate microdimples by Jet-ECM in a more efficient parallel machining process. However, the shape, size, and precision of microfeatures in Jet-ECM largely depends on the size of the nozzle: to generate smaller features, the inner diameter of nozzle must be reduced. Not only does this increase manufacturing costs, but it is extremely difficult to maintain a sustainable and stable electrolyte flow on a microscale. In addition, to generate different microsurface structures, such as triangles and squares, it is necessary to use nozzles of the corresponding shape, which poses further problems for nozzle manufacture. This is a natural limitation on the imple­ mentation of Jet-ECM for microfabrication. Through-mask electrochemical micromachining (TMEMM), another common method, employs photolithography to produce micropatterns on photoresist coated on the workpiece surface, so that a large number of desired areas dissolve in parallel. This technique is precise and rela­ tively fast, capable of generating well-defined surface textures with controlled size, location, and density. Using this method, Wang et al. [14] fabricated three-dimensional cylindrical microstructures with feature sizes as small as 40μm. Landolt et al. [15] carried out TMEMM of titanium using a laser-patterned oxide film. Qu et al. [16] developed a modified TMEMM technique for fabricating microdimple arrays with a re-useable polydimethylsiloxane mask. Ming et al. [17] used a patterned inert metal plate to fabricate metal through-hole arrays with double tapered openings. Qian et al. [18] were able to achieve a significant

* Corresponding author. E-mail address: [email protected] (Z. Guo). https://doi.org/10.1016/j.ijmachtools.2019.103471 Received 20 December 2018; Received in revised form 4 October 2019; Accepted 4 October 2019 Available online 18 October 2019 0890-6955/© 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic of mask electrolyte jet machining (MEJM).

Fig. 2. Basic principle of the multi-ion transport and reaction model.

improvement in machining localization by employing the mask as an auxiliary anode. All of these developments have widened the range of application of electrochemical micromachining processes and have improved their reliability to the extent that they are suitable for industrial imple­ mentation in surface fabrication for batch production. However, there remains the problem that the electrolyte flow direction in TMEMM is normal to the patterned photoresist. This leads to the formation of vortices in the electrolyte flow and consequently to a very low flow velocity in the mask holes [19]. This hampers the removal of electrolysis products and Joule heat by the electrolyte flow [20]. In addition, the nonuniform flow field as the electrolyte passes through the mask from one side to another can lead to poor consistency of material removal rate and to poor machining accuracy. Wang et al. [21] investigated ways to improve electrolyte flow field

during TMEMM and found that a mask with cone-shaped holes was beneficial to electrolyte flow. Chen et al. [22] investigated both the lateral and forward flow modes during TMEMM and proposed a modi­ fication to the latter mode in which the use of a multiple-slit structured cathode gave a better distribution of the electrolyte flow field. This paper presents an approach, mask electrolyte jet machining (MEJM), that combines the advantages of Jet-ECM and TMEMM, namely, high throughput and an controllable flow field, respectively. This approach also does not suffer from problems of nonuniformity, difficulties in manufacturing and assembling jet nozzles at a micro-scale, or a lack of flexibility in micropattern design caused by the need to use nozzles of fixed shape. To investigate the performance of the proposed approach, a mathe­ matical multi-ion transport and reaction model (MITReM) is constructed [23]. In particular, the effects of a mobile nozzle on electrolyte flow are 2

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Fig. 3. Two-dimensional model.

examined via numerical simulations for different nozzle travel speeds. The results show that the use of a mobile nozzle enhances the uniformity of the electrolyte flow field throughout the fabrication process. Experi­ ments on fabrication of both concave and convex microstructures are carried out. The results suggest that the proposed approach should be suitable for batch fabrication of surface microstructures.

Table 1 Parameters of model. Parameter

Value

Distance between nozzle centerline and vertical baseline Distance between through-mask hole and vertical baseline Vertical distance between electrolyte nozzle and workpiece Inside diameter of nozzle

Xn

0 mm (initial value)

Xe

7.95 mm

d

3.5 mm

Dn

2 mm

Diameter of through-mask hole

D0

100 μm

Tm

1.3 μm

Tf

1130 μm

Thickness of mask Thickness of electrolyte film layer Translational speed of electrolyte nozzle

v

2. Description of the method and theoretical model 2.1. Method Fig. 1 is a schematic of the MEJM process: 1. A photoresist is spin-coated onto the workpiece surface, which has already been ultrasonically cleaned in alcohol and acetone (Fig. 1a). 2. After soft baking, the photoresist is exposed to UV through a photomask (Fig. 1b).

800 μm/s, 2400 μm/s, 4800 μm/s

Fig. 4. Computational mesh for analysis of time variation. 3

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Fig. 5. Model of electrolyte jet shape based on the level set method.

Fig. 6. Distribution of velocity field and volume fraction of fluid. (a) Velocity field: the black line denotes the isosurface φ ¼ 0:5. (b) Volume fraction of fluid: the dark regions are electrolyte (fluid 1: φ ¼ 0), while the light regions are air fluid 2: φ ¼ 1.

Fig. 7. (a) Geometry of the jet calculated using the level set method. (b) Dimensions of the jet and electrolyte file extracted from the results of the level set method.

3. After development and hard-baking, a patterned photoresist is left on the workpiece surface (Fig. 1c). 4. The electrochemical cell for machining consists of the workpiece with the patterned photoresist, which together act as the anode, and the electrolyte nozzle, which acts as the cathode. The nozzle travels at a predetermined velocity in a predetermined path across the sur­ face of the workpiece, and electrochemical reactions take place with selective dissolution of material (Fig. 1d). 5. The photoresist is then removed to reveal the microfeatures that have been fabricated on the workpiece along the path of the nozzle (Fig. 1e).

involved depend sensitively on both electric and electrolyte flow fields. In contrast to TMEMM, both the electric field and the electrolyte flow field in MEJM are time-dependent. Therefore, a model is required that is able to take account of these differences. The focus of the theoretical analysis presented here is thus on comparison of MEJM and TMEMM, and the influence of the variations in the electrolyte flow field. The fundamental principle of the proposed multi-ion transport and reaction model (proposed by Winkelmann [20]) is shown in Fig. 2. This model regards mass transfer as a consequence of diffusion, which is a constant at a given temperature, of convection, which is determined by the electrolyte flow field, and migration, which is driven by the electric field. A two-dimensional model is set up as shown in Fig. 3, where Xn denotes the distance between the centerline of the nozzle and the ver­ tical baseline, Xe denotes the distance between the through-mask hole and the vertical baseline, and d denotes the vertical distance between the electrolyte nozzle and the workpiece (i.e., the interelectrode gap, IEG). The values of these and other parameters of the model are listed in

2.2. Theoretical model Electrochemical machining (ECM) is a very difficult process to pre­ dict, because its reaction rate and accuracy are affected by a number of physical parameters. It is well known that the electrochemical reactions 4

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Fig. 8. High-speed imaging of the electrolyte jet. The images show the nozzle and workpiece (a) without the jet at a long focus distance, (b) without the jet at a short focus distance, (c) with the jet at a long focus distance, and (d) with the electrolyte jet at a short focus distance.

Table 1. MEJM is a parallel electrochemical processing technique, with dissolution of material occurring in the numerous areas of the workpiece exposed by the through-mask holes. In this simulation, to simplify the analysis, only such area in the center of the workpiece (shown in Fig. 3) is considered. The following three assumptions are made:

of elements but is chiefly composed of iron (>70%). For an NaNO3 electrolyte, the experimental results of Rosenkranz et al. [24] indi­ cate that the ratio of ferrous ion (Fe2þ) and ferric ion (Fe3þ) gener­ ated by electrochemical dissolution is 1:2 and the average charge number is 2.67. Therefore, in this study, the charge number of the workpiece is assumed to be due to iron alone and is set as 2.67. The translational motion of the electrolyte nozzle can be modeled by a moving mesh interface based on the arbitrary Lagrange Euler (ALE) method. Given the extent of this motion, nonconvergence issues inevi­ tably arise in the calculation, and so several auxiliary nodes and domains are created to deal with these. The simulation scheme and computa­ tional mesh are shown in Fig. 4. Meshing the entire geometry with a slender electrolyte jet column and a virtual thin electrolyte film with high aspect ratio could lead to low mesh quality, and so the electrolyte jet column is slid to the electrode–electrolyte interface, with each geometrical object being meshed independently. An identical pair is used to connect the boundaries (on the border of the electrolyte jet column and the virtual thin electrolyte film) and maintain continuity of the physical fields in the solution. The electrolyte flow is governed by the laminar Navier–Stokes equations for incompressible flow, including gravity:

1. In theory, the shape of the moving electrolyte jet column will be influenced by the fabricated microfeatures, but it is assumed that in practice, owing to the low translational speed of the nozzle (800–2400 μm/s) and the fact that the features generated are on a micrometer scale, the electrolyte jet column remains unchanged throughout the machining process. 2. The electrolyte flow pressure used in the present study is extremely low (0.031 MPa) in comparison with the flow rates generally asso­ ciated with electrochemical jet techniques (0.5 MPa), and so the development of hydraulic jumps is unlikely. Hence, the electrolyte is assumed to be in constant contact with the workpiece and that a virtual thin electrolyte film (Fig. 4) of average thickness 1130 μm spreads over the latter, as will be illustrated in Section 2.3. 3. In Eq. (27), the valence (i.e., the charge number) should be based on the elements involved in the electrochemical reactions. The work­ piece used in the present study is SUS304, which contains a number

Fig. 9. Two-dimensional model of TMEMM. 5

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Fig. 10. Distribution of electrolyte flow velocity magnitude juj (m=s) at different Xn , v, and times.

8 ∂u > > < ρ þ ρðu⋅rÞu ¼ ∂t > > ∂ρ : þ ρr⋅u ¼ 0; ∂t

rp þ μΔu þ ρg;

where σ is the electrical conductivity of the electrolyte. If it is assumed that the electrolyte conductivity is so large that free charges do not exist, then X zi ci ¼ 0; (6)

where p is the pressure, ρ the density, and u the velocity of the fluid. The boundary conditions are as follows: � Inflow: a constant velocity u0 is imposed: u ¼ u0 :

where zi is the valence and ci the concentration of species i. The current density i results from ion transport: X i¼F zi Ni ; (7)

(2)

� Outflow: a gauge pressure of 0 is imposed: pt ¼ 0:

where

(3)

Ni ¼ � The electrolyte–electrode boundary interfaces are subject to no-slip conditions: u ¼ 0:

(5)

σ r ⋅ ðrϕÞ ¼ 0;

(1)

Di rci

FDi zi ci rϕ þ uci : RT

(8)

Substitution of Eq. (6) into Eq. (7) yields

(4)

i¼

The potential distribution ϕ over the IEG satisfies Laplace’s equation: 6

X F2 rϕ Di ci z2i RT

X F

zi Di rci ;

(9)

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where R is the molar gas constant and T the temperature of the sur­ roundings, which is set as 293.15 K. The boundary conditions are as follows:

� There is no current through the photoresist Γ5;6 or the electrolyte–air interface Γ3;4 : n ⋅ J ¼ 0;

� Inflow:

where n is the outward unit normal to the boundary. According to Faraday’s law, the metal removal rate vm can be described by

(10)

ci ¼ c0 : � Outflow: u ⋅ Di rci ¼ 0:

vm ¼ η (11)

(12)

2.3.1. Geometry of the electrolyte jet One of the important parameters of the theoretical study is the shape €tzgen et al. [25] constructed a model of the electrolyte jet. Hackert-Oscha using the level set method for two-phase flow to describe the dynamical generation of the jet shape. Their efforts lead to a better understanding of electrochemical machining via electrolytic free jets. In the present paper, a model is constructed, again using the level set method, but with predefined jet shape and virtual electrolyte film, as illustrated above. The level set method is used to track moving interfaces in fluid flow models in terms of a continuous function, the level set variable φ, which is obtained by solving the following equation: � � ∂φ rφ ; (19) þ u ⋅ rφ ¼ γr⋅ εrφ φð1 φÞ ∂t jrφj

(13)

� On the electrolyte–electrode interface, ions are produced or consumed by electrochemical reactions, Ri denotes the production rate of specie i: X Ni ⋅ n ¼ Ri : (14) m

The boundary conditions on the applied electric potential are as follows: � Anode (workpiece): ϕjΓ7 ¼ U0 :

(15)

� Cathode (electrolyte nozzle): � ϕ�Γ1;2 ¼ 0 V:

(16)

(18)

2.3. Simulation parameters

� On the anode (workpiece), oxidation of water occurs, and metal (Me) is dissolved: Me þ 2H2 O → Mezþ þ O2 þ 4Hþ þ ðz þ 4Þe :

M J; zF

where η is the current efficiency and is set to 78.82% in this model, and M and z are the molar mass and the valence of the workpiece material, respectively.

� On the cathode (nozzle), water molecules are dissociated into hy­ droxyl ions and hydrogen gas: 2H2 O þ 2e →H2 þ 2OH :

(17)

where γ is a reinitialization parameter and ε is a parameter determining the interface thickness, which is set as half of the maximum mesh element size in the region through which the interface passes. The values of the level set function are interpreted as follows: φ ¼ 0 means fluid 1 (air), φ ¼ 1 means fluid 2 (electrolyte), and φ ¼ 0:5 means the interface. Fig. 5 shows a schematic representation of the model. It is assumed that a jet of fluid 1 (electrolyte) emerges from the inlet and that fluid 2 (air) is removed by the electrolyte flow. The evolution of the electrolyte jet shape is shown in Fig. 6. It can be concluded that after 5.0 ms, the electrolyte jet has adapted

Fig. 11. Electrolyte flow field at different Xn with v1 ¼ 800 μm/s. 7

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Fig. 12. Electrolyte flow field at different Xn with v2 ¼ 2400 μm/s.

to the shape of the workpiece surface. As time passes, the flow field and the geometry of the electrolyte jet become stable. The key aspect of the simulation presented here is the distribution of the flow field within the through-mask hole. In contrast to a dynamic jet shape with stable electrolyte film, as illustrated in Section 2.2, a predefined jet shape has no significant impact on the solution for the flow field within the through-mask hole. As shown in Fig. 7, the dimensions of the jet and the thickness of the electrolyte film layer can be extracted from the geometry of the jet calculated using the level set method. The thickness of electrolyte film layer is found to be 1130 μm. These results can be verified by observations using a high-speed camera (Fig. 8). Not all of the elements in the same image are clear when the same optical sensor is used for a given scene, owing to dif­ ferences in depth of field. Before and after the electrolyte was ejected from the nozzle, the nozzle and the edge of the workpiece were photo­ graphed using lenses of different focal lengths. The ruler next to the edge

of the workpiece makes the thickness of the liquid film more evident (Fig. 8d). The thickness of the electrolyte film is about 1.125 mm. Thus, the geometry of the electrolyte jet as calculated by the level set method can be used as boundary conditions in the following sections. 2.3.2. Boundary settings In this simulation, aqueous NaNO3 electrolyte (1 mol/L) solution is pumped through the nozzle to the workpiece at an average inlet velocity of ju0 j ¼ 7:8 m/s. A potential difference of U0 ¼ 100 V is applied be­ tween the electrolyte jet nozzle (cathode) and the workpiece (anode). The horizontal distance between the initial position of the electrolyte nozzle and the electrode–electrolyte interface is 4.5 mm, and then the nozzle moves 16 mm along the x axis. The computational mesh consists of 12 114 domain elements and 6455 boundary elements. The model is solved using COMSOL Multiphysics software.

Fig. 13. Electrolyte flow field at different Xn with v3 ¼ 4800 μm/s. 8

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Fig. 14. Electrolyte flow field in TMEMM with different maximum depths.

2.4. Simulation results and discussion

Fig. 3.

To allow comparison of MEJM and TMEMM, a reference model of the latter is established, as shown in Fig. 9. This model has the same initial electrolyte flow rate, electric potential difference between electrodes, and electrolyte–electrode interface as in the MEJM model shown in

2.4.1. Electrolyte flow field The simulation results for the electrolyte flow field in MEJM are depicted in Fig. 10 and demonstrate a dramatic time-dependent varia­ tion of the electrolyte velocity owing to the movement of the nozzle and

Fig. 15. Concentration of O2 in different environments: (a1–a4) MEJM with different Xn ; (b1–b4) TMEMM with the same maximum depth. 9

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Fig. 16. Distribution of electric current density magnitude jJj at different Xn , v, and times.

the changes in geometry caused by electrochemical dissolution. The time-dependent transformation of the flow distribution can be divided into three stages according to the distance between the center­ line of the nozzle and the vertical baseline Xn :

respectively. In this stage, the electrolyte within the through-mask hole forms vortices with different locations and directions. The vortex cores move from left to right, and the electrolyte flow rotates anticlockwise around them when Xn ¼ 6:5 mm and 7.5 mm (Figs. 11a and b, 12a,b, and 13a,b), and then clockwise when Xn ¼ 10 mm and 12 mm (Figs. 11c and d, 12c,d, and 13c,d). The transformation of the direction and magnitude of the electrolyte flow velocity at the inter-electrode interface, which is the result of the translational movement of the nozzle over the through-mask hole (Xe ¼ 7:95 mm), facilitates the flushing away of the ferric ions, gas, and heat produced by the electrochemical reaction, preventing their accu­ mulation, which can cause nonuniformity of machining [26] and is a problem that is difficult to eliminate from the TMEMM process.

� 0 mm < Xn < 6 mm: In this stage, the main jet approaches the through-mask hole, with the electrolyte generally flowing from left to right. As the nozzle translates, the electrolyte flow velocity varies in magnitude but remains low, and its distribution remains constant in this stage. � 6 mm < Xn < 10 mm: In this stage, the main jet passes over the through-mask hole, and the electrolyte flow in the hole experiences dramatic variations in both the magnitude and distribution of its velocity. When Xn ¼ 8 mm, the nozzle is directly above the through mask-hole, and the jet impacts on the electrolyte within the hole, triggering a stirring flow condition.

� 10 mm < Xn < 16 mm: In this stage, the main jet moves away from the through-mask hole, and the electrolyte generally flows from right to left. As the nozzle translates, the magnitude of the electrolyte flow velocity varies but remained low, and the distribution of the velocity remains constant in this stage.

This dramatic variation in the flow field could be of benefit to the transport of reactants, heat, and gas. Therefore, a more detailed analysis is carried out. Figs. 11–13 show the electrolyte flow field results at different Xn with v1 ¼ 800 μm/s, 2400 μm/s, and 4800 μm/s,

In the electrochemical process, the eddy flow and the dead water 10

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Fig. 17. Electric current density on the electrode–electrolyte interface at different Xn with v3 ¼ 4800 μm/s.

zone tend to hinder heat and mass transfer and thus influence the con­ ductivity of the electrolyte and the concentration of ions involved in the electrochemical reactions. As shown by the above flow field analysis for MEJM, there is vortex motion and the velocity vector u continues to vary in both magnitude and direction. As shown in Fig. 14, the flow field of TMEMM is taken as a control simulation. It is obvious that the stream­ lines of the electrolyte flow do not cross each other, and since convection transports mass only tangent to the velocity (i.e., along streamlines), it cannot lead to mass transfer between adjacent layers of fluid: only diffusion is able to transfer mass normal to the fluid flow. It is difficult to discharge reaction products such as gas from the inter-mask hole by convection, and they accumulate on the downstream side (Fig. 15b1–b4). The gas distributions for MEJM at different Xn (with v3 ¼

4800 μm/s taken as an example) and TMEMM are shown in Fig. 15. The asymmetric accumulation of byproducts also occurs in MEJM, but the electrolyte flow is stirred and mixed because of the periodic changes in both magnitude and direction caused by the moving nozzle. Byproducts do accumulate on the downstream side, but in the whole process, to a lesser extent than in TMEMM because of the changes in flow (Fig. 15a1–a4) lead to changes in the side of accumulation. 2.4.2. Electric current density distribution The results for the electric current density distribution in MEJM are depicted in Fig. 16 (the fill colors show lgjJj for clearer visualization). The electric current density on the electrode–electrolyte interface with v3 ¼ 4800 μm/s (Fig. 17) undergoes a dramatic time-dependent 11

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Fig. 18. Electric current density on the electrode–electrolyte interface at different maximum depths.

transformation. The electric current density on the interface in TMEMM is taken as a control for comparison (Fig. 18). At the beginning of MEJM processing, the left side of the interface is closer to the cathode, and the byproducts accumulate on the right side (Fig. 15a1), so the electrical conductivity on the left will be slightly higher. It can be seen from Fig. 17c1–c7 that the left peak of the current density on the interface is higher than that on the right. Then, as the nozzle moves, everything turns around. As a consequence, there is little difference in electric charge across the interface in MEJM. However, the left peak of the current density in TMEMM (Fig. 18c1–c3) is always higher than the right peak. This eventually leads to a large asymmetry in the electric charge on the interface. The evolution of microstructures with time is of primary importance in studies of the machining process. In MEJM, different nozzle travel speeds correspond to different processing times. Results of experiments (for experimental details, see Section 4.1) and simulations are presented in Fig. 19. Fig. 20 compares the results of simulations of MEJM and TMEMM at the same maximum depth. The electric potential is relatively high where the workpiece contacts the photoresist, and therefore the local electric current density is high at the edge so microdimples fabricated both by MEJM and by TMEMM appear like islands with concave edges and convex centers. A pronounced asymmetry is observed in the simulated profile of TMEMM (red lines in Fig. 20), with the depth being greater on the upstream side. This asymmetry can be explained as follows. Ac­ cording to Faraday’s law, the depth of processing, i.e., the rate of removal of material from the workpiece, is proportional to the electric current density. As shown in Fig. 18, the electric current density on the interface in TMEMM is asymmetric, which is due to the uneven distri­ bution of ions and gas in the electrolyte, and is driven by the electrolyte flow field. Therefore, the asymmetric shape cannot be avoided in TMEMM, but it can be reduced substantially in MEJM, as depicted in Fig. 20.

continuous material removal within a large desired area of the work­ piece, ensuring uniformity of the machining process. An electrical power and control system provides the power supply to the workpiece and the electrolyte jet nozzle during machining. The electrolyte jet system comprising the electrolyte jet assembly, the electrolyte delivery system, and a pressure relief valve supplies fresh electrolyte at a high, control­ lable pressure through the nozzle and carries away byproducts. The workpiece is SUS304 stainless steel and the electrolyte solution is sodium nitrate, the use of which limits stray dissolution. There is good compatibility and strong adherence between the metallic workpiece and the positive photoresist (Shipley 1818), with high lithographic resolu­ tion. The morphologies of the patterned photoresist and the fabricated workpiece are characterized using a confocal laser scanning microscope (Olympus 2000) and a scanning electron microscope (Supra 55VP). 4. Experimental results and discussion To determine the feasibility and versatility of MEJM, investigations of accuracy and repeatability in batch fabrication of simple micro­ structures such as microdimples and microprotrusions are carried out. 4.1. Investigation of machining accuracy The electrochemical reactions responsible for metal removal are isotropic in nature, and hence there is inevitably undercutting below the photoresist. In this context, the most important index of machining localization is the etch factor EF. As shown in Fig. 22, EF is defined as the ratio of the amount of straight through cutting h to the amount of un­ dercutting ΔD: EF ¼

2h ; ΔD

(20)

with

3. Experimental setup

ΔD ¼ jD0

Fig. 21 shows a schematic of the experimental setup, which consists of a mechanical system to allow movement of the electrolyte jet nozzle and workpiece. The traveling nozzle can provide consistent and

where D0 is the radius of the microhole or disk prepared on the photo­ resist, Dt is the radius of the microdimple or protrusion fabricated on the workpiece. 12

Dt j;

(21)

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Fig. 19. Experimental and simulated profiles of microdimples. (a1-3) 3D topography of microdimples. (b1-3)Partial enlargement of the corresponding microdimples. (c1-3) Experimental and simulated profiles of corresponding microdimples.

The photolithographic process for preparing a patterned photoresist consisting of 4800 (300 � 160) microprotrusions or microdimples is illustrated in Fig. 23, and the processing conditions are listed in Table 2. The microfeatures are fabricated on the workpiece surface within every traveling path of the electrolyte jet (blue dashed line in Fig. 23) under the experimental conditions listed in Table 3. For a interpretation of the experimental data and a robust qualitative evaluation, a statistical analysis of the diameter, machining depth, and EF value is performed. A random sample of 100 is investigated in each case to study the morphological features of MEJM. The sample mean x of

data acquired for each combination of factors is given by the following expression: x¼

N 1 X xi : N i¼1

(22)

Applied voltages ranging from 50 to 200 V in steps of 50 V with nozzle travel speeds of v1 ¼ 800 μm/s, v2 ¼ 2400 μm/s, and v3 ¼ 4000 μm/s are employed to investigate their effect on the amount of material removal, diameter and machining depth of the fabricated

Fig. 20. Simulated profiles of MEJM and TMEMM at the same maximum depth. 13

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Fig. 21. Experimental setup.

Fig. 22. Basic characteristics of microfeatures fabricated by MEJM.

Fig. 23. Pattern definition for MEJM.

Table 2 Processing conditions for photolithography. Photoresist

Shipley 1818

Coat Soft bake

Spin coating/5000 rev/min 115 ∘ C/60 s hotplate

Exposure Development

150 mJ/cm 21 ∘ C/65 s double-spray puddle (DSP)

Hard bake

90 ∘ C/300 s hotplate

Table 3 Experimental conditions for fabrication of microdimples and microprotrusions. Workpiece material Electrolyte solution Electrolyte concentration Electrolyte pressure Nozzle inner diameter

3

Thickness of photoresist layer Hole diameter in photoresist Interelectrode gap Applied voltage Nozzle travel rate

14

Stainless steel c P Dnozzle

NaNO3 1 mol/L 31 kPa 2 mm

Tp

1.3 μm

D0

100 μm

IEG U v

3.5 mm 50, 100, 150, 200 V 800, 2400, 4000 μm/s

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Fig. 24. Patterned photoresist and corresponding microfeatures fabricated by MEJM: (a) microdimples of photoresist; (b) microprotrusions of photoresist; (c) microdimples fabricated by MEJM; (d) microprotrusions fabricated by MEJM; (e) confocal laser scanning microscope images of fabricated microdimples; (f) confocal laser scanning microscope images of fabricated microprotrusions.

features, as well as their EF values. Typical patterned photoresists and the microdimples and protrusions fabricated using them are shown in Fig. 24. Fig. 25 presents the results of a statistical analysis of fabricated microdimples, illustrating the variations in width, depth, and EF value. The small dots represent the experimental data. The data suggest that the mean value of diameter Dt and machining depth h of the micro­ dimples increase with increasing applied voltage, while the EF value reaches a maximum at 150 V and then levels off at higher applied voltages. Fig. 26 illustrates the variations in width, depth, and EF value of microprotrusions. It can be seen that the diameter Dt decreases and the machining depth h of increase with increasing applied voltage, while the EF value again reaches a maximum at 150 V and then levels off at higher voltages. These experimental results demonstrated the fairly high processing accuracy of MEJM, with little variation in EF value in each experimental

group. Undercutting is very small if the depth h of the microstructures is small. With some further effort, methods for reducing undercutting that have already been applied to other machining techniques [27] could be adopted in MEJM as well. 4.2. Investigation of repeatability In this work, the repeatability of the micromaching is indicated by the standard deviation vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N u 1 X (23) ðxi xÞ2 SD ¼ t N 1 i¼1 and by the extreme deviation (i.e. the range of the experimental results) ED ¼ xmax 15

xmin :

(24)

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International Journal of Machine Tools and Manufacture 148 (2020) 103471

Fig. 25. Effect of applied voltage U and nozzle travel speed v on diameter Dt , machining depth h, and EF values of microdimples.

The variations in dimension and accuracy of the shape of the fabri­ cated microstructures are quantified in terms of SD and ED in Tables 4 and 5, respectively. The processing of microdimples and that of microprotrusions are similar in that as the applied voltage V is increased or the nozzle travel speed v is decreased, the SD and ED of the diameter Dt and depth h in­ crease while those of the EF value remain unchanged. These results suggest that faster nozzle translation would improve dimensional consistency. Table 6 compares the SD of microdimples fabricated using MEJM and those fabricated using an optimized TMEMM process as reported in Ref. [22]. The ratio of the standard deviation to the width, SDratio ¼ SD= Dt , is also used here as an indicator, because of the different scales in the present study and in Ref. [22]. The comparison shows that MEJM per­ forms better in terms of the SD and SDratio of the diameter Dt and depth h, which indicate that the deviations of in size and shape are small and confirm the potential of MEJM for batch fabrication of microstructures. As mentioned above, MEJM was proposed with the aim of reducing the dimensional asymmetry that is inherent in TMEMM and is one of the main reasons for dimensional deviation. However, even with the opti­ mized electrolyte flow conditions in MEJM, variations still exist, although they show a decreasing trend with faster nozzle translation speed v. Here, an analysis of the process based on the model of Section 2 is presented. As previously mentioned, the electrochemical reactions continue throughout the traveling time of the moving nozzle and

although they are not interrupted by any hydraulic jumps, some dimensional variation of microfeatures along the nozzle travel path is inevitable. Fig. 27 shows the variation with time of the electric current I and corresponding electric charge Q for different nozzle travel speeds and in different domains (A, B, and C as indicated in the figure). Under the assumptions made in Section 2.2 and the further assumption that the interelectrode gap d remains constant, the distri­ bution of the electric potential is exactly the same in each experimental group with different nozzle travel speed. The distances qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðXn Xe Þ2 þ d2 from the nozzle to the electrolyte–electrode interface

at different nozzle travel speeds v are also exactly the same in every experimental group. It can be assumed that the current density J is inversely proportional to the distance from the nozzle to the electro­ lyte–electrode interface, and so the current density J possesses a similar gradient at a given nozzle traveling speed v. Therefore, the corre­ sponding curves of J in each experimental group must be zoomed in the direction of the horizontal (time) axis. The surface integral of J gives the electric current I: Z I ¼ J⋅dA; (25) where dA is the differential cross-sectional area vector. Therefore, for each speed v, the normal electric currents I shown by the thick red lines in Fig. 27 have identical forms and amplitudes, but have been zoomed in

16

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M. Wu et al.

Fig. 26. Effect of applied voltage U and nozzle travel speed v on diameter Dt , machining depth h, and EF values of microprotrusions. Table 4 Standard deviations of microdimples and microprotrusions. U

(μm)

50 100 150 200 h (μm) 50 100 150 200 50 100 150 200 Ntotal ¼ 100.

Dimples

Table 5 Extreme deviations of microdimples and microprotrusions.

Protrusions

U

v1

v2

v3

v1

v2

v3

0.887 1.169 1.104 2.171 0.202 0.611 0.603 0.747 0.138 0.485 0.259 0.141

0.308 0.819 1.115 1.801 0.113 0.496 0.680 0.699 0.229 0.314 0.184 0.143

0.298 0.862 0.992 1.187 0.078 0.311 0.439 0.670 1.468 0.252 0.152 0.130

0.975 1.008 1.235 2.071 0.193 0.614 0.662 0.811 0.511 0.277 0.237 0.136

0.617 0.878 1.164 1.735 0.166 0.401 0.539 0.734 0.624 0.193 0.179 0.115

0.311 0.912 0.907 1.240 0.074 0.304 0.488 0.571 0.867 0.225 0.152 0.118

(μm)

h (μm)

50 100 150 200 50 100 150 200 50 100 150 200

Dimples

Protrusions

v1

v2

v3

v1

v2

v3

4.547 6.106 4.774 9.978 0.898 2.671 3.056 3.579 0.638 2.969 1.299 0.718

1.786 4.325 5.714 9.109 0.512 2.510 3.193 3.003 1.083 1.669 0.931 0.677

1.502 4.387 4.488 5.286 0.404 1.501 2.514 3.162 9.313 1.070 0.900 0.727

4.388 5.279 6.177 9.042 1.003 3.033 2.993 4.184 4.770 1.481 1.226 0.606

2.818 3.858 5.294 7.425 0.839 1.973 2.539 3.111 3.931 0.996 0.766 0.601

1.706 4.811 4.718 6.188 0.430 1.350 2.708 2.672 5.434 1.064 0.655 0.586

Table 6 SD and SDratio comparison between MEJM and TMEMM [22].

the direction of the horizontal (time) axis. In addition, the curves of I have been discretized according to the electrochemical processing time. The homologous electric charge Qv can be calculated by time inte­ gration of I, and represented as the signed area of the region bounded by its curves:

Width Depth

17

MEJM

TMEMM [22]

0.298 0.029% 0.747 9.603%

0.98 0.925% 1.13 11.078%

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International Journal of Machine Tools and Manufacture 148 (2020) 103471

Fig. 27. Normal electric current I and corresponding electric charge Q as functions of time at different nozzle travel speeds and in different domains.

Z

tt

Z J⋅dA dt;

Qv ¼ t0

(26)

ΔM ¼ η

where t0 and tt are the times at the start and end of processing. According to Faraday’s law, the amount of the material removed (the change in workpiece mass after the machining process) is proportional to the electric charge:

M Q; zF

(27)

where M is the molar mass of the substance and z is the valence of workpiece material, Thus, the amount of material removed in every microfeature fabrication process at different nozzle speeds v can be characterized by the electric charge Qv , as calculated from Eq. (26). 18

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International Journal of Machine Tools and Manufacture 148 (2020) 103471

and the experimental work suggest that a faster nozzle translation speed can improve the uniformity of microstructures in batch fabrication. In conclusion, our results suggest the possibility of an affordable technology for batch fabrication of surface microstructures with high precision and reliability and indicate its potential for industrial application. Acknowledgment The work described in this study was supported by the National Natural Science Foundation of China (Grant No. 51575113), the Joint Funds of the National Natural Science Foundation of China and Guangdong Province (Grant No. U1601201) , the National Natural Science Foundation of China (Grant No. 51675105), and the Natural Science Foundation of Guangdong Province (2017A030313330). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijmachtools.2019.103471.

Fig. 28. Standard deviation of electric charge Qv at different nozzle speeds v.

Therefore, the variations in the dissolution process of the fabricated microfeatures can be quantified by the standard deviation of the charge Qv . For domains A and C (Fig. 27), the curves of I are symmetric, resulting in equal charges. For domain B, the values of I and therefore the charges are higher. The standard deviations for v1 ¼ 800 μm/s, v2 ¼ 2400 μm/s, and v3 ¼ 4000 μm/s from this theoretical analysis are shown in Fig. 28. There is a very clear trend of decreasing SD with faster nozzle translation speed v, which is in good agreement with the experimental results. The experimental results and theoretical analysis both demonstrate the good repeatability of the fabrication of microstructures by MEJM. With the electrolyte nozzle in uniform translation, the dimensional variation of microfeatures within each nozzle travel path is small (with an SDratio of 0.29–1.84% in width) but inevitably nonzero, since the current density curves in different regions are not exactly the same during MEJM. Nevertheless, the standard deviation can be reduced by increasing the nozzle speed. Theoretically, the deviations in dimension could be eliminated by using a nozzle with variable speed. Therefore, the development of a control strategy for the nozzle speed should be one focus of future studies.

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5. Conclusions In normal electrolyte jet machining, the minimum feature size de­ pends on the inner diameter of the jet nozzle and the need for point-topoint processing means that the method is time-consuming. Lithography has been applied in this technique to construct microfeatures, thereby transforms the sequential process to a parallel one. The processing ac­ curacy then depends strongly on the resolution of the lithographic procedure. Therefore, technically, triangular, quadrilateral features with sizes of several microns can be fabricated on various metal materials. The theoretical part of the study described here has revealed the dramatic variations in the electrolyte flow velocity vector that occur with MEJM, and the simulation results suggest that the products of electrochemical processing will be carried away efficiently. Thus, compared with TMEMM, better dimensional symmetry is achieved. The experimental part has demonstrated that microdimples and microprotrusions of the patterned photoresist will be replicated faith­ fully on the workpiece surface under certain conditions. The excellent consistency of the dimensional variation of microfeatures within each nozzle travel path, indicated by an SDratio of 0.29–1.84% in width, demonstrates the reliability of MEJM. An analysis of different regions processed by MEJM provides an explanation for the trend of variation of the standard deviation observed in experiments. Both the theoretical 19

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