Multiphysics study in air-shielding electrochemical micromachining

Journal of Manufacturing Processes 43 (2019) 124–135

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Review

Multiphysics study in air-shielding electrochemical micromachining a,⁎

a

a

a

b

M.H. Wang , W.J. Tong , G.Z. Qiu , X.F. Xu , A. Speidel , J. Mitchell-Smith

T

b

a

Key Laboratory of Special Purpose Equipment and Advanced Processing Technology, Ministry of Education & Zhejiang Province, Zhejiang University of Technology, 18, Chaowang Road, Hangzhou, Zhejiang, 310014, China Advanced Component Engineering Laboratory (ACEL), Advanced Manufacturing Technology Group, University of Nottingham, Nottingham, NG7 2RD, United Kingdom

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Air-shielding electrochemical micromachining Electrochemical micromachining Multiphysics Dynamic generation

Micro-structures on metal surface can be realized with high precision and good surface quality by air-shielding electrochemical micromachining (AS-EMM). However, fundamental understanding of the process and particularly interaction between the electrolyte jet and the surrounding gas flow is limited. In order to obtain the material dissolution process in AS-EMM, a hybrid numerical approach based on a multiphysics approach is presented to simulate the process. Firstly, a two phase flow (gas-fluid) model in the interelectrode gap is generated and the void fraction of gas in the machining gap is obtained based upon finite element analysis software Fluent Inc. Then, a multiphysics model encompassing fields of fluid flow, electric and temperature is developed to analyse the material erosion process on the workpiece surface using COMSOL software. The void fraction of the gas, the current density distribution, the electrolyte temperature in the machining gap which are known to influence material removal and machining evolution process are obtained for typical machining operations. Verification experiments were done based on the comparison results between electrochemical micromachining (EMM) and AS-EMM at different selected time step. An aggregated deviation of less than 11.8% between experimental and theoretical results has been observed.

1. Introduction Recent progress achieved in the fields of aviation, robotics, air vehicle manufacturing, and industrial chemistry has created the need for parts made of extremely hard or tough materials with meso or micro scale structured surfaces. Therefore, the fabrication of micro structures has been a research focus in recent years. A number of micro textures machining methods, such as mechanical machining methods including micro-mechanical cutting [1–2,3], micro rolling [4–5] et al and nontraditional processing methods including laser beam machining (LBM) [6-76-7], jet electrochemical machining (EJM) [8–9,10], ion beam machining (IBM) [11], micro electric discharge machining (microEDM) [12–13,14] and mask/non-mask Electrochemical micro machining (EMM) [15–16,17] have been put forward. Each machining method has its advantages and limitations. Mechanical machining methods can process the 3D complex surface, though the direct mechanical contact between the tool and workpiece induces mechanical deformation, heat generation, and distortion. LBM, IBM and micro-EDM are thermal based machining, they can machine wide range of materials, whereas there will be heat effect zone on the machined surface. EJM and EMM are based on anodic electrochemical dissolution process, the material could be dissolved in ion, there is no residual stress and ⁎

heat-affected layers on machined surface and it was proved to be an effective method for micro structure processing [8–10,18–19]. In EJM, the electrolytic current is supplied between the anodic workpiece and the cathodic nozzle via the electrolyte which is ejected from the minute nozzle. The micro structures accuracy is rely on the size of the nozzle. Unlike EJM, in EMM a micro pin is used as the cathode and even more micro structures could be realized [18–19]. In order to enhance the accuracy of EMM, the authors have demonstrated an air shielding EMM (AS-EMM) recently [20], the electrolyte is surrounded by the compressed air facing the workpiece surface, the stray corrosion reduced and the localization improved. However, fundamental understanding of the process and particularly interaction between the electrolyte jet and surrounding gas flow is limited because the material erosion process is influenced by the electric field, flow field and temperature in the machining gap [21–22]. Thus, a series of experiments is required in order to obtain the proper machining condition. With the development of the computing power, some researchers used software simulation technologies realized the number reduction of experiments required making the research process more efficient. The CAE-ECM system presented in 2001 by Kozak [23] to simulate the workpiece shape change during machining and design the tool electrode. Wang and Zhu [24] in 2009, Natsu and Mi [25] in 2016

Corresponding author. E-mail address: [email protected] (M.H. Wang).

https://doi.org/10.1016/j.jmapro.2019.05.019 Received 7 November 2018; Received in revised form 9 April 2019; Accepted 14 May 2019 Available online 28 May 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

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simulated the complex internal hole shape based on the electric field in ECM. Smets et al. [26] and Hotoiu [27] realized the material removal process in pulse electrochemical machining by simulation. Kozak and Zybura-Skrabalak [28] developed the software to simulate the evolution of surface profiles during electrochemical machining of alloys with the heterogeneous structure, changes of conductivity of electrolyte in the interelectrode gap due to heating and gas generation are neglected. Considering the complex process in ECM, a multi-domain model was presented by Deconinck et al. [29–30] to get the variation of the heat and the electrolyte flow during machining and simulate ECM stainless steel in aqueous NaNO3 electrolyte solution. Wu et al. [31] analyzed and established a micro electrolytic process model with multiphysical fields to propose a method for improving the stability of the gap flow field. Deconinck et al. [32] studied the influence of the temperature on the uniformity or copying quality of the material removal rate based on two temperature dependent electrode reaction models. Wang et al. [33] investigated the effects of multiphysics on the ECM process by using the indirect-coupling model method of thermoelectric coupling and thermal fluid coupling. In the AS-EMM, the material removal was influenced by multiphysics coupling effects. In this study, the multiphysics model combining of electric field, fluid, heat and gas generated during machining are set up. In contrast to previous studies of Jet Electrochemical Machining [8] and other simulation models in ECM [29–33], heat and gas generated during machining are considered in the presented model in addition to electrodynamics and material dissolution. A hybrid numerical approach is presented to compute and simulate the material removal process using the built theoretical model. The simulation was done based on the following assumptions, i) the electrolyte is incompressible. ii) the produced hydrogen during machining on cathode is ignore. iii) the stage of the electrolyte is turbulence. Firstly, the two phase flow field numerical model in the narrow machining gap is created and the void fraction of gas obtained based on the Fluent software. Secondly, mutiphysical coupling model of flow, electric and temperature is developed creating a hybrid of the built two phase flow field using COMSOL software. Then, the dynamic current density on the workpiece surface, the local temperature in the machining gap and the shape evolution of the workpiece in AS-EMM are obtained. Moreover, verification experiments of micro-dimples are conducted on a SS304 stainless steel plate surface with alternating conditions, including the applied air pressure voltage, the machining gap and the tool diameter. It is proposed that this method will enable the establishment of machining parameters as per the design intent of a surface without reiterative experimental work and diminish the research period.

2. Establishment of Multiphysical process models in IEG The AS-EMM is based on the electrolysis and the machining configuration is shown in Fig. 1a. The electrolyte is surrounded by compressed air jets facing the workpiece surface from a central nozzle, a micro pin protruding from the nozzle acting as the cathode with the workpiece being the anode. When a potential difference is applied between the electrodes an electrochemical cell forms, ionic dissociation occurs and material is removed from the anode. In AS-EMM, stray corrosion reduces and the localization improves due to the applied compressed air, decreasing the electrolyte conductivity around the nozzle [20]. Furthermore, the current density distribution, flow velocity and temperature will also be different from EMM due to the changing conductivity of the electrolyte and pressure variation in the machining gap. The process model spatial boundary is defined as the zone between the electrodes as described in Fig. 1b. It is assumed that electrolyte exiting the model boundary is immediately removed by the airflow. 2.1. Flow field model building in AS-EMM In AS-EMM, there exists electrolyte, sludge and gas (hydrogen and air) in the machining gap (see Fig. 1). Due to the small ratio of sludge, its influence is neglected in this study. The mixture model equations can be built based on the assumptions mentioned above. Equation for mixture continuity,

∂ (ρ ) + ∇⋅(ρm → υm) = 0 ∂t m

(1)

Equation for mixture momentum,

∂ (ρ → υm) + ∇⋅(ρm → υm → υm)= ∂t m T → υ )] − ∇p + ∇⋅[μ (∇ υ + ∇→ m

m

m

n

⎯→ ⎯ gm + F + ∇⋅( ∑ αk ρk → υdr , k → υdr , k ) + ρm→ k=1

(2)

Volume fraction equation for the secondary phase,

∂ (αk ρk ) + ∇⋅(αk ρk → υm) = −∇⋅(αk ρk → υdr , k ) ∂t

(3)

Where ⎯⎯⎯υ→ m is the quality average velocity and ρm is density of mixture, n ⎯→ ⎯ is the number of phases, F is body force, μm is viscosity of mixture, ρk υdr , k is the drift velocity of Kth is the density of Kth phase in the mixture, → phase in the mixture. And

Fig. 1. The two-dimensional axisymmetric model of the flow field. (a) Schematic of the AS-EMM (b) Research zone. 125

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Fig. 2. Description of the physical model (a) Definition of domains (b) Definition of boundaries (c) Generated mesh.

2.3. Temperature field model in AS-EMM

2

ρm =

∑ αk ρk

(4)

k=1

The temperature distribution in the inter-electrode gap in AS-EMM can be decided by the convection-diffusion equation [32],

n

μm =

∑ αk μk k=1

(5)

→ → → ρCp → u ⋅∇ T = ∇ ⋅(κ1 ∇ T ) + P J

→ υdr , k = → υk − → υm

(6)

Where Cp is the specific heat capacity and k1 is the thermal con→ ductivity, ρCp → u ⋅∇ T is the forced heat convection, PJ is the Joule heat in the electrolytic region which can be obtained by Joule's law,

The Manninen-et al. mode could be implemented to calculate the slip velocity between the flexible interaction of gas and liquid [34]. → The slip velocity υqp is defined as the relative velocity of second phase (p) to main phase (q).

→ υqp = → υp − → υq

→ →→ P J = E ⋅ J = κ ( ∇ U )2

n

∑ k=1

αk ρk → υqk ρm

(15)

Based on the discussed above, multiphysics model considering the influences of the electric field, temperature distribution and flow field in AS-EMM is noted as,

(7)

The relationship between the drift velocity and the slip velocity is

→ υdr , k = → υqp −

(14)

→ → → → ρCp → u ⋅∇ T = ∇ ⋅(κ1 ∇ T ) + κ 0 [1 + 0.016(T (x ) − T0 )][1 − β (x )]1.75 ⋅( ∇ U )2 (8)

(16)

2.2. Electric field model in AS-EMM

3. Physical model and boundary conditions

The electric field based on the conservation equations of current in AS-EMM is satisfied

3.1. Developed physical model

→ → ∇ ⋅(κ ∇ U ) = 0

The developed physical model is shown in Fig. 2. The flow of the electrolyte is assumed to be symmetric and stable. Therefore, a 2D axially symmetric physical model is selected with the parameter values a = d = 1 mm (where a is the nozzle diameter, as shown in Fig. 2a). The definitions of substance in different domain are listed in Table1.The boundaries of the developed model are set in Fig. 2b, the definitions of the boundaries are listed in Table 2. The size of the mesh elements has a significant influence on the accuracy; a higher mesh resolution reduces the discretization error and leads to a more accurate result, but long computing time. Based on the built model, different sizes of mesh elements are selected: it is dense near symmetry axis and workpiece surface, it is coarse for other domain. The generated mesh with minimum element size of 2 μm consists of 3664 triangular elements (Fig.2c).

(9)

WhereΔU is the potential difference, k is the electrolyte conductivity. The conductivity of electrolyte due to heating and gas generation is as follows [35],

κ = κ 0 [1 + 0.016(T (x ) − T0 )][1 − β ]1.75

(10)

Where k0 is the original value of electrolyte conductivity, T0 is the original temperature and β(x) is the void fraction of gas in the machining gap. In AS-EMM, the material removal on the anode can be computed based on Faraday’s law,

νn = η⋅

M ⋅n⋅J z A ⋅ρ ⋅F

(11)

Where η is the current efficiency, M is the Molar mass, ZA is the valence of substances, F is the Faraday constant, n is the normal unit vector and J is the current density.

J = κE

(12)

E = −∇U

(13)

Table 1 Definition of substance in different domain.

Where E is electric field intensity, U is the potential difference. 126

Domain

Substance

1, 2, 3, 6, 10 4, 5, 7, 8 9,11

Air Electrolyte (NaNO3) Nozzle wall

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4. Multiphysical properties and evaluation

Table 2 Boundary conditions of fluid dynamic.

4.1. Flow-field characteristics based on Fluent

Boundary

Property

1, 3, 5, 7, 9 11, 19 36 2 33, 35, 37, 39, 40

Axis of symmetry Inlet of Electrolyte (NaNO3) Inlet of compressed air Workpiece surface Movable boundary of flow field

The software Fluent was employed to analyse the gas-liquid distribution in AS-EMM and EMM based on the flow field theory and physical model built in Section 2.1. The void fraction contours of gas in the flow field model simulated were shown in Fig. 3a and Fig. 3b. The red section is gas, blue the liquid, and the fade between the blue and the red areas illustrates the different gas-liquid ratio. It can be seen that the liquid beam was suppressed due to the shielding of the compressed air in AS-EMM, the flow field around the cathode electrode becoming a mixture of liquid and gas. Fig. 6 and Fig.7 show the void fraction and velocity figures of the gas 0.1 mm above the workpiece surface in different air pressure. The void fraction and velocity of the gas increase as increasing the distance from the axis due to the liquid volume reduction. According to Fig.4, with the increasing of the applied air pressure, more air is diffused into the electrolyte, and the void fraction of gas in the machining gap increases. With the pressure of applied air increases from 0.25Mpa to 0.27Mpa, the void fraction of gas increasing from 5% to 20% at the axis and from 0 to 45% at Y = 0.2 mm, which reducing the workpiece material removal rate opposite the tool cathode clearly. However, the stray corrosion is still serious if the air pressure is smaller than 0.2Mpa because the electrolyte still maintains its original conductivity due to less void fraction of gas (see Fig.4). Fig.5 shows the electrolyte velocity increases with the increasing of surrounding applied air pressure. It increases from 1.2 m/s to 6 m/s at axis when the applied air pressure increasing from 0 to 0.27 MPa. This denotes that surrounding compressed air in AS-EMM enhances the flushing of the electrolyte in the machining gap. It can also be seen the velocity increase of the electrolyte is less when the applied air pressure increasing from 0 to 0.2 MPa. So, the suggested applied air pressure is 0.2-0.25Mpa during machining process.

Table 3 Material properties of electrolyte and gas at 20℃. material property electrolyte(NaNO3)

Compressed gas

Electrolyte concentration density Dynamic viscosity conductivity density Dynamic viscosity

symbol

value

w ρE μE k'E ρA μA

10% (wt) 1221.5 (kg/m³) 1.607 (mPa·S) 7.9 (S/m) 1.1885 (kg/m^3) 18.205(μPa·s)

3.2. Simulation boundary conditions Table 3 and Table 4 show the material properties and boundary conditions for simulations. In EMM, the turbulent state of electrolyte is expected in order to flush the sludge and eliminate concentration polarization of the electrolyte. The velocity of electrolyte keeping turbulent state can be obtained according to Reynolds equation:

R e=

ρud μ

(17)

Where ρ is the fluid density, d is the pipe diameter, μ is the dynamic coefficient of viscosity and Re = 2300 is the critical Reynold’s number from laminar to turbulent. Then, for the electrolyte and surrounding pressured air applied during machining whose properties are listed in Table.3, the critical velocity from laminar to turbulent state are uelectrolyte=3.29 m/s and uair=34.55 m/s respectively. Moreover, according to Bernoulli’s equation,

P+

ρu2 =C 2

4.2. Electric field characteristics in IEG Fig. 6 shows the distribution of potential and current density in the IEG for EMM (Fig. 6a and Fig. 6c) and AS-EMM (Fig. 6b and Fig. 6d) at machining time t = 0.1 s and t = 10 s generated from the COMSOL software. The applied voltage for the workpiece and cathode are 10 V and 0 V respectively. It shows that the current density opposite the cathode is higher than any other zone on the workpiece and material nearby was removed quickly in both EMM and AS-EMM. Moreover, the current density for the same radial range of electric field is smaller for AS-EMM than EMM, thus less material on workpiece surface will be eroded at this position in AS-EMM and improve the localization. However, the material removal rate for EMM is larger than AS-EMM, the profile of the micro-dimple being seen to be larger and deeper in EMM than AS-EMM. From the figures (Fig. 7a) of the current density at

(18)

Where P is the pressure and C is constant. Based on the experimental model, Cwater = 21412 and Cgas = 40709. Therefore, the fluid will keep in turbulent state when the pressure Pelectrolyte ≥0.016 MPa and Pgas ≥0.04 MPa. In the simulation process, the inlet pressure of electrolyte and the compressed air are 0.1 MPa and 0.25 MPa respectively.

Table 4 Computational boundary conditions of model. Physical field

domain

Flow field 1-8,10 Electrical field 1,6,7,8,10

Temperature field 1,6,7,8,10 Cathode moving

2,3

Boundary condition

boundary

property

Electrolyte inlet Compressed gas inlet outlet Electrical insulation ground potential Pulse voltage frequency Duty cycles Thermal insulation temperature heat flux Feeding speed

10,18 36 33,35,37,39-40 12-13,15,17,24-27, 30-33,35-37,39-40 4,19 2

P1in = 0.1Mpa Vf = 0 P2in = 0-0.27Mpa Vf = 1 Pout = 0 MPa n. J = 0 U0 = 0 U = 10 100KHz 0.5 -n. q = 0 Tref H'=H[1+β(T-Tref)] u = 60μm/min

2,24-27,30-33,35-37, 39-40 19 4,12,13,15,17 4,12,13

127

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Fig. 3. Void fraction contours of gas in the machining gap in (a) EMM and (b) AS-EMM.

electrolyte temperature increases from 40.32 °C to 41.18 °C for EMM and 26.92 °C to 35.76 °C for AS-EMM when the machining time increases from t = 0.1 s to t = 10 s. It increases 2.1% and 32.8% respectively. In AS-EMM, there is much more gas in IEG resulting lower heat conduction coefficient. On the other hand, the conductivity will increase when the electrolyte temperature increases, thus improves the material removal rate according to the Faraday’s law. 4.4. Dynamic generation of micro dimple process Based on the process model, material removal and geometry deformation of the micro dimple machining process based on electrolysis could be realized. Fig. 9 shows the micro dimples profile simulation result of a micro dimple manufactured by EMM and AS-EMM under the conditions of pulse voltage 10 V, 0.5 duty cycle, machining gap 100 μm, 10% mass fraction of NaNO3 electrolyte, 10 s dwell, the electrolyte and air pressure are 0.1 Mpa and 0.25 Mpa respectively. In this case, the time step of 2.5 s, 5 s, 7 s and 10 s are selected. The figure reveals the increasing of depth and diameter for micro dimple process in EMM and AS-EMM with time. Moreover, it could be seen that the depth and diameter of a micro dimple manufactured by AS-EMM are shown to be smaller than that processed by EMM due to the applied air, increasing the void fraction of gas and the lower conductivity of the electrolyte in the IEG. However, the aspect ratio of the dimple described as Eq. 17 increases with the machining time and its value is larger for AS-EMM than EMM, which was shown in Fig. 10. This indicates the localized material removal is enhanced in AS-EMM.

Fig. 4. Void fraction of gas in different air pressure(X = 0.1 mm).

γ= Fig. 5. Electrolyte velocity in different air pressure (X = 0.1 mm).

D 2D = 0. 5W W

(17)

Where γ, D and W are the aspect ratio, the depth and the width of the dimple respectively.

time step = 0.1 s and 10 s, the current density decreasing from 50.3A/ cm2 to 25.5 A/cm2 in EMM and from 40.4 A/cm2 to 22.7 A/cm2 in ASEMM. The biggest current density is 24.5% more in EMM than ASEMM, even though it decreases more quickly (49.3%) than AS-EMM (43.8%) with the machining time goes on. This can be explained according to the figure of electrolyte conductivity in EMM and AS-EMM (Fig. 7b). The electrolyte conductivity on the workpiece surface remains almost its original value for EMM. Compared with EMM, the electrolyte conductivity decreasing quickly on the workpiece surface for AS-EMM and is much less than EMM due to the surrounding of compressed air which changes the void fraction of air in the electrolyte.

5. Validation of simulation results 5.1. Experimental set-up of AS-EMM A schematic view of the experimental set-up is shown in Fig. 11, consisting of the high-pressure gas compressor, machine tool, electrolyte circulation system, power supply and time control relay. The movement of the XYZ platform was controlled by the numerical control system, the electrolyte was transported into the processing zone by the centrifugal pump, the air was compressed into the gas nozzle by the high-pressure gas compressor. Current is supplied by the power supply unit and the current is detected by the current sensor during machining. A stylus profiler (Dektak XT) was used to measure the resultant profile contour size and the surface topography was measured by the scanning electron microscope (SEM). Stainless-steel plate (SS304) is selected as the workpiece and its surface is pre-polished. The cathode is a pin with material of tungsten carbide due to its rigidity and heat resistance. Based on the theoretical principles and multiphysical model, the

4.3. Electrolyte temperature distribution in IEG Fig. 8 shows the electrolyte temperature distribution contours in the IEG for EMM (Fig. 8a and Fig. 8c) and AS-EMM (Fig. 8b and Fig. 8d) at time step t = 0.1 s and t = 10 s. It can be seen, the electrolyte temperature of the area opposite the cathode is higher than any other place and it is higher for EMM than AS-EMM. The highest point of the 128

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Fig. 6. The current density and the potential distribution in IEG at selected time step (a) EMM,Time = 0.1 s (b)AS-EMM, Time = 0.1 s (c) EMM,Time = 10 s (d)ASEMM, Time = 10 s.

comparison between simulations and experiments can be done in different parameters by AS-EMM and EMM. The parameters applied in experiments are listed in Table 5. 5.2. Erosion profile evolution of micro dimple process Experiments were done on the built set-up and the profiles of the micro-dimples were measured on a stylus profiler. The comparisons of micro dimple profiles by EMM and AS-EMM between simulation and experimental results are shown in Fig. 12 and Fig. 13. They denote the similar variation of micro dimple profile as simulations for EMM and AS-EMM. There are small deviation for the values of micro dimple diameter and depth between simulation and experimental results. According to the profile deviation figures shown in Fig. 14 and Fig. 15, the geometry deviations between simulation and experimental results increase with selected time step increasing, it is increasing from the axis of the cathode to the outside. The error is really small in the cathode opposite zone, which means the built simulation model can predict the main material removal area on the workpiece surface. Moreover, the profile error of dimple depth between simulation and experimental is decreasing. The error value is decreasing from 10.2% to 3.9% for EMM and 9.7% to 2% for AS-EMM respectively when the selected time steps is becoming from 2.5 s to 10 s. 5.3. Influence of the applied air pressure on micro-dimple process Fig. 7. Figures of (a) current density and (b) electrolyte conductivity on workpiece surface away from the symmetric axis in EMM and AS-EMM.

Fig.16 shows the SEM images of micro dimples created at different gas pressures in AS-EMM. In this case, the machining gap is 100 μm, applied voltage is 10 V, the cathode diameter is Ø100 μm and 129

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Fig. 8. The temperature distribution contours for EMM and AS-EMM in IEG at selected time step(a) EMM,Time = 0.1 s (b)AS-EMM, Time = 0.1 s (c) EMM,Time = 10 s (d)AS-EMM, Time = 10 s.

Fig. 9. Micro dimple profiles at selected time step in EMM and AS-EMM.

Fig. 10. Aspect ratio variation of micro dimple in EMM and AS-EMM.

machining time is 30 s. It shows the depth and width of the micro dimples are varied with different air pressure. The comparisons results in simulations and experiments at different gas pressures are shown in Fig. 17. The figures denote the close agreements between theoretical and actual experimental results, the errors of micro dimple depth at different gas pressure from 0 to 0.27Mpa are 8.4%, 4.6%, 10.1% and 10.7% respectively. It can be observed that the gas pressure significantly affects the form of the micro-dimple. The dimple profile area increases from 16536 μm2 to 18602 μm2 and 17823 μm2 to 21230 μm2 for experimental and simulation results with the applied air increases from 0 to 0.2Mpa. The depth of micro dimple increases about 5% (from 69 μm to th73 μm in

experiment and 75 μm to 78 in simulation) when the applied gas pressure is increasing from 0 to 0.2Mpa. According to the simulated void fraction of gas (section.4.1), there is less gas entering into the electrolyte in IEG when the applied gas pressure is less than 0.2Mpa and the electrolyte conductivity remains high. However, with the increase of applied gas pressure to be more than 0.2Mpa, the eroded dimple profile area decreasing from 18602 μm2 to 8879 μm2 in experiment and 21230 μm2 to 11421 μm2 in simulation when the applied air pressure increasing from 0.2Mpa to 0.27Mpa. With the increase of applied gas pressure to be more than 0.25Mpa, the void fraction of gas in IEG continues to increase, and the conductivity of the electrolyte decreases, thus the depth of micro-dimple decreases when the applied gas pressure 130

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Fig. 11. Experimental set-up of AS-EMM.

Table 5 Machining conditions in the experiments. parameter

value

Voltage, V Electrolyte Liquid pressure, Pl Applied air pressure, P2 IEG, d Feeding speed, UF Duty cycle, λ Frequency, f

8-10 V 10% NaNO3 g/l 0.1 Mpa 0-0.27 Mpa 50,100,150,200 μm 60 μm/min 50 % 100 KHz

Fig. 13. Experimental and simulation profiles of micro dimples in AS-EMM at selected time.

Fig. 12. Experimental and simulation profiles of micro dimples in EMM at selected time.

is more than 0.25Mpa. Fig. 18 shows the aspect ratio of micro-dimples increases with increasing of the applied air pressure. According to Fig. 4, the void fraction of gas increases with increased pressure, especially the area surrounding the nozzle. Based on equation (3), the conductivity has a linear relationship with the void fraction of gas in the electrolyte which would result in lower electrolyte conductivity on the workpiece surface in the area surrounding the cathode. Moreover, according to the distributions of the current density (Fig.6), the radial range of the same current density of the electric field becomes narrow when the pressured air is applied, thus material removal becomes more localized with increasing air pressure. Increasing the electrolyte velocity alongside increasing the air pressure will enhance the discharge of the sludge in

Fig. 14. Profile deviations between experimental and simulation results in EMM at selected time step.

IEG. Therefore, a higher ratio micro dimple can be realized in AS-EMM. 5.4. Influence of the machining gap on micro-dimple process Micro-dimples machined based on differing machining gaps are shown in Fig. 19. It can be seen that the depth and diameter rise as the machining gap is reduced due to the reduced resistance in the IEG. In this case the applied air pressure, applied voltage, the cathode diameter 131

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Fig. 15. Profile deviations between experimental and simulation results in ASEMM at selected time step.

Fig. 17. Experimental and simulation profiles of micro dimples in AS-EMM at selected applied air pressure.

and the processing time are 0.25Mpa, 10 V, Ø100 μm and 30 s respectively. The comparisons in experimental and simulation results in different machining gaps are also presented in Fig. 20. The eroded micro dimple profile area increases from 3543 μm2 to 25705 μm2 and 4362 μm2 to 28527 μm2 for experimental and simulation results respectively with decreasing the machining gap from 200 μm to 50 μm. The depth increases from 17.4 μm to 90.4 μm and 22 μm to 98.4 μm for experimental and simulation results. The material removal reduced much more due to the increasing resistance resulting the lower current density on the workpiece surface at larger inter-electrode gaps. Simulation results show good correlation to experimentally found values, the error between experiments and simulation being 7% for the diameter and 6.2% for the depth. Fig. 21 shows the aspect ratio of micro-dimples decreasing with the increasing machining gap. The electrical field is now focusing on the

Fig. 18. The aspect ratio of micro dimple in different gas pressure.

Fig. 16. SEM images of dimples created at different gas pressures in AS-EMM. (a: 0Mpa; b: 0.2Mpa; c: 0.25Mpa; d: 0.27Mpa) 132

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Fig. 19. Micro-dimples processed in different IEG. (a: 200 μm; b: 150 μm; c: 100 μm; d: 50 μm)

small area at the narrow gap and the current density being larger with the decreasing IEG. Furthermore, the applied air pressure now struggles to enter into the narrow machining gap. Thus, the aspect ratio of microdimples increases with increasing of the machining gap. 5.5. Influence of the cathode diameter on micro-dimple process In EMM, the size of the cathode may influence the distribution of electric current and therefore the material surface. In this case, different diameters of cathode with Ø100 μm, Ø200 μm and Ø300 μm are selected to fabricate the micro-dimples (shown in Fig. 22). The figures show that the depth and width of the micro-dimple rises with increasing cathode diameter. The reason for the non-symmetry of the micro dimple profile is the cathode occasionally swinging resulting the electrolyte conductivity non-uniform surrounding. Fig. 23 illustrates the comparison between experimental and simulation results with varying cathode diameter with 30 s machining time. The eroded micro dimple profile area increases from 15805μm2 to

Fig. 20. Experimental and simulation profiles of micro dimples in AS-EMM at selected IEG.

Fig. 21. The aspect ratio variation of micro-dimples in different IEG. 133

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Fig. 22. Micro dimples machined in different cathode diameter (a: Ø = 100 μm; b: Ø = 200 μm; c: Ø = 300 μm;).

6. Conclusions In this paper, the multiphysics effect on material removal in ASEMM was studied based on the multiphysical fields. The process model combined with electric, fluid and temperature was proposed. Simulations were done and verified by experiments. The following conclusions can be drawn according to the investigation: (1) It is feasible to use a hybrid method to study the multiphysics in ASEMM. Void fraction of gas based on the two-phase flow model was obtained using FLUENT. And then the void fraction of gas was entered into a multiphysical model consists of electric, fluid and temperature in COMSOL and realized erosion profile generation of AS-EMM. (2) Simulation results can provide explanation in real trial. In this study, differences of the current density, electrolyte velocity and temperature in EMM and AS-EMM verified the differences of micro dimple forming. (3) An average deviation of less than 11.8% between experimental and theoretical results has been observed by using the built model in this study. Simulation results achieve close in accordance with the experimental results.

Fig. 23. Experimental and simulation profiles of micro dimples in AS-EMM at different cathode diameter.

Acknowledgements The study was financially supported by the National Natural Science Foundation of China [grant numbers 51475428] and Zhejiang Provincial Natural Science Foundation [grant numbers LY19E050007]. This work was also supported by the Engineering and Physical Sciences Research Council [grant numbers EP/R511730/1]. The authors would like to thank the Manufacturing Metrology Team and Alexander Jackson-Crisp of ACEL for technical assistance with surface scanning.

Fig. 24. The aspect ratio variation of micro-dimples in different cathode diameter.

References

42628 μm and 13918 μm to 35402 μm for experimental and simulation results respectively when the Ø100 μm cathode is changed by Ø300 μm cathode. The width is 375 μm, the depth 62.64 μm with the Ø100 μm cathode, and they increase to be 739 μm and 109 μm when the cathode is Ø300 μm. The error between simulation and experimental results for the diameter and depth are 11.8% and 10.5% respectively. Fig. 24 shows the aspect ratio of micro-dimples decreasing with increasing of the cathode diameter. The change of diameter is more obvious due to increasing the intensity of the electric field around the cathode when increasing the cathode diameter. And the current becomes large so increasing the machining area. 2

2

2

[1] Zhang C, Bijay KS. Investigation on drag reduction performance of aero engine blade with micro-texture. Aerosp. Sci Technol 2018;72:380–96. [2] Xiang J, Yong T, Ye B, Wei Z, Yan H, Hu ZH. Compound forming technology of outside 3D integral fin of copper tubes. Trans Nonferrous Met Soc China 2009;19:335–40. [3] Chae J, Park SS, Freiheit T. Investigation of micro-cutting operations. Int J Mach Tool Manu 2006;46:313–32. [4] Ng MK, Fan Z, Gao RX, Smith III EF, Cao J. Characterization of electrically-assisted micro-rolling for surface texturing using embedded sensor. CIRP Ann Manuf Technol 2014;63:269–72. [5] Bui QT, Ro SK, Park JK. A static model for micro-pattern forming prediction in rolling-based surface texturing. The International Journal of Advanced Manufacturing Technology 2017;92(5-8):2819–29. [6] Sasi R, Subbu SK, Palani IA. Performance of laser surface textured high speed steel cutting tool in machining of Al7075-T6 aerospace alloy. Surf Coat Technol 2017;313:337–46. [7] Ahuir-Torres JI, Arenas MA, Perrie W. Surface texturing of aluminium alloy

134

Journal of Manufacturing Processes 43 (2019) 124–135

M.H. Wang, et al.

[8]

[9]

[10] [11]

[12]

[13]

[14]

[15]

[16]

[17]

[18] [19]

[20]

[21] Bhattacharyya B, Munda J, Malapati M. Advancement in electrochemical micromachining. Int J Mach Tool Manu 2004;44:1577–89. [22] Saxena KK, Qian J, Reynaerts D. A review on process capabilities of electrochemical micromachining and its hybrid variants. Int J Mach Tools Manuf 2018;127:28–56. [23] Kozak J. Computer simulation system for electrochemical shaping. J Mater Process Technol 2001;109:354–9. [24] Wang MH, Zhu D. Simulation of fabrication for gas turbine blade turbulated cooling hole in ECM based on FEM. J Mater Process Technol 2009;209(4):1747–51. [25] Mi D, Natsu W. Proposal of ECM method for holes with complex internal features by controlling conductive area ratio along tool electrode. Precision Engineering 2015;42(October):179–86. [26] Smets N, Damme SV, Wilde DD, Weyns G, Deconinck J. Time averaged calculations in pulse electrochemical machining, using a strongly non-linear model. J Appl Electrochem 2010;40(7):1395–405. [27] Hotoiu L, Deconinck J. Time-efficient simulations of nano-pulsed electrochemical micromachining. Procedia CIRP 2013;6:469–74. [28] Kozak J, Zybura-Skrabalak M. Some problems of surface roughness in electrochemical machining (ECM). Procedia CIRP 2016;42:101–6. [29] Deconinck D, Van Damme S, Deconinck J. A temperature dependent multi-ion model for time accurate numerical simulation of the electrochemical machining process. Part I: Theoretical basis. Electrochimica Acta 2012;60:321–8. [30] Deconinck D, Van Damme S, Deconinck J. A temperature dependent multi-ion model for time accurate numerical simulation of the electrochemical machining process. Part II: Numerical simulation. Electrochimica Acta 2012;69:120–7. [31] Wu J, Wang H, Chen X, Cheng P, Ding G, Zhao XL, Huang Y. Study of a novel cathode tool structure for improving heat removal in electrochemical micro-machining. Electrochim Acta 2012;75(4):94–100. [32] Deconinck D, Damme SV, Albu C, Hotoiu L, Deconinck J. Study of the effects of heat removal on the copying accuracy of the electrochemical machining process. Electrochim Acta 2011;56(16):5642–9. [33] Wang MH, Liu WS, Peng W. Multiphysics research in electrochemical machining of internal spiral hole. Int J Adv Manuf Technol 2014;74(5-8):749–56. [34] Manninen M, Taivassalo V, Kallio S. On the mixture model for multiphase flow. Technical Research Center of Finland: VIT Publications Inc;; 1996. [35] Fujisawa T, Inaba K, Yamamoto M, et al. Multiphysics Simulation of Electrochemical Machining Process for Three-Dimensional Compressor Blade. J Fluid Eng 2008;130(8):081602.

AA2024-T3 by picosecond laser: Effect on wettability and corrosion properties. Surf Coat Tech 2017;321:279–91. Hackert-Oschätzchen M, Paul R, Martin A, Meichsner G, Lehnert N, Schubert A. Study on the dynamic generation of the jet shape in Jet Electrochemical Machining. J Mater Process Technol 2015;223:240–51. Mitchell-Smith J, Speidel A, Gaskell J, Clare AT. Energy distribution modulation by mechanical design for electrochemical jet processing techniques. Int J Mach Tool Manu 2017;122:32–46. Costa HL, Hutchings IM. Development of a maskless electrochemical texturing method. J Mater Process Tech 2009;209(8):3869–78. Ariffin NHZ, Yahaya H, Shinano S, Tanaka S, Hashim AM. Fabrication of conical micropore structure on silicon nitride/silicon using focused ion beam milling for biosensor application. Microelectron Eng 2015;133:1–5. Skrabalak G, Electrochemical Stwora A. Electrodischarge and Electrochemical-discharge Hole Drilling and Surface Structuring Using Batch Electrodes. Procedia Cirp 2016;42:766–71. Yamada H, Taura H, Kaneko S. Numerical and Experimental Analyses of the Dynamic. Characteristics of Journal Bearings With Square Dimples 2018;140. 011703-1-13. Silk EA, Kim J, Kiger K. Spray cooling of enhanced surfaces: Impact of structured surface geometry and spray axis inclination. Int J Heat Mass Tran 2006;49:4910–20. Zhu D, Qu NS, Li HS Zeng YB, Li DL, Qian SQ. Electrochemical micromachining of microstructures of micro hole and dimple array. CIRP Ann Manuf Technol 2009;58(1):177–80. Raffelstetter P, Mollay B. On the modeling of shape evolution in through-mask electrochemical micromachining of complex patterned substrates. Electrochimica Acta 2010;55(6):2149–57. Wang GQ, Li HS, Qu NS, Zhu D. Improvement of electrolyte flow field during through-mask electrochemical machining by changing mask wall angle. J manuf process 2017;25:246–52. Schuster R, Kirchner V, Allongue P, Ertl G. Electrochemical micromachining. Science 2000;289:98–101. Landolt D, Chauvy PF, Zinger O. Electrochemical micromachining, polishing and surface structuring of metals: fundamental aspects and new developments. Electrochimica Acta 2003;48(20):3185–201. Wang MH, Bao ZY, Qiu GZ. Fabrication of micro-dimple arrays by AS-EMM and EMM. Int J Adv Manuf Technol 2017;93(1-4):787–97.

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