Flow Field Design in Electrochemical Machining of Diffuser

Available online at www.sciencedirect.com

ScienceDirect Procedia CIRP 42 (2016) 121 – 124

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

Flow Field Design in Electrochemical Machining of Diffuser Dong Zhua,*, Zhouzhi Gua, Tingyu Xue a, Di Zhu a a College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics & Astronautics, NO29 Yudao Street, Nanjing 210016, China * Dong Zhu. Tel.: +86-025-84892195; fax: +86-025-84895912. E-mail address: [email protected]

Abstract The diffuser which is usually made from hard-to-machine nickel-based and titanium-based alloys is a key component in the aero engine. In the electrochemical machining, the electrolyte flow mode has great effect on the machining stability and efficiency. This paper presents several flow modes for an aero engine diffuser on which the blades are long and thin. Then the flow field distributions of these flow modes are simulated by using the computational fluid dynamics software. The simulation results show that the mode in which the electrolyte flows from leading edge to trailing edge is appropriate. Furthermore, the fixture for this optimal flow mode is designed and fabricated, and the corresponding experiment is carried out. There is no flow mark on the diffuser sample surface and the process is stable. The experimental results show that this flow mode is appropriate and can be also used in other similar aero engine components. © 2016 B.V. 2016 The TheAuthors. Authors.Published PublishedbybyElsevier Elsevier B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM (http://creativecommons.org/licenses/by-nc-nd/4.0/). XVIII). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining

(ISEM XVIII) Keywords: Electrochemical machining; flow field; diffuser; electrolyte; design

1. Introduction Electrochemical machining (ECM) is an unconventional manufacturing technology, and its major advantages are high material removal rate, no tool wear, and good surface quality without any white layer as well as no mechanically affected zones. Therefore, it is an economical and effective approach to manufacturing aero craft components such as diffuser [1-2]. In ECM processes, electrolyte flow mode has great effect on the process stability and efficiency. Several new flow mode, such as W-shape mode, П-shape mode, and dynamic additional flow [3], were proposed. Various innovative measures as progressive pressure [4], and pulse electrolyte were adopted [5]. Many scholars have analyzed distribution of characteristic parameter on flow field in the inter electrode gap, Dabrowski researched on the two-dimensional electrolyte flow in ECM [6]. Fujisawa et al. [7], Van Damme et al. [8], Klocke et al. [9, 10], Westley et al. [11], and Tang et al. [12] already proposed numerical models and then delivered their findings in literature. The above researches reflected that an appropriate flow mode and even flow field distribution was very crucial in ECM. This paper proposes an ECM manufacturing method for diffuser with cathode axial feeding, and presents several flow modes to find a suitable one for processing. The flow field

distribution in flow channel is then simulated and the characteristic parameter in inter electrode gap is analyzed to obtain the best flow mode. Finally, experiments were carried out to verify the effectiveness of the flow mode. Nomenclature Ui ith components of the mean electrolyte velocity vector xi ith Cartesian coordinate ρ electrolyte density P mean electrolyte pressure ‫ݑ‬௜ᇱ ith fluctuation of the electrolyte velocity component around its mean value ν kinematic viscosity തതതതത u'i u'j Reynolds-stress tensor k turbulent kinetic energy ε dissipation rate V velocity variance n sampled point number

2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.205

122

Dong Zhu et al. / Procedia CIRP 42 (2016) 121 – 124

2. Flow mode description for ECM of diffuser An ECM manufacturing method for diffuser with cathode axial feeding is proposed for machining the channels between radial blades. The sketch of this method is shown in Fig.1. In the process, tool cathode moves towards diffuser workpiece with a certain feed rate. The metallic material dissolves when a power supply is applied on the cathode and workpiece. Meanwhile, electrolyte is continuously pumped through the inter-electrode gap with high-speed to carry away the dissolved metal and remove the Joule heat. The channel between two blades is then forming to the final profile gradually. As one channel is manufactured, diffuser is rotated by control system into a definite angle for machining another channel. The process continues until all of the channels are fabricated.

3. Simulation and discussion

Control system



(c) Mode 3 (TE to LE) (d) Mode 4 (LE to TE) Fig.2 Electrolyte flow modes The motion structure of these flow modes is entirely different, so the flow field distribution in the machining gap is diverse. In order to observe the characteristics of the flow field to choose the best mode, simulation is going to be conducted in the following part.

 3.1. Simulation process

 Feeding direction

Power supply Cathode



Leading edge

Concave part

Rotation

Diffuser sector Electrolyte

Trailing edge

Concave part

Fig.1 Sketch of the diffuser ECM process with cathode feeding in the axial direction In ECM process for diffuser, a suitable flow mode is very significant to acquire uniform flow field. While an improper flow mode may produce area with bad flushing conditions, leading to instability of the process even shortcut phenomenon. Therefore, this paper aims to obtain an appropriate flow mode. Because the blade is twisted and the channel between two blades is large, it is difficult for electrolyte to flow through the whole inter-electrode gap evenly and it is crucial to design a good flow mode in diffuser ECM. This study presents four different flow modes combining with the diffuser structure which are described in Fig.2. The electrolyte motion direction of these four flow modes are defined as below. (1) Mode 1, electrolyte inlet is at the side of convex part (VP) while electrolyte outlet is at the other side of concave part (CP). The fluid flows through VP channel, inter-electrode gap, CP channel, and goes to the electrolyte cell from the outlet (Fig.2 (a)). (2) Mode 2, electrolyte flows from CP channel to VP channel and its main motion direction is opposite to Mode 1 (Fig.2 (b)). (3) Mode 3, electrolyte inlet is at the trailing edge (TE) side while electrolyte outlet is at the leading edge (LE) side. The fluid passes through the machining gap from TE to LE (Fig.2 (c)). (4) Mode 4, the direction of electrolyte motion is from LE to TE which is contrary to Mode 3 (Fig.2 (d)).

To simplify the proposed model, several assumptions are made as follows. First, the electrolyte flow is continuous and incompressible. Second, the ECM process is in the equilibrium state. The momentum conservation and mass conservation equations, which are generally used for the turbulent flow in ECM, are described as follows [3]: డ௎೔ ൌͲ (1) డ௫೔

ܷ௝

డ௎೔ డ௫ೕ

ൌെ

ଵ డ௉ ఘ డ௫೔

൅

డ డ௫ೕ

డ௎೔

൬ߥ ൬

డ௫ೕ

൅

డ௎ೕ డ௫೔

൰ െ തതതതത u'i u'j ൰

(2)

There are many experimental results indicating that the turbulent flow is necessary for the stability of ECM process [4]. The renormalization group (RNG) k–ε turbulent model, which is suitable for curved wall flow, is then introduced to simulate the flow field in the channel with the above equations. The simulation models of presented flow modes are established when the process is in the equilibrium state (Fig.3). As described in Fig.3, simulation models of Mode 1 and Mode 2 is the same while electrolyte inlet and outlet planes are different. Plane PI represents electrolyte inlet and plane PO is the electrolyte outlet. Simulation models of Mode 3 and Mode 4 is the same, yet electrolyte inlet and outlet planes are just opposite. The boundary conditions for simulation will be determined by setting pressure values on inlet and outlet planes, and the specific values are that PI=0.8 MPa and PO=0. MPa according to the preliminary work. The inter-electrode gap is the electrochemical dissolution region, in which the flow field distribution is of most interest in this study. Plane P, whose position is in the middle of the inter-electrode gap, is then chosen to observe flow field distribution in the following article. PO(PI)

PI(PO) (a) Mode 1 (Mode 2) (a) Mode 1 (VP to CP)

(b) Mode 2 (CP to VP)

123

Dong Zhu et al. / Procedia CIRP 42 (2016) 121 – 124

Mode 4 is more appropriate to be adopted in the ECM experiment for machining diffuser. PI(PO)

( d 4)) (b) Mode 3 (Mode Fig.3 Simulation models of four flow modes 3.2. Results and discussion On the basis of the RNG k–ε turbulent model, flow field simulation for the four models in ECM for diffuser is conducted by means of CFD software ANSYS. After simulation, electrolyte velocity distributions on plane P for the four modes are obtained and they are shown in Fig.4. Some conclusions are drawn from the figure. (1) The electrolyte velocity values at intersection area of

VP and TE are smaller, the reason may be that the channel profile changes violently at this place. While it is relatively uniform as a whole in the middle of inter-electrode gap for the four flow modes. (2) Low velocity area is the least in Mode 4 while that is the most in Mode 2. CP 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

TE

LE VP

40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

(a) Mode 1 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

70

Average velocity Velocity variance Low velocity percentage

60

4 3.5 3

50

2.5 40 2 30 1.5 20

1

10

Low velocity percentage

Plane P

Average velocity (m/s) /Variance (m2/s2)

PO(PI)

0.5 0

0 Mode 1

Mode 2

Mode 3

Mode 4

Fig.5 Average velocity, velocity variance and low velocity percentage for the four flow modes 4. Experimental

(b) Mode 2 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

(c) Mode 3 (d) Mode 4 Fig.4 Velocity distribution in the machining gap for four flow modes In order to obtain the quantitative analysis of the flow field, twenty-seven thousands points are sampled on plane P and the specific velocity of every point is obtained. The average velocity UA, velocity variance V (∑(U-UA)2/(n-1), where n is the sampled point number), and low velocity percentage for four modes are calculated (Fig.5). As illustrated in Fig.5, (1) the average velocities of four flow modes are 17.42, 19.97, 15.11, and 16.17 m/s, respectively. The values of these four modes is generally large enough to take away the dissolved products. (2) The velocity variances about the four modes are 27.96, 63.61, 33.16, and 20.22 in turn. The value is the smallest in Mode 4 and this phenomenon reveals that the flow field distribution in this mode is the most uniform. (3) Blue color in Fig.4 represents low velocity. The velocity generally exceeds 10 m/s in the inter-electrode gap [3, 10, 13]. Here we used 1.5 m/s as a threshold of low velocity, which is about one order of magnitude lower than the above velocity. Low velocity percentage in the machining gap on plane P for four modes are 0.30%, 0.94%, 0.52%, and 0.18%, respectively. Because the low velocity in the inter-electrode gap may make the dissolved products carrying away difficult and reduce machining stability, the less area for the low velocity is, the more stable for process will be. As shown in Fig.5, the percentage in Mode 4 is the lowest, so the flow field distribution is the best. Therefore,

In order to verify the rationality of proposed flow modes, the fixture for the optimal flow mode is designed and fabricated, experiments are then performed on the diffuser sector in ECM process. The conditions and parameters of the experiments are illustrated in Table 1. The workpiece material is nickel-based super alloy and the tool cathode material is 1Cr18Ni9Ti stainless steel. The electrolyte is 20% NaNO3 and the pressures of electrolyte inlet and outlet are equal to the simulation boundary. The cathode feed rate changes from 0.2 to 0.9 mm/min and the feed distance is 7 mm. The process is performed on a self-developed machine tool system. Table 1 Experiment conditions for diffuser ECM process Conditions

Value

Workpiece material

nickel-based superalloy

Tool material

1Cr18Ni9Ti stainless steel

Electrolyte

20% NaNO3

Electrolyte temperature

28± 1 °C

Machining voltage

20 V

Feed distance

7 mm

Cathode feed rate

0.2-0.9 mm/min

The experiments are conducted under above machining conditions by adopting Mode 4. ECM process is stable when the cathode feeding rate increases from 0.2 mm/min to 0.9 mm/min. The machined sample are demonstrated in Fig.6. There is no remarkable flow mark on the machining surface. The surface roughness Ra of sample’s profiles are measured by a profilometer (T8000 SC, HOMMEL-ETAMIC, Germany). Ra value of concave part and convex part are 2.121 μm and 2.137 μm, respectively (Fig.7). The surface quality is similar on these two profiles. Ra value of the hub is 0.176μm, which is much smaller than that on the blade profile. The reason of this phenomenon may be that the electric density distribution in the whole machining gap is different. Electric density in inter-

124

Dong Zhu et al. / Procedia CIRP 42 (2016) 121 – 124

electrode gap between hub and cathode is intensive, leading to high consistency of dissolution for different metallic elements and good surface quality. While it is weak in side gaps between concave part and convex part profiles and cathode, causing uneven dissolution of different metallic atom and low surface quality. The result indicates that this electrolyte flow mode is reasonable and the flow field distribution is suitable in ECM for diffuser.

25mm Fig.6 Machining samples 1µm

0.35mm p Ra=2.121μm (a) Concave part 1µm

0.35mm p Ra=2.137μm (b) Convex part 1µm 0.35mm (c) Hub Ra=0.176μm Fig.7 Surface roughness of sample’s profiles 5. Conclusion 1. An ECM manufacturing method for diffuser with cathode axial feeding was proposed in this paper. 2. Four different electrolyte flow modes were presented, and the simulation results revealed that the uniformity of Mode 4 is the best. However, there was still low velocity area at local region, some new method might be adopted for further optimization. 3. Experiment of diffuser ECM was carried out and the results illustrated the machining method and flow mode is suitable. For further research, the cathode feed rate could be increased to improve surface. 6. Acknowledgement The authors wish to acknowledge the financial support provided by the National Natural Science Foundation of China (51205199), the Natural Science Foundation of Jiangsu Province (BK2012387).

7. Reference [1] Klocke F, Zeis M, Klink A, Veselovac D. Experimental research on the electrochemical machining of modern titanium- and nickel-based alloys for aero engine components. Procedia CIRP 2013; 6: 368-372. [2] Klocke F, Klink A, Veselovac D, Aspinwall DK, Soo SL, Schmidt M, Schilp J, Levy G, Kruth JP. Turbomachinery component manufacture by application of electrochemical, electro-physical and photonic processes. CIRP Ann-Manuf Techn 2014; 63: 703-726. [3] Zhu D, Zhang JC, Zhang KL, Liu J, Chen Z, Qu NS. Electrochemical machining on blisk cascade passage with dynamic additional electrolyte flow. Int J Adv Manuf Technol 2015; 80: 637-645. [4] Qu NS, Hu Y, Zhu D, Xu ZY. Electrochemical machining of blisk channels with progressive pressure electrolyte flow. Int J Adv Manuf Technol 2014; 29: 572-578. [5] Qu NS, Fang XL, Zhang YD, Zhu D. Enhancement of surface roughness in electrochemical machining of Ti6Al4V by pulsating electrolyte. Int J Adv Manuf Technol 2013; 69(9-12): 2703-2709. [6] Dabrowski L, Paczkowski T. Computer simulation of two-dimensional electrolyte flow in electrochemical machining. Russ J Electrochem 2005; 41(1): 91-98. [7] Fujisawa T, Inaba K, Yamamoto M, Kato D. Multiphysics simulation of electrochemical machining process for three-dimensional compressor blade. J Fluids Eng 2008; 130(8): 081602-1-7. [8] Van Damme S, Nelissen G, Van den Bossche B, Deconinck J. Numerical model for predicting the efficiency behavior during pulsed electrochemical machining of steel in NaNO3. J Appl Electrochem 2006; 36: 1-10. [9] Klocke F, Zeis M, Harst S, Klink A, Veselovac D, Baumgärtner M. Modeling and simulation of the electrochemical machining (ECM) material removal process for the manufacture of aero engine components. Procedia CIRP 2013; 8: 265-270. [10] Klocke F, Zeis M, Klink A. Interdisciplinary modelling of the electrochemical machining process for engine blades. CIRP Ann-Manuf Techn 2015; 64: 217-220. [11] Westley JA, Atkinson J, Duffield A. Generic aspects of tool design for electrochemical machining. J Mater Process Technol 2004; 149: 384-392. [12] Tang L, Gan WM. Utilization of flow field simulations for cathode design in electrochemical machining of aerospace engine blisk channels. Int J Adv Manuf Technol 2014; 72(9-12): 1759-1766. [13] Rajurkar KP, Zhu D, McGeough JA, Kozak J, De Silva A. New developments in electro-chemical machining. Ann ClRP 1999; 48(2): 567579.