Flow field design and experimental investigation of electrochemical

Int J Adv Manuf Technol (2014) 71:459–469 DOI 10.1007/s00170-013-5491-y

ORIGINAL ARTICLE

Flow field design and experimental investigation of electrochemical machining on blisk cascade passage Zhengyang Xu & Lunye Sun & Yuan Hu & Juchen Zhang

Received: 20 May 2013 / Accepted: 13 November 2013 / Published online: 28 November 2013 # Springer-Verlag London 2013

Abstract Electrochemical machining (ECM) provides an economical and effective way for machining heat-resistant, high-strength materials into complex shapes that are difficult to machine using conventional methods. It has been applied in several industries, especially aerospace, to manufacture blisk. The electrolyte flow field is a critical factor in ECM process stability and precision. To improve the process stability and the efficiency of blisk cascade passages, ECM with a radial feeding electrode, a rational electrolyte flow mode for electrochemical machining called “Π shape flow mode”, is discussed in the paper. Three flow field models are described separately in this report: traditional lateral flow mode, positive flow mode and Π-shaped flow mode, and the electrolyte velocity and pressure distribution vectors for each flow mode are calculated by means of a finite element fluid analysis method. The simulation results show that the electrolyte flow is more uniform with the Π-shaped flow mode. The deformation of the cathode, which is caused by the pressure difference, is also analysed in this report. The cascade passage ECM with a radial feeding electrode was experimentally tested out to evaluate the rationality of the flow field, and the fluctuation of current during the process was less than 1 %, which means that the process that uses the Π-shaped flow mode is stable. The feeding velocity of the cathode with the Π-shaped flow mode is approximately 70 % higher than that with the other two flow modes, and the incidences of short circuiting are obviously decreased. The surface roughness of the blisk hub is only 0.15 μm, and the machining error of the hub is less than

Z. Xu (*) : L. Sun : Y. Hu : J. Zhang Nanjing University of Aeronautics and Astronautics, Nanjing, China e-mail: [email protected]

0.1 mm. The results demonstrate that using the Π-shaped flow mode can enhance the quality, stability and efficiency of blisk cascade passage ECM. Keywords ECM . Flow field . Radial feeding electrode . Blisk

1 Introduction Blisk forms a very important part of a gas turbine. This part imparts kinetic energy and redirects airflow to the next stage at an optimum angle. Working under severe conditions, blisks are usually made of titanium alloys or Ni-based superalloys, which are exceedingly difficult to machine. Moreover, the shapes of these blisk profiles are very complex and the cascade passages are often narrow, so it is very difficult to achieve the required workpiece using traditional methods [1–4]. Electrochemical machining (ECM) connects the workpiece (anode) to the tool (cathode) in an electrolytic cell and an electrolyte is pumped between the gap of the anode and cathode. With the metal–electrolyte combination, the electrolysis involves the dissolution of iron from the anode and the generation of hydrogen at the cathode. The main advantages of ECM are no tool wear, the ability to produce complicated shapes that can be machined on hard metals and the rate of metal machining does not depend on the hardness of the metal. Being a non-mechanical metal removal process, ECM provides an economical and effective way for machining heat-resistant, high-strength materials into complex shapes that are difficult to machine by conventional methods [5–8]. Many studies have been carried out to improve the ECM quality for complex shapes,

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Int J Adv Manuf Technol (2014) 71:459–469

Fig. 1 The steps of blade profiles ECM

(a) The first step: cascade

(b) The second step: blade

passages machining

profiles machining

including a mathematical model for simulating the process of a blade surface and the final shape of a blade [9, 10], an ECM model that predicts the evolution of a machined surface's shape and the distribution of physical–chemical parameters inside the inter-electrode gap [11], simulations of the machining of a 3D compressor blade within which many machining parameters were taken into account [12], the control of the process parameters in machining to shape surfaces [13], titanium alloy machining with new wire electrochemical machining [14] and the spiral internal ribs machined by ECM with shaped cathode [15]. In addition to these efforts, other studies focused on improving the machining quality, efficiency and stability of ECM have included studies on the monitoring and controlling of the inter-electrode gap [16–18], optimisation of process parameters involved in machining shaped surfaces and difficultto-machine materials [19–23], the use of different compositions and concentrations of electrolyte [24–26], ultrasonically assisted electrochemical machining [27], short-pulsed voltage electrochemical machining [28] and laser-assisted jet ECM [29]. The electrolyte is pumped at a rate of approximately 5– 50 m/s through the gap between the electrodes to remove the products of machining and diminish unwanted effects, such as those that arise with cathodic gas generation and electrical heating. Thus, the electrolyte flow field is a critical factor that affects process stability, efficiency and quality. Changes in the physical conditions along electrolyte flow will cause the blade's profile to change. Moreover, the ECM process occurs in the clamping fixture, and the gap between the anode and the cathode is very small; thus, rational flow field design and analysis are necessary to hold the process stable. To improve blisk cascade passage and hub processing quality, the use of

Workpiece Cathode

Cascade passage Fig. 2 Schematic of cascade passage ECM with radial feeding electrode

ECM with a radial feeding electrode is proposed in this paper. Because of the particularity of this process, the electrolyte flow pattern is also different from the traditional flow mode. A new Π-shaped electrolyte flow mode is proposed to overcome the disadvantages of the traditional flow mode. The other two flow field patterns, which describe traditional lateral flow and positive flow, are also given in this paper. The three models of these flow patterns are built by means of a finite element fluid analysis method. The simulation result shows that the new Π-shaped flow pattern is more suitable for machining blisk cascade passages and makes up for the disadvantages of the existing flow mode such as non-uniform and random distribution of flow. Experiments on the cascade passage ECM with the radial feeding electrode were carried out to evaluate the rationality of the newly designed flow field. Our results demonstrate that, with the new flow pattern, the stability and the efficiency of blisk cascade passage ECM can be enhanced. The deformation of the cathode profile, which was caused by electrolyte pressure, was calculated and the result shows that the deformation is very tiny and will not affect processing stability and quality.

2 Flow field pattern design of blisk cascade passage ECM with radial feeding electrode 2.1 Blisk cascade passage ECM with radial feeding electrode ECM processing of blisk blade profiles often happens in two steps. As shown in Fig. 1, the first step is the cascade passage ECM processing, which creates several tens of curved tunnels with allowances as uniform as possible for the subsequent profile finishing. Because the quantity of cascade passages is large and the width is narrow, it is required that the processing efficiency and stability are of an excellent standard. The cascade passage ECM method with radial feeding electrode is examined in this paper. Figure 2 shows a schematic of this method. The workpiece is the fixed anode, and the tool is the cathode that is fed from the blade tip to the centre of the blisk along the radial direction at a certain feeding angle during the process. The electrolyte is pumped down the gap between the anode and the front-end surface of the tool, and the metal is gradually dissolved until processing is complete. With the radial feeding electrode method, the processing quality of the blisk hub, which is difficult to control with other methods, is improved because a normal blisk hub is almost parallel to


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Fig. 3 The conventional lateral flow pattern

Electrolyte flow direction

Flow direction Mussy flow lines

Non-uniform flow Leading edge

Convex part

Feeding direction

the cathode feeding direction and the gap between the anode and the cathode near the equilibrium inter-electrode gap of the ECM. Additionally, the profiles of convex and concave parts that are obtained in the process can be optimised by adjusting the cathode front-end surface and side contour lines. 2.2 Flow field pattern design A proper flow field pattern design is necessary to improve processing efficiency and stability. The traditional flow patterns of ECM are the lateral flow and the positive flow patterns, as shown in Figs. 3 and 4. With the conventional lateral flow pattern, the electrolyte flows from the blade's leading edge to its trailing edge and is separated in two parts that flow across the convex and concave parts gaps, respectively. The electrolyte separation is passive, and the flow rate across the convex and concave parts cannot be controlled precisely, resulting in non-uniform flow. Additionally, a large quantity of the electrolyte impacts the electrode with high speed and high pressure at the inlet, as shown in Fig. 3, leading to a disorderly flow field at the inlet and the increased possibility of electrode vibration; thus, the processing stability and efficiency are decreased. As Fig. 4 shows, in the traditional positive flow pattern, the cathode has an inner flow passage; the electrolyte flows towards the blisk hub along the flow passage and is separated into two parts, which flow across the convex part and concave part gaps, respectively. With the flow pattern, the residual rib after the ECM will often appear on the blisk hub area, which faces the inner flow passage of the cathode. The blisk hub is

Convex part

Concave part

Concave Trailing part edge

Cathode

very difficult to machine because it is near the root of the blade and the width of passage is very small. The residual rib makes it more difficult for any subsequent machining. Furthermore, the electrolyte separation is also passive and the flow rate across the convex and concave parts cannot be controlled precisely. The cathode must also have an inner flow passage for which the shape and placement are difficult to design and make. To overcome the above-mentioned deficiencies of traditional ECM electrolyte flow patterns, a new flow mode will be addressed in this report. The purposes of this new flow mode are as follows: 1. There is no passive flow separation and the electrolyte must flow along the designed passage; thus, the electrolyte flow can be more uniform. 2. The residual rib will be eliminated and the processing quality of the blisk hub will be improved, as the blisk hub is difficult to finish with the subsequent processing method. This study proposes a new Π-shaped electrolyte flow mode. As shown in Fig. 5, the electrolyte is pumped from the inlet on the side of fixture to the processing area; flows along the gaps between the convex part and the cathode, the blisk hub and the front-end surface of cathode; and then finally flows from the gap between the concave part and the cathode to the outlet on the other side of the fixture. The shape of the

Workpiece

Outlet

Inlet Cathode Fixture

Residual rib Fig. 4 The conventional positive flow pattern

Fig. 5 The Π-shaped electrolyte flow mode

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Int J Adv Manuf Technol (2014) 71:459–469 Electrolyte inlet

Fig. 6 The FEM discretisation meshes of the lateral flow mode

Electrolyte outlet

electrolyte flow pattern is similar to the letter Π, so it is called the Π-shaped electrolyte flow mode. The advantages of the flow mode are as follows: 1. The electrolyte flow from the inlet to the outlet along the designed passage and the flow rate can be controlled precisely; thus, there is no passive flow separation and the flow becomes more uniform. 2. There is less electrolyte impact to the cathode, and the electrolyte between the cathodes could balance the electrolyte pressure; thus, the disorder of the electrolyte flow and the possibility of electrode vibration can be reduced. 3. There is no inner flow passage in the cathode and the front-end surface of the cathode is similar to the blisk hub surface; thus, the preparation of the cathode is easier and there is no residual rib on the hub after ECM. The processing quality of the blisk hub can be improved.

P2

P1

taken to be proportional to the rate of shear in the direction perpendicular to the motion. The numerical model can be calculated using a finite element fluid analysis method. From finite element analysis, the continuity equation of the incompressible fluid flow is ∂vi ¼0 ∂xi

ð1Þ

where ν is the electrolyte velocity. The motion equation is ∂vi ∂vi 1 ∂pji þ vj ¼ fi þ ρ ∂x j ∂t ∂x j

ð2Þ

where p is the electrolyte pressure and t is the time. The constitutive equation is ∂ ∂p ∂2 v i pji ¼ − þ μ ∂x j ∂xi ∂x j ∂x j

2.3 Numerical simulation of electrolyte flow 2.3.1 Finite element equations of electrolyte flow The electrolyte motion relies on two assumptions. The first is that wherever the fluid is in contact with a solid boundary, there is no motion or slip relative to that boundary of the fluid particles adjacent to it. In the second assumption, the shearing stress between adjacent layers of fluid of an infinitesimally small thickness is

Fig. 7 The FEM discretisation meshes of the positive flow mode

ð3Þ

where μ is the Lame constant. The Navier–Stokes equation is ∂vi ∂vi ∂p ∂ 2 vi þ vj ¼ f i− þ ν ∂xi ∂t ∂x j ∂x j ∂x j where f is the gravitational force.

Outlet P4

Inlet

Outlet

P3

ð4Þ


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Fig. 8 The FEM discretisation meshes of the Π-shaped flow mode

Electrolyte outlet

P6

Electrolyte inlet

Fig. 9 The vectors and contours of the electrolyte velocity in the traditional lateral flow mode

(a) Vectors of electrolyte velocity at the end face gap

(b) Contours of velocity magnitude at the end face gap and side gap

Fig. 10 The contours of pressure at the end face gap

P5

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Fig. 11 The vectors and contours of electrolyte velocity in positive flow mode


(b) Contours of velocity magnitude at the end face gap and side gap

The numerical solution of the above-mentioned equations reveals the possibility of obtaining the velocity, ν , and the pressure, p , distribution in the gap. The unit finite element characteristic type can be obtained by the finite element discretisation of the abovementioned equations and the Galerkin integral and Green–Gauss theorem. The unit finite element characteristic types are

where: Aeij ¼∬ΩðeÞ ρ Φi Φ j d Ω ∂Φ Beijβl ¼∬ΩðeÞ ρ Φi Φl d Ω ∂xβ ∂ Φi C eαik ¼ ∬ΩðeÞ − Ψkd Ω ∂xα ∂ Φi ∂ Φl ∂ Φi Deαiβl ¼ ∬ΩðeÞ μ δαβ þ δjβ d Ω ∂x j ∂x j Z ∂xα E eαi ¼ ∬ΩðeÞ ρ f α Φi d Ω þ

Aeij v˙αj þ Beijβl vαj vβl þ C eαik pk þ Deαiβl vβl ¼ E eαi

ð5Þ

F ekβj

∂Ψ k ¼ ∬ΩðeÞ Φ jd Ω ∂xβ Z

Gek ¼

F ekβj vβj ¼ Gek


ð6Þ

ΓðeÞ

pnα Φi dΓ Γ ðeÞ

2

vn Ψ k dΓ

α; β ¼ 1; 2; i; j; l ¼ 1; 2; ⋯; I v ; k ¼ 1; 2; ⋯; I p


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where e is the sequence number of elements, Ф is the interpolation function of the velocity, ψ is the interpolation function of the pressure, I v is the velocity node total number, and I p is the pressure node total number. The general finite element equations can be obtained by assembling total unit finite element characteristic types: Anmv˙αm þ Bnmβs vβs vαm þ C αnt pt þ Dαnβs vβs ¼ Eαn F rβs vβs ¼ Gr α; β ¼ 1; 2; n; m; s ¼ 1; 2; …; N v ; r; t ¼ 1; 2; …; N p

ð7Þ

ð8Þ

where N v is the total number of velocity nodes in the calculated area and N p is the total number of pressure nodes in the calculated area. The velocity and pressure distribution in the gap can be obtained using the above-mentioned general finite element equations. Figures 6, 7 and 8 show the FEM discretisation meshes of the lateral flow mode, the positive flow mode and the Π-shaped flow mode. 2.3.2 Results of numerical simulation In the analysis of the numerical simulation results, our attention has been primarily focused on pressure, p, and velocity, v,

Fig. 14 The velocity distribution of the 3D model in the Π-shaped flow mode

distributions in the gap. Figure 9 shows the vectors and contours of the electrolyte velocity in the traditional lateral flow mode at the front-end surface. The distribution of the velocity, in this case, was very non-uniform. The highvelocity region was concentrated on the right side of the front-end surface, and the velocity on the left side was very small. The velocity on the right side was almost several times higher than that on the left side. Because of this discrepancy, the by-products of the ECM process will be concentrated in the left area and the conductivity at the end surface is nonuniform. Besides that, because the inter-electrode gap at the

Fig. 13 The vectors and contours of the electrolyte velocity in the Π-shaped flow mode


(b) Contours of velocity magnitude in the whole flow area

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Fig. 16 Deformations of cathodes of different thicknesses caused by electrolyte pressure differences

end surface of the tool is much less than side gaps, the vast majority of electrolyte will flow across side gaps and deficiency of flow in the end gap will be more serious. Thus, the machining stability at the end surface of the tool will be decreased. Figure 10 shows the pressure distribution at the end face gap. There are the great differences of pressure between the electrolyte inlet and the outlet, and the distribution of pressure is also non-uniform. Figure 11 shows the velocity vectors and contours of the electrolyte velocity in the positive flow mode. Because the inlet slit in the cathode is near the inter-electrode gap and there is no channel long enough to guide the fluid, the electrolyte flow near the slit is very disordered, which causes nonuniform dissolution and unstable machining. This problem will be more serious when the process is in equilibrium status at which the inter-electrode gap at the end of tool is very small. Additionally, the residual rib on the blisk hub will also cause an uneven electrolyte flow rate across the convex and concave parts. The pressure distribution at the end face gap is also nonuniform, as Fig. 12 shows. Figure 13 shows the velocity vectors and contours of the electrolyte velocity in the Π-shaped flow mode. The electrolyte flows from the left side to the right side; there is no disorder vector of velocity and the flow is more uniform. Furthermore, there is no inner flow passage and no residual

The thickness of root is 2mm

The thickness of root is 4mm

rib on the hub during the process; thus, the electrolyte flows evenly across the inter-electrode gap with high velocity, and the contours of the velocity magnitude are smooth and steady, which contribute to stable and uniform machining. Figure 14 shows the velocity distribution of the 3D model, and the same result is obtained using 2D analysis. The contours of pressure are shown in Fig. 15. The distribution of pressure at the end face gap with the Π-shaped flow mode is also more uniform than with other two flow modes. Therefore, the Π-shaped flow mode is more suitable for blisk cascade passage machining.

Table 1 ECM experiment conditions Conditions

Value

Workpiece material Cathode material Electrolyte Voltage Inlet pressure Outlet pressure Temperature of electrolyte Length of cathode feeding Cathode feed rate

Nickel-based super alloy 1Cr18Ni9Ti stainless steel 20 % water solution of NaNO3 20 V 0.7 MPa 0.2 MPa 30±1 °C 35 mm 0.3–1.2 mm/min


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Fixture

Cathode

Workpiece

Fig. 19 The main components of blisk ECM in the machining region Fig. 17 Schematic view of the blisk ECM machine tool

Because of the pressure difference on both sides of the cathode with the Π-shaped electrolyte flow mode, the deformation of the cathode that is caused by the pressure difference is analysed using a finite element method. Figure 16 shows the deformations of the cathodes of different thicknesses. The maximum deformation often occurs at the top of the cathode and increases as thickness decreases. When the thickness of the cathode root is less than 2 mm, the maximum deformation of the cathode will be increased to 0.025 mm. Therefore, for precision machining, the deformation of the cathode, caused by the electrolyte pressure difference, should be considered.

3 Experimental investigation Experiments were performed on the blisk cascade passage ECM with radial feeding electrode to evaluate the influence of the flow field on the processing stability and efficiency and to verify the rationality of the Π-shaped electrolyte flow mode. The conditions and parameters of the experiments are presented in Table 1. The self-developed blisk ECM machine tool includes four parts: the machining tool, DC power, the electrolyte circling system and the control system. The DC power supply unit can provide 0–40 V. The electrolyte is pumped from the tank into the inter-electrode gap at a high speed (10– 80 m/s). The control system controls and adjusts the motions

of the electrodes and acquires the current and feeding position data. Figure 17 presents a schematic view of the blisk ECM machine tool. The fixture refers to the placement of blisk machining. The workpiece is fixed, and the cathode is installed on the same horizontal plane and can move towards the workpiece during the process. The electrolyte flow from the inlet crosses the inter-electrode gap and pours out from the outlet. The designed machining fixture is shown in Fig. 18.

4 Results and discussion Figure 19 shows the main components of the process, and Fig. 20 shows the section blank and the machined sample of the blisk cascade passages that were designed using the Πshaped electrolyte flow mode. The machining process with the Π-shaped mode is more stable, and the machining efficiency improved. The current was increased rapidly at the beginning of the process, and once the process was at an equilibrium status, the current fluctuation was less than 1 %, indicating that the process had become stable. Figure 21 shows the changes in current during the process. Figure 22 shows the relationship between the cathode feeding rate and the equilibrium gap of the cathode and the workpiece. When the feeding rate was increased, the equilibrium gap between the end surface of cathode and the blisk hub would gradually decrease. A gap that is too small increases the risk of short circuit; on the contrary, the rational electrolyte flow mode

Fig. 18 Machining fixture of blisk ECM

Cathode

Inlet Outlet Workpiece

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Fig. 20 The section blank and the machined sample of blisk cascade passages

(a) the section blank

(b) the machined sample of blisk cascade passages

(c) local areas of the machined sample

The current(A)

160 120 80 40 0 1

6

11 16 21 The machining time(min)

26

Fig. 21 The current changes during the process

4

0.4

3

(a) 0.3

2

0.2

1

(c) 0.1

Times of short circuit

Inter-electrode gap(mm)

0.5

(b)

0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 Feeding rate of cathode(mm/min)

Fig. 22 The gaps and times of short circuit with different feeding rates: (a) is the relationship between gap and feeding rate, (b) is the short circuit times with the Π-shaped flow mode and (c) is the short circuit times with other flow modes

Fig. 23 The surface roughness of blisk hub with the Π-shaped flow mode

removes ECM by-products effectively and lowers the risk of short circuit. The feeding velocity of the cathode with the Πshaped electrolyte flow mode can be more than 1.2 mm/min. In contrast, the feeding velocities of the cathode for the other flow modes are less than 0.7 mm/min, as Fig. 22 shows. Thus, more uniform electrolyte flow contributes to higher machining stability and efficiency. The surface roughness of the blisk hub was measured using a surface-finish measuring instrument (Perthometer S3P). The surface of the hub, which was machined with the Π-shaped flow mode, is smooth, and the surface roughness of the hub is only Ra 0.15 μm, as Fig. 23 shows. Conversely, the surface roughness levels of the other flow modes are several times that of the Π-shaped flow mode. Furthermore, the machining accuracy of the blisk hub was measured by a coordinate measuring machine (TESA Micro-Hite 3D) and the machining error of the hub was found to be less than 0.1 mm, which fits within the machining accuracy requirement, as Fig. 24 shows. The results show that with more uniform electrolyte flow, ECM by-products, such as hydroxide precipitation, heat and hydrogen will be carried away effectively, and the conductivity of the electrolyte in the inter-electrode gap will be more uniform; thus, the surface quality and the machining accuracy can be improved.

R a =0.150µm Hub

1.0µm

250µm


469

Machining error of hub(mm)

0.12

0.08

0.04

0 0

1

2 3 4 i th cascade passage

5

6

Fig. 24 The machining accuracy of the blisk hub of every cascade passage

5 Conclusion Electrolyte flow field is a critical factor that affects ECM process stability and precision. To improve the process stability and efficiency of blisk cascade passage ECM with radial feeding electrode, three flow field models were built separately on the bases of a traditional lateral flow pattern, a positive flow pattern and an Π-shaped flow pattern, as is proposed in this paper by means of a finite element fluid analysis method. The simulation result showed that the new Π-shaped flow pattern is most suitable for machining the blisk cascade passages and makes up for the disadvantages of the existing flow mode, such as non-uniform and random flow distribution. The experiments on cascade passage ECM with radial feeding electrode were carried out to evaluate the rationality of the designed flow field. The feeding velocity of the cathode with the Π-shaped flow mode is approximately 70 % higher than that of the other two flow modes. The surface roughness of the blisk hub is only 0.15 μm, and the machining error of the hub is less than 0.1 mm. These results demonstrate that, with the new flow pattern, the quality and efficiency of blisk cascade passage ECM can be enhanced. Acknowledgments This study was sponsored by the National Natural Science Foundation of China (51005119) and Jiangsu Natural Science Foundation (BK2010506).

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