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trailing edges by electrochemical machining with tangential feeding
CIRP Annals - Manufacturing Technology 68 (2019) 165–168
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CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Improving the accuracy of the blade leading/trailing edges by electrochemical machining with tangential feeding H. Wang, D. Zhu (1)*, J. Liu Research Centre for Non-traditional Machining, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China
A R T I C L E I N F O
Keywords: Electro chemical machining (ECM) Simulation Blade
A B S T R A C T
The demand of improving the accuracy of leading/trailing edges of aero engine blades has increased continually. This paper proposes a method of electrochemical machining with tangential feeding in which the leading/trailing edges are electrochemically processed by the cathode tools feeding along the tangential direction of the mean camber line of blades. The modelling and simulation on the ECM process have been carried out. A specific experiment system has been developed. Theoretical and experimental studies have proved that the proposed technology of tangential feeding offered unique advantages such as short electrolyte path, stable machining current and so achieved high machining accuracy. © 2019 Published by Elsevier Ltd on behalf of CIRP.
1. Introduction With the development of the aviation industry, the requirement of aero engine performance is higher than before. The machining quality of blades which are core components of aero engines can influence the performances of aero engines directly. Leading and trailing edges of blades are the places where airflow divides and meets respectively. A small profile difference of leading/trailing edges of blades will lead to great influence on aerodynamic performance [1]. Therefore the improvement of the machining accuracy of leading/trailing edges has great significance on the performance of aviation engine. Due to the long, twisty and small curvature structure of blade edges, vibration and deform usually occur during milling processing and it is very difficult to control the machining accuracy of leading/trailing edges. Occasionally, subsurface cracks are observed under the action of cutting heat [2]. Electrochemical machining (ECM) is an electrochemical dissolution process that can remove electrically conductive materials regardless of their hardness and toughness [3,4]. ECM plays an important role in turbine blade production. Experts and scholars have conducted detailed research on blade ECM [5]. By the optimization of electrolyte fluid field, the stability and machining efficiency of ECM can be increased [6]. The machining accuracy of blades can be improved by some special cathode design or the optimization of tool feeding path [7]. In order to better predict the blade forming evolution, the interdisciplinary modelling of the ECM process for engine blades has also been established [8]. In conventional ECM of blades, the basin/back surfaces of blade as well as leading/trailing edges are processed at the same time as two cathode tools move towards basin and back of blade respectively. In
* Corresponding author. E-mail address: [email protected] (D. Zhu). https://doi.org/10.1016/j.cirp.2019.04.107 0007-8506/© 2019 Published by Elsevier Ltd on behalf of CIRP.
this process, it is difficult to control the profile accuracy of leading/ trailing edges while the engine designers often have higher accuracy requirement of the leading/trailing edges than the basin/back of blades. The poor accuracy of blade edges in conventional ECM is attributed to the heat and bubbles generated in the machining gap and the so-called boundary effect of electric field at the blade edges. In the ECM process, the electrolyte flows from leading edge to trailing edge through the electrode gap between tool and workpiece. The bubble and heat will accumulate inside the long flow path especially in the electrode gap of trailing edge [9] and has a significant effect on the ECM process. Additionally, the current density at the blade edges will rise rapidly in the final period of the process step, and it is also detrimental to maintaining process stability and consistency. In order to improve machining accuracy of leading/trailing edges, a new ECM method of leading/trailing edges is proposed in this paper. In the proposed method, the leading/trailing edges will be electrochemically processed with cathode tools feeding along the tangential direction of the mean camber line of blades after the basin and back of blades are processed. More steady electric field and flow field can be expected in the tangential feeding process. Due to the shorter electrolyte path, the effect of temperature rise and bubble accumulation will be significantly reduced. In order to verify the feasibility of the proposed method, the simulation and experiment were carried out in this paper. The results indicated that tangential feeding ECM achieved better machining accuracy and process stability in the machining process of leading/trailing edges. 2. Principle of tangential feeding ECM Fig. 1 shows the sketch of conventional ECM of blades. In this process cathode tools feed towards basin and back of blade and the anode workpiece is gradually dissolved as the form of ions under electrochemical reaction. The basin/back of blades and leading/ trailing edges are processed at the same time. The current on the
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Under electrochemical action, the workpiece is dissolved into salt electrolyte at the rate of vn which obeys Faraday’s law:
nn ¼ hvi
Fig. 1. Sketch of conventional blade ECM.
basin/back of blades will be constant after a period of time. Unfortunately the electric field will change dramatically at leading/ trailing edges and it is difficult to keep a stable state at the leading/ trailing edges. In addition, due to the long flow path of electrolyte which ranges from 30 mm to 50 mm, the heat and bubbles generated in ECM process will accumulate obviously from leading edge to trailing edge and dramatically affect the electric field and therefore the gap distribution. In the proposed tangential feeding ECM the leading/trailing edges are machined after the blade basin and back are processed by conventional blade ECM, as shown in Fig. 2. As shown in the Fig. 2 (b), the cathode tools connect to negative pole of power supply and the workpiece to positive pole. During machining process the anode workpiece remains static while cathode tool moves towards leading/trailing edge along the tangential direction of the mean camber line of blades while electrolyte flows from one side to another side of leading/trailing edges at a higher speed. In this process the heat and bubbles as well as other electrolytic products generated in the inter-electrode gap are quickly taken away by electrolyte flushing due to short electrolyte path. After a short period of time, usually less than 2 min, the distribution of the machining gap remains almost unchanged and the electric and flow fields in the gap will keep in a steady state. In this way, the shapes of leading/trailing edges are obtained.
ð2Þ
where, h the electrolytic processing efficiency, v the electrochemical equivalent of volume of anode material and i the current density on the surface of the workpiece. During the simulation, the entire ECM period is divided as a number of tiny time intervals Dt. At each Dt, the electric field distribution will be calculated first by Eq. (1). Then the slight displacement Dd along the normal direction of anode surface will be calculated by the Eqs. (2) and (3):
Dd ¼ v n Dt
ð3Þ
The new contour obtained will be taken to the next iteration to repeat the above calculation. After a certain number of iterative calculations, the final contour of the anode workpiece can be obtained. In this study, more attentions are given to the blade edges instead of blade basin and back and therefore initial workpiece is simplified to a plate of 1 mm thickness, as shown in Fig. 3. The simulation times of conventional blade ECM and tangential feeding ECM are set as 80 s and 120 s respectively. The process parameters and conditions are given in Table 1.
Fig. 3. Schematic of models used in simulation. Table 1 Simulation condition. Condition
Value
Electrolyte conductivity k Cathodes feeding velocity v Workpiece material Machining voltage U
14.012S/m 1 mm/min 304SS 10 V
Fig. 4 shows simulation results on the contour evolution of the workpieces during the conventional blade ECM and tangential feeding ECM. In simulations of conventional blade ECM, the contours of leading/trailing edges and basin/back surface of blade
Fig. 2. Sketch of tangential feeding ECM.
3. Simulations of blade ECM processes 3.1. Simulations of forming processes of leading/trailing edges During the ECM process, cathode tool moves towards anode workpiece at a constant speed v, and machining voltage U is applied between the tool and workpiece. The electrochemical dissolution process is dominated by the electric field which is described by Laplace Equation: r2 f ¼ 0
ð1Þ
Fig. 4. Contour evolution of workpiece at leading/trailing edges: (a) conventional blade ECM; (b) tangential feeding ECM.
H. Wang et al. / CIRP Annals - Manufacturing Technology 68 (2019) 165–168
change with the tool feeding. Usually total machining time is determined by the allowance at basin/back of blade rather than the allowance at leading/trailing edges. This means that it is very difficult to control the machining accuracy of the leading/trailing edges and the basin/back of blades at the same time in conventional ECM of blades. Differently, it can be seen from Fig. 4(b) that only the workpiece material at the leading/trailing edges is removed during the tangential feeding ECM while basin/ back surface of blade remains unchanged. After enough time, the shape of the workpiece tip will be infinitely close to the required profile of blade edges. This means that the accuracy of blade edge can be controlled in the way of tangential feeding ECM. Fig. 5 shows the current density evolutions at the tips of leading/trailing edges during the processing. The current density at the leading/trailing edges in the conventional blade ECM rises from 9.83 A/cm2 at the beginning to 95.53 A/cm2 at 80 s and there is an obvious tendency to continue to increase. The current density in the tangential feeding ECM increases from 25.75 A/cm2 at the beginning to 76.82 A/cm2 at 80 s and then keeps almost constant. It means that tangential feeding ECM can achieve better process stability and so higher repeat accuracy.
167
The distribution of hydrogen bubbles in machining gap is shown in Fig. 6. The hydrogen bubble contents of both processing methods increase from the inlet side to the outlet side. The volume fraction of hydrogen bubble at outlet side in conventional blade ECM is 0.3428. In comparison, the bubble volume fraction at outlet side in the tangential feeding ECM is only 0.009.
Fig. 6. Bubbles distribution.
Fig. 7 shows the temperature distribution in two machining processes. The temperature increases by 10.96 C from the side of inlet to the side of outlet in conventional blade ECM and only by 0.244 C in the tangential feeding ECM.
Fig. 7. Temperature distribution.
The effects of the temperature rise and bubble accumulation on the electrolyte conductivity and then gap distribution are expressed in Eq. (4). The distributions of electrolyte conductivity are shown in Fig. 8. The conductivity changes by 4.836S/m in conventional blade ECM while by 0.101S/m in tangential feeding ECM.
Fig. 5. Current density evolution at leading/trailing edges.
3.2. Simulations on distributions of bubbles and temperature Bubbles and temperature are important factors affecting ECM process. Electrolyte conductivity k which is described by Thorpe et al. [10] is affected by hydrogen bubble volume fraction bg and temperature T: Fig. 8. Electrolyte conductivity distribution.
n k ¼ k0 ð1 þ jðT T 0 ÞÞ 1 bg
ð4Þ
where, j is the temperature coefficient determined by electrolyte, n is the bubble influence coefficient, k0 is the conductivity of the electrolyte at inlet and T0 is the electrolyte temperature at inlet. The simulations have been carried out to demonstrate the distribution of bubble and temperature in the two machining processes. The simulation parameters of both two processes are listed in Table 2. Table 2 Simulation condition. Condition
Value
Electrolyte Electrolyte temperature at inlet T0 Electrolyte pressure at inlet P0 Electrolyte pressure at outlet P1 Machining voltage U Temperature coefficient j Bubble influence coefficient n
228 g/L NaNO3 20 C 0.6 Mpa 0.1 Mpa 10 V 0.2911S/(m C) 1.5
In order to quantitatively analyze the distribution of bubbles, temperature and electrolyte conductivity at inlet side and outlet side of interelectrode gap, the section lines at inlet side and outlet side in both two ECM processes are selected for analysis, as shown in Fig. 3.
To sum up, the bubble accumulation and temperature rise in the tangential feeding ECM is much less than in conventional method and negligible to the ECM gap distribution. In the above simulation, the same process parameters are used in two methods and the final machining gap is 0.17 mm. According to simulation, bubble accumulation and temperature rise in conventional machining are too serious to keep stable machining process. This has been proved experimentally. Actually, the machining gap of 0.5 mm has to be used in the following experiments of conventional ECM process for stable machining process. Therefore, the simulation on the 0.5 mm inter-electrode gap with machining voltage of 30 V has been carried out. The simulation results on bubble accumulation, temperature rise and conductivity rise are shown in the Fig. 9 in which upper results are
Fig. 9. Simulation on conventional ECM with 0.5 mm gap.
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H. Wang et al. / CIRP Annals - Manufacturing Technology 68 (2019) 165–168
inlet and lower results are outlet. The volume fraction at trailing edge is 0.03, and the electrolyte temperature rise is 9.2 C. The electrolyte conductivity increases by 1.934S/m. Compared to the tangential feeding ECM, the accumulation of bubbles and temperature rise in the conventional blade ECM with large machining gap of 0.5 mm are still serious.
4. Experimental Fig. 10 shows schematically the experimental system for both conventional blade ECM and tangential feeding ECM. In conventional blade ECM, two cathode tools move towards the basin and back of the blade at a constant speed, respectively. The thickness of the anode workpiece is 3 mm. In order to verify the simulation results, the machining voltages of 10 V and 30 V in conventional blade ECM have been carried out. During tangential feeding ECM, the cathode tool feeds along the tangential direction of the mean camber line. The thickness of the anode workpiece is 1 mm, and the machining voltage is 10 V. In both two experiments, the materials of the tool and workpiece are 304SS. The other experimental parameters are consistent with the simulation (shown in Tables 1 and 2).
Fig. 12. Deviations of workpieces at leading/trailing edges: (a) location of measured section; (b) leading edges of conventional blade ECM; (c) trailing edges of conventional blade ECM; (d) leading/trailing edges of tangential feeding ECM.
conventional blade ECM is 0.020 mm and 0.017 mm. As shown in Fig. 12(c), the maximum positive deviation and negative deviation at trailing edges are 0.025 mm and 0.032 mm. Comparatively, the maximum positive deviation and negative deviation of edges processed by tangential feeding ECM are only 0.0076 mm and 0.0065 mm. The machining accuracy of leading/ trailing edges has been improved obviously. 5. Summary A method in which the leading/trailing edges are electrochemically processed by the cathode tools feeding along the tangential direction of the mean camber line of blades is presented in this paper. Due to the short flow path and stable machining current in the proposed method, the gap distribution is less affected by bubbles and temperature rise. High machining accuracy has been achieved in machining experiments.
Fig. 10. Experimental system.
Acknowledgments The abnormal short circuit sparks were observed in conventional blade ECM when the machining voltage was 10 V, 0.17 mm gap. After that 30 V and 0.5 mm gap were used in conventional machining. The both experiments with 30 V machining voltage in conventional blade ECM and 10 V in tangential feeding ECM have carried out 5 times. Good surface quality was obtained in all of 5 samples in each machining method and the maximum Ra of workpieces was 0.177 mm. Fig. 11 shows the specimens, left by conventional ECM and right by tangential ECM. The specimens were measured by a coordinate measuring machine. A standard edge profile was obtained by averaging five contours of the machined edges. By comparing each edge profile with the standard profile, the deviation distribution of each edge was obtained. The deviations of 5 edges at the section near the root of blade are shown in Fig. 12. It can be seen from Fig. 12(b) that the maximum positive deviation and negative deviation at leading edges of specimens manufactured by
Fig. 11. Specimens processed by ECM.
This work was supported by the National Natural Science Foundation of China (no. 51535006).
References [1] Sauer H, MüLler R, Vogeler K (2001) Reduction of Secondary Flow Losses in Turbine Cascades by Leading Edge Modifications at the Endwall. Journal of Turbomachinery-Transactions ASME 123(2):207–213. [2] Aspinwall DK, Dewes RC, Mantle AL (2005) The Machining of g-TiAl Tntermetallic Alloys. CIRP Annals - Manufacturing Technology 54(1):99–104. [3] Rajurkar KP, Zhu D, Mcgeough JA, Kozak J, Silva AD (1999) New Developments in Electro-Chemical Machining. CIRP Annals - Manufacturing Technology 48 (2):567–579. [4] Kunieda M, Mizugai K, Watanabe S, Shibuya N, Iwamoto N (2011) Electrochemical Micromachining Using Flat Electrolyte jet. CIRP Annals - Manufacturing Technology 60(1):251–254. [5] Klocke F, Klink A, Veselovac D, Aspinwall DK, Soo SL, Schmidt M, Schilp J, Levy G, Kruth JP (2014) Turbomachinery Component Manufacture by Application of Electrochemical, Electro-Physical and Photonic Processes. CIRP Annals Manufacturing Technology 63(2):703–726. [6] Koyano T, Hosokawa A, Igusa R, Ueda T (2017) Electrochemical Machining Using Porous Electrodes Fabricated by Powder Bed Fusion Additive Manufacturing Process. CIRP Annals - Manufacturing Technology 66(1):213–216. [7] Xu ZY, Xu Q, Zhu D, Gong T (2013) A High Efficiency Electrochemical Machining Method of Blisk Channels. CIRP Annals - Manufacturing Technology 62(1):187–190. [8] Klocke F, Zeis M, Klink A (2015) Interdisciplinary Modelling of the Electrochemical Machining Process for Engine Blades. CIRP Annals - Manufacturing Technology 64(1):217–220. [9] Klocke F, Zeis M, Herrig T, Harst S, Klink A (2014) Optical In Situ Measurements and Interdisciplinary Modeling of the Electrochemical Sinking Process of Inconel 718. Procedia CIRP 24:114–119. [10] Thorpe JF, Zerkle RD (1969) Analytic Determination of the Equilibrium Electrode Gap in Electrochemical Machining. International Journal of Machine Tool Design and Research 9(2):131–144.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Improving the accuracy of the blade leading/trailing edges by electrochemical machining with tangential feeding H. Wang, D. Zhu (1)*, J. Liu Research Centre for Non-traditional Machining, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China
A R T I C L E I N F O
Keywords: Electro chemical machining (ECM) Simulation Blade
A B S T R A C T
The demand of improving the accuracy of leading/trailing edges of aero engine blades has increased continually. This paper proposes a method of electrochemical machining with tangential feeding in which the leading/trailing edges are electrochemically processed by the cathode tools feeding along the tangential direction of the mean camber line of blades. The modelling and simulation on the ECM process have been carried out. A specific experiment system has been developed. Theoretical and experimental studies have proved that the proposed technology of tangential feeding offered unique advantages such as short electrolyte path, stable machining current and so achieved high machining accuracy. © 2019 Published by Elsevier Ltd on behalf of CIRP.
1. Introduction With the development of the aviation industry, the requirement of aero engine performance is higher than before. The machining quality of blades which are core components of aero engines can influence the performances of aero engines directly. Leading and trailing edges of blades are the places where airflow divides and meets respectively. A small profile difference of leading/trailing edges of blades will lead to great influence on aerodynamic performance [1]. Therefore the improvement of the machining accuracy of leading/trailing edges has great significance on the performance of aviation engine. Due to the long, twisty and small curvature structure of blade edges, vibration and deform usually occur during milling processing and it is very difficult to control the machining accuracy of leading/trailing edges. Occasionally, subsurface cracks are observed under the action of cutting heat [2]. Electrochemical machining (ECM) is an electrochemical dissolution process that can remove electrically conductive materials regardless of their hardness and toughness [3,4]. ECM plays an important role in turbine blade production. Experts and scholars have conducted detailed research on blade ECM [5]. By the optimization of electrolyte fluid field, the stability and machining efficiency of ECM can be increased [6]. The machining accuracy of blades can be improved by some special cathode design or the optimization of tool feeding path [7]. In order to better predict the blade forming evolution, the interdisciplinary modelling of the ECM process for engine blades has also been established [8]. In conventional ECM of blades, the basin/back surfaces of blade as well as leading/trailing edges are processed at the same time as two cathode tools move towards basin and back of blade respectively. In
* Corresponding author. E-mail address: [email protected] (D. Zhu). https://doi.org/10.1016/j.cirp.2019.04.107 0007-8506/© 2019 Published by Elsevier Ltd on behalf of CIRP.
this process, it is difficult to control the profile accuracy of leading/ trailing edges while the engine designers often have higher accuracy requirement of the leading/trailing edges than the basin/back of blades. The poor accuracy of blade edges in conventional ECM is attributed to the heat and bubbles generated in the machining gap and the so-called boundary effect of electric field at the blade edges. In the ECM process, the electrolyte flows from leading edge to trailing edge through the electrode gap between tool and workpiece. The bubble and heat will accumulate inside the long flow path especially in the electrode gap of trailing edge [9] and has a significant effect on the ECM process. Additionally, the current density at the blade edges will rise rapidly in the final period of the process step, and it is also detrimental to maintaining process stability and consistency. In order to improve machining accuracy of leading/trailing edges, a new ECM method of leading/trailing edges is proposed in this paper. In the proposed method, the leading/trailing edges will be electrochemically processed with cathode tools feeding along the tangential direction of the mean camber line of blades after the basin and back of blades are processed. More steady electric field and flow field can be expected in the tangential feeding process. Due to the shorter electrolyte path, the effect of temperature rise and bubble accumulation will be significantly reduced. In order to verify the feasibility of the proposed method, the simulation and experiment were carried out in this paper. The results indicated that tangential feeding ECM achieved better machining accuracy and process stability in the machining process of leading/trailing edges. 2. Principle of tangential feeding ECM Fig. 1 shows the sketch of conventional ECM of blades. In this process cathode tools feed towards basin and back of blade and the anode workpiece is gradually dissolved as the form of ions under electrochemical reaction. The basin/back of blades and leading/ trailing edges are processed at the same time. The current on the
166
H. Wang et al. / CIRP Annals - Manufacturing Technology 68 (2019) 165–168
Under electrochemical action, the workpiece is dissolved into salt electrolyte at the rate of vn which obeys Faraday’s law:
nn ¼ hvi
Fig. 1. Sketch of conventional blade ECM.
basin/back of blades will be constant after a period of time. Unfortunately the electric field will change dramatically at leading/ trailing edges and it is difficult to keep a stable state at the leading/ trailing edges. In addition, due to the long flow path of electrolyte which ranges from 30 mm to 50 mm, the heat and bubbles generated in ECM process will accumulate obviously from leading edge to trailing edge and dramatically affect the electric field and therefore the gap distribution. In the proposed tangential feeding ECM the leading/trailing edges are machined after the blade basin and back are processed by conventional blade ECM, as shown in Fig. 2. As shown in the Fig. 2 (b), the cathode tools connect to negative pole of power supply and the workpiece to positive pole. During machining process the anode workpiece remains static while cathode tool moves towards leading/trailing edge along the tangential direction of the mean camber line of blades while electrolyte flows from one side to another side of leading/trailing edges at a higher speed. In this process the heat and bubbles as well as other electrolytic products generated in the inter-electrode gap are quickly taken away by electrolyte flushing due to short electrolyte path. After a short period of time, usually less than 2 min, the distribution of the machining gap remains almost unchanged and the electric and flow fields in the gap will keep in a steady state. In this way, the shapes of leading/trailing edges are obtained.
ð2Þ
where, h the electrolytic processing efficiency, v the electrochemical equivalent of volume of anode material and i the current density on the surface of the workpiece. During the simulation, the entire ECM period is divided as a number of tiny time intervals Dt. At each Dt, the electric field distribution will be calculated first by Eq. (1). Then the slight displacement Dd along the normal direction of anode surface will be calculated by the Eqs. (2) and (3):
Dd ¼ v n Dt
ð3Þ
The new contour obtained will be taken to the next iteration to repeat the above calculation. After a certain number of iterative calculations, the final contour of the anode workpiece can be obtained. In this study, more attentions are given to the blade edges instead of blade basin and back and therefore initial workpiece is simplified to a plate of 1 mm thickness, as shown in Fig. 3. The simulation times of conventional blade ECM and tangential feeding ECM are set as 80 s and 120 s respectively. The process parameters and conditions are given in Table 1.
Fig. 3. Schematic of models used in simulation. Table 1 Simulation condition. Condition
Value
Electrolyte conductivity k Cathodes feeding velocity v Workpiece material Machining voltage U
14.012S/m 1 mm/min 304SS 10 V
Fig. 4 shows simulation results on the contour evolution of the workpieces during the conventional blade ECM and tangential feeding ECM. In simulations of conventional blade ECM, the contours of leading/trailing edges and basin/back surface of blade
Fig. 2. Sketch of tangential feeding ECM.
3. Simulations of blade ECM processes 3.1. Simulations of forming processes of leading/trailing edges During the ECM process, cathode tool moves towards anode workpiece at a constant speed v, and machining voltage U is applied between the tool and workpiece. The electrochemical dissolution process is dominated by the electric field which is described by Laplace Equation: r2 f ¼ 0
ð1Þ
Fig. 4. Contour evolution of workpiece at leading/trailing edges: (a) conventional blade ECM; (b) tangential feeding ECM.
H. Wang et al. / CIRP Annals - Manufacturing Technology 68 (2019) 165–168
change with the tool feeding. Usually total machining time is determined by the allowance at basin/back of blade rather than the allowance at leading/trailing edges. This means that it is very difficult to control the machining accuracy of the leading/trailing edges and the basin/back of blades at the same time in conventional ECM of blades. Differently, it can be seen from Fig. 4(b) that only the workpiece material at the leading/trailing edges is removed during the tangential feeding ECM while basin/ back surface of blade remains unchanged. After enough time, the shape of the workpiece tip will be infinitely close to the required profile of blade edges. This means that the accuracy of blade edge can be controlled in the way of tangential feeding ECM. Fig. 5 shows the current density evolutions at the tips of leading/trailing edges during the processing. The current density at the leading/trailing edges in the conventional blade ECM rises from 9.83 A/cm2 at the beginning to 95.53 A/cm2 at 80 s and there is an obvious tendency to continue to increase. The current density in the tangential feeding ECM increases from 25.75 A/cm2 at the beginning to 76.82 A/cm2 at 80 s and then keeps almost constant. It means that tangential feeding ECM can achieve better process stability and so higher repeat accuracy.
167
The distribution of hydrogen bubbles in machining gap is shown in Fig. 6. The hydrogen bubble contents of both processing methods increase from the inlet side to the outlet side. The volume fraction of hydrogen bubble at outlet side in conventional blade ECM is 0.3428. In comparison, the bubble volume fraction at outlet side in the tangential feeding ECM is only 0.009.
Fig. 6. Bubbles distribution.
Fig. 7 shows the temperature distribution in two machining processes. The temperature increases by 10.96 C from the side of inlet to the side of outlet in conventional blade ECM and only by 0.244 C in the tangential feeding ECM.
Fig. 7. Temperature distribution.
The effects of the temperature rise and bubble accumulation on the electrolyte conductivity and then gap distribution are expressed in Eq. (4). The distributions of electrolyte conductivity are shown in Fig. 8. The conductivity changes by 4.836S/m in conventional blade ECM while by 0.101S/m in tangential feeding ECM.
Fig. 5. Current density evolution at leading/trailing edges.
3.2. Simulations on distributions of bubbles and temperature Bubbles and temperature are important factors affecting ECM process. Electrolyte conductivity k which is described by Thorpe et al. [10] is affected by hydrogen bubble volume fraction bg and temperature T: Fig. 8. Electrolyte conductivity distribution.
n k ¼ k0 ð1 þ jðT T 0 ÞÞ 1 bg
ð4Þ
where, j is the temperature coefficient determined by electrolyte, n is the bubble influence coefficient, k0 is the conductivity of the electrolyte at inlet and T0 is the electrolyte temperature at inlet. The simulations have been carried out to demonstrate the distribution of bubble and temperature in the two machining processes. The simulation parameters of both two processes are listed in Table 2. Table 2 Simulation condition. Condition
Value
Electrolyte Electrolyte temperature at inlet T0 Electrolyte pressure at inlet P0 Electrolyte pressure at outlet P1 Machining voltage U Temperature coefficient j Bubble influence coefficient n
228 g/L NaNO3 20 C 0.6 Mpa 0.1 Mpa 10 V 0.2911S/(m C) 1.5
In order to quantitatively analyze the distribution of bubbles, temperature and electrolyte conductivity at inlet side and outlet side of interelectrode gap, the section lines at inlet side and outlet side in both two ECM processes are selected for analysis, as shown in Fig. 3.
To sum up, the bubble accumulation and temperature rise in the tangential feeding ECM is much less than in conventional method and negligible to the ECM gap distribution. In the above simulation, the same process parameters are used in two methods and the final machining gap is 0.17 mm. According to simulation, bubble accumulation and temperature rise in conventional machining are too serious to keep stable machining process. This has been proved experimentally. Actually, the machining gap of 0.5 mm has to be used in the following experiments of conventional ECM process for stable machining process. Therefore, the simulation on the 0.5 mm inter-electrode gap with machining voltage of 30 V has been carried out. The simulation results on bubble accumulation, temperature rise and conductivity rise are shown in the Fig. 9 in which upper results are
Fig. 9. Simulation on conventional ECM with 0.5 mm gap.
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H. Wang et al. / CIRP Annals - Manufacturing Technology 68 (2019) 165–168
inlet and lower results are outlet. The volume fraction at trailing edge is 0.03, and the electrolyte temperature rise is 9.2 C. The electrolyte conductivity increases by 1.934S/m. Compared to the tangential feeding ECM, the accumulation of bubbles and temperature rise in the conventional blade ECM with large machining gap of 0.5 mm are still serious.
4. Experimental Fig. 10 shows schematically the experimental system for both conventional blade ECM and tangential feeding ECM. In conventional blade ECM, two cathode tools move towards the basin and back of the blade at a constant speed, respectively. The thickness of the anode workpiece is 3 mm. In order to verify the simulation results, the machining voltages of 10 V and 30 V in conventional blade ECM have been carried out. During tangential feeding ECM, the cathode tool feeds along the tangential direction of the mean camber line. The thickness of the anode workpiece is 1 mm, and the machining voltage is 10 V. In both two experiments, the materials of the tool and workpiece are 304SS. The other experimental parameters are consistent with the simulation (shown in Tables 1 and 2).
Fig. 12. Deviations of workpieces at leading/trailing edges: (a) location of measured section; (b) leading edges of conventional blade ECM; (c) trailing edges of conventional blade ECM; (d) leading/trailing edges of tangential feeding ECM.
conventional blade ECM is 0.020 mm and 0.017 mm. As shown in Fig. 12(c), the maximum positive deviation and negative deviation at trailing edges are 0.025 mm and 0.032 mm. Comparatively, the maximum positive deviation and negative deviation of edges processed by tangential feeding ECM are only 0.0076 mm and 0.0065 mm. The machining accuracy of leading/ trailing edges has been improved obviously. 5. Summary A method in which the leading/trailing edges are electrochemically processed by the cathode tools feeding along the tangential direction of the mean camber line of blades is presented in this paper. Due to the short flow path and stable machining current in the proposed method, the gap distribution is less affected by bubbles and temperature rise. High machining accuracy has been achieved in machining experiments.
Fig. 10. Experimental system.
Acknowledgments The abnormal short circuit sparks were observed in conventional blade ECM when the machining voltage was 10 V, 0.17 mm gap. After that 30 V and 0.5 mm gap were used in conventional machining. The both experiments with 30 V machining voltage in conventional blade ECM and 10 V in tangential feeding ECM have carried out 5 times. Good surface quality was obtained in all of 5 samples in each machining method and the maximum Ra of workpieces was 0.177 mm. Fig. 11 shows the specimens, left by conventional ECM and right by tangential ECM. The specimens were measured by a coordinate measuring machine. A standard edge profile was obtained by averaging five contours of the machined edges. By comparing each edge profile with the standard profile, the deviation distribution of each edge was obtained. The deviations of 5 edges at the section near the root of blade are shown in Fig. 12. It can be seen from Fig. 12(b) that the maximum positive deviation and negative deviation at leading edges of specimens manufactured by
Fig. 11. Specimens processed by ECM.
This work was supported by the National Natural Science Foundation of China (no. 51535006).
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