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Electrochemical Machining of γ-TiAl Intermetallic Blades by Using the Stainless Steel Anti-copied Tool Electrodes
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 757 – 761
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Electrochemical machining of γ-TiAl intermetallic blades by using the stainless steel anti-copied tool electrodes Jia Liua*, Hao Wanga, Di Zhua a Nanjing University of Aeronautics & Astronautics, 29 Yu Dao Jie Street, Nanjing, 210016, China * Corresponding author. Tel.: +86-025-8489-6837 ; fax:+86-025-8489-6612.E-mail address:[email protected]
Abstract Electrochemical machining (ECM) is an important technology to process γ-TiAl intermetallic components such as aero-engine blades. The anticopy method is an efficient ECM technology to process workpiece without design cathode tools. By using the transfer effect of two opposite ECM stages, the workpiece can be processed by the standard sample directly. In ordinary anti-copy method, the processing accuracy is guaranteed by controlling the consistency of processing conditions in two ECM stages. The anode material of anti-copy tool and workpiece must be consistent. This limits the generality of anti-copy method. In order to eliminate the anode material restriction, an generic ECM anticopy method with machining gaps control was presented. The processing dynamic simulation analysis was carried out. The simulation result shows that the optimum machining gap in copy workpiece stage with minimum profile different is near the machining gap in anti-copy tool stage. Through the feeding rate adjustment to control the optimum machining gap, the γ-TiAl blades were processed by using the stainless steel anti-copied tools. The test specimens have good quality and high accuracy. The experimental results proved the feasibility of the proposed method. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ©2018 2018The The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Electrochemical machining; γ-TiAl; blade; anti-copy; experiment;
1. Introduction Gamma titanium aluminium (γ-TiAl) intermetallic offer low density, high stiffness, flame resistance and strength at a wide range temperature. Such characteristics make γ-TiAl intermetallic an important material in aero-engine blades as the replacement of heavier nickel based alloys[1,2]. However, owing to its low temperature ductility and fracture toughness, γ-TiAl intermetallic is extremely difficult to process by traditional mechanical technologies such as turning, grinding and high speed milling[3,4]. Electrochemical machining (ECM) is a nontraditional process to remove material via a controlled electrochemical anodic reaction. It has such advantages as no electrode wear, good surface quality, regardless of the material hardness and ductility. Compared with the traditional mechanical technologies, ECM has significant advantages in efficiency, cost and quality[5,6]. It has become one of the important processing technologies for
γ-TiAl blades manufacturing in high thrust-weight ratio aeroengines. In ECM, the most time-consuming work in the production preparation cycle is the design and correction cathode tools. The anti-copy method is an efficient ECM technology to process workpiece without have to design cathode tools. By using the transfer effect of two opposite ECM stages, the workpiece can be processed by the standard sample directly. Due to saving the cathode design and correction cycles, the anti-copy method has significant efficiency advantage and been widely used in aero-engines blades manufacturing. In ordinary anti-copy method, the processing accuracy is guaranteed by controlling the consistency of processing conditions in two ECM stages. The anode material, electrolyte system and parameters in the two stages must be consistent. If processing of γ-TiAl blades, the materials of anti-copy tool electrodes and workpiece must be γ-TiAl intermetallic. The pool machinability and expensive material price of γ-TiAl will increase the processing cycle and cost obviously. The
2212-8271 © 2018 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 scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.133
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Jia Liu et al. / Procedia CIRP 68 (2018) 757 – 761
restrictions of anode materials have affected the application generality of anti-copy method. In order to eliminate the anode material restriction, an generic ECM anti-copy method with optimum machining gap control was presented. Assuming that the materials of anticopy tool and workpiece are stainless steel and γ-TiAl intermetallic respectively. The finite element simulation of workpiece dynamic formation process was carried out to reveal the machining gaps relation between anti-copy tool stage and copy workpiece stage. Based on the simulation results, the experiments were carried out to evaluate the feasibility of the proposed method. Nomenclature Δb η ω κ i UR U vc va
Equilibrium machining gap Current efficiency Volume electrochemical equivalent Electrolyte conductivity Current density Electrode potential difference Applied voltage Feed rate of cathode Normal corrosion rate of anode
2. Principle of ECM anti-copy method 2.1. Ordinary anti-copy method The anti-copy method includes two opposite ECM stages, anti-copy tool and copy workpiece. In first stage of anti-copy tool, the standard sample connected to power supply cathode, the two anti-copy tool electrodes connected to power supply anode. High pressure electrolyte was pumped into the interelectrode gap. Under the effect of electrochemical reaction, the material of anti-copy tool electrodes was dissolved into the electrolyte. With the anti-copy tool electrodes continuous feeding toward standard sample, the shapes of anti-copy tool electrodes were similar to the standard sample. In second stage of copy workpiece, the two anti-copied tool electrodes connected to power supply cathode, the workpiece connected to power supply anode. With the anti-copy tool electrodes continuous feeding, the workpiece can be machined without tool design. The schematic of ECM anti-copy method is shown in Fig. 1. By using two opposite ECM stages, the shape of the standard sample will transfer to the workpiece. Without tool design, the workpiece can be machined by standard sample directly. If the machining gap distributions of two opposite ECM stages are identical, the shape of sample will be perfect copied to the workpiece. The same processing conditions will lead to similar machining gaps. So in ordinary anti-copy method, the processing accuracy is guaranteed by controlling the consistency of processing conditions in two ECM stages. The anode material of anti-copy tool and workpiece must be
the same. The electrolyte system and machining parameters in two ECM stages are also to be consistent. Anti-copy tools
Stage 1 Anti-copy tool - + Standard sample
Anti-copied tools
Anti-copied tools
Stage 2 Copy workpiece + Workpiece
Processed specimen
Fig. 1. The schematic of ECM anti-copy method.
However the processes of the two opposite ECM stages are not reversible. Even if the processing conditions in two ECM stages are consistent, many factors such as opposite electric field and opposite electrolytic product distribution are still lead to change the machining gaps. The differences of machining gap in anti-copy tools and copy workpiece are inevitable exist. So instead of processing status control, through the machining parameters adjustment to control the machining gap distributions might be the effective measure to ensure the anti-copy precision. The anode materials of anticopy tool electrode and workpiece can be different. 2.2. Proposed anti-copy method In ECM, the equilibrium machining gap between the tool and workpiece is an important factors influencing the machining gap distribution. Based on the theory of Cosθ system, the equilibrium machining gap Δb can be calculated by the Equation (1). It is find that the feeding rate vc is inversely proportional to the equilibrium machining gap Δb. So the machining gap distribution can be controlled by adjusting the feed rate.
Δb =
ηωκU R vc
(1)
Based on the above analysis, the new anti-copy mode with optimum machining gap control was presented. Assuming that the tool and workpiece materials are different. The two opposite ECM stages will come into two different machining gaps, Δb1 and Δb2. If set the first stage machining gap Δb1 as the standard machining gap, there may be a optimum machining gap Δbidea to make the machining precision of anticopy method acceptable. By using feeding rate adjustment, the second stage machining gap Δb2 can be controlled equal to Δbidea. In order to find the relationship between Δb1 and Δbidea, the finite element simulation of workpiece dynamic formation process was carried out. The materials of the anti-copy tool electrode and workpiece were assumed to be stainless steel 1Cr-18Ni-9Ti (at.%) and γ-TiAl intermetallic Ti-48Al-2Nb2Cr (at.%) respectively
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3. Simulation of proposed ECM anti-copy method
The anti-copy method consists of two opposite ECM stages. The two stages used the same electrolyte flow model and the two machining gaps had similar geometry. In order to simplify the simulation model, it is assumed that the influence of flow factors in two stages can be counteracted. The differences of electrolyte flow field, electrolyte temperature field and electrochemical product distribution in two stages are neglected. The simplified simulation model is mainly based on the electric field and obeys the following assumptions: (1) The potential inside model conforms to the Laplasse equation. (2) The current density distribution on the anode surface is dominated by ohmic effects. The finite element dynamic simulation is based on the software COMSOL. 3.1. dynamic formation process in anti-copy tool stage The simulation mashed modal of first stage of anti-copy tool are shown in Fig. 2. This model was designed based on the convex surface of typical aero-engine blades. The processing parameters in simulation were determined according to the previous research of ECM stainless steel. The standard sample surface was a feeding cathode equipotential surface. The cathode potential was φc = 0 V. The anti-copy tool surface was a corroded anode equipotential surface. The anode potential was φa= 20 V. The other surfaces of the model were fixed surfaces and the normal potential difference were ∂φ/ ∂n = 0 V.
Fig. 3. (a) The anode dynamic formation result; (b) The calculated anti-copy tool surface.
3.2. dynamic formation process in copy workpiece stage The simulation mashed modal of copy workpiece stage is shown in Fig. 4. The processing parameters in simulation were determined according to the previous research of ECM γ-TiAl intermetallic. The simulation process in copy workpiece was consistent with the anti-copy tool. The cathode potential of anti-copy tool surface was φc = 0 V. The anode potential of workpiece surface was φa= 30 V. The value of ηω is according to the actual measured ηω-i curve of γ-TiAl intermetallic. It is shown in Fig. 5. The value of κ is the conductivity of 10% w/v sodium chloride solution (NaCl) at 40 . Electrolyte outlet
Workpiece surface
Electrolyte outlet
Anti-coped tool surface
Anti-copy tool surface
Electrolyte inlet Standard sample surface
Fig. 4. The mashed modal of coping workpiece stage. Electrolyte inlet
Fig. 2. The mashed modal of anti-coping tool stage.
Select a point on the anti-copy tool surface and mark it Px. The normal corrosion rate at Px is up to Equation (2). In the Equation (2), the value of ηω is according to the ηω-i curve of stainless steel[7]. The value of κ is the conductivity of 20% w/v sodium nitrate solution (NaNO3) at 30 . The current density ix is derived from transient electric field simulation result.
v px = ηωκ i px
CurRent efficiency ηω/mm3(Amin)-1
1 0.8 0.6 0.4 0.2 0 0
10
20
30
40
50
60
70
Current density i/Acm-2
Fig. 5. The ηω-i curve of γ-TiAl intermetallic.
(2)
In order to facilitate analysis, the equilibrium machining gap Δb1 in anti-copy tool stage was set to 0.3 mm. The feed rate was adjusted to meet this standard machining gap. The anode dynamic formation result and calculated anti-copy tool surface are shown in Figure 3 (a) and (b).
In order to search for the optimal machining gap Δbidea, the rough search from 0.2mm to 0.5mm with step of 0.1mm was carried out. The feed rate was adjusted to meet the assumed machining gaps. After calculations, four machined workpiece surfaces were obtained. The four results were compared with the standard sample surface. The profile difference of different
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Jia Liu et al. / Procedia CIRP 68 (2018) 757 – 761
machining gaps are shown in Fig. 6. The difference distributions are show in Fig. 7.
4. Experiments
Profile differences /mm
4.1. Experimental equipments
Machining gap b/mm
Fig. 6. The profile differences of different in rough search.
Profile error 0.2
0.15
0.15
0.1
0.1
0.05
0.05
[mm]
Profile error 0.2
0
0
[mm]
Profile error 0.2
Profile error 0.2 0.15
0.15 0.1
0.1
0.05
0.05
[mm]
The developed experimental equipments consists of feeding system, standard sample, anti-copy tool electrodes, test setup, electrolyte cell, electrolyte circulation system and power supply, as shown in Fig. 9. The anti-copy tool and copy workpiece stages used the same test setup. Fig. 10 shows the details of test setup. The materials of the anti-copy tool electrodes and workpiece were stainless steel 1Cr-18Ni-9Ti (at.%) and γ-TiAl intermetallic Ti-48Al-2Nb-2Cr (at.%) respectively. In the stage of anti-copy tool, the anodes of two anti-copy tool electrodes were fed to the fixed standard sample. At the stage of copying workpiece, the workpiece will be installed in the standard sample position. The cathodes of two anti-copied tool electrodes were fed to the workpiece.
0
[mm]
Fig. 9. (a) Feeding system; (b) Electrolyte cell and circulation system.
0
Workpiece holder
Fig. 7. (a)Profile differences of machining gap 0.2mm; (b) Profile differences of machining gap 0.3mm; (c) Profile differences of machining gap 0.4mm; (d) Profile differences of machining gap 0.5mm.
From Fig. 6, the minimum profile difference is at the machining gap 0.3mm. The profile difference of surface is 0.0438mm. In order to obtain more precise Δbidea, the fine search near 0.3mm with step of 0.01mm was carried out. The simulation results are shown in Fig. 8.The fine search result shows that minimum profile difference is at 0.28mm. The profile difference of surface is 0.0354mm. In order to verify the correctness of simulation analysis, the experiment of γTiAl intermetallic blade with optimum machining gap control was carried out. Profile difference /mm
Anti-copy tool electrode
Anti-copy tool electrode
Standard specimen
Fig. 10. The details of test setup in anti-copy tool stage.
The experiments of proposed anti-copy method were carried out to investigate the process feasibility. In the processing, the parameters of ECM are consistent with the simulation. The important parameters in anti-copy tool and copy workpiece stages such as machining voltage, electrolyte concentration, electrolyte pressure and the target machining gap are listed in Table 1. Table 1. Experimental parameters Unit
Machining gap b/mm
Fig. 8. The profile differences of different in fine search.
Electrolyte
Value Anti-copy tool
Copy workpiece
NaNO3
NaCl
Electrolyte concentrations
20%
10%
Electrolyte temperature
30
40
Electrolyte pressure inlet
0.8 MPa
0.8 MPa
Electrolyte pressure outlet
0.1 MPa
0.1 MPa
Target machining gap
0.3 mm
0.28 mm
Actual feed rate
0.72mm/min
1.13mm/min
Voltage
20 V
20 V
761
The anti-copied stainless steel tool electrodes are show in Fig. 11 (a). The machined γ-TiAl blades are show in Fig. 11 (b). The geometric profile differences between the three blades and standard sample were measured by the three coordinate measuring equipment. Three measuring section lines were selected on the convex and concave surface of the blades respectively. The position of section lines are shown in Fig. 12.
Sampling length L/mm
Roughness Ra/μm
4.2. Result of experiments
Roughness Ra/μm
Jia Liu et al. / Procedia CIRP 68 (2018) 757 – 761
Sampling length L/mm
Fig. 14. (a) Roughness in convex surface of blade 2; (b) Roughness in concave surface of blade 2.
5. Summary Anti-copied tool of blade convex surface
Anti-copied tool of blade concave surface
Blade 1
Blade 2
Blade 3
Fig. 11. (a)The anti-copied tool electrodes; (b) The machined γ-TiAl blades.
Line 3
Line 6
Line 2
Line 5
Line 1
Line 4
Fig. 12. The measured sectional line distribution. Line 1 Line 2 Line 3
Sampling point number
Line 4 Line 5 Line 6
Profile differences /mm
Profile differences /mm
In ordinary anti-copy method, the processing accuracy is guaranteed by controlling the processing conditions in anticopy tool and copy workpiece stages. The anode material, electrolyte system and parameters in the two stages must be consistent. In order to eliminate the processing condition restriction, an generic ECM anti-copy method with machining gaps control was presented in this paper. The processing dynamic simulation analysis was carried out. The simulation result shows that the optimum machining gap in copy workpiece stage with minimum profile different is near the machining gap in anti-copy tool stage. Through the feeding rate adjustment to control the optimum machining gap, the γTiAl blades were processed by the stainless steel anti-copied tool. The test specimens have good quality and high accuracy. Acknowledgements This study was sponsored by the China Natural Science Foundation (51405230) and Key Research & Development Plan of Jiangsu Province (BE2015160).
Sampling point number
Fig. 13. (a)Profile differences in convex surface of blade 2; (b) Profile differences in concave surface of blade 2.
Fig. 13 shows the distributions of the profile differences in the convex and concave surfaces of γ-TiAl blade 2. The maximum differences of convex surface are less than 0.056 mm. The maximum differences of concave surface are less than 0.071 mm. Fig. 14 shows the roughness in the convex and concave surfaces of γ-TiAl blade 2. The roughness in convex surface of blade 2 is Ra 1.246 μm. The roughness in concave surface of blade 2 is Ra 1.361 μm. The experiment result showed that the test specimens of γ-TiAl blades with optimum machining gap control have good quality and high accuracy. Therefore the feasibility of propose method was improved obviously.
References [1] Klocke F, Klink A, Veselovac D, et al. Turbo machinery component manufacture by application of electrochemical, electro-physical and photonic processes. CIRP Ann. Manuf. Technol 2014; 63: 703-726. [2] Yamaguchi M, Inui H, Ito K. High temperature structural intermetallics. Acta Mater 2000; 48: 307-322. [3] D.K Aspinwall, R.C Dewes, A.L Mantle. The machining of gamma-TiAl intermetallic alloys. CIRP Ann. Manuf. Technol, 2005; 54: 99-104. [4] Hood T, Aspinwall D.K, Sage C, et al. High speed ball nose end milling of γ-TiAl alloys. Intermetallice 2013; 32: 284-291. [5] Liu J, Zhu D, Zhao L, et al. Experimental investigation on electrochemical machining of γ-TiAl intermetallic. 15th MIC 2015, Germany. [6] Klocke F, Herrig T, Zeis M, et al. Results of surface intergrity and fatigue study of PECM and PEO processed γ-TiAl for turbine applications. 18th ISEM 2016, Japan. [7] Wang D, Zhu Z, Wang N, et al. Investigation of the electrochemical dissolution behavior of Inconel 718 and 304 stainless steel at low current density in NaNO3 solution. Electrochim Acta, 2015, 156:301-307.
ScienceDirect Procedia CIRP 68 (2018) 757 – 761
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Electrochemical machining of γ-TiAl intermetallic blades by using the stainless steel anti-copied tool electrodes Jia Liua*, Hao Wanga, Di Zhua a Nanjing University of Aeronautics & Astronautics, 29 Yu Dao Jie Street, Nanjing, 210016, China * Corresponding author. Tel.: +86-025-8489-6837 ; fax:+86-025-8489-6612.E-mail address:[email protected]
Abstract Electrochemical machining (ECM) is an important technology to process γ-TiAl intermetallic components such as aero-engine blades. The anticopy method is an efficient ECM technology to process workpiece without design cathode tools. By using the transfer effect of two opposite ECM stages, the workpiece can be processed by the standard sample directly. In ordinary anti-copy method, the processing accuracy is guaranteed by controlling the consistency of processing conditions in two ECM stages. The anode material of anti-copy tool and workpiece must be consistent. This limits the generality of anti-copy method. In order to eliminate the anode material restriction, an generic ECM anticopy method with machining gaps control was presented. The processing dynamic simulation analysis was carried out. The simulation result shows that the optimum machining gap in copy workpiece stage with minimum profile different is near the machining gap in anti-copy tool stage. Through the feeding rate adjustment to control the optimum machining gap, the γ-TiAl blades were processed by using the stainless steel anti-copied tools. The test specimens have good quality and high accuracy. The experimental results proved the feasibility of the proposed method. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ©2018 2018The The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Electrochemical machining; γ-TiAl; blade; anti-copy; experiment;
1. Introduction Gamma titanium aluminium (γ-TiAl) intermetallic offer low density, high stiffness, flame resistance and strength at a wide range temperature. Such characteristics make γ-TiAl intermetallic an important material in aero-engine blades as the replacement of heavier nickel based alloys[1,2]. However, owing to its low temperature ductility and fracture toughness, γ-TiAl intermetallic is extremely difficult to process by traditional mechanical technologies such as turning, grinding and high speed milling[3,4]. Electrochemical machining (ECM) is a nontraditional process to remove material via a controlled electrochemical anodic reaction. It has such advantages as no electrode wear, good surface quality, regardless of the material hardness and ductility. Compared with the traditional mechanical technologies, ECM has significant advantages in efficiency, cost and quality[5,6]. It has become one of the important processing technologies for
γ-TiAl blades manufacturing in high thrust-weight ratio aeroengines. In ECM, the most time-consuming work in the production preparation cycle is the design and correction cathode tools. The anti-copy method is an efficient ECM technology to process workpiece without have to design cathode tools. By using the transfer effect of two opposite ECM stages, the workpiece can be processed by the standard sample directly. Due to saving the cathode design and correction cycles, the anti-copy method has significant efficiency advantage and been widely used in aero-engines blades manufacturing. In ordinary anti-copy method, the processing accuracy is guaranteed by controlling the consistency of processing conditions in two ECM stages. The anode material, electrolyte system and parameters in the two stages must be consistent. If processing of γ-TiAl blades, the materials of anti-copy tool electrodes and workpiece must be γ-TiAl intermetallic. The pool machinability and expensive material price of γ-TiAl will increase the processing cycle and cost obviously. The
2212-8271 © 2018 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 scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.133
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Jia Liu et al. / Procedia CIRP 68 (2018) 757 – 761
restrictions of anode materials have affected the application generality of anti-copy method. In order to eliminate the anode material restriction, an generic ECM anti-copy method with optimum machining gap control was presented. Assuming that the materials of anticopy tool and workpiece are stainless steel and γ-TiAl intermetallic respectively. The finite element simulation of workpiece dynamic formation process was carried out to reveal the machining gaps relation between anti-copy tool stage and copy workpiece stage. Based on the simulation results, the experiments were carried out to evaluate the feasibility of the proposed method. Nomenclature Δb η ω κ i UR U vc va
Equilibrium machining gap Current efficiency Volume electrochemical equivalent Electrolyte conductivity Current density Electrode potential difference Applied voltage Feed rate of cathode Normal corrosion rate of anode
2. Principle of ECM anti-copy method 2.1. Ordinary anti-copy method The anti-copy method includes two opposite ECM stages, anti-copy tool and copy workpiece. In first stage of anti-copy tool, the standard sample connected to power supply cathode, the two anti-copy tool electrodes connected to power supply anode. High pressure electrolyte was pumped into the interelectrode gap. Under the effect of electrochemical reaction, the material of anti-copy tool electrodes was dissolved into the electrolyte. With the anti-copy tool electrodes continuous feeding toward standard sample, the shapes of anti-copy tool electrodes were similar to the standard sample. In second stage of copy workpiece, the two anti-copied tool electrodes connected to power supply cathode, the workpiece connected to power supply anode. With the anti-copy tool electrodes continuous feeding, the workpiece can be machined without tool design. The schematic of ECM anti-copy method is shown in Fig. 1. By using two opposite ECM stages, the shape of the standard sample will transfer to the workpiece. Without tool design, the workpiece can be machined by standard sample directly. If the machining gap distributions of two opposite ECM stages are identical, the shape of sample will be perfect copied to the workpiece. The same processing conditions will lead to similar machining gaps. So in ordinary anti-copy method, the processing accuracy is guaranteed by controlling the consistency of processing conditions in two ECM stages. The anode material of anti-copy tool and workpiece must be
the same. The electrolyte system and machining parameters in two ECM stages are also to be consistent. Anti-copy tools
Stage 1 Anti-copy tool - + Standard sample
Anti-copied tools
Anti-copied tools
Stage 2 Copy workpiece + Workpiece
Processed specimen
Fig. 1. The schematic of ECM anti-copy method.
However the processes of the two opposite ECM stages are not reversible. Even if the processing conditions in two ECM stages are consistent, many factors such as opposite electric field and opposite electrolytic product distribution are still lead to change the machining gaps. The differences of machining gap in anti-copy tools and copy workpiece are inevitable exist. So instead of processing status control, through the machining parameters adjustment to control the machining gap distributions might be the effective measure to ensure the anti-copy precision. The anode materials of anticopy tool electrode and workpiece can be different. 2.2. Proposed anti-copy method In ECM, the equilibrium machining gap between the tool and workpiece is an important factors influencing the machining gap distribution. Based on the theory of Cosθ system, the equilibrium machining gap Δb can be calculated by the Equation (1). It is find that the feeding rate vc is inversely proportional to the equilibrium machining gap Δb. So the machining gap distribution can be controlled by adjusting the feed rate.
Δb =
ηωκU R vc
(1)
Based on the above analysis, the new anti-copy mode with optimum machining gap control was presented. Assuming that the tool and workpiece materials are different. The two opposite ECM stages will come into two different machining gaps, Δb1 and Δb2. If set the first stage machining gap Δb1 as the standard machining gap, there may be a optimum machining gap Δbidea to make the machining precision of anticopy method acceptable. By using feeding rate adjustment, the second stage machining gap Δb2 can be controlled equal to Δbidea. In order to find the relationship between Δb1 and Δbidea, the finite element simulation of workpiece dynamic formation process was carried out. The materials of the anti-copy tool electrode and workpiece were assumed to be stainless steel 1Cr-18Ni-9Ti (at.%) and γ-TiAl intermetallic Ti-48Al-2Nb2Cr (at.%) respectively
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Jia Liu et al. / Procedia CIRP 68 (2018) 757 – 761
3. Simulation of proposed ECM anti-copy method
The anti-copy method consists of two opposite ECM stages. The two stages used the same electrolyte flow model and the two machining gaps had similar geometry. In order to simplify the simulation model, it is assumed that the influence of flow factors in two stages can be counteracted. The differences of electrolyte flow field, electrolyte temperature field and electrochemical product distribution in two stages are neglected. The simplified simulation model is mainly based on the electric field and obeys the following assumptions: (1) The potential inside model conforms to the Laplasse equation. (2) The current density distribution on the anode surface is dominated by ohmic effects. The finite element dynamic simulation is based on the software COMSOL. 3.1. dynamic formation process in anti-copy tool stage The simulation mashed modal of first stage of anti-copy tool are shown in Fig. 2. This model was designed based on the convex surface of typical aero-engine blades. The processing parameters in simulation were determined according to the previous research of ECM stainless steel. The standard sample surface was a feeding cathode equipotential surface. The cathode potential was φc = 0 V. The anti-copy tool surface was a corroded anode equipotential surface. The anode potential was φa= 20 V. The other surfaces of the model were fixed surfaces and the normal potential difference were ∂φ/ ∂n = 0 V.
Fig. 3. (a) The anode dynamic formation result; (b) The calculated anti-copy tool surface.
3.2. dynamic formation process in copy workpiece stage The simulation mashed modal of copy workpiece stage is shown in Fig. 4. The processing parameters in simulation were determined according to the previous research of ECM γ-TiAl intermetallic. The simulation process in copy workpiece was consistent with the anti-copy tool. The cathode potential of anti-copy tool surface was φc = 0 V. The anode potential of workpiece surface was φa= 30 V. The value of ηω is according to the actual measured ηω-i curve of γ-TiAl intermetallic. It is shown in Fig. 5. The value of κ is the conductivity of 10% w/v sodium chloride solution (NaCl) at 40 . Electrolyte outlet
Workpiece surface
Electrolyte outlet
Anti-coped tool surface
Anti-copy tool surface
Electrolyte inlet Standard sample surface
Fig. 4. The mashed modal of coping workpiece stage. Electrolyte inlet
Fig. 2. The mashed modal of anti-coping tool stage.
Select a point on the anti-copy tool surface and mark it Px. The normal corrosion rate at Px is up to Equation (2). In the Equation (2), the value of ηω is according to the ηω-i curve of stainless steel[7]. The value of κ is the conductivity of 20% w/v sodium nitrate solution (NaNO3) at 30 . The current density ix is derived from transient electric field simulation result.
v px = ηωκ i px
CurRent efficiency ηω/mm3(Amin)-1
1 0.8 0.6 0.4 0.2 0 0
10
20
30
40
50
60
70
Current density i/Acm-2
Fig. 5. The ηω-i curve of γ-TiAl intermetallic.
(2)
In order to facilitate analysis, the equilibrium machining gap Δb1 in anti-copy tool stage was set to 0.3 mm. The feed rate was adjusted to meet this standard machining gap. The anode dynamic formation result and calculated anti-copy tool surface are shown in Figure 3 (a) and (b).
In order to search for the optimal machining gap Δbidea, the rough search from 0.2mm to 0.5mm with step of 0.1mm was carried out. The feed rate was adjusted to meet the assumed machining gaps. After calculations, four machined workpiece surfaces were obtained. The four results were compared with the standard sample surface. The profile difference of different
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machining gaps are shown in Fig. 6. The difference distributions are show in Fig. 7.
4. Experiments
Profile differences /mm
4.1. Experimental equipments
Machining gap b/mm
Fig. 6. The profile differences of different in rough search.
Profile error 0.2
0.15
0.15
0.1
0.1
0.05
0.05
[mm]
Profile error 0.2
0
0
[mm]
Profile error 0.2
Profile error 0.2 0.15
0.15 0.1
0.1
0.05
0.05
[mm]
The developed experimental equipments consists of feeding system, standard sample, anti-copy tool electrodes, test setup, electrolyte cell, electrolyte circulation system and power supply, as shown in Fig. 9. The anti-copy tool and copy workpiece stages used the same test setup. Fig. 10 shows the details of test setup. The materials of the anti-copy tool electrodes and workpiece were stainless steel 1Cr-18Ni-9Ti (at.%) and γ-TiAl intermetallic Ti-48Al-2Nb-2Cr (at.%) respectively. In the stage of anti-copy tool, the anodes of two anti-copy tool electrodes were fed to the fixed standard sample. At the stage of copying workpiece, the workpiece will be installed in the standard sample position. The cathodes of two anti-copied tool electrodes were fed to the workpiece.
0
[mm]
Fig. 9. (a) Feeding system; (b) Electrolyte cell and circulation system.
0
Workpiece holder
Fig. 7. (a)Profile differences of machining gap 0.2mm; (b) Profile differences of machining gap 0.3mm; (c) Profile differences of machining gap 0.4mm; (d) Profile differences of machining gap 0.5mm.
From Fig. 6, the minimum profile difference is at the machining gap 0.3mm. The profile difference of surface is 0.0438mm. In order to obtain more precise Δbidea, the fine search near 0.3mm with step of 0.01mm was carried out. The simulation results are shown in Fig. 8.The fine search result shows that minimum profile difference is at 0.28mm. The profile difference of surface is 0.0354mm. In order to verify the correctness of simulation analysis, the experiment of γTiAl intermetallic blade with optimum machining gap control was carried out. Profile difference /mm
Anti-copy tool electrode
Anti-copy tool electrode
Standard specimen
Fig. 10. The details of test setup in anti-copy tool stage.
The experiments of proposed anti-copy method were carried out to investigate the process feasibility. In the processing, the parameters of ECM are consistent with the simulation. The important parameters in anti-copy tool and copy workpiece stages such as machining voltage, electrolyte concentration, electrolyte pressure and the target machining gap are listed in Table 1. Table 1. Experimental parameters Unit
Machining gap b/mm
Fig. 8. The profile differences of different in fine search.
Electrolyte
Value Anti-copy tool
Copy workpiece
NaNO3
NaCl
Electrolyte concentrations
20%
10%
Electrolyte temperature
30
40
Electrolyte pressure inlet
0.8 MPa
0.8 MPa
Electrolyte pressure outlet
0.1 MPa
0.1 MPa
Target machining gap
0.3 mm
0.28 mm
Actual feed rate
0.72mm/min
1.13mm/min
Voltage
20 V
20 V
761
The anti-copied stainless steel tool electrodes are show in Fig. 11 (a). The machined γ-TiAl blades are show in Fig. 11 (b). The geometric profile differences between the three blades and standard sample were measured by the three coordinate measuring equipment. Three measuring section lines were selected on the convex and concave surface of the blades respectively. The position of section lines are shown in Fig. 12.
Sampling length L/mm
Roughness Ra/μm
4.2. Result of experiments
Roughness Ra/μm
Jia Liu et al. / Procedia CIRP 68 (2018) 757 – 761
Sampling length L/mm
Fig. 14. (a) Roughness in convex surface of blade 2; (b) Roughness in concave surface of blade 2.
5. Summary Anti-copied tool of blade convex surface
Anti-copied tool of blade concave surface
Blade 1
Blade 2
Blade 3
Fig. 11. (a)The anti-copied tool electrodes; (b) The machined γ-TiAl blades.
Line 3
Line 6
Line 2
Line 5
Line 1
Line 4
Fig. 12. The measured sectional line distribution. Line 1 Line 2 Line 3
Sampling point number
Line 4 Line 5 Line 6
Profile differences /mm
Profile differences /mm
In ordinary anti-copy method, the processing accuracy is guaranteed by controlling the processing conditions in anticopy tool and copy workpiece stages. The anode material, electrolyte system and parameters in the two stages must be consistent. In order to eliminate the processing condition restriction, an generic ECM anti-copy method with machining gaps control was presented in this paper. The processing dynamic simulation analysis was carried out. The simulation result shows that the optimum machining gap in copy workpiece stage with minimum profile different is near the machining gap in anti-copy tool stage. Through the feeding rate adjustment to control the optimum machining gap, the γTiAl blades were processed by the stainless steel anti-copied tool. The test specimens have good quality and high accuracy. Acknowledgements This study was sponsored by the China Natural Science Foundation (51405230) and Key Research & Development Plan of Jiangsu Province (BE2015160).
Sampling point number
Fig. 13. (a)Profile differences in convex surface of blade 2; (b) Profile differences in concave surface of blade 2.
Fig. 13 shows the distributions of the profile differences in the convex and concave surfaces of γ-TiAl blade 2. The maximum differences of convex surface are less than 0.056 mm. The maximum differences of concave surface are less than 0.071 mm. Fig. 14 shows the roughness in the convex and concave surfaces of γ-TiAl blade 2. The roughness in convex surface of blade 2 is Ra 1.246 μm. The roughness in concave surface of blade 2 is Ra 1.361 μm. The experiment result showed that the test specimens of γ-TiAl blades with optimum machining gap control have good quality and high accuracy. Therefore the feasibility of propose method was improved obviously.
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