Electrochemical Machining of High-temperature Titanium Alloy Ti60

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ScienceDirect Procedia CIRP 42 (2016) 125 – 130

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

Electrochemical machining of high-temperature titanium alloy Ti60 Zhengyang Xua,*, Xuezhen Chena, Zesheng Zhoua, Peng Qina, Di Zhua College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing 210016, China * Corresponding author. Tel.: +86-025-8489-6304; fax:+86-025-8489-5912. E-mail address:[email protected]

Abstract The high-temperature titanium alloy Ti60 (Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd) is often used for manufacturing critical components of aero-engines. The use of this alloy has made it possible to increase the service temperature to up to 600 °C. However, machining such alloys by conventional methods is difficult, often resulting in low process efficiency, high tool wear, reduced precision, and poor surface integrity. Electrochemical machining (ECM) is an effective method to machine titanium alloys but the electrochemical dissolution behavior of titanium alloys is different from those of other difficult-to-cut materials such as nickel alloys. This study focuses on the ECM of the high-temperature titanium alloy Ti60. The anodic polarization curve, open circuit potential, and actual volume electrochemical equivalent–current density curve of Ti60 are obtained. The electrochemical dissolution behavior of Ti60 is analyzed, and the composition, concentration, and temperature of the electrolyte used for ECM are optimized. Dissolution experiments are performed at different current densities, and results show that the surface roughness of Ti60 undergoing ECM deteriorates when the current density is small. Finally, electrochemical parameters are optimized and a blisk sector made of Ti60, which is to be used in an aero-engine compressor, is machined by ECM. The process is stable and efficient. The maximum machining rate of the channels is more than 1.2mm/min. The best surface roughness is Ra 0.6 μm, and the machining accuracy of the blade profile is 0.05–0.07mm. © 2016 The Authors. © Authors. Published Publishedby byElsevier ElsevierB.V. B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM (http://creativecommons.org/licenses/by-nc-nd/4.0/). XVIII). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining

(ISEM XVIII) Keywords: Electrochemical machining; Titanium alloy; Ti60; Dissolution behavior; Blisk

1. Introduction The high-temperature titanium alloy Ti60 (Ti–5.6Al– 4.8Sn–2Zr–1Mo–0.35Si–0.7Nd), which is a near-α titanium alloy with an optimized chemical structure, has been used for manufacturing different components; the use of this alloy has made it possible to increase the service temperature of the components to up to 600 °C [1,2]. Ti60 is an important structural material often used for manufacturing critical components of aero-engines, such as the blisks or blades of high-pressure compressors, because of its low density combined with good high-temperature mechanical properties. However, machining such alloys by conventional methods is difficult, often resulting in low process efficiency, high tool wear, and reduced precision. In addition, the surface integrity is often poor because of thermo-mechanically altered or damaged rim zones [3–5]. Therefore, an alternative

manufacturing technology is required for machining such alloys. Electrochemical machining (ECM) is an electrochemical dissolution method to produce smooth surfaces with a high accuracy and without limitations to the mechanical properties of the alloys [6,7]. ECM is increasingly being considered as an important method for the production of hard-to-machine titanium alloys. Compared to traditional technologies, ECM does not cause heat-affected zones and internal stresses on the machining surface. In addition, because ECM causes no tool wear and gives high material removal rates, it is a very costeffective and highly efficient production method. In recent years, many researchers have focused on the ECM of hard-to-machine materials, especially titanium alloys. Weinmann et al. used different techniques like linear sweep voltammetry and electrochemical impedance spectroscopy to study the electrochemical dissolution behavior of Ti90Al6V4 and Ti60Al40. They chose a suitable electrolyte and

2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.206

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corresponding current parameters [7]. Klocke et al. conducted basic research on the electrochemical machinability of selected modern titanium and nickel alloys for aero-engine components [6]. They also carried out technological and economic comparisons of roughing strategies via milling, sinking electrical discharge machining (EDM), wire EDM, and ECM for titanium and nickel blisks [8]. Clifton et al. investigated the surface characteristics and integrity of γ-TiAl subjected to ECM using perchlorate and chloride electrolytes [9]. Dhobe et al. performed experiments on the ECM of commercially pure titanium using a sodium bromide electrolyte at a tool feed rate of 0.1 mm/min. The roughness of the oxide-layered machined surface was suitable for use in titanium implants without the need for further surface preparation [10]. Santhi et al. optimized the process parameters in ECM using DFA-fuzzy set theory-TOPSIS for a titanium alloy [11]. Ittah et al. evaluated the pitting corrosion of titanium in ammonium bromide solutions by electrochemical methods [12]. Qu et al. machined a titanium alloy by wire ECM (WECM) and implemented axial electrolyte flushing in WECM for removing electrolysis products and renewing the electrolyte [13]. Zaytsev et al. proposed the optimal conditions for the ECM of Ti–6Al–4V titanium alloy using a microsecond pulsed current; under these conditions, surface-layer hydrogen was not found and pitting did not occur [14]. Mount et al. carried out theoretical analysis of chronoamperometric transients in ECM and characterized titanium 6/4 and Inconel 718 alloys [15]. Lu and Leng employed jet-electrochemical micromachining to produce micro-hole patterns on titanium specimens [16]. Sjöström and Su presented a technique involving a water-free electrolyte that can be used to perform electrochemical micromachining of Ti surfaces. Pit and groove structures with approximately 50-μm diameter/width were fabricated by using a tungsten carbide tool [17]. Weber et al. studied the electrochemical dissolution of cast iron in NaNO3 electrolyte [18]. The main problem in using ECM for titanium alloys is titanium self-passivation, whereby a passive oxide layer is formed, which inhibits the dissolution process. A propriate process parameters and a suitable electrolyte are important for the machining. This study focuses on the ECM of the hightemperature titanium alloy Ti60. The electrochemical dissolution behavior of Ti60 is analyzed. The anodic polarization curves of Ti60 with sodium nitrate and sodium chloride solutions are obtained, and a suitable electrolyte is chosen. The open circuit potential and the actual volume electrochemical equivalent–current density curve of Ti60 are obtained. Dissolution experiments are performed at different current densities, and the results show that the surface roughness of Ti60 undergoing ECM deteriorates when the current density is small. Finally, electrochemical parameters are optimized and the blisk sector made of Ti60, which is to be used in an aero-engine compressor, is machined by ECM.

2. Experimental 2.1. Materials The workpiece material is the Ti60 alloy, which is made by a conventional forging method in which the forging temperature is below the phase transition point (1050– 1060 °C). Fig. 1 shows the material morphology of the Ti60 alloy. In the microstructure of the conventionally forged Ti60, a major part is occupied by lumpy equiaxed primary α grains and the dendritic secondary platelet α phase and the remaining β phase are interveined between the α grains.

Fig. 1. Microstructure of Ti60 material.

2.2. Experimental setup and sample preparation The structure and components of the samples used in the study were characterized by using a metalloscope and by energy-dispersive X-ray spectroscopy (EDX). The metalloscope is a Zeiss Axioplan 2 Imaging & Axiophot 2 microscope. The EDX investigations were performed using another microscope (JSM-6360LV, JEOL, Japan). Electrochemical measurements were performed using a three-electrode setup and an electrochemical workstation (CHI660E, CH Instruments Inc., China). The counter electrode was a platinum flat electrode, the reference electrode was Hg/Hg2SO4 in 0.5 M sulfuric acid, and the working electrode was a Ti60 cuboid (10 × 10 × 5mm) whose five surfaces were covered with resin and only one surface was exposed to pretreatment using wet abrasive papers and rinsing with distilled water. The actual volume electrochemical equivalent (ηω)–current density (i) curve was obtained by using a custom-made ECM experimental system with a single feeding axis (Fig. 2). The experiments involving the comparison of the machining surfaces roughness with different current densities were performed for the same machine tool. Cubes with dimensions 10 × 10 × 10 mm were used as samples during the experiments. Finally, a blisk sector made of Ti60, intended for use in an aero-engine compressor, was machined electrochemically using custom-made blisk ECM machine tools such as a blisk channel ECM machine tool and profile ECM machine tool (Fig. 3). The surface roughness was measured using a surface-finish measuring instrument (Perthometer S3P, Mahr GmbH, Germany). The machining accuracy of the blade profile was measured using a coordinate measuring machine (TESA Micro-Hite 3D, TESA, Switzerland).

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Fig. 3. Custom-made blisk ECM machine tools.

3. Results and discussion 3.1. Composition of material samples The composition of the Ti60 alloy samples was analyzed by EDX, as shown in Fig. 4. The results for the sample that underwent polish pretreatment are shown in Fig. 4(a). The elements are almost in accordance with the nominal composition of a Ti60 alloy. Fig. 4(b) shows the composition of the sample without pretreatment. Because titanium is a self-passivation metal and there is a passive oxide layer on the sample surface, oxygen is detected. Ti

6

Intensity/a.u.

5 4 3

Wt% 04.97 01.10 03.10 01.14 00.74 03.88 83.49 00.13 01.45



2 1 0

Al Zr Mo Si Nb Sn

At% 08.92 01.90 01.65 00.59 00.38 01.58 84.47 00.11 00.39

Ti 6 5 4 3 2

Wt% 04.02 04.43 00.88 02.80 00.96 00.62 03.46 81.14 00.66 01.03

At% 11.24 07.34 01.41 01.37 00.46 00.29 01.30 75.79 00.53 00.25



Al Zr Mo Si Nb Sn

1

O

Ta

Fe

Element O Al Si Zr Nb Mo Sn Ti Fe Ta

7

Intensity/a.u.

Element Al Si Zr Nb Mo Sn Ti Fe Ta

7

Fe

40°C NaCl (0.1% wt) 40°C NaCl (1% wt) 40°C NaCl (5% wt) 40°C NaCl (10% wt) 40°C NaCl (15% wt)

0.4 0.3 Current/A

Fig.2. Single-feeding-axis ECM experimental system.

The current–voltage curves show that the electrolyte composition as well as the concentration influence the dissolution behavior of the material. Based on the measurements for sodium chloride shown in Fig. 5, the curves can be divided into two different regions. Between 0 and 2 V, the curves show a distinct area of passivation. There is no electrochemical dissolution, and currents are almost zero because the potentials are below the dissolution voltage (Ediss). When the potentials are more than 2 V, the currents increase gradually, which means that electrochemical dissolution occurs. This area is called the transpassive region. There is a chemical interaction between chloride ions and the oxide surface. Chloride ions with small sizes and strong adsorptive properties can penetrate the passive layer and react with the base alloy, resulting in the destruction of the oxide layer. The dissolution voltages can be determined by evaluating the inflection points of the polarization curves. Electrolyte concentration is an important factor governing the dissolution behavior. In the measurements, several different concentrations (0.1%, 1%, 5%, 10%, and 15%) were chosen. For low-concentration electrolytes (0.1% and 1%), Ediss was higher than that for the other higher-concentration solutions (5%, 10%, and 15%). Furthermore, the increase in current levels is very slow when the concentration is low (0.1% and 1%). Especially, for the electrolyte with 0.1% concentration, there is almost no obvious dissolution and the material is still in passivation. In contrast, for higherconcentration solutions (5%, 10%, and 15%), the currents increase rapidly and steep ascending gradients are achieved. Therefore, for higher concentrations, faster transpassive dissolution is observed.

0.2 0.1

Ta

0

0

1

2

3

4

5

6

7

8

9

0

Energy/keV

(a)

1

2

3

4

5

6

7

8

9

Energy/keV

(b)

Fig. 4. Composition of Ti60 alloy samples: (a) sample with pretreatment and (b) sample without pretreatment.

3.2. Electrochemical characterization In a series of measurements, the influence of different electrolytes and temperatures on the electrochemical dissolution behavior of the Ti60 alloy was investigated by using the polarization curve and the time-dependent evolution of the open circuit potential (EOCP). Two kinds of electrolytes—sodium nitrate and sodium chloride solutions— were selected for the measurements. Fig. 5 shows the polarization curves for the Ti60 alloy in sodium chloride with different concentrations. The electrolyte temperature was always 40 °C during the experiments. The scan rate of voltage in the experiments is 0.01V/s, so the time of each experiment is about 1000 seconds.

0.0

0

1

2

3

4

5

6

7

8

9 10

Potential/V

Fig. 5. Polarization curves for Ti60 alloy in sodium chloride with different concentrations.

Electrolyte temperature is another important factor influencing the dissolution behavior. In the present work, six different temperatures (10, 20, 30, 40, 50, and 60 °C) were used. Fig. 6 shows the current–voltage curves for the sodium chloride electrolyte with 10% concentration and abovementioned temperatures. The curves can be divided into two groups. For the group of low-temperature electrolytes (10, 20, and 30 °C), Ediss values are quite high (around 6 V). This indicates that it is difficult for electrochemical dissolution to continue at low electrolyte temperatures. In contrast, when the temperature is more than 40 °C, Ediss values clearly decline (only approximately 2 V). The dissolution starts at lower voltages, which indicates better machinability. Electrochemical dissolution occurs more easily when the

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temperature is higher. A possible explanation for the differences among different electrolyte temperatures might be the velocity of reactive ion exchange, considering the kinetic and thermodynamic effects. At low temperatures, the velocity of reactive ion exchange could be assumed to be lower. This affects the chemical interaction between the oxide layer and chloridion; hence, the continuation of electrochemical dissolution becomes difficult. 10°C NaCl (10% wt) 20°C NaCl (10% wt) 30°C NaCl (10% wt) 40°C NaCl (10% wt) 50°C NaCl (10% wt) 60°C NaCl (10% wt)

0.4

Current/A

0.3 0.2 0.1 0.0

0

1

2

3

4

5

6

7

8

9

10

Potential/V

Fig. 6.Current–voltage curves for sodium chloride electrolyte with 10% concentration and different temperatures.

corresponds to the dissolution resistance. The influence of the oxidation power of different anions is measured by a timeresolved recording of EOCP. The Ti60 samples are immersed in sodium chloride and sodium nitrate electrolytes with 10% concentration and different temperatures, and EOCP is measured for 1000 s. Fig. 8 shows the time dependence of the EOCP of Ti60 in different electrolytes and temperatures. The increase in EOCP indicates the generation of the oxide layer. If the oxide layer is removed, electrochemical dissolution will be accelerated, and then, EOCP will decrease. From Fig. 8, it can be concluded that the formation of the oxide layer is faster in the nitrate electrolyte than in the chloride electrolyte because the EOCP of Ti60 in the nitrate electrolyte is always higher than that in the chloride electrolyte. In addition, the higher the electrolyte temperature, the lower is the value of EOCP. The abovementioned results indicate that better machinability can be obtained by using the chloride electrolyte with a higher temperature. 30°C NaNO3 (10% wt) 40°C NaNO3 (10% wt)

0.2

50°C NaNO3 (10% wt)

30°C NaNO3 (10% wt) 40°C NaNO3 (10% wt)

0.4

50°C NaNO3 (10% wt)

Current/A

0.3 0.2 0.1 0.0

0

1

2

3

4

5

6

7

8

9 10

Potential/V

Fig. 7. Current–voltage curves for Ti60 in sodium nitrate electrolyte with 10% concentration and different temperatures.

The same results can be obtained by measuring the open circuit potential (EOCP) of Ti60 in an electrolyte, which

Potential/V

Potential/V

0.0

Fig. 7 shows the current–voltage curves for Ti60 in sodium nitrate electrolyte with 10% concentration and different temperatures. Compared to the sodium chloride electrolyte, the curves of the sodium nitrate electrolyte are very different. At a low temperature (30 °C), the current is almost zero during the entire measurement. This indicates that the material is still passive and the oxide layer is not destroyed. When the temperature is 40 °C, a tiny electrochemical reaction occurs. The current increases slightly and then declines to zero. This demonstrates that there is only little corrosive pitting on the surface and then the oxide layer is generated again. At a higher temperature (50 °C), the current increases slowly, which indicates the occurrence of a hybrid reaction involving oxide layer generation and pitting dissolution. When the potential is more than 9 V, the current curve becomes serrated. A possible explanation for this is that with an increase in the potential, pitting corrosion is generated on some areas of the surface and the current increases rapidly. However, because sodium nitrate solution is an inactive electrolyte and has higher oxidation power than halide electrolytes, pitting can be oxidized immediately and hence the current declines almost linearly.

-0.2 -0.4 -0.6

0

200

400

600

Time/s

(a)

800

1000

0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

40°C NaCl (10% wt) passivation 30°C NaCl (10% wt) 40°C NaCl (10% wt) 50°C NaCl (10% wt)

0

200

400

600

800

1000

Time/s

(b)

Fig. 8. Time dependence of EOCP of Ti60 in different electrolytes and temperatures: (a) NaCl solution and (b) NaNO3 solution.

The actual volume electrochemical equivalent (ηω)–current density (i) curve for Ti60 in the sodium chloride with the 10% concentration and 40 °C temperature is obtained according to the abovementioned results using the custom-made ECM machine tool, as shown in Fig. 9. Here, η is the current efficiency, and ω is the volume electrochemical equivalent. The curve can be divided into two regions. Between 0 to 20 A/cm2, the actual volume electrochemical equivalent increases sharply. This phenomenon indicates that electrochemical dissolution becomes stronger as the current density increases. When the current density is more than 20 A/cm2, ηω remains stable and hardly changes in this measurement range. This demonstrates that the experiment is in the state of steady electrochemical dissolution. In addition, based on ECM theory, the dissolution velocity of the anode workpiece is proportional to ηω and i. Thus, increasing the current density of ECM not only enhances the dissolution velocity but also improves ηω, which can further enhance the dissolution velocity of the anode workpiece. Fig. 10 shows the experiment samples machined at different current densities in the NaCl electrolyte with 10% concentration and 40 °C temperature. When the current density is 50 A/cm2, the machined plane is very smooth. In contrast, when the current density is 10 A/cm2, the electrochemical dissolution is uneven and pitting corrosion is observed on the workpiece surface. Thus, a high current density, more than at least 20 A/cm2, is beneficial to ECM stability, efficiency, and quality.

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Table 1. ECM experimental parameters.

ηω/(cm3·A-1·min-1)

0.0018 0.0016

Item

0.0014

Workpiece material

Ti60

0.0012

Cathode material

1Cr18Ni9Ti

0.0010

Electrolyte

NaCl (10%)

0.0008

Voltage

30 V

0.0006

Inlet pressure

0.5 MPa

Temperature

40 ± 0.1°C

0

10

20

30

40

50

60

70

80

90

Current density/(A·cm-2)

Value

Initial gap Fig. 9. Actual volume electrochemical equivalent (ηω)–current density (i) curve for Ti60 in sodium chloride electrolyte with 10% concentration and 40 °C temperature.

(a)

(b)

Fig. 10. Surfaces produced by ECM at different current densities: (a) 10 A/cm2 and(b) 50 A/cm2.

3.3. ECM of Ti60 blisk sectors Experiments were performed on Ti60 blisk sectors using the custom-made blisk channel ECM machine tool and profile ECM machine tool. ECM processing of blisk blade profiles often happens in two steps. As shown in Fig. 11, the first step is channel ECM processing, which creates several tens of curved tunnels with allowances. The second step is blisk profile finishing ECM, which produces blades with good machining accuracy and surface roughness.

(a)

0.5 mm 1.2 mm/min (for channel)

Feed rate

0.5mm/min (for profile)

Fig. 12 shows the workpiece samples machined electrochemically. Fig. 12(a) shows the Ti60 workblank. Fig. 12(b) shows the sample after channel machining. The maximum feeding rate of the cathode for channel machining is approximately 1.5mm/min. When the feeding rate is increased, the equilibrium gap between the end surface of the cathode and the blisk hub gradually decreases. If the gap is too small, the risk of short circuit increases; thus, a feeding velocity of 1.2mm/min is selected for the stable ECM of the blisk channel. Each channel can be machined within 30 min. The best surface roughness of the blisk hub after ECM is Ra 0.6 μm. Fig. 12(c) shows the blisk sector sample after profile precision ECM. The concave and convex profiles of the blade can be machined electrochemically at the same time. For precision machining, the feeding rate of the cathode is reduced to 0.5mm/min. However, because the working allowance after channel machining for the subsequent finishing is only 2 mm, each blade profile can be machined within 5 min. The machining accuracy of the blisk profiles is measured using a coordinate measuring machine. Fig. 13 shows the machining error of the section lines on the blade profile. Fig. 13(a) shows the concave profile, whereas Fig. 13(b) shows the convex profile. As the figure shows, the ECM error of the concave profile is approximately 0.05mm, and the machining error of the convex profile is approximately 0.07mm.

(b) Fig. 11.Two steps of blisk ECM:(a) channel machining and (b) blade profile machining.

The main experimental parameters are listed in Table 1. The workpiece material was Ti60. The cathode was fabricated from stainless steel to make it impervious to corrosion. Based on the above mentioned electrochemical behavior of Ti60, sodium chloride with 10% concentration was used as the electrolyte because of its good activity. The temperature of the electrolyte was 40 °C. Current density plays an important role in ensuring stable machining and producing a good surface finish on titanium; thus, parameters such as temperature and voltage were set to high values for obtaining a higher current density during the process.

Tool electrode

(a)

(b)

Tool electrode

(c) Fig. 12. (a) Ti60 workblank, (b) sample after channel machining, and (c) sample after blade profile machining.

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0.15

line1 line2 line3 line4 line5 line6

Machining error/mm

0.10

0.05

0.00

-0.05

Acknowledgements This study was sponsored by the Program for New Century Excellent Talents in University (NCET-12-0627) and by the Fundamental Research Funds for the Central Universities (NE2014104). References

-0.10 0

10

20

30

40

50

60

70

80

Point number

(a) 0.15

line1 line2 line3 line4 line5 line6

Machining error /mm

0.10

0.05

0.00

-0.05

-0.10 0

10

20

30

40

50

60

70

80

Point number

(b) Fig. 13. Machining error of (a) concave part of blade and (b) convex part of blade.

3. Conclusions This study focuses on the ECM of the high-temperature titanium alloy Ti60. Because titanium exhibits the characteristic of self-passivation, choosing the appropriate process parameters and a suitable electrolyte is important for obtaining good machining results. The electrochemical dissolution behavior of Ti60 is analyzed. The anodic polarization curves and the open circuit potential of Ti60 with sodium nitrate and sodium chloride solutions are obtained. Compared to the sodium nitrate electrolyte, the sodium chloride electrolyte is more active and chloride ions can chemically interact with the oxide layer. Thus, better electrochemical machinability is achieved using the sodium chloride electrolyte. Concentration and temperature are important factors influencing electrochemical dissolution. The concentration of the electrolyte influences the dissolution voltage (Ediss) and transpassive behavior. For destroying the oxide layer effectively and stabilizing the dissolution, a high concentration is essential. When the temperature of the electrolyte is under 30 °C, Ediss clearly increases. The actual volume electrochemical equivalent (ηω)–current density (i) curve is measured using a custom-made ECM machine tool. To achieve good ECM stability, efficiency, and quality, a high current density, more than at least 20 A/cm2, is necessary. Finally, the blisk sector of Ti60 is machined by ECM. The process is stable and efficient. The maximum machining rate of channel ECM is more than 1.2mm/min. The best surface roughness is Ra 0.6 μm, and the machining accuracy of the blade profile is 0.05–0.07mm.

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