A study of electrodischarge machining–pulse electrochemical machining combined machining for holes with high surface quality on superalloy

Abstract

Noncircular holes on the surface of turbine rotor blades are usually machined by electrodischarge machining. A recast layer containing numerous micropores and microcracks is easily generated during the electrodischarge machining process due to the rapid heating and cooling effects, which restrict the wide applications of noncircular holes in aerospace and aircraft industries. Owing to the outstanding advantages of pulse electrochemical machining, electrodischarge machining–pulse electrochemical machining combined technique is provided to improve the overall quality of electrodischarge machining-drilled holes. The influence of pulse electrochemical machining processing parameters on the surface roughness and the influence of the electrodischarge machining–pulse electrochemical machining method on the surface quality and accuracy of holes have been studied experimentally. The results indicate that the pulse electrochemical machining processing time for complete removal of the recast layer decreases with the increase in the pulse electrochemical machining current. The low pulse electrochemical machining current results in uneven dissolution of the recast layer, while the higher pulse electrochemical machining current induces relatively homogeneous dissolution. The surface roughness is reduced from 4.277 to 0.299 µm, and the hole taper induced by top-down electrodischarge machining process was reduced from 1.04° to 0.17° after pulse electrochemical machining. On account of the advantages of electrodischarge machining and the pulse electrochemical machining, the electrodischarge machining–pulse electrochemical machining combined technique could be applied for machining noncircular holes with high shape accuracy and surface quality.

Introduction

Under the effect of high temperature and load, the performance and reliability of turbine rotor blades are directly related to the performance, durability, reliability, and life of aero engines. ¹ A large number of noncircular holes distributed on the surface of turbine rotor blades are the most critical for the durability of aero engine disk due to stress concentration.^2,3 The noncircular holes on high-strength parts, such as superalloy, cemented carbide, heat-resistant steel, and hardened steel, are usually machined by electrodischarge machining (EDM) instead of the traditional machining method. ⁴

EDM is an important manufacturing process for machining hard metals and alloys used in aerospace. ⁵ However, due to the rapid heating and cooling effects induced by the EDM, a recast layer will be formed on the surface of the component. This recast layer is typically very fine grained and hard and may be alloyed with carbon from the cracked dielectric or with material transferred from the tool. ⁶ It has also been called the “white layer” since it remains unaffected by etching, and it appears white in color under the optical microscope (Figure 1(a)). The recast layer has been observed to occur under different spark erosion conditions, and it contains numerous pock marks, globules, cracks, and microcracks (Figure 1(b)). It has also been proved that the recast layer has tensile residual stresses, which are detrimental to the functional behavior of the part. ⁷ Thus, the recast layer on the parts for some specific applications in aviation and aerospace fields should be removed.

Figure 1.

SEM photographs of recast layer: (a) micrograph of recast layer due to the EDM process and (b) SEM image of an EDM surface showing microcracks.

At present, abrasive flow grinding and chemical polishing are the two main methods to remove the recast layer. However, the disadvantages of the aforementioned two methods are very obvious. Abrasive flow grinding has disadvantages of low processing precision, complex clamping device, and existence of blind corners where the abrasive could not reach. Chemical polishing removes metals mainly through chemical dissolution, which are not environmentally friendly and universal.

Electrochemical machining (ECM) could produce finished surfaces on different metal matrixes. ⁸ As ECM follows the principle of anodic metal dissolution, it has the great advantage of that the surface is not affected by heat or process forces from the tool. In addition, no material removal occurs at the cathode; ⁹ therefore, the process works nearly wear-free.

Based on the advantages of ECM and the disadvantages of recast layer caused by EDM, EDM-ECM combined technology could be applied in machining holes or special-shaped microholes on high-strength parts.^10–14 For instance, Chung et al. ¹¹ and Nguyen et al. ¹² successfully machined microholes with high surface quality by EDM process using the low-resistivity deionized water as the dielectric fluid. It was found that the material removal mechanism was transformed from mere EDM process to complex EDM-ECM process when the deionized water and low feed rate were applied. Hung et al. ¹³ used the electrochemical polishing method to improve the surface roughness of the holes drilled by EDM. Kurita and Hattori ¹⁴ carried out EDM and ECM processes in sequence on the same system with the same dielectric fluid (water) and the same electrode (Cu). A smoother surface with a roughness (Ra) of 0.2 µm could be obtained by the developed ECM-EDM complex process. Chung et al.’s ¹¹ and Nguyen et al.’s processes ¹² are essential to produce microholes with high accuracy. However, it is not appropriate for parts with relatively large dimensions. Hung et al.’s and Kurita and Hattori’s methods either suffer the low efficacy or are not environment-friendly because of acid electrolyte.

Pulse electrochemical machining (PECM) is an ECM process using pulsed current, which is a valuable technique to enhance the accuracy of ECM because of the less machining gap and stirring induced by the pulsed current. ¹⁵ Therefore, in order to efficiently produce high-precision holes with no whiter layer, the EDM-PECM combined machining process is carried out in sequence on a machine tool with the electrode unaltered and the electrolyte changed from deionized water to neutral sodium nitrate solution. The removal performance of the recast layer was systematically investigated through experiments.

Experiment and theory

In this study, a computer numerical control (CNC) EDM machine was used to drill noncircular holes on the surface of the samples. The EDM perforating machine spindle was fixed to the Z-axis of the three-axis CNC ECM machine, and a set of the clamp, electrolyte slot, and electrolyte circulation system has been set up. The electrolyte slot was fixed on electrochemical machine worktable and the workpiece was clamped on the workpiece holder in the electrolyte slot. During the EDM process, the deionized water recirculating system was opened to maintain the processing area washed continuously by deionized water, and the metal removed by EDM can thereby be taken away. The schematic diagram and the photograph of the experimental setup are shown in Figure 2(a) and (b). Special tubular brass electrodes with outside diameters of 1.980 mm and inside diameters of 1.020 mm (Figure 2(c)) were used to optimize the machining parameters. The material of workpiece was superalloy GH4169, which is widely used in aero engine, and the thickness was 3 mm. The chemical composition and material properties are given in Table 1.

Figure 2.

Schematic diagram of the experimental setup: (a) Schematic illustration of the experimental setup, (b) the photography of the experimental setup, and (c) the micrograph of the EDM tool.

Table 1.

Chemical composition of GH4169.

Element	C	Cr	Ni	Mo	Al	Ti	Fe
Wt%	≤0.08	17.0–21.0	50.0–55.0	2.80–3.30	0.60–1.20	0.65–1.15	Others
Element	Nb	B	Mn	Si	P	S
Wt%	4.75–5.50	≤0.006	≤0.35	0.35	≤0.015	≤0.15

It is known that pulsed current and pulse-on duration are the principal factors which influence hole enlargement, surface roughness, thickness of the recast layer, and induced stress.^16–19 In order to conduct the subsequent PECM process under the same conditions, the EDM process parameters were held constant as follows: discharge current, 1.5 A; pulse-on time, 85 µs; frequency, 4.7 kHz; and voltage, 40 V.

After the EDM machining process, scanning electron microscopy (SEM; JSM-6360 LV, Japan) was employed to examine the surface integrity and the thickness of the recast layer. A three-dimensional (3D) surface profilometer (Zygo New View 5022, USA) was then used to measure the surface roughness.

When the workpiece was pierced, the EDM perforating machine spindle was shut down. The workpiece was held and the deionized water recirculation system was closed. Then, the electrolyte recirculation system was opened and the PECM power was turned on for electrolytic polishing.

Analysis of experimental results

The roughness and surface morphology of EDM recast layer were analyzed first, as shown in Figure 3. The surface profile and the oblique plot of the recast layer, which were measured by the 3D surface profilometer, indicate that the recast layer is very rough with a roughness as high as Ra = 4.277 µm. The energy-dispersive X-ray spectroscopy (EDS) energy spectrum diagram of recast layer section (the white dashed region in Figure 4(b)), the SEM photographs of recast layer section, and the EDS spectrum diagram of the smooth unprocessed area are shown in Figure 4. Comparing Figure 4(a) and (c), it can be seen that oxygen and carbon content of recast layer increases significantly. Besides, the content of metallic elements like Cr, Fe, and Ni are lessened, while the other metallic elements like Ti, Al, and Nb are increased. This is probably because some composition of the alloy melted and cooled again forming the oxide or elementary substance. The recast layer generated by melting and cooling is loose with a thickness of about 30 µm (Figure 4(b)).

Figure 3.

Surface profile of the recast layer: (a) Surface profile and (b) Oblique plot of recast layer.

Figure 4.

(a) EDS energy spectrum diagram and (b) SEM photographs of recast layer section: the recast layer is shown in the white dashed region. (c) EDS energy spectrum diagram of the smooth unprocessed area.

The current density is the most important quantity for the PECM process because the current density determines the electric field. Therefore, it specifies the dissolution speed and has strong impact on the surface roughness.^20–22 Appropriate current density is crucial to improve the electrolysis efficiency and machining surface quality. To optimize the ECM parameters, the quality of the holes processed under different current densities was analyzed. The electrolyte in the experiment is 5 wt% sodium nitrate solution. Figure 5 shows the micrographs of the holes processed for different time under different current conditions. When current I is 0.7 A, processing time t is 320 s and the recast layer has been completely removed. However, this recast layer was not completely removed after 400 s PECM process when another experiment was conducted, which indicates that the removal process of recast layer was unstable at 0.7 A. After 550 s PECM process, the surface qualities of holes at the entrance are smooth and there is almost no dispersive corrosion. It can be inferred probably that the current density distribution induced by the low current could not satisfy the breakdown potential of all the components in the recast layer. Therefore, the removal rate of recast layer is low, unstable, and nonuniform under low current. Nevertheless, the low current also has the advantage of low dispersive corrosion because of the low electric field intensity at the entrance. High current density has higher electric field for rapidly and uniformly dissolving the components of the recast layer; in addition, the increased gas bubbles with the increasing current are beneficial for the removal of electrolytic products. ²³ Hence, as the current I increased from 1.1 to 2.2 A, the processing time t for totally removing the recast layer decreased from 90 to 25 s and the quality of surfaces at the entrance is good, except little dispersive dissolution.

Figure 5.

Frontal micrographs of holes processed at different time under different currents.

To measure the material removal rate by PECM under different current conditions, hole diameters polished under different currents at different time are measured, and the results are shown in Figure 6. When the recast layer has been removed completely, smallest diameters of holes can be obtained, and the results are shown in Table 2.

Figure 6.

Diameters of holes with different time under different processing currents: (a) 1.1 A, (b) 1.5 A, (c) 1.8 A, and (d) 2.2 A.

Table 2.

Diameters of holes after the recast layer has been removed under different currents.

Current I (A)	0.7	1.1	1.5	1.8	2.2
Time t (s)	350	90	50	40	25
Diameter D (mm)	2.66	2.38	2.31	2.33	2.31

From Table 2, it can be inferred that when the current is small, that is, I is 0.7 A, more time is needed to remove the recast layer completely and the uniformity of the material removal is poor. So, eventually, holes with diameters of 2.66 mm are obtained after 350 s. When the current is 1.5 and 2.2 A, the acquired diameter of hole (D = 2.31 mm) is the smallest. The diameter of the hole is 2.13 mm after the EDM process.

Inner-wall roughness and surface morphology of the machined holes are also analyzed. The relationship between the roughness and the current is shown in Figure 7(a). Generally, the roughness decreases with the increasing current. To investigate the influence of the processing time on the roughness, experiments are carried out with parameters in Table 3, in which the processing time are prolonged compared to that in Table 2. Roughness changes with current are shown in Figure 7(b).

Figure 7.

Roughness with different currents: (a) roughness changes with currents (experiment as Table 2), (b) roughness changes with currents (experiment as Table 3), (c) roughness changes with currents (experiment as Tables 2 and 3), (d) surface profile of polished holes obtained under 2.2 A after 40 s, and (e) oblique plots of the polished holes obtained under 2.2 A after 40 s.

Table 3.

Diameters of holes under different currents at a certain time.

Current I (A)	0.7	1.1	1.5	1.8	2.2
Time t (s)	350	120	70	50	40
Diameter D (mm)	2.66	2.43	2.39	2.38	2.38

The results of Tables 2 and 3 are integrated into Figure 7(c). It can be seen that under the same current, surface roughness of hole reduces with the increase in processing time. When the current is 0.7 A, more time is needed to remove the recast layer, and this is the reason why the roughness of the hole surface is relatively low under the current of 0.7 A. It can also be seen from the experiments that high shape accuracy can be obtained through EDM, but the surface roughness is Ra = 4.277 µm. High surface quality can be obtained by subsequent PECM processing, and the lowest roughness is Ra = 0.299 µm, as shown in Figure 7(d) and (e). However, in order to obtain high shape accuracy, processing time should be reduced as far as possible in order to meet the requirements of surface quality.

In order to further illustrate the influence of current on the hole surface quality, the surface morphologies of the inner wall of the hole are analyzed according to Table 2. The SEM images are shown in Figures 8 (30× magnification) and 9 (500× magnification). Figures 8(a) and 9(a) are the inner-wall morphology of the hole after EDM processing. Figures 8(b)–(f) and 9(b)–(f) are inner-wall morphology of the hole after EDM-PECM.

Figure 8.

Morphologies of the inner-wall surface of the hole after EDM and PECM (30× magnification): (a) EDM only, (b) PECM at 0.7 A after EDM, (c) PECM at 1.1 A after EDM, (d) PECM at 1.5 A after EDM, (e) PECM at 1.8 A after EDM, and (f) PECM at 2.2 A after EDM.

Figure 9.

Morphologies of the inner-wall surface of the hole after EDM and PECM (30× magnification): (a) EDM only, (b) PECM at 0.7 A after EDM, (c) PECM at 1.1 A after EDM, (d) PECM at 1.5 A after EDM, (e) PECM at 1.8 A after EDM, and (f) PECM at 2.2 A after EDM.

It can be seen that inner-wall surface of the hole is rough and many micropores exist after EDM processing, while the inner-wall surface becomes smooth after PECM. From Figure 9(b)–(f), it can be inferred that when the current is 0.7 A, the processed surface is smooth. But there exist many micropores, and the number of the micropores reduced with the increasing current. When the current reaches 2.2 A, the number of micropores reduces significantly and the hole surface becomes smooth. It is likely that the anodic dissolution potential of some metal elements in the alloy is high, and low current is inadequate to dissolve the alloy materials evenly. When the current I is 2.2 A, high anodic potential leads to evenly dissolving of alloy elements. Therefore, the number of micropores decreased.

EDM for the holes in the experiment is a top-down process; therefore, inverted cone holes were easily formed (Figure 8(a)). The sequence PECM could reduce tapering of the holes because of the uniformly electrolytic dissolution. The average taper γ of the machined holes is defined as the average of α and β (Figure 10(a)). The taper of the holes machined by EDM was 1.04°. After the PECM, the average taper γ decreased obviously with the increasing current, and more importantly, γ approaches −0.015° and 0.17° at currents 1.8 and 2.2 A during the PECM process, which indicates that high PECM current is also beneficial for high precision.

Figure 10.

The taper of the machined holes: (a) the definition of the hole taper and (b) the hole taper with the current.

According to the experimental analysis, noncircular hole (Figure 11) is obtained by EDM-PECM with optimized parameters. The inner-wall surface is smooth and vertical and the entrance is flat with almost no dispersive dissolution.

Figure 11.

Noncircular hole obtained by EDM-PECM with optimized parameters: (a) Top view, (b) Top left view, and (c) Top front view of the noncircular hole.

Conclusion

EDM-PECM combined machining technique was applied for machining noncircular holes on the superalloy, which is widely used in aero engine but difficult to process. Advantages and disadvantages of EDM-PECM combined machining method were investigated through experiments. The results show that holes obtained through EDM have relatively high shape accuracy but poor surface quality with roughness of up to Ra = 4.277 µm. Recast layer with microcracks is formed on the surface during the EDM process. High surface quality can be acquired by the subsequent PECM, and the lowest roughness can reach Ra = 0.299 µm. The roughness shows general downward trend with the increase in the ECM current. Under the conditions of the same current, the roughness decreases with the increase in ECM time. Therefore, under the premise of meeting the roughness requirement, high current with short processing time should be employed. Importantly, the PECM after EDM could obviously reduce the hole taper induced by top-down EDM process. On account of high accuracy induced by the EDM and the high surface quality produced by PECM, the EDM-PECM process can provide a better way to machine superalloy component with high surface quality.