1. Introduction
Electrochemical machining (ECM) is a nontraditional machining technique that relies on electrochemical anode dissolution to remove material [1,2]. Owing to its high processing efficiency [3], the absence of tool wear [4], processing stress, a work-hardening processing zone [5], and its relatively low cost [6,7], ECM has a wide range of applications in aerospace, power industries, medicine, and other fields [4,8,9]. In particular, for complex aerospace components made from difficult-to-machine materials, ECM has significant advantages over traditional processing techniques [4,7,10].
In ECM, the properties of the electrolyte are among of crucial factors [3,11–13]. The electrolyte not only transfers the current and facilitates the electrolytic reaction, but also takes away the electrolytic products, bubbles, heat, etc. [8,14,15], and thus playing a key role in determining processing quality and stability [16]. Numerous studies have explored the relationships between the electrolytes temperature, concentration, type, and machining quality in ECM. For example, Xu et al. [17] explored the influence of electrolyte concentration and temperature on electrochemical dissolution behavior and machining quality in the case of Ti60 alloy. They obtained optimized electrolyte and process parameters for the machining of a blisk sector. Ahn et al. [18] used nanosecond pulse electrochemical micro-drilling in the acid electrolytes to minimize machining clearance and achieve a high-quality surface. Zhang et al. [19] studied the mechanism of material removal in tube electrode electrochemical discharge drilling with a low-conductivity salt solution and compared the results with those of high-speed electrical discharge drilling. Liu et al. [20] utilized a composite electrolytic liquid to process micropores with a high depth-to-diameter ratio, optimizing electrolyte composition based on inlet diameter, material removal rate, and the ratio of removal rate to current. They optimized the electrolyte composition, taking the inlet diameter, material removal rate, and ratio of removal rate to current as evaluation indices. Baehre et al. [21] studied the electrochemical dissolution behavior of titanium and titanium alloys in different electrolytes and showed that electrolytes containing halides are preferred for the machining of these materials. There have also been some studies of the effect on the machining quality of adding complexing agents to the electrolyte on the machining quality. Chen et al. [22] carried out the ECM experiments on stainless steel in sodium chlorate electrolyte added with EDTA disodium salt dihydrate (EDTA-Na2) as a complexing agent, and indicated that EDTA-Na2 is an effective complexing agent for ECM of stainless steel. Wang et al. [16] focused on the ECM behavior of Inconel 718 and its electrolytic product removal mechanism after adding C6H5K3O7 to the NaNO3 solution. In addition, nonaqueous phase electrolytes were further evaluated for using electrochemical micromachining by scholars [23]. TC4 alloy is a popular choice owing to its high performance in the aerospace industry, and there have been numerous studies of ECM using this alloy. Yang and Wang [24] investigated various electrolytes in the ECM of titanium alloys, identifying the effects of the electrolyte on spot corrosion, and proposing a dissolution mechanism. T. Mukesh [25] conducted the ECM experiments of TC4 alloy on radial overcut microporous machining in different electrolytes and found that adding EDTA to the electrolyte improved dimensional characteristics. Yang et al. [26] focused on the influence of electrolyte composition on the corrosion behavior and tribological performance of polyethylene oxide coatings in TC4 alloy and another alloy, and determining optimal electrolyte choices for preparing these coatings. Wang and Qu [27] investigated electrolyte jet machining of TC4 alloy, discovering that increasing the working area of the tool front-end face can enhance the material removal rate.
In recent years, a large amount of research has been conducted on the impact of electrolytes and flow fields on machining accuracy. Tao et al. [28] optimized the flow field structure to reduce wall resistance, increase the electrolyte flow rate, and thus reduce the pressure loss of the electrolyte during the flow process. The experiment proves that the microgroove structure facilitates the flushing of electrolytic products, which is beneficial for improving processing quality and ensuring high processing efficiency. Chen et al. [29] solved the problems related to traditional electrochemical single-pass milling by combining simulation and experimental investigation. The study found that a low electrolyte flow rate in the inter-electrode gap (IEG) weakened the removal of electrolytic products, leading to uneven material dissolution along the depth of the cell, resulting in conical sidewalls. Cao et al. [30] proposed a strategy of periodically changing the direction of electrolyte flow to solve the problem of poor machining accuracy caused by changes in physical states such as conductivity in the electrolyte process and established a multi-physics field simulation model to reveal the mechanism of counter-rotating ECM.
Despite previous extensive studies, the intricate laws between electrolyte filtration accuracy and machining quality remain inadequately understood, in particular for TC4 alloy. Electrolyte cleanliness is an important factor reflecting the performance of ECM equipment [13]. To ensure the stability of ECM, most of the larger flocculate products of electrolysis are removed by an electrolyte filtration device [14]. However, some of these products remain in the electrolyte in the form of micro-particles, with the amount depending on the filtration accuracy of the electrolyte filtration device. High precision and high surface quality in ECM require better consistency of the state of the electrolyte, however, the presence of electrolytic products can have a deleterious impact on this [28].
This paper thus presents a comprehensive study to shed light on the intricacies of the relationship between electrolyte filtration accuracy and machining quality and provides new insights to further enhance the machining quality. The results show that when the electrolyte filtration accuracy is improved, so too is the machining quality of the ECM. However, once the filtration accuracy has reached a certain value, the machining quality has extremely limited improvement. For the TC4 alloy and conditions considered in this paper, when the filtration accuracy of the electrolyte is further improved by more than 0.5 μm, there is no longer a positive correlation between electrolyte filtration accuracy and machining quality.
2. Experimental
2.1 Experimental details
The anode workpiece used in this study was a forged TC4 alloy. Its detailed chemical composition is given in Table 1. The microscopic morphology and chemical composition of the workpieces were measured by field-emission scanning electron microscopy (S4800, Hitachi, Japan) and energy-dispersive x-ray spectroscopy (XFlash 5010, Bruker, Germany), respectively, as shown in Fig. 1. For the experiments, the workpiece sample used in the experiments were cut to 20 × 20 × 20 mm3, with only one surface exposed to the electrolyte. Before the experiment, the workpiece surface was polished with wet sandpaper to remove the self-passivation oxide film. The electrolyte was 10% NaCl solution. After ECM, the surface quality of the workpiece was evaluated by a surface-finish measuring instrument (Perthometer M1, Mahr GmbH, Germany). The macro profiles of the machined surfaces were observed with an optical microscope (DVM5000, Leica, Germany). The machining accuracy of the workpiece was measured by a coordinate measuring machine (TESA Micro-Hite 3D, TESA, Switzerland). Additionally, the particle size distribution of the filtered electrolyte was determined by using a particle size analyzer (Mastersizer 2000, Malvern, England).
2.2 Electrolyte preparation and characterization
The electrolysis products of the TC4 alloy are nonconductive metal oxides that can accumulate on the workpiece surface and hinder machining [29–31]. Fig. 2(a) shows the principle of ECM and Fig. 2(b) shows the microstructure of the dried electrolytic products after the dissolution of the TC4 alloy. The formation and diffusion of these products are affected by many factors, including temperature and flow rate [32]. In an unfiltered electrolyte, the electrolytic products usually exist in a flocculent form or as large masses. To address this issue, the turbid electrolyte, containing abundant electrolytic products, was filtered using a custom-built electrolyte filtration device, with the electrolyte filtration accuracy being accurately distinguished governed by the use of ceramic membrane tubes with a different pore size of 50 μm, 5 μm, 0.5 μm, 0.2 μm, and 0.05 μm, respectively. Electrolytic products exceeding the given pore size were separated from the electrolyte, and the filtered electrolyte was used for the final experiments. The difference in electrolyte filtration accuracy was reflected in changes to the electrolyte physical properties, including electrolyte conductivity, viscosity, and particle size distribution, as shown in Fig. 3.
It was found that the particle size of the flocculent electrolysis products in the electrolyte was normally distributed. With the improvement of electrolyte filtration accuracy, the flocculent electrolysis products in the electrolyte are gradually filtered until they can not be detected, as shown in Table 2. Moreover, the kinematic viscosity of the electrolyte decreases as the filtration accuracy increases, mainly because of the reduced content of viscous flocculent and solid products.
3. Simulations
3.1 Simulation conditions
To explore the influence of electrolyte filtration accuracy on the flow field in the machining area, simulations and analyses were performed for the electrolyte with different filtration accuracies used in the experiments. The conductivity and viscosity of the electrolytes were measured, and these measured values were used as variable conditions. The electrolyte flow is governed by the turbulent k–ɛ equation and the Navier–Stokes equation, i.e.,
where μ [kg/m/s] is the dynamic viscosity of the electrolyte, p [Pa] is the pressure, u [m/s] is the flow speed, and ρ [kg/m3] is the electrolyte density.
The numerical simulations were implemented by using the COMSOL software to analyze the temperature rise and change in bubble rate in the flow field during the machining process. Fig. 4 depicts a simplified physical model for the numerical simulation of the flow field. Several assumptions were adopted for the numerical simulation [33–35] and the simulation parameters are shown in Table 3.
3.2 Analysis of simulation results
The simulation results indicated that the electrolyte maximum temperature in the processing area decreases from 315.7 K to 311 K, as the electrolyte filtration accuracy increases from 50 to 0.05 μm, as shown in Fig. 5(a), the Joule heating in the process-Joule heating in the processing area correlates certain with the electrolyte filtration accuracy.
As the simulation continues, the influence of electrolyte filtration accuracy on the bubble rate in the machining area can be seen. The results in Fig. 5(b) show that, when the electrolyte filtration accuracy is 50 μm, the bubble rate in the processing area reaches 20.2%. When the filtration accuracy is further increased, the bubble rate in the processing area gradually decreases, and when the filtration accuracy is 0.05 μm, the bubble rate is 11.6%, which represents a reduction of nearly 43.5%. The reason for this large reduction may be that when the electrolyte filtration accuracy is low, the electrolyte viscosity is higher, and the bubble’s escape speed is slower than in an electrolyte with high filtration accuracy. Moreover, in an electrolyte with low filtration accuracy, more bubbles are generated per unit of time.
4. Machining Experiments
To quantitatively analyze the influence of electrolytes with different filtration accuracy on machining accuracy and surface quality, the electrolyte with different filtration accuracies were used in the experiments. The main parameters of the experimental machining process are shown in Table 4.
The machining experiments were carried out on a custom-built platform, shown in Fig. 6, consisting of ECM equipment, a power supply system, and a ceramic membrane filtration system, together with control and auxiliary components.
In ECM, the anode workpiece material reacts with the ions in the electrolyte under the action of an applied electric field, leading to the material being removed from the workpiece in an ionic state [2,14]. The microstructure of the anode material, therefore, has a direct impact on the surface morphology and quality of the workpiece [17]. For the machining experiments described herein, electrolytes were prepared with different filtration accuracies as described above in Section 2.2. The temperature control device ensured that the electrolyte temperature remained constant throughout the experiment. Each group of experiments was repeated three times with the same parameters to obtain more accurate and reliable experiment results. The effects of filtration accuracy on machining quality were investigated from the perspectives of machining accuracy, surface quality and repetition accuracy.
5. Results and Discussion
5.1 Material dissolution
As shown in Fig. 7(a), for an electrolyte filtration accuracy of 50 μm, the dissolution of the workpiece surface was very uneven, because the flocculent products in the electrolyte led to a nonuniform electric field in the processing area [31,32,36]. In addition, the high viscosity of this electrolyte and the low filtration accuracy lead to a slow rate of removal of electrolytic products and bubbles from the processing area, and the processing surface is covered by the viscous products, resulting in an uneven dissolution of materials, flow marks and short-circuit burning of the workpiece surface. The presence of the viscous product film and the flocculent electrolytic products in the electrolyte increase the resistivity of the anode surface far above normal, and the current density was relatively low, which cause pitting of the workpiece surface, as shown in Fig. 7(b) [30,37,38]. Upon improving the filtration accuracy of the electrolyte to 5 μm, the dissolution of the workpiece material became more uniform. This is again related to the current density, which is influenced by the viscous passive film and the electrolytic products. When the electrolyte filtration accuracy was improved to 0.5 μm, the dissolution at the material surface became more uniform, and the workpiece microstructure became visible. With a further improvement in filtration accuracy to 0.05 μm, the surface of the dissolved workpiece material became flat, and the grain boundaries of the substrate became visible in Fig. 7(c).
5.2 Surface quality
Fig. 9 shows the macro morphology and surface quality of workpieces processed using electrolytes with different filtration accuracies. It can be observed that with the improvement of electrolyte filtration accuracy, the surface morphology of the workpiece gradually improves. When the electrolyte filtration accuracy is 50 μm, the obvious non-uniform dissolution areas on the surface can be observed in the workpiece. When the filtration accuracy further improves to 0.5 μm, or even higher, the surface of the workpiece becomes smoother. Meanwhile, the edges of the micro-grooves are also neater compared to 50 μm. Simultaneously, the surface roughness values of grooves processed with the electrolyte with 0.05 μm filtration accuracy are lower than those processed with the other electrolytes. The average surface roughness of the workpieces processed with electrolytes with filtration accuracies of 0.05 μm, 0.2 μm, 0.5 μm, and 5 μm are Ra = 0.753 μm, Ra = 0.872 μm, Ra = 1.004 μm, and Ra = 1.425 μm, respectively. Short circuit burns and significant flow marks are evident on the surfaces of the workpieces processed with the electrolyte with 50 μm filtration accuracy, making it difficult to accurately detect the surface roughness. These results indicate that the electrolyte filtration accuracy is more than 5 μm, and the filtration accuracy enables the production of a smoother machined surface. The current density during the processing of electrolytes with different filtration accuracies is shown in Fig. 8. A possible reason for the result is that the kinematic viscosity of the electrolyte gradually decreases as the electrolyte filtration accuracy increases, thereby facilitating heat and bubbles removal from the machining area [38]. Hence, the current density is less affected by factors such as bubbles and processing heat in the machining area. The electrolytic products generated on the anode surface are thus more easily removed and the anode material dissolved more evenly. In the case of an electrolyte with low filtration accuracy, the electrolytic products will interfere with the electric field between the anode and cathode, making the electric field nonuniform in the processing area [28,40,41]. It is evident that with the improvement of electrolyte filtration accuracy, the surface quality of the workpiece gradually improves. However, when analyzing the surface quality of each group of workpieces separately, it was found that there was a certain difference in the surface quality of each workpiece. When the filtration accuracy of the electrolyte is 0.5 μm, 0.2 μm, and 0.05 μm, the difference in surface quality of the workpiece is 0.083 mm, 0.083 mm, and 0.015 mm, respectively. In other words, when the filtration accuracy of the electrolyte is 0.5 μm, the difference in surface quality of the three processed workpieces is the smallest.
As shown in Fig. 8, with the improvement of electrolyte filtration accuracy, the current value during the processing also increases. When the electrolyte filtration accuracy is 0.5 μm, the current is approximately 157 A and the current density is 35.6 A/cm2. It is evident that in the final stage of processing, the current value remains stable. In other words, when processing enters a balanced state, the electric field distribution on the surface of the workpiece is relatively uniform, resulting in a smoother machining surface. This is consistent with the results obtained when the electrolyte filtration accuracy is 0.2 μm and 0.05 μm.
Owing to the presence of viscous electrolytic products on the anode material surface, the resistivity between the anode and cathode is higher [37]. Hence, the current density on the surface of the workpiece is low, the dissolution rate of anode material decreases, and there may even be a short circuit. A further possible reason for the poor surface quality when an electrolyte with low filtration accuracy is that the presence of electrolytic products in the electrolyte facilitates the formation of a passive film on the workpiece surface. Owing to its high electrical and chemical resistance, this dense oxide layer is difficult to dissolve in the electrolyte. This will lead to nonuniform removal of material from the workpiece surface and thus poor surface quality [20,37,40]. According to the above analysis, the most consistent surface quality of the workpiece is obtained with an electrolyte filtration accuracy of 0.5 μm.
5.3 Cross-sectional profile and repeatability
Fig. 10 shows profile accuracy curves for three workpieces processed using electrolytes with different filtration accuracies electrolytes under the same machining parameters. For the electrolyte with different filtration accuracies, it can be seen that the profile accuracy at the electrolyte flow field inlet is better than that at the outlet. This indicates that in ECM, the electrolyte flow field at the inlet is hardly affected by the generation and diffusion of electrolytic products. In addition, electrolyte filtration accuracy has the least effect on the temperature and bubble rate of the flow field near the inlet. For 0.05 μm electrolyte filtration accuracy, the maximum deviation of 0.051 mm in the workpiece profile occurs at the flow field outlet, as shown in Fig. 10(a). The workpiece profile accuracy curves for machining in the electrolyte with 0.5 μm filtration accuracy under the same parameters are shown in Fig. 10(c). The maximum deviation of 0.0617 mm again appears at the flow outlet, which is the same result as for machining in electrolytes filtered with 0.05 μm, 0.2 μm, and 5 μm filtration accuracy. Fig. 10(e) shows workpiece profile accuracy curves for machining with 50 μm filtration accuracy. The maximum deviation of the workpiece is 0.0872 mm. Near the outlet, the profile deviations are large and irregular, which may be attributed to the fact that for electrolytes filtered with low filtration accuracy, the high kinematic viscosity of the electrolyte and the low removal rate of newly generated electrolytic products lead to nonuniformities in the electric field and flow field in the machining gap, which destabilizes the machining process, and thus to poor profile accuracy [38,42].
The workpiece profile accuracy was determined by a coordinate measuring instrument. To illustrate the relationship between electrolyte filtration accuracy and machining repetition accuracy, this is expressed in terms of the mathematical variance E(xi), which is calculated as follows:
Where, xi denotes the machining accuracy, with i = 1, 2, 3, 4, and 5 corresponding to the electrolytes filtration accuracies of 0.05 μm, 0.2 μm, 0.5 μm, 5 μm, and 50 μm, respectively. The serial numbers of workpieces used in the experiment, are denoted by j = 1, 2, and 3.
| x i j m a x - x i j m i n ¯ |
The above analysis indicates that by improving the electrolyte filtration accuracy from 50 μm to 0.5 μm, the repetition accuracy of the workpiece is also significantly improved by 0.0211 mm. When the electrolyte filtration accuracy is increased from 0.5 μm to 0.05 μm, the workpiece repetition accuracy improves by 0.0075 mm, as shown in Table 5. In this range, the improvement in repetition accuracy achievable by improving the electrolyte filtration accuracy decreases sharply. The filtration accuracy of the electrolyte has been improved from 50 μm to 0.05 μm, and the repetition accuracy of the workpiece has also gradually improved. Calculating the variance of the profile accuracy of the workpiece indicates that when the electrolyte filtration accuracy is less than 0.5 μm, the variance in the profile accuracy decreases gradua positive correlation between electrolyte filtration accuracy and repetition accuracy. When the electrolyte filtration accuracy is greater than 0.5 μm, however, this correlation no longer exists, as shown in Fig. 11. From the experimental results of this article, it can be seen that when the filtering accuracy is improved to 0.5 μm, the surface quality and processing accuracy of the workpiece can meet most of the processing technology requirements. However, when the electrolyte filtration accuracy was further improved from 0.5 μm to 0.05 μm, the improvement in repetition accuracy was very limited. By analyzing the variance of repetition accuracy, it was found that the electrolyte filtration accuracy is within this range, and there is no correlation between filtration accuracy and repetition accuracy.
Further improvements in filtration accuracy are better, which leads to sharply increasing economic costs while decreasing filtration efficiency. Therefore, it can be concluded that an electrolyte filtration accuracy better than 0.5 μm will not be suitable for the wide application of precision ECM.
6. Conclusions
This paper has investigated the laws between electrolyte filtration accuracy and machining quality. TC4 alloy workpieces were processed in electrolytes with different filtration accuracies, and the surface quality, machining accuracies, and repetition accuracy were evaluated and compared. From the results of both simulations and experiments, the following conclusions can be drawn:
The simulation results indicated that the electrolyte filtration accuracy improves from 50 to 0.05 μm, and the Joule heating decreases in the processing area from 315.7K to 311K. This indicates that the Joule heating in the processing area correlates certain with the electrolyte filtration accuracy. Simultaneously, the bubble rate in the processing area decreases from 20.2% to 11.6%, i.e., by a decrease of nearly 43.5%. The results mean that the electrolyte filtration accuracy has a more significant impact on the bubble rate compared to the temperature in the processing area.
The results of the ECM experiments showed that increasing the electrolyte filtration accuracy improves the workpiece machining quality. Measurement of the workpieces found that the surface roughness decreased from Ra=1.425 μm to Ra=0.753 μm as the filtration accuracy improved from 5 μm to 0.05 μm, the workpieces surface quality has been significantly improved, reaching 89.24%. As the electrolyte filtration accuracy improved from 50 μm to 0.05 μm, the machining accuracy improved from 0.0863 mm to 0.0425 mm, and the repetition accuracy improved from 0.0689 to 0.0403 mm. When the electrolyte filtration accuracy was higher than the level range of 0.5 μm, further improvements in filtration accuracy produce only a very limited gain in machining quality.
The trade-off between the improved machining quality and increasing economic cost means, that an electrolyte filtration accuracy maintained within the level range of 0.5 μm is a reasonable choice for further large-scale development of precision ECM technology. Especially for manufacturing blade profiles or inlet and exhaust edges, the electrolyte filtration accuracy in the level range of 0.5 μm is contributed to obtaining better machining quality.
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