Simulation and Experimental Study on Improving Electrochemical Machining Stability of High Convex Structure on Casing Surface by Using Backwater Pressure


 Casing parts are regarded as one of the key components in aero-engine components. Most casing parts are attached with different shapes of convex structures, and their heights range from hundreds of microns to tens of millimeters. The use of profiling blocky electrodes for electrochemical machining of casing parts is a widely used method, especially in the processing of high convex structures. However, with the increase of convex structure height, the flow field of machining areas will become more complex, and short circuits may occur at any time. In this study, a method to improve the flow field characteristics of machining area by adjusting the backwater pressure is proposed, the simulation and experiment are carried out respectively. The simulation results showed that the back-pressure mehtod can significantly improve the uniformity of the flow field around the convex structure compared with the extraction outlet mode and the open outlet mode, and then the optimized back-pressure of 0.5 MPa was obtained according to simulation results. The experimental results showed that under condition of the optimized back-pressure parameters, the cathode feed-rate increased from 0.6 mm/min to 0.8 mm/min, and the convex structure with a height of 18 mm was successfully machined. This indicated that the back-pressure method is suitable and effective for the electrochemical machining of high convex structure with blocky electrode.

manufacturing these parts is a significant challenge for traditional machining methods due to serious tool wear, high machining cost, and long machining periods [6][7][8][9].
Electrochemical machining (ECM) is a non-contact machining that based on electrochemical dissolution of anode materials without limitations to the mechanical properties of the alloys [10,11]. Due to the advantages of high material removal rate, no tool wear, etc., ECM has recently become an important processing method for casing parts [12][13][14]. Zhu et al. proposed counter-rotating electrochemical machining (CRECM) technology [15][16][17]. Through the counter rotating movement of the rotary tool electrode and the work-piece, the one-time forming of convex structure on the casing surface can be realized, which effectively improves the machining efficiency and machining quality of the casing parts. Zhang et al. carried out the research on the technology of mask electrochemical machining (M-ECM) of casing parts, broke through the key technologies such as the preparation of the protective film of the rotary surface, and the uniformity control of the flow field, and realized the engineering application for a certain type of casing part [18,19]. However, up to now, the above methods are mainly used for machining of some casing parts with relatively low convexity height.
The use of profiling blocky electrodes for ECM of convex structures on the casing parts is a widely used method, especially in the processing of high convex structure on larger-sized casing parts, due to its high machining efficiency. Li et al. carefully designed a series of working stations and blocky cathode tools. By matching the position of the working stations and the cathode tools, the complex concave-convex structure of the casing parts surface was gradually formed [20]. Sheng et al. carried out the experimental research of ECM of convex structure on the casing surface by using blocky electrode. Based on the experimental results, the optimized flow field and process parameters were obtained, and the machining quality of the casing parts was improved [21,22]. However, it is well known that the flow field is an important factor affecting the machining performance of ECM [23,24]. Compared with other ECM methods such as CRECM and M-ECM, the machining area of blocky electrode method is larger, and the uniformity of flow field within the machining gap is more difficult to control. With the increase of the feed depth of the blocky electrode, the flow field of machining gap will become more complex, and short circuits may occur at any time. Therefore, ensuring the uniformity and stability of the flow field in the machining gap and avoiding the occurrence of "dead water area" has become the most important factor in the process of ECM of high convex structure with a blocky electrode.
In this paper, the flow field characteristics during ECM of high convex structure with blocky electrode are emphasized, and the back-pressure method is proposed to improve the flow field of machining area. Three modes of extraction, open and backpressure on the backwater outlet were studied in detail through the CFD software, and the optimized flow field state was obtained. Afterwards, experiments were conducted and the results indicated that the back-pressure method is suitable and effective for ECM of high convex structure with a blocky electrode.  To realize the back-pressure machining method proposed in this paper, a special block electrode is carefully designed, as shown in Fig. 2. The red area is the main machining area and the rhombic window is the non-machining area to ensure that the anode work-piece surface forms a convex structure. The upper end of the blocky electrode is provided with inlet and back-water outlet. The inlet is responsible for delivering high-speed electrolyte to the entire machining area. The back-water outlet is connected with the window area to adjust the flow field state of the window area. The rhombic window is an important area to ensure the formation of convex structure, and the flow field state is also more complicated. Therefore, the pressure mode of backwater outlet has an important influence on the flow field state of machining gap and the formation of the convex structure. Fluid flow follows the physics conservation law of momentum conservation and mass conservation equations, which can be described as follows: In the simulation model, the electrolyte flow path is complex and the size changes greatly, the flow field is in a complex turbulent state. In this paper, the renormalization group (RNG) k-ε turbulence model, which is very suitable for flows that have high streamline curvatures and strain rates, is adopted.

Machining principle and flow field characteristics
The turbulence kinetic energy equation and dissipation rate equation for the RNG k-ε turbulence model can be described as follows: Where t is the time,

Simulation and results
In this paper, the adopted CFD model is shown in Fig. 3. The light blue area is the inlet of electrolyte, which provides the necessary pressure for the machining process.
In order to reduce the electrolyte velocity in the convex structure area where does not require machining, the backwater outlet is set to release the electrolyte pressure.
The parameters used in the simulation were set as follows: the inter-electrode gap was 0.5 mm, the pressures of inlet and outlet were 1 and 0 MPa respectively, the extraction outlet pressure was 0.4 MPa, the open outlet pressure was 0 MPa, the backwater pressure were 0~1 MPa.

Influence of flow mode on the flow field
In this paper, three modes of extraction outlet, open outlet and back-pressure outlet are designed to obtain a better flow field state. Fig.4

Influence of backwater pressure on the flow field
In this section, the flow field of machining gap under different back-pressures are studied in detail. Fig. 5 shows the electrolyte velocity distribution of the machining gap at back-pressures of 0.2, 0.6 and 1 MPa, respectively. It can be seen that with the increase of back-pressure, the flow field around the convex structure is also improved.  To better explain the above phenomena, this paper establishes the spline curves of the top area and the surrounding area of the convex structure (as shown in Fig. 3), and extracts the electrolyte velocity parameters. Fig. 6 (a) displays the electrolyte velocity distribution of the spline curve 1 around the convex structure under different backpressure. It can be seen that when the back-pressure is less than 0.4 MPa, the electrolyte velocity around the convex structure is less than 4 m/s, which can be considered as a dangerous area that may cause short circuit. With the increase of back-pressure, the electrolyte velocity of dangerous area is improved, and the overall velocity of curve1 is also increased accordingly. Fig. 6 (b) shows the electrolyte velocity distribution of the spline curve 2 on the top of convex structure with different back-pressure. It is obviously that, when the back-pressure is greater than 0.6 MPa, the electrolyte velocity on the top area of the convex structure is already greater than 5 m/s, which indicates that there may be large stray corrosion on the top of convex structure. In summary, the back-pressure method can improve the flow field state of the machining gap, but too high back-pressure will cause excessive stray corrosion in the top area of convex structure that is undesirable during processing. Therefore, the back-pressure of 0.5MPa will be applied preferentially in the following experiments.

Experimental verification
To further verify the effectiveness of the proposed method in this paper, experiments of ECM of high convex structure on a certain casing part surface were also carried out. The experimental set-up consists of tool motion control system, electrolyte circulation system, power supply system, and current direction system, as shown in Fig.   7(a). The cathode fixture is made of fiber-reinforced plastic material, the work-piece fixture is made of stainless steel material, and the assembly site for machining process is shown in Fig. 7(b). In the machining process, the outlet is an open outlet with a constant pressure of 0 MPa, the backwater pressure outlet is an adjustable outlet. The conditions and parameters of the experiments are given in Table 1 Fig. 8 (a). The blocky electrode tool is also damaged as shown in Fig. 9. Fig. 10 shows the current signal collected during machining. It can be seen that the machining time is about 14 minutes, and the corresponding convex structure height is about 8.4 mm, which is significantly higher than that obtained by other electrochemical machining methods such as CRECM and M-ECM etc [10,11]. This illustrates that the blocky electrode method has a significant advantage in the machining of the high convex structure on the surface of

Conclusions
This paper focuses on the flow field characteristics during the electrochemical machining of high convex structure on the surface of casing parts. According to the simulation and experiment results, some conclusions can be drawn as follows: 1)The simulation experiments of three modes were carried out respectively. The results show that the back-pressure mode can significantly improve the uniformity of the flow field around the convex structure and eliminate potential dangerous areas compared with the extraction outlet mode and the open outlet mode.
2)The back-pressure mode was specially studied, and the results show that with the increase of the back-pressure, the flow field around the convex is significantly improved, but the velocity at the top of the convex is also significantly increased, which may aggravate the unnecessary stray corrosion. Then, the optimized backpressure of 0.5 MPa is obtained according to simulation results, which can realize uniform and stable distribution of flow field in machining area.
3)Experiments were conducted to verify the effectiveness of the simulation results.
The experimental results show that under condition of the optimized back-pressure parameters, the cathode feed-rate increased from 0.6 mm/min to 0.8 mm/min, and the convex structure with a height of 20 mm was successfully machined. The results indicated that the back-pressure method is suitable and effective for the electrochemical machining of high convex structure with blocky electrode.