Counter-rotating Electrochemical Machining Research Articles

Improvement on profile accuracy of convex structure during counter-rotating electrochemical machining by periodically changing electrolyte flow direction

Counter-rotating electrochemical machining (CRECM) is an innovative electrochemical machining (ECM) method, which can be used to machine thin-walled casings. However, due to the complex electrode movement and the special flow field structure, there are some challenges in the uniformity control of the profile accuracy of convex structure during CRECM. In this paper, to improve machining accuracy of convex structures, a strategy of periodically changing electrolyte flow direction is proposed. A multi-physical field simulation model is established to reveal the CRECM mechanism. The simulation results show that the one-sided electrolyte supply causes the uneven distribution of flow rates, bubbles and temperature on both sides of the convex structure, mainly because the convex structure forces the electrolyte flow path to bend and increase electrolyte flow resistance. The conductivity near the inlet is higher than that at the outlet, which causes the profile of the convex structure to be asymmetric. However, this problem can be effectively solved by changing electrolyte flow direction periodically, because the average conductivity at the symmetrical positions on both sides of convex structure is consistent, the average material removal rate is the same, and the manufactured convex structure is axisymmetric. CRECM experiments are carried out, and the processed convex structure has good symmetry, and the deviation is <0.05 mm. This fully demonstrates that periodically changing electrolyte flow direction can significantly improve the forming accuracy of the convex structure.

Localized sinking electrochemical machining of pre-shaped convex structure on revolution surface using specifically designed cathode tool

Counter-rotating electrochemical machining (CRECM) is an effective method for machining thin-walled revolving parts with complex structures. However, it is difficult to achieve a desirable uniform straight sidewall during the CRECM of isolated convex structures with variable cross-sectional profiles, so a subsequent local machining process is necessary. In this paper, a localized sinking ECM method is proposed to locally machine a pre-shaped convex structure by CRECM. A mathematical model is established to analyze the anode shaping process. In addition, a special cathode tool is designed to control the junction mark between the subsequent dissolution regions and the surrounded non-machined areas. The simulation and experimental results indicate that the sidewall profiles of the convex structure can be improved remarkably. In lateral view, convex side wall taper is reduced from 21.81° to 3.05°, In front view, convex side wall taper is reduced from 6.45° to 0.28°, Furthermore, the junction mark can be effectively controlled to within 0.07 mm.

Research on Multi-Physical Field Coupling Simulation of Local Electrochemical Machining

Casing is one of the most important components of an aircraft engine. However, due to its thin wall thickness and difficult-to-cut materials, it is difficult to process with a conventional mechanical method. Counter-rotating electrochemical machining (CRECM) is a special electrochemical machining method, which is very suitable for machining aircraft engine casing parts. However, for the convex structure with large surface height and a complex shape of the casing, is sometimes difficult for CRECM to obtain the desired design accuracy. Local electrochemical machining is proposed under this background, which is used for after-machining of the pre-shaped convex structure by CRECM. In order to predict the local electrochemical machining result accurately and improve the machining precision, this paper establishes a multi-physical field coupling simulation model of the local electrochemical machining considering the influence of gas–liquid two-phase flow and temperature field. The influence of a gas–liquid two-phase flow field and temperature field on the conductivity distribution were simulated and analyzed, the reason for simulation error with pure electric field and the influence of cathode end width L on machining accuracy was analyzed, and it was found that the gas–liquid two-phase flow field played a major role in the simulation results of local electrochemical machining. The experimental results show that there is a significant error between the pure electric field simulation results and the experimental results, and the multi-physical field coupling simulation results are basically consistent with the experimental results. The multi-physical field coupling simulation can predict the results of local electrochemical machining with high accuracy and has important significance for improving the precision of local electrochemical machining.

Investigation of the Shaping Process of Island-like High Convex Structure on Thin-walled Revolving Part during Counter-rotating Electrochemical Machining

Investigation of the Shaping Process of Island-like High Convex Structure on Thin-walled Revolving Part during Counter-rotating Electrochemical Machining

Improvement of Surface Quality for Titanium Alloys During Counter-Rotating Electrochemical Machining Using an Auxiliary Cathode

Counter-rotating electrochemical machining (CRECM) is an innovative ECM method that can be used to manufacture revolving parts, especially titanium alloy casings. Because titanium alloys are easily passivated metals, sufficient electrical quantity is required to achieve the transition from pitting to smoothing with a lower speed. However, a low rotational speed means more material is removed in a machining cycle, which is not conducive to wall thickness control. Therefore, achieving fast smoothing at a relatively high rotational speed for titanium alloys is focused. In this paper, the proposed methodology applies an auxiliary cathode to accelerate the transition from pitting corrosion to smoothness of titanium alloys at a relatively high rotational speed. Experimental results show that titanium alloy can be smoothed regardless of rotational speed with an auxiliary cathode. The auxiliary cathode can weaken the off-current density region and accelerate the breakdown of the oxide layer, and the layer-by-layer dissolution of micron-thick materials is achieved at a relatively high rotational speed. The grid-like convex structure is produced at a relatively high rotational speed with an auxiliary cathode, and the surface roughness is 1.808 μm. This fully demonstrates the reliability of the auxiliary cathode for achieving the smoothness of titanium alloys.

Improvement on the machining accuracy of titanium alloy casing during counter-rotating electrochemical machining by using an insulation coating

Counter-rotating electrochemical machining (CRECM) is an innovative ECM method that can be used to manufacture revolving parts, especially thin-walled casings with complex convex structures. As titanium alloys are easy to passivate and difficult to machine, active NaCl solution is often used to obtain a good surface quality from ECM. However, the top surfaces of convex structures will be subjected to severe over cutting using NaCl solution, resulting in poor machining accuracy. Therefore, an insulation coating is employed to protect the non-processed area by shielding stray currents in this paper. A simulation model is established to investigate the evolution of convex structures with an insulation coating. According to the simulated convex structures, the top of the convex structure suffers from serious stray corrosion without the protection of insulation coating, and the sidewall inclination angle is 25.42°. However, the insulation coating enables convex structures without over cutting, achieving a sidewall inclination angle of only approximately 1.1°. Under the protection of insulation coating, the width and height decrease linearly with time, and the radius of the root fillet increases towards an approximately constant value. The experimental results indicate that stray currents can be shielded completely with an insulation coating, and no electrochemical dissolution occurs on the top of the convex structure. The deviation between simulation and experimental results does not exceed 10 %, demonstrating reliability of the proposed model. Cylindrical and conical parts with a grid-like convex structure are successfully produced with a surface roughness value of Ra1.8 μm. This fully verifies the effectiveness of an insulation coating and demonstrates the excellent processing capability of CRECM.

Anodic dissolution mechanism of TA15 titanium alloy during counter-rotating electrochemical machining

Anodic dissolution mechanism of TA15 titanium alloy during counter-rotating electrochemical machining

Analysis of the roundness error elimination in counter-rotating electrochemical machining

Roundness error plays an important role in numerous science and engineering fields, and it seriously affects the surface machining accuracy of revolving parts. Counter-rotating electrochemical machining (CRECM) has huge advantages in eliminating the roundness error of the blank, especially for thin-walled casings. In this study, a simulation model is established using the finite element method to analyze the mechanism of the elimination of roundness errors during CRECM. The rotation and feed motion of the electrodes are realized by kinematic equations, and the characteristics of material removal on the workpiece surface are explained. The simulation results show that due to the uneven surface of the revolving part, the current density presents a varying distribution, and the electric field is concentrated at raised points. The material removal rate at raised points is thus much greater than that at sunken points, and the raised points require more power during the material removal process. By optimizing the simulation parameters, the minimal interelectrode gap can be maintained substantially constant. The reliability of the simulation methodology is verified by experiments. The machined thin-walled parts are found to have good consistency of wall thickness, and an error is only around 0.028 mm.

Counter-rotating electrochemical machining of intensive cylindrical pillar array using an additive manufactured cathode tool

Pillar arrays on metallic surfaces gain important applications in industry, however the machining of them still remains great challenges for conventional methods. In this paper, counter-rotating electrochemical machining (CRECM) using a small cathode tool with multiple hollow windows was proposed to fabricate intensive cylindrical pillar array on revolving surface. The cathode tool was designed on basis of the anode shaping process in CRECM. The results indicated that in order to obtain desired circular pillar structures, the hollow windows on the cathode tool should be designed to be non-circular shapes and their sidewalls needed to be electrically insulated. These multiple specific-designed hollow windows bring challenges for conventional methods.To machine the specific-designed cathode tool, a novel hybrid additive manufacturing process combining 3D printing and electrodeposition was presented. A cylindrical cathode tool with multiple non-circular insulating hollow windows was successfully fabricated through electrodeposition on a 3D printed base part with partial conducive treatment. CRECM experiment of intensive pillar array was conducted using the additive manufactured cathode tool. An intensive (60 × 7) cylindrical pillar array with average dimensions of 4.3 mm diameter and 2.4 mm height was machined on the revolving surface. The profiles of different pillar structures exhibited favorable uniformity.

Optimize the flow field during counter-rotating electrochemical machining of grid structures through an auxiliary internal fluid flow pattern

Flow field distribution plays a vital role in electrochemical machining (ECM) because it can directly affect the machining stability and accuracy of ECM. In counter-rotating electrochemical machining (CRECM), the uniformity of the flow field is difficult to control due to the complicated and changeable flow channel shape. Through the simulation of the conventional lateral fluid flow pattern, it is found that the complexity of the flow channel with grid structures makes the flow field of machining area strongly disordered, which leads to the low velocity zones and dead zones. Based on the simulation results, a new electrolyte flow pattern with an auxiliary internal fluid is proposed, which can remarkably improve the uniformity of flow field by apply supplementary electrolyte to the machining area. Experimental results show that the new flow pattern effectively improves the machining stability of CRECM, and enhances the machining precision of grid structures, the sidewall taper angle is reduced from 29.3° to 7.7°.

Evolution of convex structure during counter-rotating electrochemical machining based on kinematic modeling

Counter-rotating electrochemical machining (CRECM) is a novel electrochemical machining (ECM) method, which can be used to machine convex structures with complex shapes on the outer surface of casings. In this study, the evolution of the convex structure during CRECM is studied. The complex motion form of CRECM is replaced by an equivalent kinematic model, in which the movement of the cathode tool is realized by matrix equations. The trajectory of the cathode tool center satisfies the Archimedes spiral equation, and the feed depth in adjacent cycles is a constant. The simulation results show that the variations of five quality indexes for the convex structure: as machining time increases, the height increases linearly, and the width reduces linearly, the fillets at the top and root fit the rational function, and the inclination angle of the convex satisfies the exponential function. The current density distributions with different rotation angles is investigated. Owing to the differential distribution of current density on workpiece surface, the convex is manufactured with the cathode window transferring into and out of the processing area. Experimental results agree very well with the simulation, which indicates that the proposed model is effective for prediction the evolution of the convex structure in CRECM.

Modeling and experimental validation of interelectrode gap in counter-rotating electrochemical machining

Counter-rotating electrochemical machining (CRECM) holds the unique advantages to machine thin-walled revolving parts, especially for thin-walled engine cases. The electric field intensity distribution on the revolving anode surface is non-uniform, which is difficult to calculate accurately. In this work, a novel equivalent model for analytic solution of the electric field in CRECM is proposed based on complex variable function, and anode material dissolution model is established for CRECM. The variations of the material removal rate (MRR) and interelectrode gap (IEG) at different machining parameters have been fully investigated. The results indicate the MRR can reach a relative equilibrium state after a transition stage. The relative equilibrium MRR is higher than the feed rate, leading to a linear increase in the IEG. Moreover, machining parameters are optimized numerically to shorten the time reaching the relative equilibrium state. Experiments are conducted to verify the theoretical results. The experimental results fit well with the theoretical values. It means that the proposed model is effective for the prediction of the material dissolution process in CRECM.

Investigation of the anode shaping process in counter-rotating electrochemical machining of a convex array

This paper presents the formation processes of convex arrays on revolving parts by using synchronous and differential counter-rotating electrochemical machining (CRECM) methods, in which cylindrical cathode tools with concave cavities rotate relatively to the anode workpiece at same and multifold rotational speeds. The anode shaping processes are numerically simulated on basis of a mathematical model. The profiles of the convex structures at rotating ratios of 1:1 and 5:1 are compared, and the shape evolution process at different feed depths of cathode tool is analyzed. The simulation results indicate that differential CRECM can achieve an approximate machining performance of a convex array with synchronous CRECM by using suitable diameter of the cathode tool. The profile of the convex structure changes from a tapered shape to an inverted cone shape with the increase of the feed depth of cathode tool. Experiments are conducted to verify the numerical analysis. An equal-size cathode tool with five concave cavities at the same rotation speed and a small cathode tool with a single concave cavity at a rotating ratio of 5:1 are used, respectively. It is seen that five array distributed convex structures are machined successfully on the anode surface by using both synchronous and differential CRECM methods. Furthermore, convex structures with tapered, straight and inverted conical sidewalls can be obtained by changing the depths of cathode tool in differential CRECM.

Experimental research on the counter-rotating electrochemical machining of 304 stainless steel and Inconel 718 alloy

Experimental research on the counter-rotating electrochemical machining of 304 stainless steel and Inconel 718 alloy

Analysis and control of inter-electrode gap during leveling process in counter-rotating electrochemical machining

The inter-electrode gap (IEG) is an essential parameter for the anode shaping process in electrochemical machining (ECM) and directly affects the machining accuracy. In this paper, the IEG during the leveling process of an oval anode workpiece in counter-rotating ECM (CRECM) is investigated. The variation of the minimum IEG is analyzed theoretically, and the results indicate that rather than reaching equilibrium, the minimum IEG in CRECM expands constantly when a constant feed speed is used for the cathode tool. This IEG expansion leads to a poor localization effect and has an adverse influence on the roundness of the machined workpiece. To maintain a small constant IEG in CRECM, a variable feed speed is used for the cathode based on a fitted equation. The theoretical results show that the minimum IEG can be controlled at a small value by using an accelerated feed speed. Experiments have been conducted using a specific experimental apparatus in which the cathode tool is designed as a combined structure of two sectors and a thin sheet. By detecting the machining currents flowing through the minimum IEG, how the latter varies is obtained indirectly. The results indicate that using an accelerated feed speed is effective for controlling the IEG, thereby improving the roundness of the machined workpiece.

Formation of a Convex Structure During the Counter-Rotating Electrochemical Machining of TC4 Titanium Alloy in NaCl Solution

Formation of a Convex Structure During the Counter-Rotating Electrochemical Machining of TC4 Titanium Alloy in NaCl Solution

Research on counter-rotating electrochemical machining of convex structures with different heights

Counter-rotating electrochemical machining (CRECM) is an alternative method for the manufacture of aero-engine casing parts with complex convex structures. In previous studies, the revolving part with hollow windows was used as the cathode tool. However, only convex structures with the same height could be machined. In practice, there are usually differences in height between different convex structures on the surface of most casing parts. To machine convex structures of different heights by using CRECM, a specific cathode with concave cavities has been designed. The bottom surfaces of concave cavities are conductive, and the depths of concave cavities are changed according to the height of convex structures. The material of the workpiece in this study is Inconel 718. Forming processes of convex structures with the cathode are simulated by using the finite element method. Results show that during the processing, the top surface of the convex structure is gradually changed from stray corrosion at low current density to normal dissolution at high current density. The height of the convex structure increases and then approximately keeps constant with increasing amount of feed, and it also can be controlled by changing the depth of the concave cavity. Finally, convex structures of different heights on the cylindrical workpiece are machined experimentally, verifying the feasibility of the method.

Analysis of the flow field in counter-rotating electrochemical machining

In electrochemical machining (ECM), the electrolyte flow field has marked influence on the processing stability, efficiency and surface quality. This paper presented a new electrolyte flow mode to improve the flow field distribution in counter-rotating electrochemical machining (CRECM), in which the electrolyte flows into the processing zone from both sides and top of the fixture, and flows out from the bottom of the fixture. The flow field distributions were analyzed and simulated via computational fluid dynamics (CFD) software. The simulation results showed that the low-velocity area in CRECM was eliminated and the velocity distribution was more uniform by using the optimized flow mode. Experiments were also conducted using a specific fixture. The experimental results indicated that a stable machining process with a small inter-electrode gap could be achieved, and the ECM accuracy was remarkably improved. The sidewall taper angles and rounded corners of the machined convex structure were reduced to be only about -0.8° and 0.6 mm.

Investigation of Pulsating Electrochemical Dissolution of Nickel in Rotating Processes

In counter-rotating electrochemical machining, the current density at a certain point on the workpiece surface presents a pulse characteristic due to the rotation of the workpiece. The waveform and the period of the current for material dissolution are different from that in conventional pulse ECM, resulting in different dissolution performances. In this paper, the pulsating dissolution behaviors of nickel in rotating processes are investigated. Nickel sheets are particularly used to investigate the dissolution processes during one rotation cycle and at different rotational speeds. The machined nickel surfaces are examined by using scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The results indicate that the pulsating dissolution of nickel depends strongly on the rotational speed. Passivation, pitting, high rate dissolution and selective dissolution occur successively during one cycle at low rotational speeds. While, when the rotational speed is over 50 rpm, passivation and pitting alternately happen due to the shortening of the pulse period, which results in rapid decrease in material removal weight. Based on the above research, it can be concluded that the low rotational speeds are more favorable for the effective machining of nickel in CRECM.

Counter-Rotating Electrochemical Machining of a Convex Array Using a Cylindrical Cathode Tool with Multifold Angular Velocity

A modified counter-rotating electrochemical machining (CRECM) method is proposed for machining a convex array on rotary part, in which a small cylindrical cathode tool rotates in the opposite direction to and at n times the angular velocity of the anode workpiece. The trajectories of the cathode tool and the profiles of the convex structure were analyzed numerically. The electrolyte flow distributions in immersed and enclosed flow mode were simulated. The results indicated that tapered, straight, and inverted tapered sidewalls can be obtained for cathode tools with different radii, and the enclosed flow mode can give a better flow of electrolyte. Experiments were conducted using small cathode tools with different radii in a specially designed electrolyte cell. Five convex structures were fabricated on the surface of the anode workpiece using a small cathode tool with a single concave cavity. The sidewalls were near straight with small taper angles of 0.71° and −0.61°. Moreover, a cylindrical anode workpiece with an extensive convex array on the surface was fabricated successfully using a small cathode tool with multiple convex cavities. This indicated that the proposed method can offer a superior machining ability for the rotary part with array structures.