An electrochemical machining method for aeroengine blades based on four-directional synchronous feeding of cathode tools

Jia LIU, Libing HUI, Dongqian JIA, Yan LIU, Di ZHU

College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

KEYWORDS

Abstract Blades are critical components of modern aero-engine.Among the many characteristic structures of the blade,the leading/trailing edges are key structures that have the greatest influence on the aerodynamic effect and power conversion of the blade.Electrochemical machining(ECM)is regarded as one of the most important techniques in blade manufacturing due to its process characteristics of high material removal rates, virtually no tool wear, and no areas of thermal or mechanical damage to the workpiece rim zones.Herein, an ECM method based on the fourdirectional synchronous feeding of four cathode tools is proposed to improve the machining accuracy of the leading and trailing edges of the blade.During blade ECM using this method,four cathode tools feed toward the basin/back surfaces and leading/trailing edges respectively.The dynamic processing simulation and flow field simulation results of the ECM process show that the proposed method eliminates the sharp changes in the electric field and electrolyte flow field at the leading and trailing edges seen in traditional machining methods.Thus, the electric field and flow field stability of the leading and trailing edges at the final stage of machining is greatly improved.Experimental comparison of the conventional and proposed ECM methods showed that four-directional synchronous feeding results in improved profile accuracy over repeated machining processes and good surface quality.

1.Introduction

Blades are core components of aero-engines and take the pole of energy transformation.The leading and trailing edges are the most important functional areas of a blade.During the operation of an aero-engine, the airflow divides and merges at the leading and trailing edges, respectively.A small profile error in the leading or trailing edge will significantly affect the aerodynamic performance of the aero-engine.1–3Therefore,among all areas of the blade, the leading and trailing edges have the highest requirements for profile accuracy.However,the leading and trailing edges are complex, weak, and rigid structures;they are also ultra-thin and twisty with small curvature.4In addition, aero-engine blades are usually made of materials that are difficult to cut(e.g.,nickel-based superalloys and titanium alloys).5–7In the process of milling,the vibration and deformation of the workpiece can seriously affect the machining accuracy.Occasionally, subsurface cracks caused by cutting heat also affect the surface integrity of the blade.8Thus, it is very difficult to control the machining accuracy of the leading and trailing edges of blades.

Electrochemical machining (ECM) is a non-traditional machining technique9in which materials are removed via a controlled electrochemical anodic reaction.ECM has the advantages of no tool wear and good surface quality regardless of the material hardness and toughness.10,11Based on these characteristics, ECM has become one of the main processing methods in aero-engine blade manufacturing.12Experts and scholars have conducted detailed research on blade ECM.Demirtas et al.studied the effects of two ECM parameters,feed velocity and the electrical conductivity of the electrolyte,on freeform surface machining.13Brusilovski analyzed the influence of pulse voltage, pulse width, electrolyte pressure,and other parameters on machining accuracy in pulse ECM.14Paczkowski et al.established a mathematical model of ECM and evaluated the distribution of the machining gap.15Fujisawa et al.established a three-dimensional simulation model of an ECM blade and predicted the distribution of machining clearance.16Li and Niu proposed a model and a finite element numerical approach for cathode design based on the potential distribution of the inter-electrode gap.17Ernst et al.developed a rapid method for design method based on simulation and optimization.18Liu et al.proposed a multidirectional auxiliary electrolyte flow mode for the ECM of blisk.19Klocke et al.established a multi-physical coupled material removal model20and a surface micro-morphology prediction model for ECM21and numerically simulated the ECM process of the leading and trailing edges of the blade.22Zhu et al.studied the design strategy of cathode tool trajectory for the ECM of blisk23and proposed a tool design method to improve the machining accuracy of the leading and trailing edges of the blade.24Wang and Zhu proposed an ECM method based on tangential feeding in which the leading/trailing edges are electrochemically processed by the cathode tools,which are fed along the tangential direction of the mean camber line of the blade.25.

In the conventional ECM of blades, the processing mode usually involves two cathode tools feeding toward the basin and back surfaces of the blade.The basin/back surfaces and leading/trailing edges of the blade are thus processed at the same time.In this processing mode, the ECM accuracy of the basin and back surfaces can be well controlled.However,the leading and trailing edges are at both ends of the opening gap between the two cathode tools.The continuous feeding of the two cathode tools causes drastic changes in the electric field and electrolyte flow field at the leading and trailing edges,which seriously affects the accuracy of the ECM.In addition,the current density at the leading/trailing edges increases rapidly in the final stage of the ECM process, which is detrimental to maintaining process stability and consistency.For aero-engine blades, the accuracy requirement for the leading and trailing edges is often higher than that for the basin and back surfaces.To improve the ECM accuracy of the leading and trailing edges, a new ECM method for blades based on the four-directional synchronous feeding of four cathode tools is proposed in this study.In the proposed method, four cathode tools are arranged in a cross shape to form a closed curved surface.A specially designed slip surface is located between each pair of adjacent cathode tools.When the four cathode tools are fed according to the set feed velocity ratio,the sliding surface between any two adjacent cathodes always remains in contact,and the surface surrounded by the four tools is always closed.During blade ECM,four cathode tools feed toward the basin/back surfaces and leading/trailing edges.All the profile surfaces of the blade are processed simultaneously with the contraction of the closed surface surrounded by the tools.The proposed method eliminates the opening electric and flow field in the leading/trailing edges, which improves the stability of the electric field and electrolyte flow field,thereby improving the controllability of machining accuracy.In order to verify the feasibility of the proposed method, the simulation and experiment were carried out.

2.Principle of four-directional synchronous feeding method

The schematic of the traditional ECM method for blades is shown in Fig.1.In this method, the workpiece is connected to the positive pole of power supply, and the cathode tools are connected to the negative poles.A high-velocity, highpressure electrolyte passes through the machining gaps between the cathode tools and the basin/back surfaces.During processing, the two cathode tools feed toward the basin and back surfaces, respectively.The material of the workpiece is gradually dissolved into ions via an electrochemical anodic dissolution reaction.After a period of time,the dissolution velocity of the workpiece material in the basin and back surfaces is roughly balanced with the feed velocity of the cathode tools.In this processing mode,the ECM accuracy of the basin and back surfaces can be tightly controlled.The leading and trailing edges, however, are located at the opposite ends of the gap between the two cathode tools.The leading and trailing edges do not have cathode tools feeding along their direction.Due to the large machining gap,the materials at the leading and trailing edges are always in a free dissolution state.It is hard to precisely control the shape of the leading and trailing edges without the cathode tool profile constraint.In addition, with the continuous face-to-face feeding of the two cathode tools,the shape of the machining gaps at the leading and trailing edges changes dramatically, especially in the final stage of machining.This will cause significant changes in the electric field and the electrolyte flow field in this region, resulting in random fluctuations of current density in the machining gaps.In ECM,the corrosion removal rate of the material is proportional to the current density on the workpiece surface.Thus,continuous and drastic changes in the electric field and electrolyte flow field will seriously affect repeatability.As a result,in the traditional ECM process, it is difficult to control the ECM accuracy of the leading and trailing edges.

Fig.1 Schematic diagram of traditional blade ECM process.

In order to improve the ECM accuracy of the leading and trailing edges, a new ECM method for blades based on the four-directional synchronous feeding of four cathode tools is proposed.A schematic of the proposed ECM method is shown in Fig.2.The workpiece is connected with the positive pole of the power supply.Four cathode tools are arranged in a cross shape and connected to the negative pole of the power supply.The four tools form a closed surface,and a sliding surface separates every-two adjacent tools.When the four tools are fed at a certain velocity ratio, sliding motion only occurs between adjacent tools, and the surface surrounded by the tools remains closed at all times.During blade ECM, the four tools are fed toward the basin/back surfaces and the leading/trailing edges, respectively.All the surfaces of the blade are processed at the same time.In this ECM method,two specially designed cathode tools are fed to the leading and trailing edges respectively.With the continuous feeding of the cathode tools, the electric field distribution in the machining gap of the leading and trailing edges will enter the equilibrium state.In this ECM state, the controllability of the leading and trailing edge shapes will be greatly improved.Furthermore, since the surface surrounded by the tools remain closed during the ECM process, the drastic changes in the electric field and electrolyte flow field of the leading and trailing edges are caused by the gap between the cathode tools, which exist in the traditional ECM method,are eliminated.The proposed method improves the stability of the electric field and electrolyte flow field,which helps to improve the controllability of the machining accuracy.

Fig.2 Schematic diagram of proposed blade ECM process.

To ensure that the sliding surfaces of two adjacent tools remain in contact at all times during the synchronous movement of four tools, the relationship between the dip angle of the sliding surface and the feed velocity ratio of two adjacent tools must be determined.The method used to determine the dip angle of the sliding surface is shown in Fig.3.Take the sliding surface between the basin-surface tool and the leading-edge tool as an example.The feed directions of the two tools are V1and V2,the angles between the sliding surface and the two tools are α and β, the angle between α and β is γ,and the feed velocity ratio V1/V2is K.The included angles α and β can be calculated using the Eqs.(1)and(2),respectively.By using this method to design four sliding surfaces,the structures of the four synchronous feeding tools can be determined.

3.Simulation of the forming processes in blade ECM

3.1.Simulation model and boundary conditions

The simulation geometric models of the blade machining gaps for the traditional and proposed ECM methods are shown in Fig.4.The workpiece blank is a rectangle with a length of 48 mm, a width of 8 mm and a height of 36 mm.The cathode tool profiles of the two methods are designed according to the typical stator blade model of an aero-engine.In the proposed four-directional feeding method,the cathode tools are perpendicular to each other and are fed at the same velocity, and the four dip angles of the sliding surfaces are all 45°.In both ECM modes, the electrolyte flows from the tip surface to the root surface of the blade,and the direction of electrolyte flow is perpendicular to the simulation section.Therefore, the effects of electrolytic products on electrolyte conductivity were ignored in the simulation, and the conductivity distribution in the machining gap was assumed to be uniform and constant.

Fig.3 Method for determining dip angle of a slip surface.

Fig.4 Simulation geometric models of blade machining gap.

In the simulation models shown in Fig.4, the boundaries Γc1and Γp1are the workpiece surfaces,and the boundary conditions are φa= U.The boundaries Γc2to Γc3and Γp2to Γp9are the cathode tool surfaces,and the boundary conditions are φc= 0.Other boundaries such as Γc4and Γp5are the insulation surfaces of the fixture, and the boundary conditions are?φ/?n = 0.In the ECM process, the dissolution of the workpiece material is dominated by the electric field, which is described by the Laplace equation:

In ECM, the material dissolution velocity νnobeys Faraday’s law, as shown in Eq.(4):

where η is the electrolytic processing efficiency,ω is the electrochemical equivalent volume of the workpiece material,and i is the current density on the surface of the workpiece.

During the simulation of the traditional and proposed ECM methods, all tools were fed to the workpiece at the constant velocity of ν.The entire ECM processing period was divided into small time intervals (Δt).For each time interval Δt,the electric field distribution in the machining gap was first calculated using Eq.(3).Next,according to the distribution of current density on the anode surface, the displacement of the anode boundary Δl was calculated using Eqs.(4) and (5).At the same time, the tool boundary also translates the distance Δd along the feed direction (Fig.5), as shown in Eq.(6).

Fig.5 Simulation calculation method of ECM process.

The new workpiece and tool boundaries are then taken to the next iteration, and the above calculation is repeated.Repeat the above simulation steps until the profile of the workpiece enters the tolerance range of the theoretical contour,and the final profile of the workpiece is obtained.The simulation parameters and conditions are shown in Table 1.The current efficiency curve of the workpiece material (SS304 stainless steel) used in the simulations was obtained from the study by Wang et al.26.

3.2.Simulation results and analysis

The total time of the simulation process is 1200 s.Since the leading and trailing edges molding occurs at the last stage of the machining process,the most drastic changes in the leading and trailing edges profiles occur during this time, so the simulation results within the last 120 s are selected for processing and analysis.

Fig.6 shows the simulation results (current density distribution and anode formation process)for the leading and trailing edges of the traditional ECM method at 1080 s,1100 s,???,1200 s during the machining process.With the continuous feeding of the two cathode tools,the electric field distributions and profiles at the leading and trailing edges of the blade change drastically during traditional ECM.In order to quantify the simulation results, two measurement boundaries were intercepted at the leading edge (points A to C in Fig.6) and trailing edge (points D to F).Eleven reference points were evenly selected within these two boundaries, where points B and E represent the vertices of the leading and trailing edges respectively, as shown in Fig.7.

Fig.8 shows the current density distribution of the leading and trailing edges at the moments (t = 1160 s, 1180 s and 1200 s) during ECM.In the final stage of ECM, the electric field is obviously concentrated near the vertices, and the current density near points B and E increases rapidly.The rapid increase in local current density will accelerate the dissolutionof local workpiece material.This will significantly reduce the controllability of machining accuracy.

Table 1 Simulation conditions of forming processes.

Fig.6 Simulation results of current density distribution for traditional ECM method.

Fig.9 shows the simulation results of the anode formation process based on the proposed ECM method.With the synchronous feeding of the four cathode tools, the feed velocity of the cathode boundaries is approximately equal to the corrosion velocity of the anode boundary.Thus,the machining process occurs in an approximately equilibrium state, and the electric field distribution in the entire machining gap is relatively stable.As for the traditional ECM simulation,two measurement boundaries were selected in the areas of the leading and trailing edges (points U to W and points X to Z, respectively, where points V and Y are vertices).During the last 40 seconds of ECM, the current density at each point in the two research boundaries basically remains stable, as shown in Fig.10.The uniform variation in the anode profile will significantly improve the controllability of ECM accuracy at the leading and trailing edges.

Fig.7 Location of sampling points.

Fig.8 Current density distribution in final stage of traditional ECM method.

Fig.9 Simulation results of current density distribution for proposed ECM method.

4.Simulation of the electrolyte flow field in blade ECM

4.1.Simulation model and boundary conditions

To analyze the effect of different processing modes on the stability of the electrolyte flow field, the simulation of the flow field in traditional and proposed methods during the last 40 seconds was carried out.In the simulation,the electrolyte flow fields at two moments(t=1160 s and t=1200 s)were chosen as the object of analysis.Since the velocity of electrolyte is much higher than the velocity of tool feed and workpiece material dissolution,the motion of tool and workpiece boundary is neglected.Based on the simulation results of the forming process, the geometric models of the electrolyte flow channel for these two moments are designed, as shown in Fig.11.

Fig.10 Current density distribution in final stage of the proposed ECM method.

In ECM,both the traditional and proposed methods adopt the flow mode of electrolyte from the blade tip surface to the root surface of the blade.In the simulation model, the inlet and outlet boundaries of the electrolyte are set as pressure type.The pressures of electrolyte inlet and outlet are set to 0.8 MPa and 0.1 MPa,respectively.Other surfaces in the electrolyte flow channel are set to be walls.The RNG k-ε model is used to accurately describe the turbulent flow of electrolyte.Its turbulent kinetic energy k and dissipation rate ε are shown in Eq.(7) and Eq.(8).27.

Fig.11 Geometric models of electrolyte flow field.

where ρ is the density of the electrolyte; t is the time; u is the flow velocity; μ is the effective viscosity coefficient; Gkis the turbulent kinetic energy generation term caused by the average velocity gradient; and μtis the turbulent viscosity.Additionally, the constant= C1ε= 1.42, C2ε= 1.68,Cμ= 0.0845, αk= 1, αε= 1.

4.2.Simulation results and analysis

Fig.12 shows the simulation results of the electrolyte velocity distributions at the leading and trailing edges of the traditional and proposed methods at the two moments.In the traditional ECM method, the electrolyte velocity at the leading and trailing edges of the blade decreases significantly as the gap between the two tools decreases.By contrast, the apparent shape change of the electrolyte flow channel near the leading and trailing edges is eliminated in the proposed ECM method due to the synchronous feeding of the four cathode tools.

In order to quantify the simulation results, eleven measuremnt lines were selected on the cross-section of the electrolyte flow channel at the leading and trailing edges.1100 reference points were set uniformly on these lines, as shown in Fig.13.Using the flow velocity distribution cloud maps of the leading and trailing edges at two moments (t = 1160 s and t = 1200 s) of both methods, the flow velocity data of 100 reference points on each measurement line were extracted and the mean values were taken for analysis, and the results were shown in Fig.14.In the traditional ECM method, the average flow velocity is about 16 m/s.Additionally, the flow velocity fluctuates very sharply at the leading and trailing edges.The flow velocity at the leading edge ranged from 20.6 m/s to 15.14 m/s at 1160 s, however, it is only 16.37 m/s to 15.81 m/s at 1200 s.On the contrary,in the proposed ECM method, the average flow velocity is about 19 m/s and there is little fluctuation in the flow velocity at the leading and trailing edges.The flow velocity at the leading edge varies from 19.46 m/s to 18.36 m/s at 1160 s and from 19.18 m/s to 17.92 m/s at the last moment.The flow velocity fluctuation at the trailing edge is similar to those at the leading edge.Furthermore, in the traditional ECM method, the variance of the electrolyte flow velocity for the leading edge is 23.5 m2/s2and 21.7 m2/s2, respectively, at 1160 s and 1200 s.However, in the proposed method,the variance of the flow velocity is 6.5 m2/s2and 6.8 m2/s2,respectively.The stable electrolyte flow field can reduce the random error in the ECM and significantly improve the consistency of repeated processing.

Fig.12 Simulation results of electrolyte velocity distribution.

Fig.13 Location of measurement line.

Fig.14 Electrolyte velocity analysis results.

Table 2 Experimental conditions.

Fig.15 ECM system and corresponding process devices.

Fig.16 Specimens machined using traditional and proposed ECM methods.

Fig.17 Blade middle profiles of traditional and proposed ECM methods.

Fig.18 Deviation distribution of each profile with respect to average profile.

5.Experiment

To verify the feasibility of the proposed method, comparison experiments of the two ECM methods were carried out.The experiments for both methods were simplified as two halfblade tests on the leading-edge and trailing-edge parts.Fig.15 shows the ECM system and corresponding devices for the traditional and proposed methods.In the newly proposed ECM method, three cathode tools are fed at a constant velocity toward the basin, back, and leading-edge (or trailingedge) surfaces of the blade, respectively.A high-speed, highpressure electrolyte flows in from the blade tip and out from the blade root.During traditional ECM, two cathode tools are fed toward the basin and back surfaces of the blade,respectively, and the gap above the leading edge (or trailing edge) is filled with insulating blocks.The workpiece is a rectangle with a length of 28 mm, width of 8 mm, and height of 36 mm.The workpieces and tools are made of SS304 stainless steel.The fixture is made of glass fiber-reinforced plastic.The experimental parameters are consistent with the simulation parameters, as shown in Table 1.The experimental electrolyte and other experimental parameters are given in Table 2.

Fig.16 shows the leading- and trailing-edge parts of the specimens machined using the traditional and proposed ECM methods.Each experiment was repeated three times.On each specimen, three section lines located at the blade tip,middle of the blade,and blade root were selected for measurement and analysis.In ECM, the profile repeatability between multiple machined specimens is an important index for evaluating the accuracy of the machining method.Therefore, the analysis of the experimental results primarily focuses on the repeatability of the profiles of the machined specimens.

The blade middle profiles near the leading and trailing edges of the specimens are shown in Fig.17.The three profiles obtained for the same ECM method and position were averaged to obtain the average profiles at the leading and trailing edges (shown as dotted lines in Fig.16).The distributions of deviation from the average profiles are shown for both ECM methods in Fig.18.For the traditional ECM method,the deviation from the average profile ranges from 0.036 to - 0.025 mm (from the positive direction to the negative direction) for the leading edge and from 0.061 to-0.040 mm for the trailing edge.In comparison, the deviations obtained using the proposed ECM method range from 0.011 to - 0.013 mm at the leading edge and from 0.012 to - 0.017 mm at the trailing edge.Thus, the proposed method significantly improved the machining accuracy at the leading and trailing edges.These results demonstrate that the deviation in the profile at the middle of the blade from the average profile is obviously smaller for the proposed method than for the traditional method.

Fig.19 Repeated deviation distributions for traditional and proposed ECM methods.

To evaluate the repeated ECM accuracy of the three section lines on the blade,the difference between the positive deviation and negative deviation of the measuring point from the average profile was defined as the evaluation index.Fig.19 shows the repeated deviation distributions for the blade tip, blade middle, and blade root of the specimens processed using the traditional and proposed ECM methods.For the traditional ECM method, the maximum deviations on the leading and trailing edges were 0.064 and 0.086 mm, respectively.For the proposed ECM method,the maximum deviations on the leading and trailing edges were 0.036 and 0.038 mm, respectively.Thus,the four-directional synchronous feeding method significantly improved the machining accuracy at the leading and trailing edges compared to the traditional ECM method.

Fig.20 shows the measured surface roughness values obtained for the two ECM methods.The surface roughness of the specimens processed using both methods were better than Ra0.4 μm.Overall, the experimental results demonstrate the feasibility and effectiveness of the proposed ECM method.

Fig.20 Surface 3D profile and roughness.

6.Conclusions

A method for the ECM of blades based on the four-directional synchronous feeding of four cathode tools is presented in this paper.In this method, four cathode tools are fed toward the basin/back surfaces and leading/trailing edges, respectively,and all profile surfaces of the blade are processed at the same time.The new method was evaluated through simulation and experiment, and the conclusions are summarized as follows:

1) In the conventional two-directional feeding method for the ECM of blades, the materials at the leading and trailing edges are always in a free dissolution state,and the electric field changes dramatically during the ECM process.The simulation results indicate that the distributions of current density and electrolyte velocity at the leading/trailing edges change sharply in the final stage of the ECM process, which is detrimental to the control of machining accuracy.

2) The proposed ECM method eliminates the sharp changes in the electric field and electrolyte flow field at the leading and trailing edges.In the final stage of blade ECM,the distributions of electric field and flow field are approximately in a steady state,which improves the controllability of machining accuracy.

3) The experimental comparison of the traditional and proposed ECM methods demonstrated that the profiles of the leading and trailing edges have higher accuracy when machined using four-directional synchronous feeding compared to traditional ECM.The surfaces obtained using the proposed ECM method also have good surface quality.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant 52075253) and the National Science and Technology Major Project (Grant 2017-VII-0004-0097).