Improving profile accuracy and surface quality of blisk by electrochemical machining with a micro inter-electrode gap

Jingtao WANG, Zhengyang XU, Di ZHU

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

KEYWORDSElectrochemical machining;Blisk;IEG;Machining quality;Surface quality

AbstractElectrochemical machining(ECM)has emerged as an important option for manufacturing the blisk.The inter-electrode gap(IEG)distribution is an essential parameter for the blisk precise shaping process in ECM,as it affects the process stability,profile accuracy and surface quality.Larger IEG leads to a poor localization effect and has an adverse influence on the machining accuracy and surface quality of blisk.To achieve micro-IEG(<50 μm)blisk finishing machining,this work puts forward a novel variable-parameters blisk ECM strategy based on the synchronous coupling mode of microvibration amplitude and small pulse duration.The modelling and simulation of the blisk micro-IEG machining have been carried out.Exploratory experiments of variable-parameters blisk ECM were carried out.The results illustrated that the IEG width reduced with the progress of variable parameter process.The IEG width of the blade’s concave part and convex part could be successfully controlled to within 30 μm and 21 μm,respectively.The profile deviation for the blade’s concave surface and convex surface are 49 μm and 35 μm,while the surface roughness reaches Ra=0.149 μm and Ra=0.196 μm,respectively.The profile accuracy of the blisk leading/trailing edges was limited to within 91 μm.Compared with the currently-established process, the profile accuracy of the blade’s concave and convex profiles was improved by 50.5 % and 53.3 %, respectively.The surface quality was improved by 53.2%and 50.9%,respectively.Additionally,the machined surface was covered with small corrosion pits and weak attacks of the grain boundary due to selective dissolution.Some electrolytic products were dispersed on the machined surface, and their components were mainly composed of the carbide and oxide products of Ti and Nb elements.The results indicate that the variable-parameters strategy is effective for achieving a tiny IEG in blisk ECM, which can be used in engineering practice.

1.Introduction

Blisks (bladed integrated disks), as a core component in aeroengines,have received considerable attention due to their high thrust-weight ratio and working efficiency.1–3Blisks are generally made of difficult-to-cut materials,such as Ni-based superalloys and titanium alloys,and its blade profile is an extremely distorted three-dimensional (3D) surface.One of the nickelbased superalloys (Inconel 718), a typical precipitation hardened material, has been widely used in blades (bladed integrated disks) of the aero engines because of its excellent mechanical properties as well as its structural stability from cryogenic to elevated temperatures.4–5For most traditional machining methods of blisk, such as linear friction welding,6electron beam machining,7electrical discharge machining,8and numerical control milling,9the traditional machining methods of blisk present high tool costs and long process times.Besides, the access of the milling tools to the cavity can be difficult or even impossible.10–11Electrochemical machining (ECM) is a typical non-contact machining process governed by Faraday’s law.The process is based on the anodic electrochemical dissolution at the ionic level in the presence of an electrolyte to remove material and realize parts manufacturing,which can be used in any difficult-to-cut conductive material.Aqueous solutions are generally selected as the electrolyte solution, whose role is to provide the electrolyte conductivity required for the machining process and expel byproducts from a specified inter-electrode gap (IEG).Compared with traditional mechanical cutting methods, ECM affords many inherent advantages,such as the absence of tool wear,low cost,and high machining efficiency and surface quality.12Furthermore,ECM is also an economical process for mass production of components with complex profiles.Recently, Xu13gave a detailed overview of the current development,trends,and technological advances of the ECM process on the machining of complex structural components of aero-engines.There is no doubt that ECM has become quite a mainstream machining method for the fabrication of various sophisticated parts such as turbine blades, blisks, and micro-components.

In blisk ECM, machining process includes two steps: blisk cascade passage machining and profile machining.14Blisk cascade passage machining is a rough machining process, which processes many channels with certain allowances.The profile machining is a precisely shaped process.Accompanied by the movement of the tool electrode toward the anode workpiece,the workpiece blank with certain allowances forms a final blade profile at a certain IEG width level in an electrolyte solution,which can be a water-based, neutral salt.In recent years,there has been a considerable amount of research on ECM for blisks with the aim of achieving higher machining accuracy and surface quality.Lei et al.15investigated feeding strategy optimization for blisks with twisted blades to improve the machined allowance.Ernst et al.16proposed an algorithm to optimize the profile of the cathode, which significantly shortened the tool development procedure of jet engine vanes in ECM.Wang et al.17presented an ECM approach for blisk channels with rotations of the cathode and the workpiece to reduce the allowance differences in the concave and convex parts.Zhang et al.18proposed a vibration-assisted blisk channel ECM method to achieve fine machining stability and surface quality.Wang et al.19proposed a method of ECM with tangential feeding, in which the cathode tools fed along the tangential direction of the mean camber line of blades to improve the accuracy of the blade leading/trailing edges.Recently, hybridization techniques were applied to ECM to improve its machining performance.One of the important breakthroughs is that the periodic reciprocating vibration movement with a frequency from tens of Hz to hundreds of kHz has been well employed in ECM process.In vibration cycle,when the tool electrode moves towards the closest point of the anode workpiece, the electrochemical reaction occurs,on the contrary, when the tool electrode moves away from the anode workpiece, the electrochemical reaction stops.This helps to enhance convective mass transport inside the IEG,accelerating product removal and potentially enabling the micro-IEG machining in ECM.Several investigations have been carried out to research the effect of vibration parameters on ECM performance.Hewidy et al.20established a mathematical model to assess the mechanism of metal removal of ECM assisted by low-frequency vibrations of the tool electrode.Natsu et al.21carried out the influence of vibration direction, amplitude, and the tool feed-rate on replicating accuracy, and obtained the relationship between replicating accuracy and vibration parameters.Xu and Pan22researched the ECM process using a small pulse voltage applied in synchronization with tool vibration, and the machining localization of a workpiece was significantly increased.Bhattacharyya et al.23determined the influence of vibration frequency on machining performance in electrochemical micro-machining.They suggested that using low-frequency vibration in ECM, the machining accuracy and material removal rate could be increased significantly.Kurogi et al.24researched the influence of the electrolyte concentration and the effect of ultrasonic vibration on the machining characteristics of the WC alloy, and found that the replicating accuracy was further improved.Liu et al.25manufactured some crosschannel array boards with a better rib flatness and surface quality of the channels using the pulsed current coupled with a small tool vibration.Ghoshal and Bhattacharyya26used vibration in microtools with small amplitude in micro-ECM,and found that a low-frequency and small-amplitude vibration of the microtool significantly improved the micromachining stability.For these studies, the ECM process, which combines a short pulse current and a periodic reciprocating vibration of tool electrode with a small amplitude, and provides a promising alternative for manufacturing various sophisticated parts(such as turbine blades, blisks, and micro-components).It is known that ECM is a ‘‘copy”manufacturing technique.The IEG is essential for ECM accuracy because the profile accuracy and surface quality of anode workpiece are determined by its corresponding IEG distribution.Many studies have analyzed the IEG distribution rules.De Silva et al.27carried out the relationship between the IEG width and the machining accuracy,and the results showed that a narrow IEG could further improve the machining accuracy.Wang et al.28reported a theoretical analysis of the IEG in counter-rotating ECM, and found that an accelerated feed speed strategy could control the IEG width within a small value, and the machining accuracy was improved.Wang et al.29proposed a constant-IEG machining process to restrict IEG fluctuation to a small value,improving the machining accuracy and surface quality.During the ECM,the IEG is a critical factor for obtaining the desired machining accuracy and surface quality of blisk, with a small IEG helping to localize the anodic dissolution and improve the leveling ability.For these reasons, maintaining a stable,tiny IEG is one of the means of high-precision machining and surface quality in ECM.At present, most of the reports about micro-IEG machining (<50 μm) focus on the microscopic scale field.However, in macroscopic scale field, especially in the blisk ECM process,the IEG width of ECM is still large(>150 μm),which hinders the development of ECM process towards high accuracy and high surface quality.Thus,how to accomplish a stable micro-IEG in blisk ECM is an urgent problem to be solved.At the same time, another striking problem encountered in the processing is that the highefficiency mass transport in the tiny IEG.This is because the accumulation of electrolytic products in the IEG at micrometer scale may affect the stability of blisk ECM, and decrease the machining accuracy and surface quality.

Therefore, the aim of this work is to overcome the above key problems and manufacture some blisks with high surface quality and profile accuracy via a micro-IEG machining mode(IEG < 50 μm).Firstly, a novel variable-parameters blisk ECM strategy that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration was proposed to implement the micro-IEG machining in blisk ECM.Furthermore, the modelling and simulation of the variable-parameters blisk micro-IEG electrochemical machining have been carried out to analyze the mass transport characteristics of the electrolyte fluid flow.It is found that the electrolytic products, bubbles, and Joule heat can be expelled effectively from the micro-IEG,ensuring that the conductivity of the electrolyte is updated in time.Finally, a series of variable-parameters blisk ECM verification experiments were performed.The results indicate that the IEG width of the blade’s concave part and convex part can be successfully restricted to within 30 μm and 21 μm, respectively, which greatly improved the surface quality and profile accuracy of the blisk.To sum up,the variable-parameters blisk ECM strategy proposed here is feasible in the blisk micro-IEG ECM process, the surface quality and machining accuracy of the full profile of machined blisk are enhanced to a higher level,which can be used in engineering practice.

2.Description of variable-parameters blisk ECM strategy

Blisk ECM is a ‘‘copy”manufacturing technique in which the profile of the tool electrode is similar to that of the blade.The micro-IEG machining of blisk was realized based on pulse ECM assisted by a vibrating electrode.Fig.1(a) illustrates the principle of the pulse ECM of blisk with a vibrating electrode.As shown in the figure, when ECM begins, the convex cathode and the concave cathode slowly moves towards the anode workpiece at a constant feed-rate,respectively,while being simultaneously assisted by periodic reciprocating vibration movement.The constant feed-rate movement of the tool electrode,S(t),and periodic reciprocating vibration movement of the tool electrode, Z(t), can be given by Eq.(1) and Eq.(2), respectively.21,25.

where A,f,φ,and v0are the vibration amplitude,vibration frequency, phase angle, and constant feed-rate, respectively.The vibration velocity of the tool electrode, v(t), is expressed as:

In the case of ECM assisted by the periodic reciprocating vibration movement of the tool electrode, the IEG varies according to the feeding rate of the tool electrode and its sinusoidal vibrating motion.The feed rate of the tool electrode is the sum of the constant feed-rate, v0, and the vibration velocity, v(t), that is:

In blisk ECM, the blisk’s concave and convex cathodes are connected to the negative pole of a power supply,and the anode,namely the blisk,is connected to the positive pole.In each vibration cycle, when the tool electrode moves towards the closest point of the blisk, the power supply is turned on, and when the tool electrode moves away from the blisk, the power supply is turned off.The pulse duration occurs only at the closest point between the tool electrode and the blisk, and remains off at all other times.Hence, the electrochemical reaction is confined to a small area at the on-time of the pulse voltage, and the electrolytic products will be removed from the IEG at the off-time of the pulse voltage.This method helps enhance convective mass transport and diffusivity inside the IEG, ensuring that the conductivity of the electrolyte in the extremely narrow gap is updated in time, which is essential for blisk micro-IEG machining.The high-speed flow electrolyte flows into machining region,divides into two ways at the blade tip,flows respectively through the convex and the concave, and flows out at the blade root.Under the action of applied electric field, the workpiece blank is dissolved rapidly and the blisk profile is formed gradually.Supposing that the pulse blisk ECM assisted by a vibrating electrode is in a state of equilibrium,based on Ohm’s law and Faraday’s law, the front IEG could be expressed as:

where η is the current efficiency, ω is the volume electrochemical equivalent, κ is the electrolyte conductivity, U is the electrical potential between blisk and tool cathode, δEis the over-potential, and tonand tdwellare the pulse duration and dwell time, respectively.

Fig.1 Schematic diagram of variable-parameters blisk ECM.

It is known that ECM is a‘‘copy”manufacturing technique in an electrolyte solution at a certain IEG width level.In blisk pulse ECM assisted by a vibrating electrode, IEG exhibits a varying periodicity, which differs distinctly from the steady IEG that arises after reaching equilibrium during sinking ECM.An extremely narrow gap is highly efficient in providing a very high degree of localization of the anodic dissolution and the leveling ability.28From Eq.(5),it can be seen that the pulse duration, tool feed-rate, vibration frequency, amplitude, electrolyte conductivity and applied voltage will influence the IEG status in ECM.Generally speaking, the electrolyte conductivity and applied voltage are constant in the process of ECM.From this point of view, the vibration frequency, pulse duration, and vibration amplitude become important factors that affect the change of IEG.In the work by Bhattacharyya et al.23,they suggested that the low-frequency vibration of tool electrode was beneficial for improving the process performances compared with high-frequency vibration.For a fixed low vibration frequency,the pulse duration is essential for regulating the IEG width,and a small IEG can be obtained via a short pulse duration.In addition, in the work by Hewidy et al.20, it was reported that higher amplitude brings about a severe increase in the pumping action of the electrolyte, which leads to the presence of bubbly flow.This finding indicates that high amplitude vibration not only does not reflect the advantage of vibration electrochemical machining to enhance the discharge of electrolytic products, but also causes the surface quality and profile accuracy of the workpiece to become worse.Considering the above two situations, a novel process method that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration was proposed to implement the micro-IEG machining in blisk ECM.It should be pointed out that if only the micro vibration amplitude and small pulse duration parameters were used in entire blisk ECM process, the machining efficiency was extremely low.In order to take into account the machining efficiency and profile accuracy of blisk, the distributed control and analysis of the blisk ECM process are necessary factors.Here,the pulse duration, vibration amplitude, and tool feed-rate are changed in vibration cycle according to the needs of the IEG at different feed depths.The blisk ECM process can be described with two distinct stages according to their machining status.The schematic diagram of two-step variable parameter blisk ECM strategy is shown in Fig.1(b).In the first stages,namely preform process, ultra-long pulse current machining was adopted to homogenize the allowance distribution of the blade blank,with its IEG width being restricted to a range of 250 to 350 μm,so as to the high efficiency of machining was ensured.In the second stages, a novel variable-parameters blisk ECM method was used to realize the micro-IEG precision machining, with its IEG width being restricted within 50 μm.Thus,the forming accuracy and surface quality have effectively improved.In particular, it should be pointed out that the second processing steps can be further subdivided according to the IEG state.In this paper, the variable-parameters blisk ECM method can be described in three distinct stages according to their machining status: 1) the blisk ECM assisted with large-amplitude vibrating electrode (the vibration amplitude was ‘‘A”) and long pulse current to improve the leveling ability,and its IEG width was restricted within the range of 100 to 250 μm; 2) the blisk ECM assisted with medium-amplitude vibrating electrode (the vibration amplitude was ‘‘2A/3”) and short pulse current to further improve the leveling ability,and its IEG width was restricted within the range of 50 to 100 μm; 3) the blisk ECM assisted with small-amplitude vibrating electrode (the vibration amplitude was ‘‘A/3”) and ultra-short pulse current to realize micro-IEG precision machining of full profile for blisk, and its IEG width was restricted within 50 μm.All machining stages are carried out continuously, that is, only some processing parameters need to be adjusted when each step of the processing occurs.

A certain type of blisk, as shown in Fig.2, is used as an example in this paper.Fig.2(a),2(b),and 2(c)show the geometry of the blade,the radial section and the axial section of the blade, respectively.In these figures, the blade is made up of eight cross-sectional profiles (i.e., profile 1 to profile 8), and the blade is 6.55° in twisted angle from the blade tip to the blade root.L1is the deviation between the center of blade tip and the center of blade root, which is 3.85 mm.L2, L3,and L4are the widths between the concave and convex parts in blade tip, blade middle, and blade root, respectively, which are about 24.38, 21.83, and 19.14 mm, respectively; L5is the length of the blade, and its value is 40 mm.

3.Multiphysics coupled simulation for variable-parameters blisk ECM

3.1.Mathematical mode

Fig.2 Schematic diagram of a certain type of blisk.

In variable-parameters blisk ECM, the machining quality is determined by the effect of multiphysics coupling.During the blisk ECM, the pulse voltage is loaded on the electrodes,the electrochemical dissolution of the blisk is dominated by the electric field, which is described using Laplace’s equation as follows:30.

where φ is the electrical potential between blisk and tool cathode.

In electric field, the current density of the blisk surface can be described as:

where j is the current density,and κ is the electrolyte electrical conductivity.

In blisk ECM, both the hydrogen bubbles and Joule heat generated in the electric field play important roles on the distribution of electrolyte electrical conductivity, which will affect the current density distribution and the material removal of the blisk.At the same time, the electrolyte velocity in the flow field affects the effect of the convective heat transfer and the distribution of hydrogen bubbles.The mathematic relation between them and the electrolyte electrical conductivity is as follows:19

where βgis the gas volume fraction, n is the bubble influence coefficient, and is taken to be 1.5,31κ0is the initial electrolyte electrical conductivity, Teis the electrolyte temperature, Τ0is initial temperature, and α is the temperature coefficient.

The electrolyte flow is a gas–liquid–product mixing flow in the blisk ECM.As the ratio of flocculated products is small,their influence can be neglected.32The effects of gas and liquid are studied in the simulations.The gas–liquid two-phase flow can be governed by the following the governing equations:33.

where βlis the liquid volume fraction, βgis the gas volume fraction,qlis the liquid density,qgis gas density,μlis the liquid velocity,p is the pressure,G is the gravity vector,F is the arbitrary increase in the volume force, μgis the gas velocity, μTis turbulent viscosity.

Hydrogen gases produced at the tool cathode in the blisk ECM would adversely influence the electrolyte electrical conductivity.The amount of the hydrogen gases can be described by Faraday’s law, and can be calculated according to Eq.(11).30.

where F, and j are Faraday constant, and current density,respectively.

The hydrogen bubbles are transported with the flow of electrolyte.The transport equation of gas phase can be described according to Eq.(12).34.

where Γ is the source term for hydrogen gas on cathode, δ is the diffusion coefficient of hydrogen gases.

During the migration of gas phase, the pressure affects the bubbles volume in the electrolyte.The variation of bubble volume with pressure is expressed by the ideal gas state equation.31,35–37Hence, in the simulation, the gas density was set according to the ideal gas state equation:

where r is the ideal gas state constant,t is electrolyte temperature, and p is the pressure.

The Joule heat is produced owing to the current heating effect in the electric field.The temperature distribution in the inter-electrode gap can be obtained by solving convection–diffusion equations with heat sources, respectively as:38–39.

where Cp, q, q, λ, Q, E represent the specific heat capacity of the mixed gas–liquid, the density of the gas–liquid mixture,the heat flux,the thermal conductivity,the heat source(ohmic heating), and the electric field intensity, respectively.The Cp,and q satisfy: Cp= Clβl+ Cgβg, q = qlβl+ qgβg.

The temperature rise ΔT along the flow path of the electrolyte satisfies:32.

where Clis the specific heat capacity of the liquid,L is the flow path.

Based on what discussed above, the multiphysics model considering the influences of the electric field,temperature distribution and flow field in blisk ECM can be considered as the following equation:

3.2.Simulation model

A multi-field coupled simulation of the blisk ECM in micro-IEG machining mode was carried out.Fig.3 illustrates the geometrical mode used for simulation, as well as the coupling mode for pulse duration and tool vibration.The pre-position coupling mode of pulse duration and tool vibration was used for the whole process stage in this work.This is because that the best machining localization and highest feed rate are obtained by the pre-position coupling mode when compared to the symmetry coupling mode and the post-position coupling mode.40The duty cycle is defined as the proportion of the pulse duration time to the vibration period.In the simulation model, the yellow area represents the tool electrode, and the grey area represents the workpiece.The simulation model consists of the blisk’s convex and concave electrodes(with boundaries of Γ1–Γ4and Γ5–Γ8, respectively) and the anode workpiece(boundary Γ9).The convex boundaries and concave boundaries of the blisk are assigned to superimposed motion with both constant feeding and a periodic reciprocating vibrated motion,which is described by Eq.(4).The electrolyte flows through the IEG from the inlet (boundary Γ11) to the outlet (boundary Γ10).The length of electrolyte path is 40 mm.The boundaries of diversion section at the electrolyte inlet are Γ13and Γ14.The boundaries of diversion section at the electrolyte outlet are Γ12and Γ15.

In electric field, the boundaries satisfy:

The material removal rate of the blisk in ECM can be computed based on Faraday’s law.In simulation, the material removal rate of the blisk and the vibration movement of tool electrode are simulated by a moving mesh, the boundaries satisfy:

where, η is the electrolytic processing efficiency, ω is the electrochemical equivalent of volume, and j is the current density.A multi-field coupled simulation of the variable-parameters blisk ECM process was conducted in COMSOL Multiphysics.The simulation conditions were listed in Table 1.Simulation of variable-parameters blisk ECM was performed to study the effect of mass transport on micro-IEG machining once the boundaries and simulation conditions had been set.

Fig.3 Schematic diagram of blisk ECM used for simulation.

Table 1 Summary of simulation conditions.

3.3.Simulation results

The analysis focused on high-efficiency mass transport of electrolytic products in micro-IEG machining when the proposed mode was applied to blisk ECM.Notably, the mass transfer characteristics are not analyzed during the ultra-long pulse current machining stage,because it belongs to preform process stage used to homogenize the allowance distribution of the blade blank.The variable-parameters blisk ECM process is blisk precision forming stage.Here,a series of simulations during each vibration cycle were carried out with the following machining parameters: a vibration amplitude of 0.3, 0.2, and 0.1 mm; a pulse duration of 7.2, 4.4, 2.7 ms; a duty cycle of 13: 72, 1: 9, and 5: 72; and a tool feed-rate of 0.18, 0.15, and 0.1 mm/min, respectively.It is known that the Joule heat and bubbles generated in the ECM process will gradually accumulate along the flow path of the electrolyte,so the bubble rate and temperature rise are most serious near the electrolyte outlet.Hence,region C(see Fig.3)near the outlet was selected as the sample point to investigate the mass transport effects on the IEG for different process stages.Fig.4 illustrates the variation of the bubble rate,temperature rise,and electrolyte conductivity at region C for the different process stages.From the trend of the curves,it is clear that the bubble rate,temperature rise and conductivity follows a sinusoidal relationship.Note that, in a vibration cycle, when the tool electrode moves towards the blisk workpiece, the bubble rate gradually increases and reached the maximum at the closest point, and on the contrary, the bubble rate gradually decreases, and decreases to the minimum at the farthest point.In addition,for the different machining stages, when the tool cathode vibrates near the closest point of the blisk workpiece,the bubble rates are 12.2%,9.3%,and 8.1%,respectively,as shown in Fig.4(a).The bubble rate shows a decreasing trend as the machining stages progress.This is because the electrochemical reactions become milder with the decrease of the pulse duration, so that fewer electrolytic products were produced.It can be seen that the bubble rate on the blisk surface decreased from 12.2 % in the initial process stage to 8.1 % in the final process stage.This is beneficial to the precision machining of the blisks.In the meanwhile, the value of the temperature rise in the IEG also reaches minimum at the final process stage,as can be observed from Fig.4(b).In addition to this, the timedependent changes of electrolyte conductivity at the different machining stages are shown in Fig.4(c).It can be seen that the variation of electrolyte conductivity decreases from 1.09 S/m in the initial stage to 0.64 S/m in the final stage, a drop of approximately 41.3%.The variation of electrolyte conductivity is very small between the electrolyte inlet position and outlet position in the final stage.This is crucial for the precision machining of the blisk profile.In particular, the IEG rapidly decreases from 217 to 30 μm through a variableparameters blisk ECM strategy.Taken together, the simulation results verify the rationality of the variable-parameters blisk ECM strategy used for achieving the micro-IEG in blisk ECM.

To obtain details of the mass transfer properties in micro-IEG, a simulation for the blisk ECM process that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration was carried out under the following parameters: an applied voltage of 20 V, pulse duration of 2.7 ms,duty cycle of 5:72,vibration amplitude of 0.1 mm,tool feed-rate of 0.1 mm/min, NaNO3electrolyte conductivity of 9.5 S/m, inlet pressure of 0.9 MPa, and outlet pressure of 0.05 MPa.Fig.5 depicts the changes of electrolyte velocity,bubble rate, temperature rise, and electrolyte conductivity in the sampling regions (A, B, and C, which are distributed near the inlet,blade body,and outlet,respectively)over time during the reciprocating vibration of the cathode tool.As seen in Fig.5(a), the electrolyte velocity in the channels declines to its lowest value when the cathode vibrates near the closest point of the workpiece.Correspondingly, it is found that the velocity of electrolyte along the electrolyte path decreased from 16.8 m/s in the inlet region to 14.9 m/s in the outlet region, which is mainly due to the presence of flow resistance along the flow direction.The velocity difference of the electrolyte between regions A and C is only 1.9 m/s, where the ECM processing stability and surface quality are supposed to be improved significantly.Fig.5(b) shows a distribution of gas bubbles in the IEG during one vibration cycle.It can be seen that the change of gas volume fraction shows a sinusoidal relationship by varying the periodic vibration of the tool cathode,which changes from 0%to maximum and then back to 0%.Specifically,the gas volume fraction started to increase as the cathode approached the anode workpiece, and the gas volume fraction maximum occurred near the closest point.In contrast, in the case of the tool electrode oscillating in the backward direction (far from the blisk workpiece), the IEG increases, a large amount of electrolyte solution is supplied to the IEG and the electrolyte velocity in the IEG increases to a value higher than the average velocity.Thus, the electrolyte solution is renewed in the IEG, and the gas volume fraction begins to decrease.Additionally, the bubbles were stacked gradually along the flow path from the inlet to the outlet.The values at the region A and the region C were 0.2 and 8.1 %, respectively.The contour plot of simulations of blisk ECM for a gas volume fraction also shows the same result,as shown in Fig.6(a).Red means high gas volume distribution,blue means low gas volume distribution.As can be seen from Fig.6(a), the red area gradually increases when the tool electrode moves towards the closest point of blisk workpiece.The gas volume distribution is the highest when the machining time is 0.01 s.However,the red area gradually decreases when the tool electrode is away from the closest point of blisk workpiece, which indicates that the gas volume distribution gradually decreases in the interelectrode gap.It can also be found that the red area gradually increases along the flow channel,indicating that the accumulation of gas is becoming more and more serious.Similar to the distribution rules of gas volume fraction, it is found that electrolyte temperature also follows a sinusoidal relationship, as shown in Fig.5(c).In addition, the contour plot of simulations of blisk ECM for electrolyte temperature is shown in Fig.6(b),where Red means high electrolyte temperature, blue means low electrolyte temperature.As can be seen from Fig.6(b),the red area gradually increases when the tool moves towards the workpiece in each vibration cycle, indicating that the electrolyte temperature increases, and reaches its highest electrolyte temperature(310.1 K, see Fig.5(c)) at the closest point.In contrast, the red area gradually decreases when the tool is far from the workpiece, indicating that the electrolyte temperature decreases, and reaches its initial electrolyte temperature(298.15 K, see Fig.5(c)) at the farthest point.The effects of temperature and gas accumulation on electrolyte conductivity is expressed in Eq.(8),and the distributions of electrolyte conductivity are shown in Fig.5(d).The electrolyte conductivity only changes by 0.64 S/m along electrolyte path during pulse on-time with periodic reciprocating vibrated motion of the tool electrode in blisk ECM.However,electrolyte conductivity distribution becomes uniform during pulse off-time,which proves that electrolyte can be effectively renewed during the pulse offtime.To summarize,the variation of electrolyte conductivity is very small along the electrolyte path in blisk ECM process.Simulation results indicate the ECM process that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration is reasonable to realize micro-IEG blisk machining, which help reduce the influence of bubbles and byproducts on processing stability.

Fig.4 Time-varying curve of parameters in blisk ECM at variable-parameters blisk ECM stage.

Fig.5 Time-varying curve of parameters in blisk ECM process that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration.

Fig.6 Contour plot results of the simulations for blisk ECM process that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration.

4.Experiments and discussion

4.1.Experimental procedure

To gain further insight into the feasibility of the variableparameters blisk ECM process under micro-IEG mode in engineering practice, a series of blisk ECM experiments were carried out.To better characterize the effect of the proposed process, pulse ECM was used for comparison in this paper.The machining parameters of comparative experiments are as follows: applied voltage is 20 V, pulse frequency is 1000 Hz, duty cycle is 80 % and feeding speed is 0.55 mm/min.A schematic diagram and an experimental environment of the experimental setup are shown in Figs.7 and 8, respectively.Some of the key experimental equipment used in our experiments are shown in Fig.7: a multi-axis linkage CNC large precision machine tool, an electrolyte management system, and a power supply.The machine tool mainly consists of two horizontal axes (X1and X2) that can move left and right, and a vertical axis (Z) that can move up and down,and a vertical axis(Y)that can move in and out,and two rotation axes (B and C).The ECM fixture, blisk workpiece, and other main components of the blisk ECM are displayed in Fig.8(a).According to the characteristics of blisk ECM and flow field, a special clamping fixture and combined cathodes were designed and fabricated, as shown in Fig.8(b).The outside of the fixture was made of epoxy materials and a stainlesssteel plate.The epoxy components played specific watersealing and insulation roles to reduce stray corrosion in blisk ECM, and the stainless-steel plate was used to improve the rigidity of the fixture.The mechanical movements of the blisk’s concave and convex cathodes included a periodic reciprocating vibrated motion and linear feed motion,which were controlled by a Siemens numerical control system.Electrolyte management system supplies the electrolyte solution to the machining region at a set temperature and pressure,and washes away the electrolytic products and Joule heat over time.The filtration accuracy is 0.5 μm,which can effectively filter out the insoluble electrolytic products.An aqueous NaNO3solution of a constant pressure was used for blisk ECM.The electrolyte was pumped into the inlets through the blade’s concave and convex surfaces, and then ejected from the blisk hub.A twodimensional (2D) model of the flow path is shown in Fig.8(c).A pulse power supply was used in this work.The ECM power supply supported constant-voltage mode,which ranged from 0 to 36 V, and the maximum forward output peak current was 15,000 A.The blisk workpiece and the tool electrode were made from Inconel 718 and 1Cr18Ni9Ti, respectively.After many trials, the detailed machining parameters for twisted blades in our experiments were shown in Table 2.

4.2.Results and discussions for variable-parameters blisk micro-IEG ECM

4.2.1.Machining current and IEG

The average machining current and IEG width were recorded during the machining process to evaluate the real-time machining status.Fig.9 shows typical machining current and IEGvarying trends from the initial to final processing stages.Note that in the figure, different scales are used for the machining current (bottom x-axis and right y-axis) and IEG varying trends(top x-axis and left y-axis).It can be seen that the initial IEG width of the blisk’s concave and convex electrodes were 590 μm and 316 μm, respectively.The difference of the IEG width for blisk’s concave and convex electrodes is 274 μm.This is due to the uneven distribution of allowances between these surfaces after blisk cascade passage machining.In the preform process stage, ultra-long pulse current blisk ECM was carried out to homogenize the channel allowances of the blisk’s concave and convex surfaces.It was found that the machining current rapidly increased with the tool electrode feed distance from 0 mm to approximately 0.97 mm, which indicated that the IEG width of the blisk’s concave and convex surfaces decreased.Correspondingly, the IEG width of the blisk’s concave and convex electrodes were regulated to 440 μm and 290 μm, respectively.At this moment, the first step machining finishes and the distributed variable-parameters machining process starts.Firstly,a long pulse current blisk-ECM assisted by a large-amplitude vibration was carried out to improve the leveling effect of the blisk’s concave and convex surfaces.Compared with the initial stage, the average machining current of blisk ECM was obviously reduced.This is because the effective current used to process the blisk’s concave and convex surfaces decreased as the pulse duration decreased.This is helpful to improve the profile accuracy of blisk.Note that the machining current gradually increased in this processing step when the tool electrode was fed from 0.97 to 1.47 mm, which indicated that the IEG width was further reduced via process parameter adjustment.As a result, the IEG width of the blisk’s concave and convex electrodes were regulated to 225 μm and 116 μm,respectively.Then, a short pulse current blisk ECM assisted by a medium-amplitude vibration was carried out to further improve the leveling effect.It can be seen that the machining current increased by degrees when the tool electrode was fed from 1.47 mm to approximately 1.77 mm, and the IEG width was further reduced;the IEG width of the blisk’s concave and convex electrodes were regulated to 98 μm and 83 μm, respectively.Finally,an ultra-short pulse current blisk ECM assisted by a small-amplitude vibration was carried out to realize the precision machining of a full profile for the blisk.Note that there were obvious linear downward trends for IEG when the tool electrode was fed from 1.77 mm to approximately 2.22 mm.By contrast, the IEG width of the blisk’s concave and convex electrodes could be maintained at 30 μm and 21 μm, respectively, which was obviously smaller than its initial value of 590 μm and 316 μm, respectively, helping to improve the leveling ability.Comparatively speaking, the IEG widths of the blade’s concave part and convex part in the comparative experiment were 451 μm and 558 μm, respectively.Overall, no sudden fluctuation in machining current occurred during blisk ECM from beginning to end.This indicated that the electrolytic products generated during micro-IEG were quickly washed away, meaning the dissolution of the blisk’s concave and convex surfaces was very uniform and stable.The IEG width can be successfully restricted to a small value in blisk ECM.Thus,the forming accuracy and surface quality of the blisk have effectively improved.

Fig.7 Schematic diagram of blisk ECM equipment.

Fig.8 System image for blisk ECM.

Table 2 Main experimental parameters and their values.

Fig.9 Machining current and IEG of blisk ECM with a variable-parameters blisk ECM strategy.

4.2.2.Surface quality

Fig.10 shows photographs of blisk workpieces processed by the variable-parameters blisk ECM process that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration, as well as the surface quality of the blisk.It can be clearly seen that there was no short circuit phenomenon on the machined surface,and the machining process was very stable.The whole processing time lasted for approximately 9 min.However, if the processing parameters only used the micro-vibration amplitude and small pulse duration in the whole blisk ECM process, the process lasted for approximately 22.2 min.The machining efficiency of the blisk was improved by 59.5 % via variable-parameters blisk ECM strategy.Note that the machined surface of the blisk’s convex and concave parts is very smooth, and does not present any obvious flow mark in whole blade area.Meanwhile,the surface quality of the blisk’s convex and concave parts was analyzed via a surface-finish measuring instrument (Perthometer M1,Mahr GmbHs, Germany).The position of the cross sections of the machined blade used for profile measurement, as well as the surface roughness measurement results, are listed in Fig.10.The measured values of the surface roughness of the blisk’s convex and concave surfaces reached Ra= 0.196 μm and Ra= 0.149 μm via variable-parameters blisk ECM process with a micro-IEG mode.On the contrary, the surface roughness of the blisk’s convex and concave surfaces reached Ra= 0.419 μm and Ra= 0.304 μm via pulse ECM process.Compared with the pulse ECM process, the variableparameters blisk ECM process that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration is conducive to improving the surface quality.This is because the oscillating electrolyte flow is beneficial to reducing the Ra value.To sum up,the surface quality of the convex and concave surfaces of this study’s blisk was improved by 53.2 % and 50.9 %, respectively.These results indicated that variable-parameters blisk ECM process was reasonable, and the blisk with twisted blades processed via this strategy had a good surface quality.

Fig.10 Photographs of machined blisk and surface roughness measurement results.

4.2.3.Surface morphology

The surface morphology and product composition were observed via a SEM (S4800, Hitachi, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS) system (Xflash 5010, Bruker, Germany).Secondary electron image(SEI) mode was selected to carry out SEM observations.The acceleration voltage and the working distance were set to 20.0 kV and 14.7 mm,respectively.Meanwhile,the magnification of SEM images was set to 200 × and 500 ×.Fig.11(a)shows the surface morphology of blisk after ECM process.It can be seen that the blisk surface was very smooth, indicating that the electrochemical dissolution of the blisk was uniform.This was also consistent with the surface roughness measurement results of the blisk.Upon closer inspection,the machined surface was covered with small corrosion pits and weak attacks of the grain boundary as the grain boundaries were still visible.This is attributed to the selective dissolution of the Inconel 718 material.41–42Moreover, it was clear that some electrolytic products were dispersed on the blisk surface.The composition of the products was determined by EDS.In EDS analysis, the acceleration voltage and the working distance were set to 20 kV and 14.7 mm, respectively.The point sweep mode was selected, while the sweep time was 82 s.Note that from the green wire frame in Fig.11(a), the Ti content (～57.65at%)and Nb content (～10.17at%) in the electrolytic products were higher compared with the chemical composition of the matrix materials (see Fig.11(b)), and the products contained a large content of C and O elements.Taken together, the data indicated that the product was composed mainly of the carbide and oxide products of Ti and Nb elements.This is because the standard electrode potentials of Nb and Ti are - 1.1 and - 1.63 V, respectively, much lower than those of Ni, Cr,Fe, or Mo elements in Inconel 718, facilitating the occurrence of electrochemical reactions to generate corresponding carbides and oxides.Similar conclusions were also reported in other works.41.

4.2.4.Profile accuracy

By using a coordinate measuring machine (CMM, TESA Micro-Hite 3D, Switzerland) to scan the surface profile of the machined blisk, we obtained the profile deviation of the concave and convex parts between the machined blisk and the ideal blisk profile.For the CMM measurements, sampling points were chosen on the blisk’s concave and convex surfaces from the blisk leading edge to the blisk trailing edge.Fig.12(a) and 12(b) show the profile accuracy of the blisk machined via different ECM processes.It can be seen that the measured values of the maximum deviations of the blisk’s concave and convex profiles were 99 μm and 75 μm via pulse ECM process.On the contrary, the profile accuracy of the blisk processed by the variable-parameters blisk ECM was improved.The maximum deviations of the concave part and convex part were 49 μm and 35 μm.The profile accuracy of the blisk’s concave and convex profiles was improved by 50.5 % and 53.3 %, respectively.Further, for the variableparameters blisk ECM process that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration, the repeatability accuracy of the blade’s concave and convex parts for five blades reached 47 μm and 38 μm,respectively.The above results demonstrate that new process can gain more high machining accuracy while gaining high processing efficiency.

Fig.11 SEM and EDS measurement results.

Fig.12 Comparison of profile accuracy of machined blisk.

Fig.13 Machining accuracy of leading/trailing edges for blisk.

In addition, the profiles of the leading/trailing edges of the machined blisk were also measured via CMM.As shown in Fig.13, the machining accuracy of the blisk leading/trailing edges was limited to within 91 μm via variable-parameters blisk ECM process that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration.At the same time, cross-sections of the leading/trailing edges contours were observed using a digital microscope (VHX-6000, Keyence, Japan) after cutting the blade at the position of line‘‘2′′.In digital microscope analysis,the working distance was set to 26 mm.The magnification of digital images was set to 100 ×.It can be seen that there were smoother arcs, there was no cusp, blunt, inclined and dramatic shape-changes for the blisk leading/trailing edges.Thus, the formation accuracy of blisk was effectively improved.Based on analyses of the experimental results above, it is clearly seen that the variable-parameters blisk ECM process developed in this paper is reasonable.The processed blisk has good machining accuracy and surface quality in terms of blade profile, and blade leading/trailing edges.

5.Conclusions

This paper reports the results of investigations on the variableparameters blisk ECM strategy, which aims to achieve micro-IEG (<50 μm) blisk finishing machining, and obtain a profile accuracy and surface quality that are as better as possible.The experimental results showed that the variable-parameters strategy in the blisk ECM process was feasible, the IEG could be controlled effectively at a tiny value, thereby improving the surface quality and machining accuracy of the machined blisk.The conclusions of this work are given as follows:

1) To achieve micro-IEG(<50 μm)blisk finishing machining, a novel variable-parameters blisk ECM strategy that adopts the synchronous coupling mode of microvibration amplitude and small pulse duration was proposed.The IEG width reduced with the progress of variable parameter process, the IEG width of the blade’s concave part and convex part could be successfully restricted to within 30 μm and 21 μm in the final process stage,respectively.The surface quality and profile accuracy of the blisk were greatly improved.

2) Simulation results indicated bubble rate, temperature rise, and other byproducts could be expelled effectively from the micro-IEG using a variable-parameters blisk ECM process, and the variation of the electrolyte conductivity along the electrolyte path was only 0.64 S/m.The simulation results demonstrate the efficacy of the variable-parameters blisk ECM strategy for achieving the high-efficiency mass transport in the tiny IEG.

3) A blisk with good dimensional accuracy and surface quality was fabricated successfully.The maximum profile deviation for the blade’s concave and convex surfaces was 49 and 35 μm, while the surface roughness reached Ra = 0.149 μm and Ra = 0.196 μm, respectively.Additionally, the profile accuracy of the blade leading/trailing edges was controlled within 91 μm.Compared with prevalent methods, the profile accuracy of the blade’s concave and convex profiles was improved by 50.5%and 53.3%,respectively.Meanwhile,the surface quality was increased by 53.2 % and 50.9 %,respectively.Therefore, it can be concluded that the blisk produced via the variable-parameters blisk ECM strategy that adopts the synchronous coupling mode of micro-vibration amplitude and small pulse duration has good machining accuracy and high surface quality,which can be used in engineering practice.

4) Additionally, the machined surface was covered with small corrosion pits and weak attacks of the grain boundary due to the selective dissolution.Some electrolytic products were dispersed on the machined surface, and their components were mainly composed of the carbide and oxide products of Ti and Nb elements.

6.Outlook

A novel method of electrochemical machining with a micro inter-electrode gap model was proposed for improving the profile accuracy and surface quality of blisk.Further research can consider using auxiliary electrode to reduce stray corrosion of non-machined surfaces of blisk.

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 sponsored by the National Science and Technology Major Project (Grant No.2017-VII-0004-0097), and National Natural Science Foundation of China for Creative Research Groups(Grant No.51921003),and the Postgraduate Research & Practice Innovation Program of Jiangsu Province(Grant No.KYCX21_0191).