Improvement of trailing edge accuracy in blisk electrochemical machining by optimizing the electric field with an extended cathode

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Abstract

In order to obtain high machining accuracy in the electrochemical machining (ECM), an appropriate cathode design is necessary. Previous cathode employing in radial ECM (called equal-thickness in this paper) leads to considerable distortion of workpiece shape at the trailing edge in machining blisk cascade passages. To improve the accuracy of the trailing edge, electrical field distribution in the inter-electrode gap is simulated in this paper, and a specially designed cathode (called extended cathode in this paper) is developed. The equal-thickness and extended cathodes are compared in experiments. Use of the extended cathode allows fabrication of a workpiece with a shape similar to that of the designed blisk. Compared with the equal-thickness cathode, the allowance difference at the trailing edge is decreased by 31.6% and the surface roughness Ra is decreased by 29.6%.

Introduction

A blisk, which is a key component in aero engines, consists of a rotor disk and blades constructed as a single substance to substance bond component. The friction and wear that occur at the junctions between the hub and blades of a conventional structure are thereby avoided and additionally gas pressure loss is reduced remarkably. Consequently, the working life, thrust-to-weight ratio, security and reliability of aero engines are increased significantly (Lu et al., 2014). Blisks are usually made of difficult-to-machine materials such as titanium alloys or Ni-base superalloys and have complex shapes (Xu et al., 2014). Therefore, blisk machining presents a great challenge to the aerospace industry. Conventional machining methods meet with difficulties in blisk machining, but electrochemical machining (ECM) has shown promise, with advantages including a high rate of material removal, absence of tool wear and induced residual stresses (Klocke et al., 2014).

To improve machining accuracy, a considerable amount of research has been done into modelling the ECM process. Li et al. (2011) described the actual machining process by a mathematical model considering the influence of a nonlinear electrolyte. Wu et al. (2012) studied on a multi-physics (electrical field, flow field and heat transfer) model that takes account of bilateral interactions using a customized partial differential equation and the model shows main factors in heat removal during the EMM process. Sun et al. (2006) proposed an approach using the finite element method to design tools for ECM. This method was capable of designing a three-dimensional freeform surface tool from the scanned data on a known workpiece. Li and Niu (2007) also proposed a finite element numerical approach to cathode design based on the potential distribution in the inter-electrode gap. Cathode deformation has a negative influence on machining quality and Zhu et al. (2012) performed optimization of the cathode thickness to minimize deformation through simulation. Tang and Gan (2014) verified that electrolyte flow field simulation was an effective method to optimize cathode design. Lu et al. (2014) presented a numerical approach to optimizing tool shape using a simplified potential model.

Generally, blisk machining using ECM involves two steps in sequence: blisk cascade passage machining and profile shape machining (Xu et al., 2014). According to the memory effect in ECM, the unevenness of workpiece may be transferred to the final product, leading to difficulties in tolerance control (Rajurkar et al., 1998). This means that the machining accuracy of the cascade passages after the first process will have a very strong influence on the processing stability and precision of the subsequent profile shaping process.

To achieve a good sealing in blisk or blade ECM, including the machining of cascade passages by the blisk ECM method of radial ECM, a cathode has generally been used (Qu et al., 2014) (such a cathode will be called equal-thickness cathode in the following). However, for an equal-thickness cathode, there is a weak current density distribution adjacent to the trailing edge, which leads to distortion of the workpiece shape and decreased accuracy. As a result, the electrolyte flow field and electric field distribution during machining of the profile shape are unsatisfactory, with consequent reductions in process stability and efficiency. In order to address this problem, a model of radial ECM is presented here based on the theory of ECM. Electric field distribution is simulated and the machined workpiece shape is obtained. Results show that the current density near the trailing edge at the suction side of the blade is much lower than in other regions. On the basis of the simulation results and the subsequent analysis, an extended cathode is designed with the aim of changing the electric field distribution. Finally, experiments are performed and a workpiece is machined with the extended cathode.

Section snippets

Development of extended cathode

Fig. 1(a) shows a sketch of blisk cascade passage machining with radial ECM. The workpiece (anode) remains static while the tool (cathode) is fed towards the centre of the blisk at constant speed. Electrolyte flows through the inter-electrode gap at high speed, taking the generated products as well as produced heat and electrolytic products away. Cathode shape is especially shown in Fig. 1(b). Lateral walls of the cathode tool are electrically insulated being covered with a dielectric coating,

Electric field physical model for radial ECM

The following assumptions are made in the model:

  • 1.

    The electrical conductivities of both cathode and anode are much larger than that of the electrolyte, and the electrodes can be treated as equipotential surfaces (Shenoy et al., 1996).

  • 2.

    The machining rate at the anode satisfies Faraday’s law (Shenoy et al., 1996).

  • 3.

    The electrolyte is homogeneous and its electrical conductivity is constant (Zhou and Derby, 1995).

When ECM is processing, lots of heat is produced and temperature of electrolyte in the

simulation setup and calculation

Simulation is carried out using COMSOL Multiphysics. In the simulation, the relationshipη=η(i) is entered into COMSOL as a function.

Values of the parameters used in this paper are listed in Table 1.

The mesh was not fixed in time but remeshed using automatic remeshing method. Automatic remeshing does not remesh at each time step but only when the meshing quality do not meet request.

As mesh was not fixed in time, number of elements changes when remeshing is carried out. The maximum value of

Experimental investigation

The extended cathode described above has been employed experimentally to verify its effect on machining shape and accuracy. Experimental conditions are listed in Table 2.

A diagram of the machining device with the extended-cathode modification is shown in Fig. 11. Workpiece remains still while the cathode is fed towards the anode during machining. The upper and lower fixtures are made from insulating material and provide a sealed flow channel with electrodes. DC power is employed to supply a

Conclusions

An extended cathode design has been developed to improve machining accuracy in radial ECM by removing the small radius that appears in the trailing edge. The conclusions are listed as follows:

  • 1.

    A 2D modelling is established and simulations are carried out to study the whole ECM process. Current density distribution in side machining gap are achieved through simulation.

  • 2.

    A cathode structure called extended cathode is proposed. Simulations show that this cathode structure improves current density

Acknowledgments

This study was sponsored by the National Natural Science Foundation of China (51205199) and the Natural Science Foundation of Jiangsu Province (BK2012387).

References (19)

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