Blisk (bladed integrated disk) is construction used for advanced heavy-duty compressor rotors in military and also civil turbine aircraft engines . Blisk has essential advantages, such as weight reductions, improved aerodynamics and reductions in fuel consumption. However, since blisk usually is made by difficult-to-machine materials and requires high machining accuracy, it brings a great challenge to manufacturing. Therefore, diverse advanced machining methods, such as Linear Friction Welding (LFW) , Electro Discharge Machining (EDM) , and Electrochemical Machining (ECM)  are introduced into blisk process. Especially, due to characteristics of non-existent tool wear and short process-time, ECM plays a prominent role in the blisk process . Lots of efforts have been devoted to blisk ECM. A multi-physical approach is presented for modeling the ECM material removal process by coupling all relevant conservation equations . Flow models such as Π-shaped flow mode and dynamic additional electrolyte flow are developed to raise quality of electrolyte flow field [7, 8]. An extended cathode is employed to improve trailing edge accuracy by optimizing the electric field .
Since blisk channel is narrow and twisted, interference between cathode and anode may happen during blisk process with ECM method. To avoid this interference, this paper studies a promising blisk ECM, namely the spiral ECM. Due to cathode's complex movement, spiral ECM succeeds in improving machining accuracy and surface quality . This complex movement also increases interference possibility between electrodes. To eliminate interference, some researchers focus on changing cathode trajectory with a trajectory control strategy . However, changing cathode trajectory is not suitable for spiral ECM, because the trajectory of cathode is optimized on the aim of obtaining a high machining accuracy and could not be changed . Therefore, a cathode designing method is developed to avoid the interference between electrodes.
This paper develops a cathode designing method to avoid interference. First, according to theory of electrochemical machining, machined channel is predicted by calculation. Second, with this channel, interference analysis is carried out and a cathode is designed. Finally, the cathode is employed in experiment and no interference appears.
2.1. Description of Spiral ECM
Sketch of spiral ECM is illustrated in Fig. 1. While the workpiece is fixed, the cathode has a complex movement. The cathode feeds from the tip of blade to the bottom with a rotation motion simultaneously. Thus, the cathode actually has a spiral movement. During machining, cathode is connected to the negative pole of power and workpiece is connected to the positive pole. Owing to continuously pumped electrolyte, electrolytic products are taken away and electrolyte is kept clean in the machining gap. Most of the areas in the side surfaces are covered with insulated materials except the exposed regions, illustrated in Fig. 2a. Thus, only exposed regions are involved in the reaction.
2.2. Prediction of Blisk Channel
To avoid the interference, the channel must be predicted. It can be made basing on electrochemical theory.
Structure of cathode is given in Fig. 2a. If width of exposed region is h, the value of the side gap G can be calculated with the following relationship :
(1) in which, Δ is inter-electrode frontal gap determined by the following equation:
(2) where η is the current efficiency, ω is the volume electrochemical equivalent of the material, κ is the conductivity of the electrolyte, U R is the potential difference in the machining gap, and v f is the cathode feed speed. By offsetting exposed region with a distance of G, channel outline can be obtained (Fig. 2b).
2.3. Cathode Designing
It is necessary to obtain positions of channel relative to cathode. Since the cathode is moving in process, the relative positions are changing all the time. Thus, while an absolute coordinate C is located in the initial position, a relative coordinate C' is located at the cathode, shown in the Fig. 3. Due to the movement of cathode, the relative coordinate has an angle difference of θ around Z axis combined with position difference F in the direction of Z axis. A plane P is built in the relative coordinate and plane P has distance of D from X'- Y'. A point A is located in the intersection curve I of plane P and blisk channel. Thus, if the coordinate of A is (X, Y, Z) in coordinate C, the coordinate of A in coordinate C' (X', Y', Z') is given as follows:
Then, keeping the value of D constant and changing the value of F, a cluster of curves I can be obtained (Fig. 4). If the cathode is designed inside all the curves I, just as the green curve S, it should not collide with channel shown in Fig. 4. Since the cathode has a thickness of insulated coating L and a width of flow passage W, the outline curve of side surface Q is obtained by offsetting curve S with a distance of L+W.
Selecting D with different value, a series of outline curves of side surfaces in the cathode can be obtained. Through linking the curves, the two side surfaces of cathode are finally constructed (Fig. 5a). To avoid undesirable erosion in the side machining gap, the side surfaces is usually coated with insulated materials, such as ceramics and epoxy resin (Fig. 5b).
Interference detection is carried out in the Computer Aided Design (CAD) software (Fig. 6). Fig. 6a shows cathode employed previous cathode design method, in which the side surface of cathode is a ruled surface. The ruled side surface has a difference relative to twisted channel, which leads to large width difference of side gap. And interference between cathode and anode even appears in some areas, marked by yellow line. Fig. 6b shows the detection result employed cathode designed with method presented in this paper. The width side gap is almost uniform and no interference happens.
3. Experiment and Results
The designed cathode is employed under the experimental conditions shown in the Table 1. The cathode is made from stainless steel 304 by CNC machine center (Fig. 7). Side surfaces of cathode are covered with high-hardness ceramic. Experimental system is shown in Fig. 8a. During machining, machining gap is pumped with clean electrolyte of 228 g/L NaNO3. A potential difference of 20 V is applied to the electrodes by a DC power supply. Cathode is fixed to the main spindle and feeds with a constant feed speed of 1.2 mm/min. Fig. 8b shows final position of the process, which illustrates that side surfaces of cathode suits the channel exactly. There is a large space between blisk channel and cathode side surfaces. Thus, the design is verified to be reasonable and successful.
This paper develops a cathode designing method, which is verified by the experiment. The method contains three steps:
Firstly, inter-electrode frontal gap is calculated based on the electrochemical theory. Blisk channel shapes are obtained by offsetting the cathode outline with a distance of inter-electrode frontal gap.
Secondly, a relative coordinate system is built on the cathode. Through transformation of coordinates, blisk channel shape is converted to the relative coordinate system. Shape of cathode is finally calculated based on the relationship between of cathode and channel.
Thirdly, an experimental system is employed to verify that the design allows avoiding interference completely.