# A Study of Precision Current Efficiency Curve Measurement with a Casing-Type Anode
## 1 Introduction  
Electrochemical machining (ECM) is a non-traditional machining technology whereby material removal is accomplished through the controlled anodic dissolution of the workpiece. It has the advantages of high efficiency, no tool wear, and good results in terms of surface quality, regardless of the hardness and toughness of the processed material [1, 2, 3]. Thus, ECM has become an important processing method for difficult-to-cut material parts and is widely used in the aviation industry [4, 5, 6]. The current efficiency is an important characteristic parameter that describes the relationship between the dissolution rate of the anode material and the current density in ECM [7]. The accurate measurement of the current efficiency is the basis for both anode shape prediction and cathode tool design [8, 9, 10]. However, the dissolution process of alloy materials is affected by elemental composition, electrochemical reaction valence, and material peeling action. Therefore, it is difficult to obtain the current efficiency of alloy materials by theoretical calculations, and it is usually measured by experimental techniques [11, 12, 13, 14]. Several experimental measurement methods used to obtain the current efficiency of target alloy materials at different current densities have been reported in the literature. One of these methods is the constant current method. In this measurement technique, the target alloy material is dissolved into electrolyte under a constant current by adjusting the processing voltage; however, it is without any tool feed action. The voltage, the machining time, and the dissolved mass of the anode are recorded. The current efficiency of the target material can be calculated on the basis of the assumption of uniform currentdistribution on the anode surface [15; 16; 17]. This method is easy to implement; however, as the ECM process usually adopts a constant voltage machining mode, the cathode tool gets continuously fed during the processing. The variation of the applied voltage may cause a change in the material dissolution behavior. For example, in [PERSON]'s study, it was shown that the machining current of Mn presented three machining states with an increase of machining voltage, and it had the same processing current density at five different processing voltages [18]. As such, it is difficult to maintain the stability of the measured state in the constant current method. In [PERSON]'s study, the dissolving properties of steel 100 Cr6 in NaCl and NaNO\({}_{3}\) were measured. The polarization curve indicated a fluctuation in the anode potential when the current density was lower than 3 A/cm\({}^{2}\)[19]. The above studies show that the anode potential fluctuates significantly at a low current density, which results in errors in the measurement of current efficiency. Another method used to measure current effiency is the constant voltage method. In this method, the processing voltage is kept constant. The target alloy material is dissolved in the electrolyte under the constant current, which is obtained by adjusting the cathode tool feed rate. This helps the ECM processing to reach the equilibrium state. Given that the tool feed rate is proportional to the material dissolution rate under the ECM equilibrium state, the current efficiency can be calculated from the recorded parameters of the material dissolution rate and the equilibrium current. [PERSON] et al. measured the current efficiency of S-03 material in NaCl, NaNO\({}_{3}\), and mixed solutions, and reported that the NaCl electrolyte had the highest current efficiency [20]. [PERSON] and [PERSON] obtained the current efficiency curve of GH4169 in NaNO\({}_{3}\) using the constant voltage method [21; 22]. [PERSON] et al. explored a constant flow measurement method based on the conventional constant voltage method [23]. Compared with the constant current method, the measurement conditions of the constant voltage method are more consistent with the ECM process, so this paper mainly discusses the constant voltage method.