Effects of pulsating electrolyte flow in electrochemical machining

Electrochemical machining (ECM) is a promising and low-cost process for yielding various components of difficult-to-machine materials, and has been well established in diverse applications. Distributions of gas and temperature affect the electrolyte electrical conductivity and determine the machining accuracy in ECM. Attempts have been made to generate the pulsating flow via a servo-valve in the electrolytic supply pipe, which is introduced to improve the heat transfer, material removal rate and surface profile in ECM. A multi-physics model coupling of electric, heat, transport of diluted species and fluid flow is presented. Simulation results indicate that pulsating flow has a significant impact on the distributions of velocity, gas fraction, and temperature near the workpiece surface along the flow direction. Experiments are conducted to verify the feasibility of the proposed process and study the effects of pulsating flow on material removal rate. The experimental results agree well with the simulations. Using optimal pulsating parameters, the material removal rate and surface profile are enhanced.     

Wire electrochemical discharge machining (WECDM) of quartz glass with titrated electrolyte flow

Electrochemical discharge machining (ECDM) is a promising technique with great potential for application to machining non-conductive brittle materials. In this study, wire electrochemical discharge machining (WECDM) was applied to processing quartz glass with electrolyte supplied in titrated flow. During WECDM, under surface tension and gravity, the electrolyte flows down in droplets, over the graphite auxiliary electrode and brass wire, producing oxygen and hydrogen bubbles, respectively. The fluid inside the droplet forms eddies on the two sides of the quartz, removing chips and electrolysis resultants. Rapid replenishment of electrolyte to constant concentration can ensure even distribution of current density and a stable insulation gas film can thus be formed. Experimental results show that quartz glass processed by WECDM with titrated electrolyte flow yielded long straight slits of small mean width. In addition, with the electrolyte supplied in droplets, less electrolyte is consumed in the process. The proposed droplet titration approach to WECDM of quartz glass incurs less cost and pollution, making it both cost effective and environmental friendly.     

Improving machining performance of wire electrochemical discharge machining by adding SiC abrasive to electrolyte

The use of wire electrochemical discharge machining (WECDM) to slice hard brittle materials has recently been studied because its effectiveness is independent of the mechanical characteristics of the machined materials. Therefore, materials with high hardness, brittleness, strength and electrical insulation, which are difficult-to-cut, can be machined. In ECDM, the electrochemical reaction produces hydrogen bubbles, which accumulate around the cathode. A thin gas layer forms on the surface of the electrode and isolates the electrode from the electrolyte. When a voltage that exceeds the critical voltage is applied, continuous discharge occurs. The material near the electrode is removed by the discharge erosion and chemical etching. The use of WECDM to cut electrically insulating materials has only recently been investigated. However, the breakdown of the gas in the bubbles and the vibration of the wire in WECDM strongly affect the shape accuracy. This work aims to improve the over cut quality by adding SiC abrasive to the electrolyte. A mechanism that combines discharge, chemical etching and abrasive cutting is studied. The effects on expansion, roughness and material removal rate (MRR) are discussed. The experimental results reveal that adding abrasive reduces the slit expansion because it increases the critical voltage. The particles disrupt the bubble accumulation to form an isolating layer around the wire, increasing the critical voltage and reducing the discharge energy. The surface roughness is improved because the abrasive helps to refine the micro-cracks and melted zone that is formed by discharge heat erosion. Meanwhile, smaller grit produces lower roughness. The quality of the slit can be controlled; its expansion and roughness of the slit are 0.024 mm and 0.84 um Ra, respectively.     

Current efficiency during the electrochemical machining of iron and nickel

Current efficiency determinations from weight-loss measurements were made on pure iron and pure nickel anodes in 4M NaClO₃ solution in a flow cell at flow rates between 500 and 3000 cm/s in a current range from 5 to 50 A/cm². The current efficiency for metal removal was virtually independent of current density and flow rate on iron anodes. On nickel anodes the current efficiency increased strongly with current density. In the high current density region, the current efficiency decreases with flow rate up to 2000 cm/s and then increases with higher flow rates. This behavior was accounted for by differences in the nature and properties of the anodic films formed on iron and nickel anodes.     

钨钴硬质合金电化学蚀刻加工研究

对YG6硬质合金电化学蚀刻工艺进行了研究，提出了采用光致抗蚀剂作保护膜，在生成一定钝化膜的情况下进行电化学蚀刻的方法，在一定的蚀刻深度范围内，该工艺的可控性好，腐蚀区表面质量较好。     

Study of gas film quality in electrochemical discharge machining

Electrochemical Discharge Machining (ECDM) has been demonstrated to be an alternative spark-based micromachining method for fabricating microholes and microchannels in non-conductive brittle materials. However, the mechanism for attaining accurate control of the contour shape and dimensions remains to be explored. In ECDM process, the gas film on the electrode surface is used as the dielectric medium required for discharge generation. Quality of gas film is the dominant factor that determines the machining qualities such as geometric accuracy, surface roughness and repeatability. Nevertheless, it is difficult to assess the gas film quality of ECDM. In this study, current signals and machined contours were taken as indexes of gas film quality. Experimental results showed that a stable and dense gas film could be obtained when the applied voltage exceeded the critical voltage and reached a specific level, which is called the “transition voltage” in this study. At the transition voltage, a stable electrochemical discharge activity could be generated, thus producing the smallest deviation of contour dimensions. Moreover, when the drilling process reached a certain critical depth, bubbles inside the hole could not easily escape. In order to reduce the interface energy between bubbles, a thicker gas film is formed at the hole entrance, resulting in unstable discharge performance that undermined machining results. In summary, information provided by current signals can shed light on the changes in gas film structure, which serve as useful reference for varying process parameters to achieve better efficiency and accuracy.     

数控展成电解加工技术的研究进展

评述了数控展成电解加工技术的优点，介绍了近年来国内外在该技术领域的研究进展及其所取得的研究成果，包括阴极设计、成形规律研究、加工软件的开发和工艺试验等。     

The effect of electrolyte stirring and laminar flow on measured electrochemical noise

In the present paper the influence of various types of electrolyte movement on the characteristics of measured electrochemical noise (EN) has been investigated. For this reason the measurements were performed during different phases: in a still solution, during stirring, as well as in laminar flow. In order to relate the measured EN to the development of corrosion processes, digitized images of the electrodes were recorded continuously during these measurements. After the tests were finished, the corroded electrode surfaces were examined also by the scanning electron microscope (SEM). It was established that the characteristics of EN, in general, change significantly with the movement of the solution. On the other hand, no clear difference was observed between the EN signals measured during stirring and those obtained during laminar flow. This observation, combined with the results obtained in a passive environment and in a very aggressive still solution, confirmed that the main source for the change of EN characteristics during electrolyte movement is the transformation of corrosion processes. The direct effect of the electrolyte movement on the measured EN (modulation of signals due to spatio‐temporal flow disturbances) was found to be insignificant compared to the influence of the corrosion processes themselves.     

The prediction of workpiece shape during electrochemical machining by the boundary element method

The authors aim to improve the reliability and speed of workpiece shape prediction for electrochemical machining. A review of some previous mathematical modelling work is followed by a resume of the Boundary Element Method. Linear and quadratic elements are used herein to represent the boundaries and, because the workpiece shape changes as machining progresses, an automatic re-noding procedure is adopted after each iteration. The effect of element and time step size on the accuracy of the workpiece profile is studied, accuracy being measured by comparing converged parts of the workpiece shape with exact equilibrium solutions. The paper reveals considerable promise for boundary element simulation particularly when it is reasonable to assume that homogeneous physical conditions exist in the inter-electrode zone.     

硬脆金属的超声电解复合加工研究

硬脆金属材料采用普通切削加工相当困难。本文对硬脆金属材料进行了超声电解复合加工工艺试验与研究，该复合加工方法使加工速度、精度有表面质量较单一工艺有显著改善。     

On the analysis of the electrochemical spark machining process

The electrochemical spark machining (ECSM) process has been proved as a potential process for machining of low machinability high-strength electrically non-conducting materials, but the mechanism of material removal during the process, by and large, is not yet understood. In the present work, the electrochemical discharge is modelled as a phenomenon similar to that which occurs in arc discharge valves. This phenomenon is used to explain various experimental results, on the basis of circuit and arc discharge valve characteristics. The spark energy and the approximate order of hydrogen gas bubble diameter are computed by the proposed valve theory. Material removal rate is evaluated by modelling the problem as a 3-D unsteady state heat conduction problem. The problem is solved by the finite element method to compute the temperature distribution which is post-processed for estimating material removal per spark, overcut obtained in the machined cavity, and attainable maximum penetration depth. The conclusion drawn is that the application of valve theory to the ECSM process seems to be realistic. Estimated material removal rate, overcut and maximum penetration depth show a good agreement with experimental findings.     

Ultrasonic measurement of the inter-electrode gap in electrochemical machining

During the process of electrochemical machining the dependency of the inter-electrode gap with time and process parameters can be used to determine process characteristics and to define the shape of the workpiece surface relative to the tool surface. Defining process variables to map out the required gap-time function requires the use of time-consuming iterative trials. In-line monitoring of the gap would enable process control and tool to workpiece transfer characteristics to be achieved (for ideal conditions) without the requirement to generate such parameter maps. This work explores the use of ultrasound applied as a passive, non-intrusive, in-line gap measurement system for ECM. The accuracy of this technique was confirmed through correspondence between the generated gap-time and current time data and theoretical models applicable to ideal conditions. Gap measurements are also used to demonstrate and quantify the degree of departure from ideal behaviour for an In718/chloride system as the electrolyte flow rate is reduced from 16 to 4 l min⁻¹. The monitoring of the gap size has also been shown to be effective when determining shape convergence under ideal conditions, for the example case of a 2D sinusoidal profile.     

Prediction of chip flow direction during machining with self-propelled rotary tools

This paper presents a model for chip flow prediction during tube-end machining process using self-propelled rotary tools. The analysis performed is based on the transformation of the relative kinematic relationships in rotary cutting to that of the conventional cutting with large nose radius. Both relative and absolute chip flow angles are investigated. Tests were performed to measure the absolute chip flow angle, insert self propelled motion, and the deformed chip thickness during machining with self-propelled rotary tool under different cutting conditions. The predicted values of the absolute chip flow angle were in good agreement with that measured experimentally.     

Development of a process signature for electrochemical machining

Electrochemical machining (ECM) principally enables a highly productive and virtually wear-free production of components with simultaneously high surface quality. However, the process generates changes concerning both the geometry as well as the rim zone of manufactured components, so that the entire process design currently runs through several heuristic cycles. As a result, the cost-effectiveness of the process is often only given in large-scale production. The paper therefore mechanistically links the material modifications and the process-induced material loads for electrochemical processes to predict rim zone properties. Inverted components of the resulting process signature can finally be used for virtual process design.     

Modelling of the electrochemical machining process by the boundary element method

This paper describes the development and application of the boundary element method to model the machining of simple milling and turning features. The 3D model uses linear triangular elements to discretise the workpiece and tool surfaces. Highlights of the program include the use of analytical integration to calculate the element matrices rather than numerical, and the automatic refinement of the mesh as the workpiece is progressively machined. The program has been tested for milling slots using a rectangular tool and for turning a thin-walled tube. It is shown that there is good agreement between the predicted and experimental results.     

Kinetics of the Cu2O chemical decomposition during the copper electrochemical oxidation in the sulfuric-acid-based electrolyte systems

Electrochemical copper oxidation in sulfuric acid aqueous solutions and sulfate electrolytes thickened with the silicon-dioxide powder is studied potentiodynamically. Based on the current theoretical concepts, the polarization-rate dependences of the anodic current density maximum is analyzed. It is shown that these dependences can be adequately described using a porous-layer resistance model that takes into account chemical decomposition of the passivating Cu₂O film formed via electrochemical mechanism. The film decomposition rate constants are calculated for the studied electrolytic systems at different temperatures.     

聚丙烯酸对钒电池正极热稳定性和电化学性能影响的研究。

通过原位热稳定性、紫外-可见光谱、拉曼光谱、X射线光电子能谱(XPS)、循环伏安法和充放电方法,研究了聚丙烯酸(PAA)对全钒氧化还原流电池(VRFB)正极电解液的热稳定性和电化学性能的影响。结果表明,PAA添加剂可以提高V(V)电解液的热稳定性。在室温条件下,少量的PAA添加剂能轻微的提高正极电解液的电化学性和VRFB的能量效率。此外,以PAA添加量为3%的正极电解液组装电池,该电池在50 ℃ 时表现出良好的充放电循环性能,其充电容量保持率高于没有添加PAA的电池。     

Evaluation of shaping accuracy upon electrochemical machining of metals

This work investigates the influence of the pulse repetition frequency on the profile generated within pulse electrochemical dimensional machining of nickel foil using a cylindrical electrode tool. Geometrical criteria for the shaping accuracy characteristics are introduced. It has been demonstrated that, irrespectively of the criterion calculation method, the significant influence of the pulse repetition frequency on the shaping is observed. Herewith, the frequency variation makes it possible to determine the extreme criterion point corresponding to the best replication of the electrode tool. It is established that the variation of the pulse repetition frequency influences first of all the region of generation of the outer corner of the formed profile.     

Experimental investigation on the influence of electrochemical machining parameters on machining rate and accuracy in micromachining domain

Non-conventional machining is increasing in importance due to some of the specific advantages which can be exploited during micromachining operation. Electrochemical micromachining (EMM) appears to be a promising technique, since in many areas of application, it offers several special advantages that include higher machining rate, better precision and control, and a wider range of materials that can be machined. A better understanding of high rate anodic dissolution is urgently required for EMM to become a widely employed manufacturing process in the micro-manufacturing domain. An attempt has been made to develop an EMM experimental set-up for carrying out in depth research for achieving a satisfactory control of the EMM process parameters to meet the micromachining requirements. Keeping in view these requirements, sets of experiments have been carried out to investigate the influence of some of the predominant electrochemical process parameters such as machining voltage, electrolyte concentration, pulse on time and frequency of pulsed power supply on the material removal rate (MRR) and accuracy to fulfil the effective utilization of electrochemical machining system for micromachining. A machining voltage range of 6 to 10 V gives an appreciable amount of MRR at moderate accuracy. According to the present investigation, the most effective zone of pulse on time and electrolyte concentration can be considered as 10–15 ms and 15–20 g/l, respectively, which gives an appreciable amount of MRR as well as lesser overcut. From the SEM micrographs of the machined jobs, it may be observed that a lower value of electrolyte concentration with higher machining voltage and moderate value of pulse on time will produce a more accurate shape with less overcut at moderate MRR. Micro-sparks occurring during micromachining operation causes uncontrolled material removal which results in improper shape and low accuracy. The present experimental investigation and analysis fulfils various requirements of micromachining and the effective utilization of ECM in the micromachining domain will be further strengthened.