Precise Electrochemical Machining Research Articles

Optical in Situ Analysis of Gas Bubble Evolution in Electrochemical Machining Based on Similarity Theory Using Particle Image Velocimetry

Numerical simulations are used to shorten the tool development process for precise electrochemical machining. However, these simulations are often based on simplified models that disregard bubble transport or have not been validated. Therefore, this paper presents optical in situ measurements of gas bubble transport during precise electrochemical machining. These measurements, by means of particle image velocimetry on an upscaled working gap in accordance with the principle of dynamic similarity, are then compared to simulation results. It was observed that due to the coalescence of the gas bubbles, the bubble size grows rapidly and obstructs the fluid flow.

Micro structuring tool steel components using Precise Electrochemical Machining (PECM)

Surface structuring offers great potential for modifying frictional properties for various applications, such as complex forming processes like sheet-bulk metal forming. The production of surface structures in micrometre range is challenging for manufacturing processes in particular when machining hard-to-cut materials like hardened tool steels. Precise electrochemical machining (PECM) has great potential for surface structuring and shaping of metallic materials regardless of their hardness with high surface quality and comparatively very short process times, especially when structuring large areas and batches. Micro structuring of hardened tool steel surfaces using PECM is investigated in this paper. Surfaces of high-speed tool steel and hot-work tool steel are structured using a commercial PECM machine with neutral solution of NaNO_3 as electrolyte. In a process sequence, PECM tools were manufactured in a first step producing selected structures by high-feed milling (HFM) and micromilling (MM). In a further process step, the negative shape of these complex structures was machined using the PECM process. Through this process chain, new types of structures can be generated which have different tribological properties than their corresponding negative shapes of HFM and MM structures. Tribological behaviour and wear properties of the structured surfaces are investigated through ring compression test (RCT).

Towards in-process evaluation of the precise electrochemical machining (PECM)

Increasing environmental and economic concerns have increased the use of high-strength materials and superalloys, such as the well-known nickel-based alloys. Due to their very high strength, such materials push established machining methods to their economic limits. As a result, non-traditional machining methods such as precise electrochemical machining (PECM) are gaining importance. PECM has shown significant advantages such as high material removal rates and lower surface roughness. Despite these advantages, some challenges still need to be overcome, such as the implementation of an in-process monitoring system and the determination of optimal process parameters to achieve maximum material removal rates and good surface roughness. While the analytical determination of the optimal parameters is difficult due to the high complexity of the process, the empirical determination shows better results. However, a hitherto very unused approach in this respect is the data-driven model. Based on process data, such models have shown great results in other fields, especially for highly complex systems. The first step in developing such a model is to figure out what process data are needed. In this paper, the potential of a data driven model for PECM is presented. Furthermore, current waveforms for different PECM setups are analyzed and various parameters for the in-process monitoring of the process are formulated. The ability of these parameters to differentiate between different processes was investigated. It was found that the formulated parameters could be used to monitor the efficiency of flushing in the working gap.

Concept for the experimental and numerical study of fluid-structure interaction and gas transport in Precise Electrochemical Machining

The biggest challenge in precise electrochemical machining (pECM) remains the highly time and cost consuming tool and process design. The development of deterministic and automated design methods requires a physics-based understanding of the individual phenomena influencing the removal rate and structural vibration of the workpiece during the machining process and the exact geometry of the final product. A concept study for the quantitative prediction of the fluid-structure interaction (FSI) between the multiphase electrolyte flow and the workpiece electrode is presented. This includes a novel experimental setup for the pECM process, in which the machining gap is upscaled using the principles of dynamic similarity. This enables the application of 3D particle image velocimetry for detailed measurements including a tracking of the workpiece deflections. The experimental results are used for the validation of a computational fluid dynamics method especially developed for the multiphase electrolyte flow. First results of numerical simulations focusing on the gas transport in the turbulent multiphase electrolyte flow are discussed.

Optimization of the Anodic Dissolution of the Superalloy Inconel 718

Due to its particular mechanical and thermal properties, nickel-based superalloys such as Inconel 718 are widely used for aerospace industry. They are known to be the among the most difficult-to-cut- materials[1], so that conventional machining methods such as milling are inefficient and hard to proceed[2]. Because of its ability to generate high quality surfaces and to reproduce complex shapes. Pulse or Precise ElectroChemical Machining (PECM), an unconventional machining process (no cutting tools), can be useful to manufacture those alloys. This technic is based on the anodic oxidation of metals through a strong applied pulse current. The electrical signal is synchronized to the oscillation of the cathode corresponding to a rectilinear translational movement. Throughout the process, the electrolyte is continuously renewed that allows the removing of the solid residues of the dissolved metal and guarantees a good dissolution efficiency. The PECM is characterized by a very low inter-electrode distance (gap), from 0.01 to 0.2 mm, which ensures the dimensional accuracy. First step was to understand the physical-chemical phenomena related to the anodic dissolution of the Inconel 718 in the usual electrolyte NaNO3. Thus, polarization curves associated with current constant dissolution have been studied in 2,35 mol/L (20 % wt) NaNO3. Energy dispersive X-ray spectroscopy analysis of cross-sections were used to highlight the solid anodic products formed during anodization. They are mainly constituted of Ni and Nb oxides as well as γ '' (Ni3Nb) inclusions. From ICP analyses of the electrolyte, the dissolved elements in solution are mainly those of the austenitic matrix. Consequently, in order to optimize the dissolution process, the individual anodic behavior of both main components forming the superalloy, namely the matrix and the γ'' phase were studied in a mixture of NaNO3 and EDTA as a chelating agent. Linear Sweep Voltammetry (LSV) was performed to study the global behavior of each phase in the chelating medium. Chronopotentiometry was carried out to determine the dissolution yield. Finally, electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the electrochemical interface following the anodization process. It appears that, in presence of EDTA, the hydroxides formation in the electrolyte is limited, whereas the charge transfer resistance and the anodizing layer resistance decrease. As a final step, the electrolyte containing the chelating agent was subsequently tested for Inconel 718 in PECM configuration using a home – made experimental set up in order to determine the influence of the gap, the electrolyte flow and the pulse time on anodic dissolution. In addition, metallographic sections and dissolution products analysis were also made to identify the material integrity. [1] D. Dudzinski, A. Devillez, A. Moufki, D. Larrouquere, V. Zerrouki, et J. Vigneau, « A review of developments towards dry and high speed machining of Inconel 718 alloy », Int. J. Mach. Tools Manuf, vol. 44, no 4, p. 439-456, mars 2004. [2] M. Burger, L. Koll, E. A. Werner, et A. Platz, « Electrochemical machining characteristics and resulting surface quality of the nickel-base single-crystalline material LEK94 », J. Manuf. Process, vol. 14, no 1, p. 62-70, janv. 2012.

Multi-physics Coupling Simulation of Precise Electrochemical Machining(PECM) with Pulsed Current and Vibrating Cathode

A new multi-physics model is presented for numerical simulation of the precise electrochemical machining process with pulsed current and vibrating cathode. The method includes the change of electric field, flow field, temperature field and geometric deformation field. Meanwhile, the effects of cathodic vibration on electrolyte velocity is obtained by transient simulation of flow field and the velocity distribution obtained from the multi-step steady state flow field is compared with that of transient flow field. To solve the problem that machining process simulation needs a lot of computing time, a multi-step steady state flow field solution and a loop iterative solution algorithm based on COMSOL are presented, and the error analysis was carried out. The results show that the algorithm can ensure the error within a reasonable range and greatly reduce the calculation time.

A Novel Modeling Approach for the Simulation of Precise Electrochemical Machining (PECM) with Pulsed Current and Oscillating Cathode

Simulation in electrochemical machining (ECM) offers the capability to distinctly reduce time and costs of the design process and, thereby, increase the economic potential of the manufacturing technology. In recent years, different models for the simulation of ECM with direct current (DC-ECM) have been introduced and successfully applied to varying geometries. Furthermore, first approaches to model the effect of ECM with pulsed current have been presented. However, state of the art machine tools additionally apply an oscillation of the cathode to further improve surface quality and machining precision. To date there exists no validated model for this precise ECM (PECM) process.Therefore, this paper outlines the challenges emerging from considering cathode oscillation in ECM simulation models. Based on an overview of existing models for ECM with pulsed current, conclusions are drawn which concepts can be adopted for the modeling of precise ECM and which phenomena require the introduction of new approaches. Lastly, a model for the simulation of precise ECM is presented. This model accounts for the flow field of the electrolyte, the evolution of temperature in the machining gap as well as the resulting electrical field and finally the material removal.

Experimental Study on the ECM and PECM of Pressed and Casted γ-TiAl Alloys for Aero Engine Applications

Based on low density and excellent high-temperature performance, Gamma Titanium Aluminide (γ-TiAl) is adequate for high temperature structural material in aero engine components, such as turbine blades, compressor casings etc. However, due to low temperature ductility and fracture toughness, problems occur by machining these alloys with traditional cutting techniques. Electrochemical machining (ECM) is a reasonable alternative because of its specific process characteristics. Therefore, this paper focuses on the experimental research of electrochemical removal behaviour of typical new materials in aero engine applications. The theoretical calculation was used to compare and analyze the test results. Furthermore, experiments with precise ECM with vibration (PECM) were performed to improve surface roughness. From the results of the examined pressed and casted γ-TiAl alloys, having almost the same chemical composition, it is concluded that the more homogeneous the phase, the higher is the reachable material dissolving rate (va) and less energy is needed in order to get a same value of va. The cross-sections attest that the electrochemical process has no influence on the rim zone.

Experimental Research on Improving ECM Accuracy and Stability for Diamond-hole Grilles

The demand for high-quality diamond-hole grilles, characterized by a voluminous and closely spaced array distribution of holes, has stretched traditional machining to the limit. For this reason, electrochemical machining is considered to be one of the better processing technologies for diamond grid holes. In order to improve the accuracy of electrochemical machining (ECM) for diamond-hole grilles, factors affecting the ECM process based on the electrochemical anodic dissolution theory were investigated in this study. The importance of small-gap processing for achieving higher machining accuracy was highlighted. In small-gap processing of diamond holes, the ECM mode was optimized using cathode side insulation, pulse power supply, passivating electrolyte, and other measures. A test-based comparative analysis of two differing machining modes, i.e., composite-feed ECM mode and precise ECM mode, was performed. The results showed that precise ECM mode, which precisely synchronizes the composite feed vibration with the pulse power supply, offered better machining accuracy and process stability in the ECM process for diamond-hole grilles. Precise ECM mode is envisaged to play a prominent role in the development of ECM technology.

A study of the current density analysis for two type method on the invar alloy in electrochemical machining

Invar alloy is important material used for the OLED (Organic light emitting diode) shadow-mask in the mobile display industry due to its characteristics of smallest thermal expansion coefficient. Consumers in modern society demand higher display resolution. Electrochemical machining is one of the methods for obtained these high-resolution requirements. Electrochemical machining is a non-contact method that has advantages regarding defects like thermal strain and micro burrs compared to other non-conventional machining methods. For precision electrochemical machining using different shape electrodes, the current density should be controlled precisely. The purpose of this study is to apply the FEM (Finite elements method) analysis in order to investigate the current density in the electrochemical machining. And the current density distribution between invar alloy and electrode are carried out with shape electrode type and micro array film type under the same conditions by simulation methods. FEM analysis results show that using shape electrode type, current density distribution is very concentrated. And also by micro array pattern film, desired current density which is needed for electrochemical machining could be obtained easily. More precise electrochemical machining can be available by controlling the current density between the suitable type and invar alloy.

The Effects of Different Electrode Types for Obtaining Surface Machining Shape on Shape Memory Alloy Using Electrochemical Machining

Shape memory alloy (SMA) is important material used for the medicine and aerospace industry due to its characteristics called the shape memory effect, which involves the recovery of deformed alloy to its original state through the application of temperature or stress. Consumers in modern society demand stability in parts. Electrochemical machining is one of the methods for obtained these stabilities in parts requirements. These parts of shape memory alloy require fine patterns in some applications. In order to machine a fine pattern, the electrochemical machining method is suitable. For precision electrochemical machining using different shape electrodes, the current density should be controlled precisely. And electrode shape is required for precise electrochemical machining. It is possible to obtain precise square holes on the SMA if the insulation layer controlled the unnecessary current between electrode and workpiece. If it is adjusting the unnecessary current to obtain the desired shape, it will be a great contribution to the medical industry and the aerospace industry. It is possible to process a desired shape to the shape memory alloy by micro controlling the unnecessary current. In case of the square electrode without insulation layer, it derives inexact square holes due to the unnecessary current. The results using the insulated electrode in only side show precise square holes. The removal rate improved in case of insulated electrode than others because insulation layer concentrate the applied current to the machining zone.

Results of Surface Integrity and Fatigue Study of PECM and PEO Processed γ-TiAl for Turbine Applications

Gamma Titanium aluminides (γ-TiAl) will play an important role in designing future aero engine components such as turbine blades and blisks. Due to the high heat resistance and low densities these materials are highly suitable for replacing nickel-based alloys in the high pressure compressor and low pressure turbine. Electrochemical machining (ECM) is an attractive process for manufacturing these materials because of the capability of this process to machine materials independent of their mechanical properties. Major advantages of ECM are its process specific characteristics of high material removal rates in combination with almost no tool wear. A possibility to enlarge the field of operation is a heat and corrosion protection layer made out of ceramics for the electrochemical machined turbine blades. One way to produce such a layer is the plasma electrolytic oxidation (PEO).Aero engine components are subjected to static and dynamic stresses. Component life and failure is greatly affected by aspects of the surface integrity of stressed parts, such as surface roughness values. Precise electrochemical machining (PECM) is capable of producing excellent surface integrity qualities on γ-TiAl without thermally and mechanically affecting the rim zone. In this paper the impact of different processing strategies by ECM as well as by a combination of ECM and PEO on fatigue strength with regard to component life is investigated.

Replication of microstructured tools for electrochemical machining applications

Electrochemical machining (ECM) is a powerful method to machine metals independently of their mechanical properties. Micro-sized structures can be formed with high precision. For this purpose, it is necessary to manufacture tools with adequately fine structures. Conventional ECM tools were manufactured by micro-drilling, -milling, -turning, etc. Depending on the complexity of the structures, these techniques are very elaborate and expensive. Recently, a new procedure, which combines photolithography and electroforming, the so-called PhoGaTool process (photolithographic electroforming of ECM tools), was published. In the present work, we demonstrate how this method can also be used for copying ECM tools that were conventionally manufactured. A copy of an industrial PECM tool (precise electrochemical machining) was manufactured and used for the structural characterization and determination of the replication accuracy. The electroforming parameters like deposition conditions, bath composition as well as physical and chemical parameters were optimized for model systems. The inclusion of bath additives in the metal matrix during the deposition leads to micro-stresses, hence in this contribution, an alternative additive-free electrolyte was used. The accuracy of the process was investigated by means of confocal laser scanning microscopy. Average deviations of the structure depth and the lateral structure dimensions are in the range of 5 %.

Electrochemical machining characteristics and resulting surface quality of the nickel-base single-crystalline material LEK94

Nickel-base single-crystalline materials such as LEK94 possess excellent thermo-mechanical properties at high temperatures combined with low density compared to similar single-crystalline materials used in aero engines. Since the components of aero engines have to fulfil demanding safety standards, the machining of the material used for these components must result in a high geometrical accuracy in addition to a high surface quality. These requirements can be achieved by electrochemical and precise electrochemical machining (ECM/PECM). In order to identify proper machining parameters for PECM the electrochemical characteristics dependent on the microstructure and the chemical homogeneity of LEK94 are investigated in this contribution. The current density was found to be the major machining parameter affecting the surface quality of LEK94. It depends on the size of the machining-gap, the applied voltage and the electrical conductivity of the electrolyte used. Low current densities yield inhomogeneous electrochemical dissolution of different microstructural areas of the material and lead to rough surfaces. High surface qualities can be achieved by employing homogenous electrochemical dissolution, which can be undertaken by high current densities. Furthermore, a special electrode was developed for the improvement of the quality of side-gap machined surfaces.

Precise pulse electrochemical machining by bipolar currentAspects of effective technological application

Abstract This paper describes a method of pulse electrochemical machining (ECM) by bipolar current. The phenomenology of the main aspects of its efficient technological application is considered: • Ensuring long-time stability of geometric and physical-and-chemical properties of the working surface of the electrode-tool (ET) through the removal of cathode sedimentations. • Creating workpiece surface layers of special physical-and-chemical properties (change of the chemical composition, corrosion resistance, etc.) due to the creation of the chrome-concentrated layer on the surface of the chrome steel workpiece. • Reducing the height and changing the shape of microroughness on the machined surface, giving it mirror like brightness. • Increasing machining precision due to greater non-equipotentiality of the machined surface. • Improving ecological cleanliness of the ECM technology due to the reduced content of toxic ions of the six-valent chrome in the electrolyte when manufacturing workpieces of chrome steels.