Electrochemical Discharge Drilling Research Articles

A sustainable, high-quality method for machining film cooling holes: Cryogenic airflow-assisted electrochemical discharge drilling

Film cooling hole is an essential part of aero-engines, necessitating without recast layer, no stray corrosion, and high machining efficiency. Its sustainable manufacturing has been at the forefront of industrial focus. Existing machining methods have problems such as low efficiency and environmental hazards. In order to achieve the production requirements of precision, quality, and green simultaneously, an external cryogenic airflow-assisted electrochemical discharge drilling (ECDD) is proposed. Compared with the current electrochemical drilling (ECD) or electrical discharge drilling (EDD), a glycol working fluid with ultra-low conductivity neutralizing salt replaces the high conductivity of acidic solution or spark oil to realize green machining without a recast layer. Compared to existing ECDD, the introduction of cryogenic gas with glycol can significantly reduce stray corrosion and achieve exceptionally high surface quality. In the theoretical part, the influence mechanism of a low-temperature environment on the electrochemical interface was analyzed by using the electrochemical analysis technique. In addition, a multi-physical field simulation model was established, which was finally verified by experiments. Finally, the surface characteristics were analyzed by using TEM. The results demonstrate that the cryogenic environment enhances the passivation effect, as evidenced by a 35% reduction in taper at 258 K and a 28.2% decrease in stray corrosion. The hole side has nano-roughness (Ra 0.512 μm). Notably, the material removal rate does not significantly diminish at low temperatures and achieves 0.1 mm/s, superior to EDD. As there is no need for an additional machining step after drilling, this method proves its characteristic to attain sustainability, efficiency, and high quality, indicating promising application prospects.

Self-adaptive micro-hole breakout detection in the electrochemical discharge drilling process based on CNN-BiLSTM

Electrochemical discharge machining (ECDM) is an effective and promising technology for the micro-hole drilling of hard and brittle insulating materials such as glass, quartz and ceramics. Hole exit quality is an important part of through-hole machining quality in ECDM. The hole exit quality is influenced by two different machining states before and after the hole breakout. Therefore, it is necessary to identify hole breakout occurrence to provide a basis for achieving a machining strategy that matches the changing machining state and thus improves the hole exit quality. However, currently, there is no effective method to detect the hole breakout in the ECDM of micro-holes. In this paper, a self-adaptive micro-hole breakout detection method based on CNN- BiLSTM was proposed. The hole breakout detection can be expressed as a binary classification problem according to the analysis of the current signals collected before and after the hole breakout. The sufficiency of the raw current signal information was discussed. A CNN-BiLSTM model was established to detect hole breakout using the current signals as the training dataset. The CNN-BiLSTM model showed better performance compared to CNN, LSTM, BiLSTM and CNN-LSTM by comprehensive considering of accuracy, F1 score, AUC value, training time, predict time, breakout detection results and Dunn test results. Furthermore, the length of current signals used for each single detection and the threshold value used for breakout detection were discussed. An optimal threshold value was calculated, and the delay of detection was within 66 ms. The proposed method was successfully applied to the hole breakout detection in the through-hole drilling experiments and the success rate was 100 %. The hole exit area just after breakout were measured and compared to the hole exit area after hole completion, the average ratios of the former to the latter were 11.69 %, 27.29 %, 38.43 % and 44.41 % for the voltage of 39 V, 41 V, 43 V and 45 V respectively.

Investigation of external capacitance effects on electrochemical discharge drilling

ABSTRACT Electrochemical discharge drilling (ECDD) involves a combination of electrical discharge drilling (EDD) and electrochemical drilling (ECD) to remove the recast layer while drilling at high speed. Here, a T-RC power supply similar to the EDD method is adopted. However, the solution resistance characteristics change as a result of utilizing a weak conductivity solution instead of deionized water with insulating properties. Different external capacitances will impact ECDD. In this study, through experiment and simulation, the influence of capacitance on the ECDD performance of Ni-based single-crystal alloy is investigated compared to EDD. In the open-circuit stage, the ECDD current shows an exponential relationship, and the voltage drops slightly. The highest line material removal rate reaches 326 μm/s, which is higher than EDD, and the tool wear ratio drops by 40%. The aperture and taper continue to ascend. Finally, no recast layer can be realized over 680 nF.

Experimental investigation of electrochemical discharge drilling (ECDM-D) performance characteristics for N-BK7 glass material

Experimental investigation of electrochemical discharge drilling (ECDM-D) performance characteristics for N-BK7 glass material

Breakthrough detection in electrochemical discharge drilling to enhance machining stability

Film cooling holes are widely used in aero-engine turbine blades. These blades feature large numbers of holes with complex angles and require a high level of surface integrity. Electrochemical discharge drilling (ECDD) combines the high efficiency of electrical discharge drilling (EDD) with high quality of electrochemical drilling (ECD). However, due to the existence of a variety of energy for material removal, accurate and timely detection of breakthroughs is fraught with difficulties. An insufficient preset setting distance results in a tiny exit aperture, influencing the structure’s shape. In addition, the electrode is prone to bending at a large overfeeding distance, causing secondary discharge damaging sidewall surface integrity. This paper compares and analyzes the characteristics of processing waveforms using EDD and ECDD. A novel breakthrough detection method is proposed based on the variance signal of average voltage (VSAV) to increase machining stability and achieve fabrication without a recast layer. This method extracts the fluctuation transformation by calculating the variance of the average. Following signal detection, the overfeeding distance is quantified. An experiment is used to validate the breakthrough detection with 100% accuracy in all tests. The optimum overfeeding distances for hole angles of 0 °, 30 °, and 60 ° are obtained, and the stable removal of the recast layer is realized. Finally, the effectiveness of the method is verified on a typical workpiece with a double-wall structure and a nickel-based single crystal blade.

A novel backflushing method for improving machining quality in electrochemical discharge drilling

Film cooling holes work in an extremely high temperature, high pressure, and alternating load. The recast layer is not allowed on the wall side. Electrochemical discharge drilling (ECDD) is a promising nontraditional method in aerospace manufacturing to obtain a high surface quality, offering high efficiency in theory. However, a shortage of working fluid near the hole exit after penetration is inevitable, leading to poor stability and surface quality, and the current processing methods make it difficult to drill small holes with high quality. Hence, a novel backflushing ECDD (BF-ECDD) method has been proposed. Backflushing is a certain conductivity liquid. It flows continuously along the backside of the workpiece during the entire process. When the hole is penetrated, the backflushing liquid flows from the exit to the entrance along the inter-electrode gap, improving the flow field and effectively solving the problem of liquid solution shortage. It helps to strengthen the electrochemical dissolution near the hole exit and improve the hole quality. Here, the method is shown to be feasible, and a conductivity matching relationship between the working and backflushing liquids is studied with simulation and comparison experiments on a nickel-based single crystal material. The results show that BF-ECDD can reduce the occurrence of short circuits by 68 % and enhance the stability of the penetration stage. The flow rate in the machining gap is more than doubled, and the taper angle is eliminated. Finally, it meets the extreme manufacturing needs of the entire hole without recast layer with high efficiency.

Experimental investigations of magnetic field-assisted high-speed electrochemical discharge drilling

ABSTRACT Owing to its promising physical, chemical, and optical properties, glass is the most widely used material in sophisticated micro-electro-mechanical systems (MEMS) devices. These devices are finding large number of applications in the fields of medical electronics, biotechnology, energy, automobiles, aerospace, etc. While manufacturing MEMS devices, micro-holes are needed to be produced in glass with high levels of accuracy. Micro-drilling in glass using the electrochemical discharge drilling (ECDD) method is an emerging technique. Enhancing the material removal rate (MRR) and lowering the entrance hole overcut in ECDD are the major challenges during this process. To address this problem, significant attention is focused on blending two or more techniques known as hybridized methods to improve the quality of the ECDD process. In the present study, a combination of magnetic flux density (MFD) and high-speed tool rotation was employed during the ECDD process for the glass. Additionally, the effect of the process variable viz. the voltage, tool rotation speed, and feed rate of the MFD around the workpiece was analyzed. Finite element analysis was employed to obtain the optimal MFD over the workpiece. For different voltages, tool rotation speeds, and feed rates; the performance analysis of MRR and entrance hole overcut were compared in the absence and presence of magnetic field-assisted high-speed ECDD.

Grinding aided electrochemical discharge drilling (G-ECDD): A theoretical analysis and mathematical modelling of material removal rate

Grinding aided electrochemical discharge drilling (G-ECDD): A theoretical analysis and mathematical modelling of material removal rate

An improvement in drilling of SiCp/glass fiber‐reinforced polymer matrix composites using response surface methodology and multi‐objective particle swarm optimization

AbstractThe growing dominance in terms of industrial applications has helped polymer‐based composite materials in conquering new markets relentlessly. But the presence of fibrous residuals and abrasive particles as reinforcement in polymer matrix composites (PMCs) affects the output quality characteristics (OQCs) of microdrilling operations. The OQC aims at reducing overcuts and momentous material removal rate (MRR). In such perception, multi‐objective particle swarm optimization (MOPSO) evident to be a suitable optimization technique for prediction and process selection in manufacturing industries. The present paper focuses on multi‐objective optimization of electrochemical discharge drilling parameters during drilling of SiCp and glass fiber‐reinforced PMCs using MOPSO. The response surface methodology (RSM)‐based central composite design was used for the experiment planning. Electrolyte concentration, interelectrode gap, duty factor, and voltage were used as process parameters, whereas MRR and overcut were observed as OQCs. The obtained experimental results were initially optimized by RSM‐based desirability function and later with multiresponse optimization technique MOPSO to achieve best possible MRR with lower possible overcut. The comparative analysis proves that OQCs can be effectively improved by using MOPSO.

Three-Dimensional Finite Element Modeling and Response Surface Based Multi-response Optimization During Silica Drilling with Closed-Loop ECDM

Electrochemical discharge machining (ECDM) is recognized as a successful method for processing non-conductive materials through thermal erosion and chemical dissolution. This article aims at the development of a finite element modeling (FEM) based three-dimensional (3D) thermal model for estimating the temperature distribution for scrutinizing the material removal rate (MRR) in electrochemical discharge drilling of silica glass material. The predicted FEM results are verified against existing literature and experimental results during micro-drilling operation. Simulations for evaluating the parametric studies such as the effect of applied voltage, electrolyte concentration, electrolyte type, energy partition on MRR is performed. Results revealed that the experimental values of MRR exhibit fare agreement with the predicted FEA results. Experiments are performed using adaptive tool feed based closed-loop ECDM machining that retracts the tool in an upward direction once its contact is detected with the work material. Tool contacts often result in a high tool wear rate (TWR). Moreover, the response surface methodology (RSM) based mathematical models are established to correlate the input variable’s relationship with the response parameters. Tool feed rate, applied voltage, and electrolyte concentration are chosen as numerical input variables while electrolyte type and tool material are chosen as categorical variables. MRR, TWR, and a number of tool contacts (NTC) are chosen as response parameters. A number of tool contacts are selected since it influences both MRR and TWR. The model’s adequacy is checked using the analysis of variance (ANOVA) and found significant for all the responses. The multi-response optimization of distinctive input variables is acquired by utilizing the desirability function (obtained 0.8646) for maximizing the MRR and minimizing the both TWR and NTC.

Study of the effect of process variables on output response in electro chemical discharge drilling of borosilicate glass

Electro Chemical Discharge Drilling (ECDD) process is a latest erosion based hybrid machining process which merges the notion of drilling electro-chemical machining (D-ECM) and drilling electro-discharge machining process (D-EDM). The ability of ECDD process is apposite for drilling the electrically non-conductive materials using a rotating tool electrode. Present article is focused on the development of tabletop ECDD setup based on various indigenous subparts like power arrangement, electrolyte feeding, workpiece feeding and machining compartment system. After assembling and testing of each subparts of ECDD, the exhaustive experiments were carried out on borosilicate glass workpiece fruitfully and examined the effect of process variables namely applied voltage, inter electrode gap, electrolyte concentration, speed of rotating electrode on output response like MRR. Moreover, it was examined that material removal rate (MRR) increases with rise in applied voltage and decreases with enhance of interelectrode gap as well as speed of rotating electrode. It is also seemed that MRR increases with varying the electrolyte concentration but beyond certain value it decreases. Micro graph study of machined workpiece material has also been scrutinized using scanning electron microscope (SEM).

Surface integrity of holes machined by electrochemical discharge drilling method

Micro-hole structures are widely used in aircraft engines, but they are difficult to fabricate because of stringent machining quality requirements. Electrochemical discharge drilling (ECDD) is a hybrid machining method for fabricating small holes in nickel-based superalloys that uses low-conductivity salt solution as the working fluid. Electrical discharge and electrochemical dissolution occur simultaneously in the ECDD process. ECDD is expected to achieve micro-holes with high machining efficiency and without any recast layer, but little research has been carried out on the surface integrity. In this study, the surface morphology, element composition, residual stress, microhardness, and recast layer properties of the ECDD machined holes were analysed. The results show that the surface integrity of the holes improves by the ECDD method, and there is no recast layer, no melted debris, and no tensile residual stress. Moreover, with an increase of the working fluid conductivity from 0.005 to 3.6 mS/cm, the residual stress of the ECDD machined hole becomes compressive, the microhardness of the wall no longer decreases significantly, and the surface roughness of the wall decreases to Ra = 1.69 μm. Furthermore, the average diameter and the taper angle of ECDD machined hole increase with the increase of machining time. In addition, the entrance of the ECDD machined hole has a round corner without a sharp edge, and its radius-of-curvature is approximately 150 μm.

Measurement and analysis of the geometric characteristics of microholes and tool wear for varying tool-workpiece gaps in electrochemical discharge drilling

In the last decade, the Electrochemical Discharge Drilling (ECDD) has emerged as an alternative to conventional laser ablation to create blind and through-holes in non-conductive materials such as fused silica, quartz, and glass workpiece. The initial tool electrode and workpiece (T-W) gap play a vital role in microhole formation and tool wear. Therefore, an experimental investigation into the effect of the initial T-W gap with different machining time and electrolyte concentrations on the blind microhole formation in the glass and tool wear in the velocity-feed ECDD is reported. The microholes having superior surface quality and lower overcut were fabricated with KOH electrolyte and lower concentration. A novel numerical model based on inverse heat flux reduction for varying initial T-W gaps was studied for the first time. The obtained experimental results were compared to the numerical results predicted with the finite element based numerical simulations. The opening size of the microholes increased up to a critical T-W gap and then decreased with a further increase in the gap. The critical gap is found to be 20 µm and 10 µm for NaOH and KOH electrolyte, respectively. The depth of the microholes was the maximum at a zero-gap condition; however, the tool wear was severe. The highest average depth of 175 µm was obtained at 30 wt% NaOH for the machining time of 60 s. Tool wear tends to be decreasing with increasing the T-W gap. The least average tool wear of 9 µm was observed for 20 wt% KOH electrolyte for machining time of 60 s. The experimental results indicated that the conventionally used gravity-feed mechanism is not suitable from the tool wear point-of-view, and a particular initial T-W gap should be maintained during the ECDD process. The T-W gap between 10 and 30 µm for the NaOH electrolyte and 5–20 µm for the KOH electrolyte is recommended.

Variable-parameter high-precision electrochemical discharge drilling method for 440C-Nb without recast layer

Engine nozzle orifices are broadly used in the automotive field, and their manufacture entails high drilling accuracy, consistency, and excellent machined surface quality. Electrochemical discharge drilling (ECDD) is an innovative compound machining technique, which can be used to machine small holes of difficult-to-cut materials. Electrical discharge machining (EDM) as well as electrochemical machining (ECM) can occur simultaneously in ECDD, owing to which it can obtain not only good surface quality but also high processing speed. However, the machining accuracy, taper, and surface quality achieved using ECDD do not satisfy the requirements of engine nozzle orifices, which are of a higher standard. For improving the machining accuracy and surface quality, a variable-parameter electrochemical discharge drilling (VPECDD) has been proposed. The machining process is divided into two consecutive stages, including perforation and shape modification, while different parameters are used in different stages. In this study, the finite element method was applied to study the mechanism of VPECDD, and comparative experiments were performed among EDM, ECDD, and VPECDD. The results indicate that VPECDD can achieve very high machining accuracy and consistency, obtain an inlet and outlet diameter difference at 0.002 mm, maintain an average hole taper at 0.0004°, and remove the recast layer completely. Finally, it is verified that VPECDD is suitable for machining microholes with high precision and no recast layer.

Influence of tool electrode feed rate in the electrochemical discharge drilling of a glass substrate

ABSTRACT The tool electrode feed rate (TEFR) is a parameter of vital importance for velocity-feed electrochemical discharge drilling (ECDD). The tool electrode will hit the workpiece and may even cause to crack if higher TEFR is used, while a lower TEFR will curtail the material etch rate. Hence, a comprehensive experimental study into the TEFR effects on the ECDD performance is presented. The top and bottom sizes of micro-holes, eccentricity, circularity, heat-affected zone, and tool electrode wear are the important features considered in this experimental work. Cylindrical-shaped tungsten carbide tool electrodes with 70 μm and 300 μm sizes were utilized. The TEFR in the range of 1–9 μm s−1 were selected. The stable and uniform EC discharges were observed for the TEFR < 5 μm s−1. The minimal hole size and HAZ were obtained for the TEFR in the range of 3–4 μm s−1. The eccentricity and circularity of micro-holes were found to be lower for the TEFR < 5 μm s−1. Higher tool wear was noticed for the TEFR < 2 μm s−1. Based on the overall experimental evaluation, TEFR in the range of 3–4 μm s−1 is recommended.

Reduction of stray corrosion in electrochemical discharge drilling by changing the electric field distribution

Electrochemical discharge drilling is an effective way to machine micro-hole without recast layer. During this process, stray corrosion caused by stray current will damage the surface quality around the entrance of micro-holes. Insulation coating and auxiliary anode, which can change the electric field, have been applied to reduce stray corrosion. As shown in the simulation, if non-machined zone of the workpiece is insulated, stray current will concentrate around the entrance of the micro-hole and high current density will reduce intergranular and secondary corrosion. Apart from that, using auxiliary anode whose voltage is higher than the workpiece can not only change the direction of the current flow, but also decrease the current density and stray corrosion in non-machined zone of the workpiece. Furthermore, the dimension of the auxiliary anode and the gap width between workpiece and auxiliary anode determine the effect of the auxiliary anode. Experiments prove that the use of auxiliary anode and insulation coating can change the distribution of electric field, eliminate the stray corrosion caused by stray current, and improve the surface quality of the workpiece.

Developments of non-conventional drilling methods—a review

Demand for drilled micro-holes on difficult to machine materials have increased over the past years and non-traditional drilling processes are commonly used to fabricate such micro-holes on difficult to machine materials. This research investigates different non-traditional drilling processes, such as electro discharge, laser beam, abrasive water jet, electrochemical and electrochemical discharge drilling methods. Drilling mechanism, material removal rate/machining speed and surface finish have been analysed for every process. These analyses clearly show that vaporisation, melting, chemical dissolution and mechanical erosion are dominant material removal mechanism during non-traditional drilling. The understanding on electro discharge, laser beam and abrasive water jet drilling are more developed than that of electrochemical, electrochemical discharge and hybrid drilling processes.

Investigation of Mechanism of Tool Electrode Wear in Tube Electrode High-Speed Electrochemical Discharge Drilling

Investigation of Mechanism of Tool Electrode Wear in Tube Electrode High-Speed Electrochemical Discharge Drilling

Experimental investigations, empirical modeling and multi objective optimization of performance characteristics for ECDD with pressurized feeding method

Electrochemical discharge drilling (ECDD) is a hybrid micromachining process that can be used to drill micro holes on all kind of materials, irrespective of their mechanical and electrical properties. In ECDD, the continual penetration of tool electrode having electrochemical discharge (ECD) energy along the work material generates the holes. The relative movements between tool electrode and work material controls the energy penetration rate, and subsequently effects the performance characteristics. Therefore, the effect of process parameters associated with feeding motion and ECD energy are important to investigate simultaneously with respect to the performance characteristics. In the current study, a Buckingham’s dimensional method was used to determine the relation between input process parameters (that includes simultaneous interaction of ECD energy based parameters and feeding motion based parameters) and performance characteristics. The material removal rate (MRR) and hole overcut (HOC) were considered as the performance characteristics. MRR and HOC symbolize the productivity and accuracy of the process, respectively. During predictive modeling, the constant terms and coefficients were estimated from the non-linear experimental data. The outcomes manifested by the predictive models are in good agreement with the experimental results. The underlying process mechanism for ECDD with the pressurized feeding system has been explored by considering the simulation results as an evidences. Microscopic images of machined holes and discharge characteristics were used to develop the fundamental background of ECDD process mechanism with pressurized feeding method. Additionally, genetic algorithm (GA) was used to perform the multi objective optimization of ECDD process.

Experimental investigations into rotary mode electrochemical discharge drilling (RM-ECDD) of metal matrix composites

In the present work, rotary mode electrochemical discharge drilling (RM-ECDD) was employed to machine the Al-6063 SiCp metal matrix composites (MMC’s). The effect of various process parameters such as applied voltage, pulse on time, tool rotation rate, electrolyte concentration and current on quality characteristics (hole over cut and depth of penetration) of the drilled holes was investigated. The underlying material removal mechanism during the machining of MMC’s with RM-ECDD is also proposed by considering the scanning electron micrographs (SEM) and simulation results as an evidences. A comparative analysis between RM-ECDD and ECDD was performed during the machining of MMC’s. The experimental results reveal that the integration of tool rotations in ECDD successfully evacuates the sludge/debris from the machining zone, and it also replenishes the electrolyte effectively. Therefore the use of RM-ECDD improves the aspect ratio (L/D), and it exhibits 23.07 times higher L/D ratio than the ECDD process. In ECDM, the discharge characteristics are very important outcome that explores the significant details of the involved mechanisms and phenomena of the process. In the present research, a DSO recorded voltage signals were used to understand the discharge behavior and their characteristics at different parameter combinations. This paper also presents a second-order mathematical models to determine the relation between input process parameters and quality characteristics. Desirability based multi objective optimization approach was employed to optimize the process parameters.