Assisting Electrode Research Articles

Machining of ZrO2 using wire EDM: an experiment based investigation via assisted electrode

ABSTRACT Machining of electrically insulating ceramic materials has been a challenging task for ceramic industries. Assisted electrode method (AEM) based wire electrical discharge machining (WEDM) process emerged as an advanced option to machine insulating ceramics. This study deals with the machining of yttrium (Y2O3) stabilised zirconia (ZrO2) ceramic using copper (Cu) and stainless steel (SS) assisted electrode (AE) in presence of alumina (Al2O3) powder, ethylene glycol and demineralised water mixed dielectric fluid. The effect of different AE materials, pulse on time (TON), peak current (IP) on length of cut (LC), surface roughness (Ra), and average micro-hardness (MH) were investigated. Further, the comparison was made between minimum and maximum LC, and Ra (case-(i) and case-(ii)), respectively. SEM and EDS were carried out to analyse the surface characteristics and distribution of elements of the machined surface. Experimental results show that machining with SS AE at high levels of TON and IP provides high spark energy that results in higher value of LC (1300.910 µm), Ra (24.520 µm) and MH (1401.713 HV). Increasing TON and IP significantly increases the LC and Ra. Moreover, hardness near the lower junction was higher than the hardness near the upper junction and middle region of the machined surface.

Study on analysis of plasma resistance variation in WEDM of insulating zirconia

ABSTRACT Wire electrical discharge machining (WEDM) is a specialized electrothermal machining process capable of producing complex shapes with close tolerances irrespective of material hardness. Zirconia (ZrO2) ceramic has higher fracture toughness and flexural strength which can be useful in a variety of applications, but the principle limiting factor is its electrical conductivity. Assisting electrode (AE) method is one of the methods to overcome the above limitation that serves the purpose without altering the basic properties of ceramic material. In the present research, the Zirconia (ZrO2) is coated with a copper layer. The kerosene dielectric suggested in past literature for machining of insulating ceramics is replaced by the graphite additive mixed distilled water to provide an environment-friendly approach. The electrical resistivity of ZrO2 imposing some restrictions for a normal machining operation, so the discharge voltage variations are analyzed at three different machining stages, namely, Assisting electrode (AE), AE-ceramic interface and insulating base ceramic. Also, the nature of the spark and its variation concerning the discharge voltage is studied with the help of images captured by a high-speed camera. The combined study of the discharge voltage pulse and the spark image helps to understand the machining phenomena.

A study of material removal and surface characteristics in micro‐electrical discharge machining of carbon fiber‐reinforced plastics

This article presents micro electrical discharge machining (μEDM) of carbon fiber‐reinforced plastics (CFRP) with rotating tool electrode and assisting electrode (AE). The experimental investigation majorly focused on establishing the mechanisms of material removal and surface integrity of the machined blind holes. Taguchi and regression analysis were carried out to assess the material removal rate (MRR). Analysis of variance (ANOVA) was applied to understand the significance of input parameters and their contribution toward achieving maximum MRR. The surface integrity of the machined hole and damage modes of composite constituents were also examined through micrographic images. The experiments were carried out varying the voltage, pulse duration, and tool speed at three levels. The investigations showed that μEDM process can be applied for fabricating micro holes in CFRP if assisting electrode (AE) is used to initiate the spark. From the analysis of variance (ANOVA), it was observed that the percentage contribution of voltage, pulse duration, and tool rotational speed on MRR are 75.09, 5.20, and 16.09%, respectively. The optimal condition for the highest MRR was found to be V3/T1/S3 (voltage of 170 V, pulse duration of 10 μs, and tool speed of 800 rpm). The calculated value of MRR (341,264.94 μm3/s) at the optimum level of input parameters was found to be close to the experimental MRR (340,727 μm3/s). The percentage error between the experimental and calculated value of MRR at the optimum level is very low as 0.15%. This indicates that the developed statistical model is quite significant in predicting MRR during μEDM of CFRP. POLYM. COMPOS., 40:4033–4041, 2019. © 2019 Society of Plastics Engineers

The Effect of the Electrical Discharge Machining Process on the Material Properties of Nonconductive Ceramics

Electrical discharge machining (EDM) is widely used to manufacture complex shaped dies, molds and critical parts in conductive materials. With the help of an assisting electrode (AE), EDM process can be used to machine nonconductive ceramics. This paper evaluates the mechanical properties of three high-performance nonconductive ceramics (ZrO2, Si3N4, and SiC) that have been machined with the EDM process using AE. Mechanical properties such as Vickers hardness (HV 0.3), surface roughness (Sq), and flexural strength of the machined and the nonmachined samples are compared. The EDM process causes decrease in Vickers hardness, increase in surface roughness, and decrease in flexural strength.

Analyzing the Electrical Pulses Occurring during EDM of Non-Conductive Si<sub>3</sub>N<sub>4 </sub>Ceramics

With the help of an Assisting Electrode (AE), non-conductive ceramics can be machined using Electrical Discharge Machining (EDM) process. The AE helps start the EDM process and the intrinsic conductive layer (ICL) ensures that the electrical contact between the workpiece and the generator is maintained. However, the understanding of EDM process of non-conductive ceramics is limited due to the typical characteristic of the process: the geometric dimension of the discharge region is limited to few micrometers and the duration of the dielectric discharge is in the range of few microseconds making it difficult to directly observe the spark and the associated crater formation phenomenon. Analyzing the electrical signals during the EDM process could provide an insight of the process. This paper presents a study on analyzing the electric pulses occurring during the EDM process of a conductive workpiece (copper) and a non-conductive workpiece (Si3N4 ceramics). Typical pulses occurring during the EDM of a metal and a non-conductive ceramics are identified with the signal recorded at a sampling rate of 2.5 GS/s. Sampling rate of 25 MS/s is used to monitor the pulses occurring while machining a hole with depth of 1 mm. The pulses during EDM of non-conductive materials are significantly different than those during EDM of metals. Four most outstanding differences have been identified in terms of the ringing behavior of the voltage signal, the recharge time required to reach the set value of open voltage, the presence of the reverse current and the value of the peak current detected in case of Si3N4. The pulses monitored with lower sampling frequency was characterized and discriminated into sparks, arcs and short circuits. The percentage of the discriminated pulses for varying machining depth of the test structure has been presented.

Parametric Analysis of Micro Electrical Discharge Milling Process of Non-Conductive Silicon Carbide

Silicon carbide (SiC) is a high performance ceramic material which is increasingly used in applications which demand for high temperature stable materials. The microstructuring of this material in its sintered state is a challenge for conventional machining methods due to its extraordinary mechanical properties. To overcome these limitations the process of electrical discharge machining (EDM) was modified with a so called assisting electrode (AE) to enable the structuring of non-conductive ceramics such as SiC. This paper presents a parametric analysis of micro electrical discharge milling process of non-conductive SiC for two different tool materials, tungsten carbide (WC) and copper (Cu). A full factorial design with four factors was conducted. The significant factors are shown with main effects plots. Two sets of optimized parameters for maximum material removal rate (MRR) and minimum tool wear rate (TWR) for both tool materials are presented. Hardness and surface roughness are measured and compared to the non-machined material.

Effect of Gap Voltage and Pulse-On Time on Material Removal Rate for Electrical Discharge Machining of Al<sub>2</sub>O<sub>3</sub>

Electrical discharge machining (EDM) is a non-conventional machining technique which can be used to machine non-conductive ceramics. This technique removes materials from the workpiece by thermal energy exerted from series of electrical sparks. Using copper foil as assisting electrode (AE), machining of Al2O3 is done successfully. In this investigation, experiments were performed to study the effect of gap voltage and pulse-on time on material removal rate (MRR) for EDM of Al2O3. The results showed that the lowest and the highest values of gap voltage were 12 V and 14 V, respectively, with a fixed peak current of 1.1 A and pulse-on time of 8 μs. Beyond these two voltage values, material cannot be removed due to insufficient pyrolytic carbon layer generation. Similarly, pulse-on time is varied from 6 μs to 8 μs when gap voltage is fixed at 14 V and peak current at 1.1 A. MRR, in this case, is increased almost 20 times from a lowest value of 0.006 mm3/min to a highest value of 0.119 mm3/min for the specified gap voltage and pulse-on time.

INVESTIGATION OF THE MACHINABILITY OF NON-CONDUCTIVE ZrO2 WITH DIFFERENT TOOL ELECTRODES IN EDM

Electrical discharge machining (EDM) is a non-conventional process where complex and difficult-to-cut materials can be machined. Adhesive copper foil as an assisting electrode (AE) is used to cover the zirconia (ZrO2) surface to start the primary spark between the tool electrode and workpiece. Kerosene is dissociated and produces a carbon layer on the workpiece surface when machining of the initial copper foil is completed. Thus machining continues although ZrO2 is a non-conductive material. In this study, the EDM of ZrO2 is investigated with graphite, copper and brass tool electrodes. Material removal rate (MRR) and surface characteristics are analysed. Experiments are performed by varying the parameters peak current and pulse-on time with different tool electrodes. From the experiments, MRR on ZrO2 has been compared for three different tool electrodes. It is found that the graphite tool electrode performs the highest MRR for EDM of ZrO2. The least MRR is obtained by the brass tool electrode. However, better surface quality is observed with the copper tool electrode than EDM with brass or graphite electrodes. This investigation with varying machining parameters and different tool electrodes can be helpful in finding an effective use of the EDM process.

Micro-electro discharge machining of non-conductive zirconia ceramic: investigation of MRR and recast layer hardness

Increasing material removal rate (MRR) and minimizing recast layer hardness are critical issues in machining non-conductive ceramic using micro-electro discharge machining (micro-EDM). This paper presents the analysis of MRR and recast layer hardness of zirconium oxide (ZrO2) due to micro-EDM using EDM-3 dielectric fluid and tungsten tool electrodes. The two main parts of this research are process development and the analysis of MRR and recast layer hardness. In process development, the appropriate use of assisting electrode (AE), polarity, flushing, feed rate, gap voltage, and tool electrode rotational speed are identified. The better machinability of ZrO2 was found to be with copper adhesive as AE, positive workpiece polarity, 3-μm/s feed rate, and workpiece submerged in dielectric fluid with one-way circulation. Empirical models are developed for the estimation of MRR and recast layer hardness. The optimum parameters for maximum MRR and minimum recast layer hardness are found to be at a 370-rpm rotational speed and at 80-V gap voltage.

Parametric Analysis of μ-Electric Discharge Machining of Non-Conductive Si<sub>3</sub>N<sub>4</sub>

Electric discharge machining (EDM) is an electro-thermal non-traditional machining process used to machine conductive materials. EDM process can be used to machine non-conductive ceramics by applying an assisting electrode (AE) on top of the insulating ceramics. The initial discharges occur between the tool electrode and the assisting electrode. During the spark erosion process an intrinsic conductive layer is continuously produced on the insulating ceramic. This transitional conductive layer ensures that the electric contact is sustained. However, result of parametric analysis of the EDM of non-conductive ceramics is not available. This paper presents a parametric study to evaluate the influence of five major EDM process parameters (peak current, open-circuit voltage, servo, pulse width and gap voltage) on two response variables (material removal rate and the tool ware rate) when structuring non-conductive Si3N4 ceramics using the die-sinking μ-EDM process with the help of the AE. A set of experimental trials planned with fractional factorial design with a resolution of V was performed to evaluate the primary, two-factor and quadratic effects.

Electrical Discharge Milling of Silicon Carbide with Different Electrical Conductivity

Silicon Carbide is a ceramic material with extraordinary properties. Not only does it excel in mechanical properties, such as very high hardness and flexural strength, but also shows excellent thermal properties with a low thermal expansion and operating temperatures above 1000°C. These properties predestine silicon carbide for applications in harsh environments. Structuring silicon carbide in the sintered state with conventional methods is not feasible due to its hardness. A non-conventional process to structure materials independent of their mechanical properties is electrical discharge machining (EDM). A certain conductivity is however required for this process. To fulfill this requirement, the method of an assisting electrode (AE) is used. During the process, an intrinsic AE is generated from the cracked dielectric oil and deposited on the ceramic surface. The process can therefore continue even after the applied AE has been penetrated. For a deeper understanding of the present removal mechanism EDM of non-conducting ceramics, especially in the area of micro EDM, an investigation of the influence of conductivity is necessary. Therefore three silicon carbide ceramics with different electrical conductivity (S-SiC: 1 10-7S/cm; LR-SiC: 10 S/cm; HO-SiC: 5 10-9S/cm) have been microstructured and analysed. It is found that the conductivity of the silicon carbide materials has no influence on the machinability, all samples can be microstructured. The microsections of the machined samples show that the near-surface structure of the SiC materials is not negatively influenced by the EDM process. The analysis of the surface revealed indications that for S-SiC and for HO-SiC, thermal spalling is the present removal mechanism. The LR-SiC surface shows melting structures. The material removal rate of LR-SiC is 8 × 10-3mm3/min, whereas the material removal rate of the S-SiC and the HO-SiC ranges at 3 × 10-3mm3/min. The high MRR of the LR-SiC indicates a removal mechanism analog to Silicon infiltrated Silicon carbide (SiSiC), with removal of a conductive phase.

Effect of Peak Current on Material Removal Rate for Electrical Discharge Machining of Non-Conductive Al<sub>2</sub>O<sub>3</sub> Ceramic

Electrical discharge machining (EDM) is a non-conventional machining process where materials are removed by the thermal energy exerted from series of electrical sparks. This process is applied for machining of non-conductive alumina (Al2O3). The workpiece is covered with the adhesive copper foil to initiate the initial spark between the workpiece and the tool electrode. A pyrolytic carbon (PyC) layer is generated on workpiece surface by dissociating kerosene dielectric after the machining of initial copper assisting electrode (AE) layer. In this study, experiments were performed by varying the peak current and keeping other parameters constant in order to investigate the effect of peak current on material removal rate (MRR) in EDM of Al2O3. The results showed that the lowest and the highest values of peak current were 1.1 A and 1.3 A, respectively. Material cannot be removed due to insufficient PyC layer generation for any values of peak current less than 1.1 A or more than 1.3 A. From the results, it is also observed that the MRR is increased when higher peak current values are used. MRR was found to be 0.052 mm3/min at peak current 1.1 A and it was found to be 0.132 mm3/min at peak current 1.3 A.

Analysis of Surface Reaction Mechanisms on Electrically Non-Conductive Zirconia, Occurring within the Spark Erosion Process Chain

Electrically insulating high performance ceramics like zirconia are increasingly used for high technological applications due to their extraordinary properties. Meanwhile even the spark erosion process can be used to structure non-conductive ceramics by using an assisting electrode (AE). This conductive AE is placed on top of the work piece to enable the process. The electrical contact is sustained during the process by carbonic residua of cracked oil used as a dielectric. For an enhanced understanding of the removal mechanisms during the spark erosion process of such non-conductive ceramics, we analysed the surface of ceramic samples during the entire spark erosion process chain. In our investigation a zirconium oxide with yttrium oxide stabilisation was chosen. For this purpose X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis was performed. The XRD results showed that the zirconium oxide reacts under the presence of carbonic residua during processing into zirconium carbide. This material supports sustaining the conductivity additionally, as it is conductive itself. In a subsequent thermal cleaning step with oxide atmosphere (T = 750°C), remaining carbonic residua are oxidised and thus removed. The XRD measurements after cleaning showed that the zirconium carbide was no longer detectable. Instead a monoclinic zirconium phase was found. This shows that a reverse reaction of zirconium carbide into zirconium oxide took place. To prevent the formation of a non-stabilised monoclinic phase, the oven process was adapted to higher temperatures of 900°C with higher heating and active cooling rates of up to 10 K/s. This adjustment shows that the monoclinic phase can be supressed and the reverse reaction leads to a tetragonal zirconium oxide like the bulk material.