Florian Zeller

Analyzing the Electrical Pulses Occurring during EDM of Non-Conductive Si3N4 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.

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.

Comparative Study on Parametric Analysis of μEDM of Non-Conductive Ceramics

Characterized by excellent material properties such has high mechanical, thermal and chemical stability technical ceramics such as ZrO2, SiC, Si3N4 and AlN are increasingly being used for various applications. Traditional means of machining sintered ceramics are expensive and limited by geometry. Electrical discharge machining (EDM) is an electro-thermal machining process used to structure conductive materials. By applying a conductive layer (denoted as assisting electrode) on top of the non-conductive material, the EDM process can also be used to structure insulating ceramics. This paper presents a comparative study on the major machining parameters affecting the µEDM process of non-conductive SiC, ZrO2, Si3N4 and AlN ceramics. The influence of five major machining parameters (current, open-circuit voltage, gap voltage, duty-cycle and servo) over two responses (material removal rate (MRR) and tool wear rate) is investigated for each ceramics material. The underlying reason for the variation in the MRR among the different ceramics is examined by comparing the material properties. Melting point of the ceramics material has an effect on the MRR for the µEDM of different ceramics. The bulk resistance value of the ceramic material does not have an influence on the MRR for the µEDM of different ceramics. Scanning electron microscope (SEM) images of the cross section of the unprocessed and µEDM processed surface of these ceramics have been analyzed. The SEM micrographs show that the µEDM process does not affect the ceramics bulk. It also confirmed spalling as one of the dominant material removal mechanism for ZrO2 ceramics.