Effect of electro discharge machining (EDM) on the AISI316L SS white layer microstructure and corrosion resistance

The localised corrosion resistance of austenitic stainless steels is strongly influenced by the quality of finished surface. EDM machining induces substantial changes by the high thermal gradients generated by electric sparks. Experimental techniques such as roughness measurement, scanning electron microscopy (SEM), energy dispersive microanalysis (EDX) and X-ray diffraction technique, reveal microgeometrical, microstructural, chemical and mechanical changes. These changes lead to white and heat-affected layers with a depth less than 100 μm. The white layer is a melted material characterised by dendritic structure and constituted by austenite, chromium carbide and ε-carbide. The heat-affected layer is characterised by very large grain size comparatively to the bulk material. Electrochemical test coupled with metallographic examinations using SEM reveals a weakening of the resistance to pitting and intergranular corrosion comparatively to diamond polished surface. This weakening is correlated to differences in structure and chemical composition of white layer. Susceptibility to stress corrosion cracking has been attributed to the field of tensile residual stresses resulting from thermal effects. The removal of the white layer material by polishing or wire brushing restores the corrosion resistance of the AISI316L SS

Study and analysis of residual stresses in electro-discharge machining(EDM)

Technological advances have led to an increasing use of high strength, high hardness materials in manufacturing industries. In machining of these materials, traditional manufacturing processes are increasingly being replaced by more advanced techniques such as electro-discharge machining (EDM), ultrasonic machining (USM), electric chemical machining (ECM) and laser machining. EDM has found widespread application in MEMS, tool and mould industries and aerospace industries. Therefore, promoting the quality of the EDM process by developing a thorough understanding of the relationship between the EDM parameters and the machined surface integrity has become a major research concern. Electric discharge machining removes materials by melting and vaporizing caused by the high heat within the discharge column. Furthermore, EDM can easily fabricate the precision and complicated parts by choosing the appropriate machining conditions to effectively control the amount of removed materials. Although EDM can obtain fine surface integrity and precise dimensions under finishing condition, the rough machining condition produces larger and deeper discharge craters since the great quantity of the melted material is removed. Furthermore, the melted material is not removed completely because the impulse force is insufficient to flush away the melted material at the end of discharge interval. The remaining melted material is solidified to form a recast layer that distributes micropores and cracks due to the effect of thermal stress during cooling. Thus, the microscopic feature of machined surface is severely coarse that significantly deteriorates the usage life and precision of machinery parts. EDM involves the complex interaction of many physical phenomena. The electric spark between the anode and the cathode generates a large amount of heat over a small area of the work-piece. A portion of this heat is conducted through the cathode, a fraction is conducted through the anode, and the rest is dissipated by the dielectric. The duration of the spark is of the order of microseconds and during this time, a plasma channel is formed between the tool and the work-piece. Electrons and ions travel through this plasma channel. The plasma channel induces a large amount of pressure on the work-piece surface as well. This pressure holds back the molten material in its place. As the plasma starts forming it displaces the dielectric fluid and a shock wave passes through the fluid. As soon as the spark duration time is over and the spark collapses, the dielectric gushes back to fill the void. This sudden removal of pressure results in a violent ejection of the molten and vaporized material from the work-piece surface. Ejected molten particles quickly solidify in contact with the colder fluid and are eventually flushed out by the dielectric. Small craters are formed at locations where material has been removed. Multiple craters overlap each other and the machined surface that is finally produced consists of numerous overlapping craters. Although molten material ejection is not the only means of material removal in EDM, it is, however, the dominant mode of material removal in case of metals. During machining the local temperature in the workpiece gets close to the vaporization temperature of the material. Thus, phase transformation from solid to liquid as well as liquid to vapor occurs during the heating cycle. Part of the transformed material is removed but the rest re-solidifies on the surface of the workpiece. This re-solidified layer is usually called the white layer, as it is not easily etchable. EDM processes carried out in hydrocarbon dielectrics lead to the partial breakdown of dielectrics and this further leads to some diffusion of carbon Below the re-solidified white layer lies a second layer that does not melt but is still affected by heat. For steels, during the cool-down cycle, solid-state transformations occur in this heat-affected zone because the highest temperature reaches beyond the austenite transformation temperature. Finally, all the non-uniform heating and cooling give rise to transient and residual stresses in the workpiece. As a result of these residual stresses surface cracks may be formed in the white layers. Usually, residual stresses are not high enough to cause sub-surface cracks in the parent material but may lead to detrimental effects when the machined work-piece is used in applications. This work is intended on analyzing the cause of residual stress in EDM process. It is also showing how current variation brings about a change in the surface characteristics and how the microstructure variation occurs because of subsequent sparks with constant magnitude. This is studied to draw a relationship between the micro structural change and the generation of residual stresses. Scanning Electron Microscope (SEM) images taken from the samples show the surface variation at different currents. A comparative study shows current variation is a factor for the craters developed at the EDMed surface, and that higher magnitude of current changes the grain structure of the sample drastically and intensifies the magnitude of residual stresses generated in EDMed sample. The solid-solid transformation is brought about at a higher temperature (at higher current) and sample EDMed at higher current is seen to have greater surface roughness

Temperature Distribution in Copper Electrode during Electrical Discharge Machining Process

This paper reports on a new method used to estimate the spark radius in the gap during electrical discharge machining (EDM). This method combines the heat flux and energy equations of the copper electrode. The energy partition between workpiece and electrode (tool) due to EDM process was estimated using the ratio of thermal conductivity of the workpiece to that of the electrode. Using the energy partition between the electrode and the workpiece, temperature distribution in the electrode was established and Gaussian heat distribution was used to analyze the energy released from a single spark. The energy released due to a single spark was used to calculate the fraction of energy received by the electrode based on its thermal conductivity. The 3-D temperature distribution in the electrode was carried out using ANSYS version 11.0 and the estimated temperature of the electrode from a single spark was validated by thermal diffusivity of the electrode material. The difference of 6% was recorded between the simulated and calculated temperatures of the copper electrode. Based on the achieved percentage error, the simulated temperature on the copper electrode can be accepted as EDM process

Experimental Studies on Machinability of Inconel Super Alloy during Electro-Discharge Machining: Emphasis on Surface Integrity and Metallurgical Characteristics of the EDMed Work Surface

Inconel alloys are Nickel-Chromium based high temperature super alloys widely applied in aerospace, marine, nuclear power generation; chemical, petrochemical and process industries. Execution of traditional machining operations on Inconel super alloy is quite difficult due to its very low thermal conductivity which increases thermal effects during machining operations. Inconel often exhibits strong work hardening behavior, high adhesion characteristics onto the tool face, and thereby alters cutting process parameters to a remarkable extent. Additionally, Inconel may contain hard abrasive particles and carbides that create excessive tool wear; and, hence, surface integrity of the end product appears disappointing. The extent of tool life is substantially reduced. Thus, Inconel super alloys are included in the category of ‘difficult-to-cut’ materials. In view of the difficulties faced during conventional machining, non-traditional machining routes like Electro-Discharge Machining (EDM), Wire Electro-Discharge Machining (WEDM), micro-machining (micro-electro-discharge drilling) etc. are being attempted for processing of Inconel in order to achieve desired contour and intricate geometry of the end product with reasonably good dimensional accuracy. However, low material removal rate and inferior surface integrity seem to be a challenge. In this context, the present dissertation has aimed at investigating machining and machinability aspects of Inconel super alloys (different grades) during electro-discharge machining. Effects of process control parameters (viz. peak discharge current, pulse-on time, gap voltage, duty factor, and flushing pressure) on influencing EDM performance in terms of Material Removal Rate (MRR), Electrode Wear Rate (EWR) and Surface Roughness (SR) of the EDMed Inconel specimens have been examined. Morphology along with topographical features of the EDMed Inconel work surface have been studied in view of severity of surface cracking and extent of white layer depth. Additionally, X-Ray Diffraction (XRD) analysis has been carried out to study metallurgical characteristics of the EDMed work surface of Inconel specimens (viz. phases present and precipitates, extent of grain refinement, crystallite size, and dislocation density etc.) in comparison with that of ‘as received’ parent material. Results, obtained thereof, have been interpreted with relevance to Energy Dispersive X-ray Spectroscopy (EDS) analysis, residual stress and micro-indentation hardness test data. Effort has been made to determine the most appropriate EDM parameters setting to optimize MRR, EWR, along with Ra (roughness average), relative Surface Crack Density (SCD), as well as relative White Layer Thickness (WLT) observed onto the EDMed work surface of Inconel specimens. Moreover, an attempt has been made to examine the ease of electro-discharge machining on Inconel work materials using Deep Cryogenically Treated (DCT) tool/workpiece. A unified attempt has also made to compare surface integrity and metallurgical characteristics of the EDMed Inconel work surface as compared to the EDMed A2 tool steel (SAE 304SS) as well as EDMed Titanium alloy (Ti-6Al-4V)

Aplikasi android pembelajaran abjad Bahasa Jepun (JingoCat Apps)

Bahasa asing menjadi salah satu bahasa yang perlu dipelajari oleh pelajar Ijazah Sarjana Muda di Universiti Tun Hussein Onn Malaysia (UTHM). Pelajar yang mempelajari Bahasa Jepun sebagai satu kursus bahasa ketiga mempunyai masalah menguasai perbendaharaan kata (leksikal), struktur bahasa serta tatabahasa. Faktor masalah ini berlaku adalah disebabkan, perbendaharan kata (leksikal) yang asing bagi pelajar, menyebabkan sukar untuk dihafal dan diaplikasi dalam latihan sama ada latihan komunikasi atau latihan untuk membuat ayat (Noor Aizah Abas, 2009). Kajian ini bertujuan untuk membangunkan satu aplikasi android pembelajaran abjad bahasa jepun aras satu dan dua kepada pelajar UTHM khususnya dan umumnya boleh digunakan kepada mana-mana pengguna yang berminat. Aplikasi ini merupakan pembelajaran awal dalam mempelajari abjad Jepun di samping memudahkan pelajar untuk menguasai huruf Hiragana dan Katakana melalui aplikasi pembelajaran mudah alih berasaskan multimedia. Aplikasi android pembelajaran abjad jepun tahap 1 dan 2 (JingoCat Apps) mempunyai tiga menu utama iaitu Learn, Game dan Review di mana setiap menu mengandungi pembelajaran yang berbeza kandungan. Reka bentuk maklumat adalah berdasarkan Teori Kognitif, Teori Behaviorisme dan Teori Konstruktivisme. Model reka bentuk produk adalah berdasarkan Model Hannafin dan Peck (1988) terdiri daripada fasa Analisis, Reka Bentuk, Pembangunan dan Pengulangan dan Penilaian. Oleh itu, semua fasa akan mengandungi proses penilaian dan menyemak semula setiap fasa. Reka bentuk yang terlibat dalam pembangunan aplikasi JingoCat adalah informasi, interaksi dan antara muka. Metodologi kajian melibatkan temubual berstruktur untuk menilai kebolehfungsian aplikasi android yang dibangunkan. Penilaian terhadap pakar yang terdiri daripada empat orang pakar iaitu dua orang pakar dalam bidang Multimedia Kreatif dan dua orang pakar dalam bahasa asing (Bahasa Jepun). Hasil dapatan menunjukkan, keseluruhan pakar bersetuju bahawa reka bentuk informasi, interaksi dan antara muka memenuhi keperluan dan bersesuaian. Kesimpulannya, aplikasi pembelajaran abjad Jepun JingoCat menjadi satu Alat Bahan Bantu Mengajar (ABBM) dan mudah digunakan oleh semua pihak yang memerlukan

Electro-Discharge Machining of Ceramics: A Review

Conventional machining techniques of ceramics such as milling, drilling, and turning experience high cutting forces as well as extensive tool wear. Nevertheless, non-contact processes such as laser machining and electro-discharge machining (EDM) remain suitable options for machining ceramics materials, which are considered as extremely brittle and hard-to-machine. Considering the importance of ceramic machining, this paper attempts to provide an insight into the state of the art of the EDM process, types of ceramics materials and their applications, as well as the machining techniques involved. This study also presents a concise literature review of experimental and theoretical research studies conducted on the EDM of ceramics. Finally, a section summarizing the major challenges, proposed solutions, and suggestions for future research directions has been included at the end of the paper

Role of Heat Transfer on Process Characteristics During Electrical Discharge Machining

This book comprises heat transfer fundamental concepts and modes (specifically conduction, convection and radiation), bioheat, entransy theory development, micro heat transfer, high temperature applications, turbulent shear flows, mass transfer, heat pipes, design optimization, medical therapies, fiber-optics, heat transfer in surfactant solutions, landmine detection, heat exchangers, radiant floor, packed bed thermal storage systems, inverse space marching method, heat transfer in short slot ducts, freezing an drying mechanisms, variable property effects in heat transfer, heat transfer in electronics and process industries, fission-track thermochronology, combustion, heat transfer in liquid metal flows, human comfort in underground mining, heat transfer on electrical discharge machining and mixing convection. The experimental and theoretical investigations, assessment and enhancement techniques illustrated here aspire to be useful for many researchers, scientists, engineers and graduate students

An investigation and analysis of Process Parameters for EDM Drilling machine using Taguchi method

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Study of orifice fabrication technologies for the liquid droplet radiator

Eleven orifice fabrication technologies potentially applicable for a liquid droplet radiator are discussed. The evaluation is focused on technologies capable of yielding 25-150 microns diameter orifices with trajectory accuracies below 5 milliradians, ultimately in arrays of up to 4000 orifices. An initial analytical screening considering factors such as trajectory accuracy, manufacturability, and hydrodynamics of orifice flow is presented. Based on this screening, four technologies were selected for experimental evaluation. A jet straightness system used to test 50-orifice arrays made by electro-discharge machining (EDM), Fotoceram, and mechanical drilling is discussed. Measurements on orifice diameter control and jet trajectory accuracy are presented and discussed. Trajectory standard deviations are in the 4.6-10.0 milliradian range. Electroforming and EDM appear to have the greatest potential for Liquid Droplet Radiator applications. The direction of a future development effort is discussed