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

Abstract

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.