Abstract
The resulting recast layer from EDM often exhibits high levels of residual stress, unacceptable crack density and high surface roughness; all of which will contribute to diminished surface integrity and reduced fatigue life. Previous studies have shown that the surface of EDM'd components can be successfully enhanced through the use of large-area pulsed electron beam surface modification, which, through a rapid remelting process, results in a net smoothing of the workpiece surface. It has also been shown that cracks created by EDM are repaired within the region molten by EB irradiation, and therefore the process is proposed to reduce the impact of EDM on fatigue life and deleterious surface properties. In this work the complex multilayers of the near surface are interrogated by TEM and XRD. A FIB-TEM study of the entire remelted layer produced by the irradiation process has been performed for the first time. The characterisation of these layers is necessary for predicting the performance of the material in application. Pulsed EB irradiation was shown to be capable of creating several distinct surface layers of nanostructures which consist of varying grain sizes and grain orientations. Austenite was revealed as the dominant phase in the remelted layer, with a grain size as small as 5nm produced at the very top surface. A needle-like phase also present in the layer is thought to be cementite.
Highlights
Unlike conventional machining processes which rely upon shear based mechanisms for material removal, electrical discharge machining (EDM) liberates material from the bulk workpiece through rapidly heating and melting localised regions
Proskurovsky et al [6] and Pogrebnjak [7] have examined transmission electron microscopy (TEM) images of a ferritic steel and single crystal copper respectively, revealing nanostructures down to 30 nm induced by the irradiation process for example in steel, there is still no clear cross-sectional TEM analysis of the entire remelted layer from the top surface revealing the evolution of microstructures with depth
The near-surface lamellar was first analysed using TEM in order to determine the grain size, grain orientation as well as depths of sub-layers associated with the transformations induced by the pulsed electron beam irradiation process
Summary
Unlike conventional machining processes which rely upon shear based mechanisms for material removal, electrical discharge machining (EDM) liberates material from the bulk workpiece through rapidly heating and melting localised regions. The EDM process involves the repeated expulsion of molten and vaporised material at the end of every discharge, typically several thousand times per second, causing some material to resolidify on the workpiece surface and leaving a recast layer to remain This region is characterised by cracks and a rough surface morphology consisting of pits and asperities limiting corrosion resistance, and these defects combined with high tensile residual stresses impede mechanical behaviour such as fatigue and tribological performance. Proskurovsky et al [6] and Pogrebnjak [7] have examined TEM images of a ferritic steel and single crystal copper respectively, revealing nanostructures down to 30 nm induced by the irradiation process for example in steel, there is still no clear cross-sectional TEM analysis of the entire remelted layer from the top surface revealing the evolution of microstructures with depth Such a study is necessary to clearly summarise the range of structures expected after EB irradiation. In this study, the optimum electron irradiation parameters which result in the crack repair process on such surfaces will be examined here using TEM and XRD
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