Ultra-fined grain materials are thermodynamically unstable and when they are exposed to a high external thermomechanical energy, such as electrical discharge machining process, many microstructural changes will occur in them. However, in the electrical discharge machining process, the recast layer and heat affected zone are the undesired and inevitable consequences of this process, which have several adverse effects on the surface layers of the workpiece including microstructural changes, grain growth, alternation of hardness, initiation of micro-cracks and changing the composition. All of which deteriorate the surface integrity. In this article, the effects of the electrical discharge machining process on the ultra-fined grain steel samples have been studied through investigating the microstructure of the recast layer and heat affected zone via scanning electron microscopy, optical microscopy and X-ray diffraction technique. The thickness of the recast layer and heat affected zone as well as the cracks’ density and the hardness profile of the ultra-fined grain samples was measured and the results were compared with coarse grain samples. Results show that the undesired effects of electrical discharge machining process on the ultra-fined grain samples are more considerable than the coarse grain ones; for instance, by comparison with coarse grain samples, relatively thicker recast layer and heat affected zone are formed in the ultra-fined grain samples, in which the microstructure changed more considerably. In addition, on one hand, the more extended cracks on the electrical discharge machined surface of the ultra-fined grain samples were observed, and on other hand, the hardness profile of the ultra-fined grain samples varies more noticeably from the surface to the depth.

Copper alloys, due to their proper ductility, proper thermal conductivity and low electrical resistivity, are very applicable in various industries. Ultra-fined grain materials undergo severe plastic deformation processes, in which the microstructure of the material is drastically changed as well as enhancing the level of the energy stored in the grain boundaries. These processes such as equal channel angular pressing, considerably change the material's properties including strength and hardness. Although some specifications of the ultra-fined grain copper obtained via equal channel angular pressing process is improved, the high level of stored energy makes it thermodynamically unstable and susceptible to microstructural changes during secondary thermo-mechanical processes. In the present paper, the effects of the electro-discharge machining on the ultra-fined grain copper alloy has been studied, and the results have been compared with coarse grain copper. The thickness of the recast layer and heat affected zone of the electro-discharge machined samples was investigated as well as the cracks density and the micro-hardness through optical microscopy, scanning electron microscopy and micro-hardness tester. The results show that the ultra-fined grain samples have thicker recast layer and heat affected zone and higher cracks density, comparing to coarse grain samples. However, the micro-hardness of the electro-discharge machined surface of both groups is approximately identical.

Ultra-fine grained materials with high strength and low weight are eventually considered to be used in industries. To produce ultra-fine grained materials, equal channel angular pressing is a functional method, imposing severe plastic deformation on the workpiece. Electrical discharge machining is an indispensable process in manufacturing industrial parts with high accuracy and precision. However, electrical discharge machining has thermo-physical consequences, damaging the surface layers of the workpiece. On the other hand, the ultra-fine grained materials are thermodynamically unstable and tend to microstructural evolution. Thus, electrical discharge machining process affects the ultra-fine grained materials more than coarse grain materials. In this study, the effects of electrical discharge machining on the ultra-fine grained steel were investigated and the undesirable influences of the electrical discharge machining were diminished by adjusting the electrical discharge machining parameters. The ultra-fine grained steel samples were electrical discharge machined in two methods including Iso-pulse (roughing mode and finishing mode) and with resistance–capacitance-type generator. The surface integrity parameters, including thickness and microstructure of the recast layer and heat-affected zone, the cracks density and hardness, which for all three types of samples, were investigated by scanning electron microscopy, optical microscopy, X-ray diffraction technique, and micro-hardness tester. The results show that electrical discharge machining with resistance–capacitance-type generator has the minimum effects on the surface integrity of the ultra-fine grained samples because of the different material removal mechanism of resistance–capacitance-type electrical discharge machining.