Recast layer and micro-cracks in electrical discharge machining of ultra-fine-grained aluminum

Aluminum alloys, due to lightweight, are widely used in aerospace and automotive industries. However, the low strength of aluminum has hindered its application. The strength of aluminum can be improved in many ways. One of them is decreasing the average grain size of metal by applying sever plastic deformation methods. Equal channel angular pressing is the most functional technique of sever plastic deformation producing ultra-fine-grained metals. Using post-process methods such as electrical discharge machining to manufacture industrial parts of ultra-fine-grained material is very conventional. The recast layer which is the consequence of electrical discharge machining process may cause undesirable influence on the surface of ultra-fine-grained aluminum. In this article, the recast layer and the heat-affected zone of electrical discharge machining of ultra-fine-grained aluminum are investigated. The thickness of recast layer, heat-affected zone and micro-cracks is observed using scanning electron microscopy and optical microscopy. In addition, the phase composition and the hardness of the recast layer and heat-affected zone are investigated by applying X-ray diffraction technique and micro-hardness test. These experiments are also repeated for the coarse-grain aluminum, and the results are compared with ultra-fine-grained aluminum. Results show that the electrical discharge machining deteriorates the surface integrity of the ultra-fine-grained aluminum rather than coarse-grain aluminum.