A study of stored energy in ultra-fined grained aluminum machined by electrical discharge machining

Due to light weight and high strength, industrial applications of ultra-fined grain materials are becoming prevalent. Ultra-fined grain materials are produced by severe plastic deformation techniques such as equal channel angular pressing, which impose large strains on ultra-fined grain microstructure, resulting in accumulation of the lattice defects and dislocations and consequently increasing the stored energy. Due to high stored energy of ultra-fined grain materials, they are thermodynamically unstable and prone to microstructural evolutions. In order to manufacture industrials parts, applying machining methods such as electrical discharge machining is necessary. Electrical discharge machining is a thermo-electrical process that erodes the surface of the workpiece by high temperature sparks. The surface of the workpiece is melted and then suddenly quenched into dielectric, and eventually a very hard and brittle layer, known as recast layer, is formed. The recast layer and heat-affected zone are the source of microstructural changes that affect the special properties of the ultra-fined grain materials and their stored energy. In this study, the bulk and local stored energy of electrical discharge machined ultra-fined grain aluminum samples are measured via differential scanning calorimetry technique and micro-hardness test. In the following, the effects of the electrical discharge machining process on ultra-fined grain aluminum are investigated. The results show that electrical discharge machining process alters the stored energy of the ultra-fined grain materials.