Effects of surface layer of AISI 304 on micro EDM performance

Micro electrical discharge machining (micro EDM) provides an alternative in generating complex 3-D micro features in difficult-to-machine materials. The micro EDM performance depends not only on machining parameters but also on the microstructures and micro-characteristics of materials. The effect of free surface layer of austenitic stainless steel on the micro EDM performance is investigated in this paper. Experiments are carried out by machining multilayer stainless steel workpieces without and with surface treatment (acid pickling and oxidation treatment). Surface layer model is built to demonstrate the increasing influence of surface layer on micro EDM performance when the size of workpiece is decreased. The surface free energies of AISI 304 workpieces are estimated by measuring the contact angles. The results show that the steel workpiece with oxidation treatment has the highest surface free energy and the highest material removal rate compared with those without treatment and with acid pickling treatment. The influence of surface layer of steel workpiece on micro EDM performance is more significant when the discharge energy is lower due to size effects. Taper ratio of machined holes on steel workpiece without treatment is much higher than those of holes on steel workpieces with acid pickling and oxidation treatment. As a conclusion, surface treatment of steel workpiece before machining is an effective method to improve the machining efficiency and machining precision of micro EDM, especially when involving the sub-micro or nano-machining where machining is conducted on the workpiece surface.