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Electrical discharge machining (EDM) is one of the most widely used non-conventional methods to machine electrically conductive materials in the manufacturing industry because of its strong capability in machining difficult-to-cut materials irrespective of their strength and hardness. Electrical discharge drilling (EDD) is an important variant of EDM. Due to the limitation of conventional drilling processes, special holes, particular those with high aspect ratios on hard-to-cut materials, can only be drilled by EDD. Extensive research has been carried out to improve the efficiency and quality of the EDD process by using different approaches, such as assisted EDD and powder-mixed EDM drilling aiming to improve the material removal rate (MRR), tool wear rate (TWR), surface quality and accuracy. This paper provides a comprehensive review of the EDD process. Different methods were compared; the advantages and disadvantages of each process were summarised; state-of-the-art technologies and the latest development were introduced, and research trends and new directions were presented.

Titanium and its alloys have been widely used in various areas due to their superior mechanical properties. However, Titanium and its alloys have low elastic modulus and low thermal conductivity, which lead to extreme high temperature during the machining process and result in poor workpiece quality. The main objective of this paper is to investigate the effectiveness of graphene oxide (GO) nanofluids in drilling process of Ti-6Al-4V by using tungsten carbide tools. The spiral chips were obtained when GO nanofluid was used and squeezed string chips were obtained when conventional coolant was applied. By analysing the back surface of the chips, it was found that the high coolant pressure resulted in better chip formation, which had less scratch and plunking. In addition, by using higher concentration of GO nanofluids, the adhesion phenomenon was found significantly reduced.