# Micromachining in Powder-Mixed Micro Electrical Discharge Machining
## 1 Introduction  
Electrical discharge machining (EDM) is a process where spark erosion is used to fabricate a complex shape through an electrically conductive workpiece by using a tool electrode. The workpiece material is removed by thermal erosion process with repetitive spark discharges produced by pulsating DC power supply [1]. The main cause for material removal from the workpiece is the temperature above the melting points, which is generated in the discharge channel [2]. In the field of material science and engineering, compared to other machining processes, EDM has been widely used to fabricate difficult to machine materials [3; 4; 5; 6]. This is due to the fact that EDM has good control in providing high surface quality and machining accuracy without sacrificing machining efficiency [7; 8; 9; 10]. There are challenges for developing the device in the field of biomedical and automotive industries, which is the miniaturization of the system [11; 12; 13]. In order to answer these challenges, where a precise and an accurate fabricating process is necessary, a smaller scale of EDM, known as micro-electrical discharge machining (\(\mu\)-EDM) becomes the solution [14; 15; 16]. Other non-conventional fabricating processes, such as wet etching and electropolishing, are incapable of providing high-accuracy machining geometry, due to the difficulty in controlling chemical machining processes [17; 18].  
Having a machining process without any direct contact between the tool and the workpiece, \(\mu\)-EDM is able to fabricate miniature parts without distorting the workpiece from any mechanical stress and machining vibrations [19; 20; 21]. Another capability of \(\mu\)-EDM is its ability to process any electrically conductive materials regardless of their hardness by using melting and vaporization as a material removal mechanism [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. The detail of material removal mechanism is illustrated inFigure 1[28]. Therefore, \(\mu\)-EDM is considered as an effective method to machine high aspect ratio microholes and 3D cavities [29].  
In recent years, researchers have been developing a method to improve the performance and machining efficiency of \(\mu\)-EDM [30; 31]. Some of these methods are utilizing high electric resistance electrodes, vibration-assisted \(\mu\)-EDM and the utilization of powder-mixed dielectric in \(\mu\)-EDM.  
The utilization of powder-mixed dielectric in EDM started in 1980 [26]. Semi-conductive powder was introduced into the dielectric fluid to improve the material-removal rate and surface quality of the workpiece machining [32; 33]. Figure 2 explains the mechanism of powder-mixed EDM. The same method was applied to \(\mu\)-EDM processes, which has similar characteristics to that of EDM [28; 34]. However, there are some differences between the two in terms of the size of tool electrodes and discharge energy [35]. The differences yield different effects, where a certain powder size which gave optimum results in EDM may not provide the same result in \(\mu\)-EDM. In addition, the introduction of powder in \(\mu\)-EDM requires a full understanding of the mechanism and the machining performance of the powder based on their mechanical properties.  
This paper presents a comprehensive and critical review of micromachining carried out by powder-mixed \(\mu\)-EDM. The focus was given on the mechanism of powder-mixed \(\mu\)-EDM and the impact of powder properties to the machining removal rate, surface quality and surface modification. The variation of the powder-mixed \(\mu\)-EDM process, current status, future perspectives and the potential of powder-mixed \(\mu\)-EDM are also discussed.  
Figure 1: Mechanism of the material-removal rate in electrical discharge machining (EDM): (**a**) the pre-ignition phase; (**b**) the ignition phase; (**c**) the plasma formation; (**d**) the discharge phase; and (**e**) the ejection phase.