# Principles and Characteristics of Different EDM Processes in Machining Tool and Die Steels
## 4 Research in EDM of Tool Steel
### Effect of EDM Process on the Surface Integrity
The authors found that the favorable machining conditions for material transfer by EDM were low discharge current (less than 5 A), shorter pulse-on time (less than 10 \(\upmu\)s), longer pulse-off time (more than 50 \(\upmu\)s), and negative polarity of the tool electrode. The most significant factor for the phenomenon of surface modification was peak current. [PERSON] et al. [174] utilized a scanning profilometer and SEM to study discharge traces and their profiles and to determine the volume of craters and flashes. SEM was used to assess the surface microstructure, whereas a scanning profilometer was utilized to determine the crater dimensions and volumes. The authors reported that the flash volumes were 50% to 90% of the crater volumes and crater diameter, and they were half of the flash diameter. Figure 7 shows the types of craters during individual discharge for different parameters. Figure 7: Types of craters during individual discharge (**a**) for small values of peak amperage and short pulse time, (**b**) for intermediate parameter values, and (**c**) for high amperage and long spark discharge time [174]. [PERSON] et al. [172] studied the surface integrity of ASP2023 tool steel machined by EDM. The surface finish, microstructure, micro hardness, residual stress, and element distribution were compared for CH- and water-based dielectrics. The authors reported that surface finishes of 0.1 \(\upmu\)m and 0.2 \(\upmu\)m were achieved for CH- and water-based dielectrics, respectively. The formation of cracks on the surface of workpieces machined by EDM was also studied in many researches. For example, in one study [146], the effect of EDM processing parameters including pulse current and pulse-on duration on the formation of surface cracks in the machined surface of SKD11 tool steel was investigated. The authors reported that increasing the pulse current or reducing the pulse-on duration suppressed the formation of surface cracks in the SKD11 machined surface, thereby improving the fatigue life. Figure 8 shows the cracks formed on the SKD11 surface machined using a pulse current of 4 A and pulse-on duration of 16 \(\upmu\)s, while the propagation of the fatigue crack is shown in Figure 9.  
[PERSON] et al. [149] studied the effects of heat treatment on the surface of a die steel SKD11 machined by wire EDM. The study compared heat treatment following four manufacturing methods, namely,  
Figure 8: SEM image showing severe cracks formed on SKD11 surface machined using a pulse current of 4 A and pulse-on duration of 16 \(\upmu\)s [146]. Figure 9: SEM cross-sectional image showing propagation of fatigue crack [146]. milling and then grinding, only wire EDM, low-temperature heat treatment after wire EDM, and high-temperature heat treatment after wire EDM. The authors reported that milling and then grinding produced the best surface among the four methods. For all methods other than milling and then grinding, a small amount of Cu was transferred from the brass electrode to the workpiece surfaces. The results also showed that the microstructure of the surface of the first method which was not EDM processed was completely different from the other microstructures. [PERSON] et al. [134] studied the surface integrity generated in AISI O1 tool steel using four different machining processes, namely, conventional hard turning, production grinding, roughing by wire EDM, and roughing and finishing by wire EDM. The authors reported that the plastic deformation due to the pressure between tool and workpiece was the main factor that generated surface integrity in production grinding. In hard turning, the main factor that generated surface integrity was the thermal effect. The results of the study showed that the EDM process generated the most damaged surface due to the high tensile stress, as well as the hard and brittle white layers generated by EDM. [PERSON] [184] studied the effect of dielectric fluid and the type of electrode on white layer structure in EDM machined surfaces in terms of retained austenite and residual stresses.