# Parametric Investigation of Die-Sinking EDM of Ti6 Al4V Using the Hybrid Taguchi-Rams-RATMI Method
## 4 Results and Discussions
### Surface Morphology  
When machining Ti6 Al4V using a copper tool in EDM, several main effects on the surface can be observed. In Figure 18a, small craters are present while Figure 18b shows large craters. EDM involves spark erosion, which results in crater formation on the machined surface due to the localized melting and evaporation of the material. R12 has higher surface roughness compared with R1 as Figure 18b shows a relatively high number of irregular craters and molten metal droplets solidified on the surface compared with Figure 18a. The roughness can vary depending on the EDM parameters. Higher discharge energy generally results in higher surface roughness.  
Figure 17: The surface plot of the RATMI majority index (E\({}_{i}\)) vs. peak current and voltage.  
Figure 18: Optical microscope images (10\(\times\) Zoom at 2.3 mm) of (**a**) R1 (T\({}_{on}\) of 500 \(\upmu\)s, duty cycle of 8%, peak current of 40 Amp, and voltage of 20 V) and (**b**) R12 (T\({}_{on}\) of 750 \(\upmu\)s, duty cycle of 8%, peak current of 45 Amp, and voltage of 30 V).  
The SEM image in Figure 19 shows how the recast layer (white layer) forms on the surface due to the molten material solidifying again. The EDM settings have an impact on the recast layer's thickness. In this instance, a thinner recast layer is usually the consequence of decreased energy levels. Nonhomogeneous heat flow, plastic deformation, and metallurgical transformations are frequently encouraged by EDM operation. This leads to residual tension being created inside the machined object. It is commonly recognized that surface cracking is caused by residual tension that arises from fast cooling and phase shifts in the white layer. Under the recast layer, there is a heat-affected zone where the material undergoes structural alterations and thermal pressures without melting [23].  
SEM micrographs showing the surface integrity of EDMed Ti6 Al-4V specimens made with copper electrodes show poor surface morphology, including big surface fractures, microcracks, globules, pockmarks, and a white layer (Figure 20). Nevertheless, the degree of the aforementioned surface imperfections varies depending on the electrode material and spark energy input. The residual tension created while the EDM procedure is carried out is what causes surface cracking at the EDMed work surface. According to the literature on the EDMed part component's residual stress [23], the residual stress seems to be the greatest at the surface. Additionally, it shows that when pulse energy increases, residual stress rises as well. Because of the way its size encourages surface cracking, such residual stress is detrimental. The process of EDM causes spark erosion, which leaves craters on the surface that has been machined. The material melts and vaporizes locally, creating these craters. Surface microcrack development can result from rapid cooling and solidification [15]. The machined part's mechanical characteristics and surface integrity may be impacted by these  
Figure 19: The SEM image revealing the existence of the white layer in the R1 sample.  
microcracks. The machined surface's microstructure may change as a result of the EDM's heat cycles. This includes modifications to the phase composition and grain size, which can have an impact on the material's characteristics. Because of the liquid metal droplets that settle on the surface and the uneven craters, the surface roughness of Ti6 Al4V machined by EDM is often very high. The discharge current, duty cycle, and other EDM parameters may all affect how rough the surface is. Surface roughness often increases with discharge energy. Copper particles are deposited on the machined surface as a result of material transfer from the copper tool to the Ti6 Al4V surface. Generally speaking, the elevated temperatures experienced during EDM might lead to the machined surface's oxidation and carburization, which may result in the creation of titanium carbides or oxides.