## 3 Results and Discussion
### Morphological Aspects of the Surface
Figure 1: SEM images of Ti-6 Al-4V workpieces after machined at (**A**) negative and (**B**) positive polarity using DF1-F, and (**C,D**) machined with DF2-Ca in the negative and positive polarities, respectively (100\(\times\) magnification). Inset: Rippled structures (solid circle) and molten and re-solidified material (asterisk) can be observed. Figure 1A,C show the surfaces machined under negative polarity conditions, while Figure 1B,D correspond to the surfaces obtained under positive polarity. To better understand these results, Figure 2 illustrates the schematic representation of the mechanisms involved in EDM machining with two different dielectric fluids and electrode configurations. In the negative polarity configuration, represented in Figure 2A,C, a greater number of electrons is emitted from the tool electrode and accelerated toward the workpiece, resulting in more intense localized heating. Consequently, a larger volume of workpiece material is melted and evaporated, which leads to a higher material removal rate (MRR) and increased molten pool formation on the surface. These effects contribute to the development of a rougher topography, with more pronounced surface irregularities when compared to positive polarity [65]. Although the surfaces in Figure 1A,C exhibit a visually more uniform appearance in the top-view SEM images, this impression may not accurately reflect the true morphological complexity. The homogeneous distribution of deep pores and re-solidified material can result in a surface that appears smooth but conceals the presence of more pronounced features. In contrast, the positive polarity configuration, represented in Figure 2B,D, results in lower energy transfer to the workpiece, as fewer electrons are directed toward its surface. This leads to less intense localized heating, producing a smaller molten pool and a lower material removal rate (MRR). Consequently, the thermal effects are milder, which favors the formation of a modified layer with a less rough surface [65]. However, this characteristic is not easily noticeable in the top-view SEM images of Figure 1B,D, which display a visually more irregular surface morphology, with the presence of clusters and cracks. This is because the top-view perspective limits the ability to assess the actual depth of surface features. A  
Figure 2: Scheme of the ions in the inter electrode space with (I) DF1-F and (II) DF2-Ca fluids, in negative (A,C) and positive (B,D) polarities. cross-sectional view may be more suitable to reveal the reduction in roughness associated with the lower thermal input under positive polarity. The energy-dispersive X-ray spectroscopy (EDS) spectra and the corresponding elemental composition tables for the samples are presented in Figure 3.  
Titanium, oxygen, and aluminum were detected on the machined surfaces of all five samples. Notably, the oxygen peak was more prominent in the sample machined un  
Figure 3: EDS spectra and elemental composition at top surface for samples machined using tool polarity (**A**) negative and (**B**) positive with DF1-F, and (**C**,**D**) machined with DF2-Ca in the negative and positive polarities, respectively. The untreated Ti-6 Al-4V sample is shown in (**E**) for reference. der negative polarity (Figure 3A) compared to the one processed under positive polarity (Figure 3B). Furthermore, the atomic percentage values obtained from the EDS analysis indicate 35.39% oxygen in the DF1-F(\(-\)) condition versus 25.62% in the DF1-F(+) condition, suggesting a higher degree of surface oxidation. The presence of titanium oxides was expected, as electrical discharges increase local temperature and oxygen availability, promoting oxide formation [3], which can enhance the biocompatibility of the alloy. The EDS results in Figure 3C,D confirmed the presence of calcium on the surfaces machined with DF2-Ca.