# Effect of ElectroSpark Process Parameters on the WE43 Magnesium Alloy Deposition Quality
## 3 Results and Discussion
### Analysis of Defects of WE43 Deposits  
The built-up deposits, unfortunately, are affected by defects (Figure 8). The main defects detected in the deposits occur as voids of different morphology: spherical or random shaped defects (n\({}^{\circ}\)1 e n\({}^{\circ}\)2, respectively) and laminar defects (n\({}^{\circ}\) 3) that extend both perpendicularly and parallel to the substrate/deposit interface. The spherical or random shaped defects are believed to be the result of the amount of material brought to melting and its distribution [25; 31; 32; 37]. While generally, the formation of laminar shaped defects was considered as a normal consequence of thermal stress induced by the high thermal gradients inherent to the ESD process. Especially, the perpendicular cracks to the interface are due to shrinkage during cooling and solidification of the deposited material. The perpendicularly cracks can arise in the first deposited layers, in the subsequent ones or nucleate on random shaped defects inside the deposits [38]. Usually, during the deposition process these cracks propagate upwards owing to repeated thermal stress [31; 32]. Instead, the parallel cracks to the interface, also known as delamination cracks, can be generated by poor mixing of the electrode material with the substrate or layer previously deposited (weakly bounded interfaces) or by the mechanical impact of the electrode tip to the workpiece [31; 39]. Figure 9 presents the percentage area fraction of defects, calculated as the ratio between the area of voids and the area of the deposit, as a function of pulse energy (E\({}_{\mathrm{s}}\)). The plot (Figure 9) illustrates  
Figure 8: SEM micrograph showing the typical voids of the ESD coating: small spherical voids (n\({}^{\circ}\)1), large, random shaped voids (n\({}^{\circ}\)2) and laminar porosity (n\({}^{\circ}\)3). Figure 7: Microstructure near the interface between the substrate and deposit: (**a**) OM and (**b**) SEM micrographs of WE43 deposit, (**c**) magnification of interface (as marked “B” in Figure 7b). that the increase of E\({}_{\rm s}\) reduces the percentage area fraction of defects. At low energy values (s1 and s2) the deposits display a greater percentage area fraction of defects, while at higher E\({}_{\rm s}\) values (s3-s5) the percentage area fraction of each deposits is smaller and does not significantly change with E\({}_{\rm s}\). It can also be observed that the percentage area fraction of defects remains small for all the samples and is well below 11%. The defects diameters dispersion is a function of the shape factor for the lowest (s1), highest (s5), and intermediate (s3) value of deposition energy is shown in Figure 10. As shown, the laminar and spherical defects are characterized, respectively, by the maximum and by minimum extension. Moreover, the size of the spherical defects is contained and less than 30 \(\upmu\)m, while the size of the random and laminar defects exhibits a greater variability. A better understanding of deposits defectiveness, as a function of process parameters can be obtained by calculating the percentage number of defects and area defects for classes of shape factor. Therefore, Figure 11a,b represent the percentage number of defects and defects area percentage for each class of the shape factor (laminar, random and spherical), respectively. Percentage number is defined as the ratio between the defects number for each class and the total defects number in the deposit, while the defects area percentage is defined as the ratio between defects area for each class of shape factor and the total area of the deposit. Results (Figure 11a) showed that for E\({}_{\rm s}\) value higher than 0.75 J, the defects percentage number decreases for the laminar shaped defects (0 \(<\)_fcircle_ \(<\) 0.4) and increases for the spherical shaped defects (0.7 \(<\)_fcircle_ \(<\) 1). No significant differences were observed between the percentage number of random shaped defects (0.4 \(<\)_fcircle_ \(<\) 0.7) as E\({}_{\rm s}\) increases.