# Study of the Direct Metal Deposition of AA2024 by ElectroSpark for Coating and Reparation Scopes
## 3 Results and Discussion
### Microstructure  
Deposit microstructure is independent of process parameters. Figure 6 shows the ESD morphology in the deposit longitudinal section (perpendicular to the ESD buildup direction). The surface is characterized by the impact of a single splat having a well-defined interface, which forms between them by fusion and diffusion reactions [32]. Some of those interfaces are highlighted by dashed lines in Figure 6a. The microstructure at interface, see the arrows in Figure 6a, was resolved at higher magnification in Figure 6b,c. It is non-uniform and contains mainly equiaxed dendrites (Figure 6b) with an unclear dendritic morphology (Figure 6c). Dendritic morphology is highlighted by the second phase segregation (light color) at the interspacing. The splats microstructure is also not uniform. In A and B areas in Figure 6b,c, the microstructure is characterized by cellular dendrites of assorted size. Figure 4: SEM micrograph of the AA2024 deposit surface following a manual ESD deposition: (**a**) morphology of 50 layer deposits in higher magnification; (**b**) enlarged image of the spot fringe at 500\(\times\). The microstructure of each splat can result not uniform as shown in the areas D and E in Figure 6b. The solidification microstructure depends on the local solidification conditions, i.e., the rate of growth V [mm/s] and the temperature gradient G [\({}^{\circ}\)C/mm] at the solid/liquid interface [33]. The ratio of G to V, namely G/V, determines the features of the solidification microstructure [23]. The smaller is the ratio of G/V, the greater is the tendency to equiaxed dendritic growth. While, high values of the ratio of G/V lead to cellular dendritic or cellular growth until to planar growth mode. Owing to manual process of deposition, the deposition rate, the distance of the electrode to work-piece surface and the force on the electrode are difficult to keep constant. As a result, the local heat input and the thickness of the splat vary at different points, which affect the solidification conditions and the resulting microstructure. Moreover, since the product G \(\times\) V (equivalent to cooling rate) governs the scale of the solidification structure (the greater the cooling rate and the finer the scale of microstructure), the size of the microstructure would vary at different points. Generally, the microstructure scale is quite fine as each splat is always characterized by small volume of molten metal (even if variable due to manual process) and undergoes very high cooling rates in contact with the cold substrate. The cross-section morphology of AA2024 electrospray deposit (parallel to the build direction) is shown in Figure 7. Figure 7a,b show optical and SEM micrograph, respectively. The deposit consists of several stacked splats [32]. The layered structures can be seen in Figure 7a, where layer-layer and layer-substrate interfaces are shown. The microstructure of the deposit cross-section is characterized by grains that have the same crystallographic orientation. The grains are columnar (parallel to the growth direction of the deposit) and nucleate epitaxially on the grains of the substrate [9; 34; 35]. For this reason, the columnar grain boundaries of the deposit are along the grain boundaries of the substrate and form a continuous network (Figure 4a). The grain boundaries have been highlighted in Figure 7a,b. Usually, columnar grains contain more than one layer and grow through the whole deposit from the surface of the substrate (Figure 7a) [36]. Figure 5: SEM image of multiple-layer deposit (cross-section) produced using 50 deposition passes and shield gas Ar: spherical voids (gas porosity n\({}^{\circ}\)1), large, random shaped voids (bridging porosity n\({}^{\circ}\)2) and thin void layers (laminar porosity n\({}^{\circ}\)3). Figure 8 shows SEM micrographs for some different areas inside of the deposit cross-section. The microstructural scale and morphology are non-uniform but strongly variable (Figure 8), as it was discussed in the above paragraph (Figure 6).