# Study of the Direct Metal Deposition of AA2024 by ElectroSpark for Coating and Reparation Scopes
## 3 Results and Discussion
### Effect of the Parameters Electric on Substrate/Deposit Interface Behavior  
It is known that the AA2024 alloy is susceptible to solidification cracks or liquation cracks being characterized by a large solidification range, high thermal expansion, large solidification shrinkage, and a large quantity of alloying addition [9; 35]. Solidification cracking occurs during the solidification of melt. It occurs along grain boundaries, and it is due to the shrinkage stress from both molten metal solidification and non-uniform thermal contraction [34; 35]. Liquidation cracking occurs at a temperature lower than solidus temperature during non-equilibrium heating of the alloy, for example in the HAZ of weld, where the peak temperature of the welding cycle is just below the solidus temperature. Figure 14: Analysis of the percent distribution of the defect area observed in the deposit at **(a)** 0.5 J (V1, C2); **(b)** 0.9 J (V2, C1) and **(c)** 1.125 J (V2, C2) for different values of frequency F1 \(<\) F2 \(<\) F3. Figure 13: Percentage defectiveness in deposits cross-section as a function of spark pulse-energy (_Es_) for three values of frequency (F1 \(<\) F2 \(<\) F3). Liquation cracking is due to the combination of constitutional liquation phenomenon [35], (localized grain boundaries melting at temperature significantly below the solidus T of the alloy) and tensile stresses that act on constitutionally liquated areas to produce cracks called liquation cracks. In fact, with the grain boundary liquated, the alloy has little strength to resist tensile stresses. All the ESD parameters set lead to cracks extending into the substrate along the grain boundaries (Figure 15a). Higher magnification SEM images suggest the delineation of the crack paths by re-solidified products, typical of liquation cracking (Figure 15b). During the ESD process, the temperature of the area close to the substrate/deposit interface can promote the melting of some areas (usually, located at intermetallic particle surface and/or at the grain boundaries) where a higher solute concentration develops and locally alters the liquidus (solidus) T of the alloy. Simultaneously, the rapid cooling from the ESD process induces a great thermal strain that opens the liquated zones. Therefore, due to ESD process, those cracks are inevitable on the heat sensitive AA2024 substrate [9; 12; 39; 40]. Once initiated, the cracks propagate along the substrate grain boundary and may also easily propagate in the deposited layers giving rise in a continuous network (Figure 15a). The liquation cracks dramatically reduce the mechanical properties of the substrate, and moreover, limit the feasibility of producing ESD deposits/repair in the 2024 alloy. So, the cracks number, length, and length distributions as a function of ESD process parameters have been investigated in the following. Figure 16 shows the average cracks length that propagates within the substrate from the substrate/deposit interface as a function of spark pulse-energy (_Es_). The analysis of the average cracks length in function of the _Es_ (Figure 16), shows that the average cracks length increase with _Es_. In fact, as _Es_ increases, a higher heat input is supplied to the substrate favoring both hot cracking and grain boundary liquation of 2024 alloy [6]. Thus, this trend is opposite to that of the defects within the deposits. Specifically, the increase of average crack length is more sensitive to voltage increase in respect to capacity increase. After all, both voltage and capacity increases lead to higher heat input, but the role of voltage on _Es_ is more significant (_Es_ = 1/2\(CV^{2}\)). Furthermore, it was observed that, for the same _Es_ the average cracks length increased with the frequency increase. Figure 17 shows, for all three pulse energy conditions, the cracks length distributions that extend into the base material at increasing frequencies. Specifically, each graph shows the cracks number having a length comprised in classes of fixed amplitude.