# Application of Directed Energy Deposition-Based Additive Manufacturing in Repair
## 3 Identified DED Capabilities in Repair  
In recent years, there has been an increasing amount of literature focused on the application of DED process in the repair and remanufacturing of damaged components [71; 72; 73]. This high level of interest in DED technologies is due to its advantages, such as high precision, narrow melting, and heat-affected zones (HAZ), that result in the low part distortion. According to literature, the critical concerns in the repair of metallic components by DED studied by various researchers are the type of repair, the microstructure of components, repaired part and their interface, the mechanical properties of the repaired parts in comparison with the as-built and conventionally processed parts, the microstructure and mechanical performance of the HAZ, and the feasibility of using different materials to repair the components and improve its performance. For instance, [PERSON] et al. studied the microstructure and mechanical response of AISI 4340 steel components repaired by DED [74]. In their research, they showed that austenite dendrites were the only present phase in the deposited layer, which had been grown in the solidification direction. Depending on the cooling rate and consequently the temperature gradient, two types of dendritic, cellular and columnar, consisting of a fine martensitic phase, had been detected in the microstructure of the deposited part. The combination of phases in the deposited material was formed as a consequence of fast cooling of the melt pool, leading to 30-40% higher hardness concerning the base alloy. However, it is revealed that the martensite, which was formed in the HAZ area as a consequence of high cooling rate, is generally brittle and reduces the ductility and the toughness, and thus further thermal treatment is necessary. Moreover, it was found that by increasing dilution, a mother hardness transition happened at the melted zone/HAZ interface, and consequently the stress concentrator sites that can deteriorate the fatigue properties of the material were eliminated. Table 3 compares the mechanical properties of as-built and as-repaired Inconel(r) 625 alloys. As can be seen, the mechanical properties of repaired Inconel(r) 625 were in line with the features of annealed bars, confirming the potential of DED technology as a repair one. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Type of Repair**} & **Repairing Corroded** & **High-Value Al Parts** & **Steel Molds That** & **Corroded Gas** \\  & **Steam Generator [12]** & **That Underwent** & **Underwent** & **Turbine Blade [70]** \\ \hline Laser & 1000 W, CO\({}_{2}\) & 1.0 KW, YAG & CO\({}_{2}\) & N/A \\ \hline Materials & NA & Al alloy & Tool steel & IN 738 \\ \hline Gas & He & Ar or He & Ar or He & Ar \\ \hline \multirow{6}{*}{Control} & \multirow{3}{*}{
\begin{tabular}{c} Operated through an \\ umbilical cord from a \\ trailer by an operator \\ using commands to a \\ computer, which \\ calculated the angles \\ and position of each \\ axis of the arm. \\ \end{tabular} } & A six-axis assembly robot, which had special end-of-arm & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of some applications of DED as repair and dedicated systems for repairing. [PERSON] et al. have investigated the effect of using a dual-beam laser for repairing surface cracks in Ni-based superalloy parts [13]. Indeed, in their work, the first laser beam was employed for melting the materials around the crack, and the second one was used to fill the generated gap. Their outcomes showed that the surface cracks in the range of 0.1-0.3 mm could be successfully repaired and consequently, the part could be reused before the final failure [13]. However, with DED technique, the repair of internal cracks is more difficult than the surface ones.