# Application of Directed Energy Deposition-Based Additive Manufacturing in Repair
## 3 Identified DED Capabilities in Repair
This difficulty is because of the difference in the accessibility to the crack and, in the case of internal cracks, it is necessary to mill a lot down to eliminate the crack and surrounding area and then refilling with the same material. [PERSON] et al. have evaluated the type of materials removal, for instance, machining a volume rectangular and triangular in cross-section, and their influence on the quality of the repaired part made of H13 hot-work tool steel parts [16]. In both cases, the final microstructure was tempered martensite surrounded a HAZ of untempered martensite. It is found that in both geometries, despite achieving a uniform deposition with fine microstructure and good fusion bonding, the formation of porosity is a critical problem. [PERSON] et al. employed the DED process to coat Stellite(r) and Tribaloy(r) families, which offer good impact resistance, corrosion, and erosion resistance at high temperatures, on the repaired steam circuit parts of thermal power station [14]. They have shown that, in comparison with conventional welding processes, DED of Co-based alloys has some advantages, such as low thermal strains, smaller dilution zone between the deposition and base materials, and smaller HAZ, that can result in the final properties equal to those of base part. [PERSON] et al. studied the feasibility of repairing of an Invar alloy mold using DED technology [76]. They showed that the microstructure of the repaired specimen was divided into three different regions, the deposited layer, the HAZ, and the base metal. Their microstructural analysis confirmed that the super-cooling degree of liquid/solid interface affected the microstructure of the deposited layer. It means, from the bottom to the surface of the deposited layer, the grains are transformed from irregular to regular ones as a consequence of the change of super-cooling. Moreover, it is found that the typical grain growth direction is perpendicular to the liquid/solid interface that has the maximum temperature gradient. [PERSON] et al. have studied the effect of HAZ on the mechanical properties of gray cast iron parts repaired by the traditional welding process and DED technology as an advanced one [77]. For this reason, the hardness of repaired parts was measured from the top surface to base metal, and the results are reported in Figure 3. As can be seen, the maximum hardness of the HAZ was achieved in the samples repaired by DED but even more important, concerning the tensile results, is that the HAZ size of samples repaired by welding was 21.5 times larger than the size when repaired by DED. The comparison between the tensile results of the parts repaired by both technologies revealed that there was no difference in tensile strength between the two methods (Figure 4). However, it was revealed that the elongation of repairing by DED had a 62% increase in average, mainly owing to its smaller HAZ size, which is a high hardness zone. \begin{table}
\begin{tabular}{c c c c c} \hline
**Process** & **UTS (MPa)** & **YS (MPa)** & **E (\%)** & **Ref.** \\ \hline Annealed bar & 841 & 403 & 30 & [75] \\ DED as-Build & 815 & 487 & 69 & [18] \\ Repair by DED & 793 & 482 & 56 & [18] \\ \hline \end{tabular} UTS: Ultimate Tensile Strength, YS: Yield Strength, E: Elongation.  
\end{table} 
Table 3: Mechanical properties of manufactured or remanufactured Inconel® 625. [PERSON] et al. studied the laser cladding of CPM 9V on H13 tool steel that has superior dimensional stability, excellent hot working properties, and high-quality surface toughness at high temperatures to repair the damaged H13 components and improve its combination of toughness, hardness, and wear resistance (Figure 5) [78]. According to their microstructure analysis (Figure 5a,b), in the deposition zone, rod-like and globular carbides were scattered all around the martensite and retained austenite matrix. The interface zone showed a quenched martensite and retained austenite matrix. The structure of HAZ was formed of chromium carbides embedded in a tempered martensite matrix.