# Application of Directed Energy Deposition-Based Additive Manufacturing in Repair
## 4 Industrial Case Studies
In particular, their work demonstrated the successful repair of defective voids in turbine airfoils according to a novel semi-automated geometric reconstruction algorithm and DED. Figure 14a shows an undamaged turbine blade, and Figure 14b indicates that, in turbine blades, damage commonly happens in their tip. This defect can affect the performance of the engine, and so the reconstruction of this area is vital. For this purpose, after scanning, the blade is digitized and transformed into a meshed representation that provides the net shape of the blade. The process of finding a parameterized geometric model required to repair a turbine blade developed by [PERSON] et al. is schematized in Figure 15. In this model, firstly the point clouds of the damaged blade were acquired and meshed (Figure 15a). After that, the Prominent Cross Section (PCS) algorithm was run on the mesh surface and the resulting PCS data, lying strictly over the non-defective region of the blade, was imported into a software. In this software, the PCS points in the non-defective area were read and used for the construction of splines that represent incremental blade cross-sections of the non-defective area (Figure 15b). Afterward, a solid body was built from the resulting combination of surfaces (Figure 15c). Once the extrapolated surface was created, a Boolean (Figure 2) operation was executed to extract the difference between the reconstructed blade and the original meshed surface of the defective blade model, leaving us with the desired repair volume, as shown in Figure 15d [18]. Ultimately, [PERSON] et al. demonstrated that the DED process lent itself to repair high-value parts in an environmentally friendly manner. Moreover, their outcomes confirmed the effectiveness of DED process in the repair and remanufacturing of a wide range of damages. Figure 14: (**a**) Turbine blade produced by DED process, (**b**) damaged turbine blade, (**c**) restored blade by DED [18]. High-pressure single-crystal turbine blades made of Ni-based superalloys have superior creep and fatigue properties with respect to the polycrystalline ones and, accordingly, they can sustain temperatures up to 1100 \({}^{\circ}\)C. However, as a consequence of the extreme conditions in the engine of commercial airplanes, they undergo erosion and cracking [90; 99; 100]. [PERSON] et al. demonstrated that the DED process had promising results in the building-up of a single crystal or directionally solidified structures, while laser remelting could be used to extend this monocrystalline height [99]. However, in this work, due to the varying geometry and consequently different thermal story during the deposition, two types of deposition for repairing had been used: notch and flat. The multilayer deposition on the blade tips was found challenging so that as the number of layers increased, the susceptibility to porosity and crack formation increased. In the case of notch repair, it was found that the highest quality of deposited material could be achieved by using a remelting speed higher than deposition speeds. [PERSON] et al. studied a repair and overhaul methodology to recover the complex geometry of expensive parts for aerospace industries [91]. They successfully implemented their repair process on a critical aero-engine component like thin-curved compressor blade (Figure 16). Their repair approach included the 3-D non-contact digitization, free-form surface reconstruction using DED process, and milling operations in an automatic hybrid process. It was found that the total repair time through their approach was 30% lower than the conventional methods. According to [PERSON] et al., the proposed repair methodology could be taken into account as a reliable and time-cost effective approach in comparison with the conventional repair methods.