# Application of Directed Energy Deposition-Based Additive Manufacturing in Repair
## 4 Industrial Case Studies
These shafts are already repaired by 420 stainless steel, and this repair results in the formation of a harder layer (50 RC higher) with higher corrosion resistance with respect to the as-produced parts or the repaired ones that are recovered by thermal spray methods. Moreover, from the economic point of view, it is interestingly found that the cost repairing of drive shafts via DED is 50% of the production of a new drive shaft [87]. Figure 9: DED crankshaft segment (**a**) around oil bore, (**b**) on the bottom side at the lower dead center position [86]. Another critical part that can be successfully repaired using DED process is a four-stroke marine piston. During the working condition, high level of forces and temperature damage the surface of pistons. Wear, which is the main defect, happens on the edges of the grooves under the working environment and, thus, the pistons should be repaired or replaced as soon as the grooves reach the tolerance limit [88]. Kimi company has repaired these pistons by DED process as an economical and technical solution that not only repairs the part but also improves the resistance of groves against the erosion and corrosion (Figure 12). Figure 11: A drive shaft repaired by thermal spray (top) and DED (bottom) processes [87]. Figure 10: (**a**) Microstructure, (**b**) hardness distribution and location of both in DED flat specimen (bottom, the Starting point (S), Endpoint (E)) [86]. Maintenance and repair of aero-engine parts, such as gas turbine, low and high-pressure turbine blades, and so on, are of the critical concerns in aerospace industries as a consequence of the increase in the raw material and manufacturing costs. Gas turbine Inconel(r) 718 compressor seal is a critical and high-value component that can be repaired using the DED process. The application of this seal in the gas turbine aims to prevent gas leakage in gas turbine engines. In this component, the main damage is the excessive wear in the labyrinth seal that negatively affects the performance of the gas turbine. Figure 13 shows a very finesse repair of a damaged gas turbine compressor seal [87]. Interestingly, it was found that the cost of a repaired compressor seal was 45% of a new one, and this confirmed the great potential of DED as a repair technology from the economic point of view. From a technical point of view, during the inspection process, it was revealed that the repair deposit resulted in a minor shrinkage of two inside diameters. Thereafter, those diameters were machined to reach the standard tolerances, and ultimately, the repaired seal had been approved by the customer [87]. In general, it is found that to be in the acceptable tolerance, further machining after repairing seems to be essential. [PERSON] et al. have investigated the residual stress in a low-pressure turbine blade after repairing by DED [89]. A low-pressure turbine blade in powder station operated at 3000 rpm, and this speed generated almost supersonic speed near the blade tips [89]. The main issue in these blades was the erosion in the leading edges that happened as a consequence of condensing water impingement and resulted in the premature retirement of these expensive blades. Thus, to extend their life, DED  
Figure 12: Four-stroke pistons repaired by DED (Courtesy of KIMI) [88]. Figure 13: Gas turbine Inconel®718 compressor seal repaired by DED process [87]. process was used to repair the eroded edges. The results demonstrated that significant tensile and compressive stresses were generated both in the blade and in the repaired area. It was also found that in both regions, tensile stresses were located on the surface and could promote crack initiation in the fatigue condition. However, it was reported that the magnitude of these stresses and, accordingly, the increasing resistance to cracking of these regions could be minimized using a post-heat treatment [89]. [PERSON] et al. have used the DED process to repair or remanufacture the turbine airfoil [18].