# Application of Directed Energy Deposition-Based Additive Manufacturing in Repair
## 2 Directed Energy Deposition (DED)  
Additive manufacturing systems are divided into two broad categories; (i) powder bed systems and (ii) powder/wire feed systems [32]. In the powder bed systems, a powder bed is created by spreading powder across the building platform area. The energy source (electron beam or laser beam) delivers energy to the surface of the powder bed and melts the powder into the desired shape according to the CAD file (Figure 1a). Afterward, the new powder is spread out across the work area again, and the process is repeated to produce a solid three-dimensional component. The advantages of this system include its ability to create high resolution features, internal channels, and maintain dimensional control [33]. Whereas, in the powder or wire feeding systems, the material is delivered directly into a melt pool that is generated by a heat source on a substrate or previously deposited layers. These technologies are based on a process where the laser beam melts an alloy added onto a substrate. In particular, the delivered material and a layer of the substrate are fused using a laser beam (Figure 1b). This melting process ensures a metallurgical bonding between layer and substrate. The capability to produce fully dense, as well as gradient objects, makes the DED more attractive, concerning the powder bed systems, in the manufacturing of large and/or functionally graded components. Despite the advantages of deposition by wire, such as full materials efficiency, its requirement regarding the difficulties in delivering the wire through complex geometries and the higher surface roughness limits its applications in different fields [34; 35; 36]. For these reasons, the powder is preferred, and it is the most common and popular form of material used in the DED process. Spherical particles in the range of 50-200 \(\upmu\)m have the best feeding properties in this process. Regarding the heating source, different types of laser can be used, depending on various factors, such as the laser beam spot size, the thickness of deposition, and the complexity of the part. For instance, CO\({}_{2}\) lasers are suitable for deposition of thick layers (several millimeters) with regular geometry, while a robot-mounted diode laser beam or Nd-YAG (neodymium-doped yttrium aluminum garnet) laser can be used to deposit thin, precise layers (less than one millimeter) with very high precision [34]. It has also been revealed that in DED, an appropriate energy density, concerning the bulk materials delivered in the form of powder, should be used for deposition. Selection of a low energy density results in limited melting  
Figure 1: Schematic of (**a**) powder bed fusion, (**b**) powder-based directed energy deposition processes [4]. of provided material, while very high energy density leads to excessive melting of the substrate, undesirable dilution of deposition, and evaporation of some alloying elements from the melt pool. A sufficient beam interaction time should also be employed for deposition to provide enough time for mixing and homogenizing the deposition, and for fast cooling of the melt pool to obtain a fine microstructure with superior characteristics in comparison with corresponding materials processed through the traditional methods. In DED process, which is also known as Laser cladding (LC), Laser Energy Net Shaping (LENS(tm)), or Laser Metal Deposition (LMD), since a thin layer of the substrate is melted, it would also be possible to create a coating layer with a nominal composition. Thus, this newly coated layer with different surface characteristics in terms of wear, corrosion, or oxidation resistance properties would increase the lifetime of some high-value parts. Owing to the disadvantages of traditional methods of coatings, DED processes have been developed to deposit some wear-resistant and high corrosion-resistant coatings obtaining a strong metallurgical bonding between the deposited material and substrate, with a high deposition rate and low wasting of material and minimal distortion.