# Application of Directed Energy Deposition-Based Additive Manufacturing in Repair
## 2 Directed Energy Deposition (DED)
It is reported that ceramic coatings, owing to their high microhardness and wear resistance features, have been employed in many advanced technologies, such as aerospace, defense, chemical, and machinery field [37, 38]. It is found that the wetting problem, which is the main challenge in the ceramics coating, through conventional manufacturing has been solved using DED process, and the overall performance of the coating has been improved [39, 40, 41]. Apart from the application of DED as a coating technique, this technology has intensively been employed as a manufacturing technology [42, 43, 4]. Over the past decade, DED could attract more attention regarding its capability in the fabrication of parts with complex geometries to near-net-shape, with the high cost and material savings. In comparison with the other AM processes, DED has a unique position due to its potential to produce monolithic metal and functionally graded parts, as well as their repair [44]. Monolithic stainless steel, aluminum, titanium, nickel, and copper alloys have been already fabricated by DED process [45, 46, 47, 48, 49]. However, functionally graded parts or bimetal joining are the main purposes of engineering technology science to obtain a predominated quality of products. Some engineering structures could achieve the best efficiency by applying the parts combined of several dissimilar materials that fully realize the economic and performance advantages of each material. The bimetallic component is fabricated by joining dissimilar metals, which have particular mechanical, physical, and chemical properties, such as mechanical strength, wear resistance, thermal and electrical conductivity, corrosion resistance, etc. [50, 51]. The most well-known metal couples are steel-copper, steel-aluminum, steel-nickel, and steel-titanium [52, 53, 54, 55]. Bimetals are widely used in many areas as reservoirs, radiators, bearings, and gaskets for motor-heads in the automobile industry, as cutting tools in the paper, metal, wood, textile, food industries, kitchen utensils, heat exchangers, electro contact, first wall/blanked material for reactor, etc. [56, 57, 58]. Conventionally, various techniques have been used to fabricate bimetals, casting [50], explosive welding [59], rolling [60], diffusion bonding [58], powder metallurgy [61], etc. However, their limited capability to specific shapes and materials, multiple process steps, non-ideal bonding ability, and also their high manufacturing expenses have limited their applications [51]. On the contrary, DED shows great potential to be used in the manufacturing of bimetal materials without the limitation of conventional processes. Indeed, DED can be used as a manufacturing method of any weldable metallic materials [62]. For instance, Figure 2a-c show the interface between two deposited materials in as-fabricated bimetallic structures of Ti-6 Al-4V/Inconel 718, Ti-6 Al-4V/stainless steel, and Inconel 718/copper [63, 64, 65]. Moreover, [PERSON] et al. indicated that due to the variation in build geometry and temperature during the repair process, the consistency of the quality of the components should be monitored [66]. Therefore, in their work, melt pool geometry was recorded via a camera and then verified with the results obtained by an optical profilometer. Thereafter, the experimental results were compared with the numerical simulations. They confirmed that through the real-time regulation of measurable process parameters via a feedback control system, it would be possible to enhance the repair quality. [PERSON] et al. showed that sensing and control systemsto monitor the process variables, such as melt pool geometry, temperature, and layer build height, played key roles in the final quality of the parts, including dimensional tolerance, microstructure, and properties [67]. Last but not least, the capability of DED for repairing and remanufacturing of damaged high-value parts need also to be considered.