## 2 AM Categories and Common Metal AM Processes
### Directed Energy Deposition (DED)  
DED covers 16% of the metal AM market (as shown in Figure 2), and can be grouped into two main categories from an energy perspective: cold spray and thermal energy [46]. Cold spray, also known as kinetic energy, adds material in the form of fine particles to a substrate with sufficient kinetic energy to create a dense coating or layer [47]. The other group of DED systems focuses on thermal energy, through the use of a laser beam, an electron beam, plasma, or arc. This group selectively melts the feedstock material, which is either wire or powder, and successively add it onto the build platform [46]. DED technologies are primarily used for fabricating metal components [48]. This AM group applies robotic welding processes for printing at higher deposition rates, but with lower resolution [34]. Table 1 shows common DED processes: Electron Beam Additive Manufacturing (EBEAM), Wire and Arc Additive Manufacturing (WAAM), Wire-based Joule printing, Laser-Engineered Net Shaping (LENS), Laser Clading (LC), and Hybrid Systems (HS). In EBEAM, which is also called EBAM, the energy source and the material are an electron beam and in wire form, respectively (Figure 3d) [49]. WAAM technology is a wire-based DED process approach where the wire feedstock is melted through an electrical arc as an energy source, and the melted material is deposited layer upon [35]. Three common welding technologies that might be employed in WAAM systems are Gas Metal Arc Welding (GMAW), Gas Tungsten Arc Welding (GTAW) or Plasma Arc Welding (PAW) [35]. For instance, Rapid Plasma Deposition (RPD) is a WAAM system that uses a plasma arc to melt Titanium Alloy wire in an inert, argon gas environment [5]. Another technology is the Joule Printing system, which has been supported by Boeing, Lincoln Electric, Khosla Ventures and G20 Ventures [50]. In this technology, electrical current passes through the wire feedstock to melt and fuse it to the previous layer to fully bond layers. Since no arc is created, and positioning and melting the raw material is performed in one step, the Joule printing process is a simple and rapid process. Furthermore, any metallic material in wire form can be printed by this technology [50]. Figure 3e shows another DED technology in which the substrate is melted and metal powder is placed on the surface of the molten substrate. This technology is called Blown Powder Technology (BPT) [51]. Unlike other powder AM processes, these technologies are not on the basis of a powder bed, but the powder and the laser beam are focused on to the substrate to deposit the metallic material. Indeed, the metal powder is entered into a pool of molten metal, and the heat source is a focused laser beam enabling manufacture of near net shape parts with high precision [52]. In addition, an inert shroud gas is used to cover molten metal and powders from oxygen. This technology is mainly used to produce coatings with defined properties for repairing worn or damaged components as well as to add features on a part surface [53]. LENS is one of the BPT technologies where a focused, high powered laser beam is used to melt metallic powder [52]. This technology was created at Stanford  
Figure 3: Metal AM Processes; (**a**) VAT Photopolymerisation System [41], (**b**) Material Extrusion Process, (**c**) Power Bed Fusion Process, (**d**) Wire DED Process [40], (**e**) Powder DED Process [42], (**f**) Material Jetting Process, (**g**) Binder Jetting Process, and (**h**) Sheet Lamination Process [43]. University and US Sandia National Laboratories, and commercialised by Optomec [54]. Since then, the same technology has also been manufactured by POM group under the name Direct Metal Deposition (DMD) [5]. In the LENS process, the metal powder is applied only where powder is being added to the substrate at that moment.