# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 2 Laser Irradiation and Material Addition Mechanisms  
The study of the powder stream process is of paramount importance as it defines the initial conditions for the generation of the melt pool [35; 36]. In the LP-DED process, powders with a particle size between 30-150 \(\upmu\)m are used [37]. Powders for AM can be produced mainly by three different technologies: Gas-Atomization (GA), Water-Atomization (WA), and Plasma Rotating Electrode Process (PREP) [38]. Powder particles produced by WA are characterized by an irregular shape elongated along a direction and dimpled surface textures [39; 40]. Instead, powder particles produced by GA are characterized by a predominantly spherical shape with smooth surface textures and some satellites [40; 41]. Finally, powder particles produced by PREP are characterized by an extremely spherical shape without satellites or defects [42]. Figure 3 compares the morphology of 4130 steel powder produced via GA and WA, respectively. Different experiments have been conducted to analyze the effect of powder production technologies on the characteristics of the produced parts. For instance, [PERSON] and [PERSON] [44] compared the effect of WA and GA 316L stainless steel powder on LP-DED walls. The results showed that although the use of WA powder significantly reduces the efficiency of the LP-DED process, a finer microstructure and smoother surfaces are observed. As a consequence, WA powder can be considered a viable and cheaper alternative for production [39]. Later, the same authors, during the deposition of WA and GA H13 tool steel powders, demonstrated that using GA powders resulted in higher values of deposition rate and hardness [41; 45]. [PERSON], et al. [46] and [PERSON], et al. [47] compared the use of GA and PREP powder particles, respectively, for the production of IN718 and Ti6 Al4V samples. They revealed a higher porosity content and a lower deposition rate on the tracks produced with GA powders. Further improvements in the powder production methods could open up new opportunities for production efficiency. As mentioned earlier, the powder particles are driven by the deposition head to produce a powder stream that interacts with the melt pool generated by the laser. Therefore, the importance of the deposition head is evident. The deposition head is mainly composed of laser optics, powder feed nozzles, shielding gas nozzles, and sensors [20]. Several deposition head configurations have been developed so far [22]. The first deposition head was the lateral one (Figure 4a), which uses a single off-axis nozzle. However, this type of deposition head has some limitations. The most important limitation is that during the deposition process with this configuration, the geometry and the characteristics of deposited tracks are direction dependent. To overcome this limitation, the coaxial configuration was introduced. Two types of coaxial deposition heads have been developed. The first uses a discrete number of symmetrically positioned nozzles (Figure 4b), while the second uses a conical nozzle (Figure 4c). The coaxial deposition head configuration is the most widely  
Figure 3: Morphology of 4130 steel powders obtained using the two systems: (**a**) gas-atomized and (**b**) water-atomized (reproduced with permission from [43]; Copyright © 2021 Elsevier). used [37] due to its high powder capture efficiency and independence of deposited tracks from the deposition direction [48]. However, it should be considered that the lateral deposition head is more economical due to the simplicity of the equipment. Moreover, it is possible to deposit into small locations, such as inside tubes and channels [20]. Deposition head configurations are illustrated in Figure 4. Table 2 summarizes the main advantages and disadvantages of the different deposition head configurations. The powder stream could be analyzed considering two aspects: the powder flow and the interaction between the powder and the laser beam.