## 2 Overview of the L-PBF Process
### Process Signatures  
The process signatures describe the dynamic characteristics of the melting and solidification process during the laser/powder interaction [33]. There are several classification methods for the L-PBF process signatures in the literature. [PERSON] et al. [33] classified them into observable and derived signatures, while [PERSON] et al. [3] classified them according to their level of detail (size of the smallest feature). Figure 2 describes the four levels considered in this review article based on the level of details used for in situ monitoring for the L-PBF process. The signatures considered in this review article are meltpool, laser tracks, scanned layer, and powder bed. Meltpool sizes depend on many factors, such as laser power, laser type, and powder absorptivity, for example. The laser tracks size also depends on meltpool size, as they are essentially a combination of consecutive meltpool. The scanned layer and powder bed are usually measured across the build-plate dimension, which depends on the type and model of the L-PBF machine.  
The meltpool is produced due to the laser interaction with the powder metal [34], illustrated in Figure 2a. The interaction occurring in this area is considered the lowest level of detail among the L-PBF process signatures. The meltpool shape, size, temperature, and stability depend on the VED delivered to the powder [34]. Plume and spatter are also considered characteristics of meltpool stability. Plume is generated when the meltpool surface temperature reaches a melting point. The metal partially evaporates and might alter the laser beam profile and affect the quality of the manufactured part [35]. It is also reported that as plume condenses, it might form nanoparticles ejected out of the  
Figure 1: A typical setup of an L-PBF process.  
meltpool [36]. Spatter is known to have a negative impact on the performance of metallic parts manufactured using L-PBF processes. [PERSON] et al. [37] identified three types of spatter: metallic jet, droplet, and powder spatter. The high laser power causes the metallic jet spatter; it increases the meltpool instability, causing more liquid to be stricken out and leak out in the form of a spatter. Droplet spatter is attributed to the convection transport of the liquid or vaporized metal out of the meltpool [33], while the powder spatter is caused by nonmelted metal powder blown away due to the metallic vapor.  
Laser tracks are a combination of simultaneous meltpool coming together to form one scan vector of the part being printed. The laser track's size, temperature, and continuity highly affect the quality of the printed parts, as shown in Figure 2b [38]. The layerwise signature considers the printed part at a larger level of detail and can be divided into two partitions. The first partition is the scanned geometry, designating layers after the laser scans the geometry, as shown in Figure 2; this slice can be compared to the original geometry to check for errors. Besides this, the temperature can be monitored, as well as the surface quality. The second partition is related to the powder bed, after a fresh layer of powder is spread, as illustrated in Figure 2d. The errors related to the recoater include hopping, where the recoater has many chipped ends, and streaking, when parts of the debris are dragged by the recoater. In addition, the presence of debris on the surface might indicate the presence of a defect. The superelevation of melted parts may appear or incomplete powder spreading may occur.