## 1 Introduction  
Additive manufacturing (AM) is the fabrication process of generating precise geometrical shapes through the sequential deposition of incremental layers. The application range of AM is incredibly vast and continues to garner increased industry attention due to its rapid prototyping potential. It offers flexible design freedom, short lead times, and reliable manufacturability of dense parts. AM of metals is accomplished by fully melting either powder or wire feedstock using the energy input from a laser or electron beam. The energy strategically lays the melted metal in a specific configuration for the current layer. The deposition of each layer is repeated until the physical part is completed. The various AM methodologies can be taxonomized by the binding mechanism(s) between the joined layers of material and the aggregate state and nature of the feedstock material composition. Metal AM binding mechanisms can be classified into three main processes: laser beam melting (LBM), electron beam melting (EBM), and laser metal deposition (LMD). Other names for the LBM process include laser powder bed fusion (LPBF), selective laser melting (SLM), direct metal laser sintering (DMLS), and laser metal fusion [1]. The LBM and EBM methods are powder bed-based processes, where the product is developed by melting metal powder layer by layer using a high-performance laser (LBM) and electron beam gun (EBM). The powder bed platform is lowered after each layer of exposure to create vertical space for the new layer of powder. LBM operates in a protective inert gas chamber environment, while EBM operates under vacuum conditions. The LMD process applies metal powder through a nozzle while simultaneously melting the feedstock using a laser. In LMD processes, the deposition heat is typically repositioned for each layer while the part is kept stationary. A stationary deposition head and moving base plate have also been proven to work. The proof-of-concept for each AM process has been established, though some technologies have been more popular than others, including LPBF. For the LPBF printing process, the metal powder substrate is deposited from an industrial hopper or an adjacent powder supply and smoothed using a leveling system to achieve a uniform distribution. Once the scan has been completed for the current height, the build plate is slightly lowered, and the next layer is deposited and smoothed. This process is repeated until the part is completed. The laser beam sources for all current LPBF technologies are ytterbium or Nd-YAG fiber lasers with a power range between 200 and 1000 W that can generate a laser spot diameter range between 50 and 100 \(\upmu\)m [2]. The variability of other parameters, such as scan speed, hatch spacing, and layer thickness, usually depends on the particular machine's limitations. Though the LPBF process can produce highly accurate and complex parts, it is not immune to defects. The types of defects can be mainly attributed to the material's (thermophysical and microstructural) properties and the manufacturing process parameters. Theoretically, combining optimized process parameters and appropriate material properties should provide synergistic conditions to produce parts with no defects. In reality, however, the appropriate conditions for a one-part build may be unacceptable for another, producing unforeseen defects not observed in prior prints. Significant research efforts have been reported to understand and quantify the triggers and origins of LPBF defects by investigating the material properties and process parameters for various geometries. These investigations into defect detection and monitoring are accomplished through three primary sources: sensing technologies (optical, thermal, acoustic, and ultrasonic), numerical methods (finite element, finite volume, analytical coding), and the integration of artificial intelligence (AI) models (supervised, unsupervised, and semi-supervised).