# Roles of Modeling and Artificial Intelligence in LPBF Metal Print Defect Detection: Critical Review
## 2 Types of Common LPBF Part Defects
### Surface Quality Defects  
Surface quality defects are common in LPBF and could lead to poor part quality and reduced mechanical properties. Severe balling, surface oxidation, denudation, surface roughness, and metallic vaporization are surface quality defects. Figure 1 provides a visual representation of the common surface quality defect types.  
Balling occurs due to low wettability during the melt track deposition, forming pores, rough surfaces, and irregular melt tracks [6]. This phenomenon is primarily influenced by fabrication-controlled parameters (laser beam power level, scan speed, hatch spacing, and layer thickness) and the substrate material's physical properties. For instance, a higher level of laser power generates more heat, reducing viscosity and enhancing the molten pool liquid wettability. Surface oxidation happens in the presence of oxygen; even though most LPBF processes are executed under an inert gas environment, unwanted oxygen (with a content around 0.1-0.2%) remains [7]. The oxidation layer thickness is crucial: thin films (nanometer) cause nearly no damage, whereas thick films (<100 \(\upmu\)m) can cause deformed geometries [8]. Subsequent oxide layers can create porosities, leading to part cracks and the degradation of its mechanical properties. Using fresh and dry metal powders is recommended to minimize surface oxidation. Several factors, including beam power, scan speed, layer thickness, and hatch spacing, determine the severity of surface roughness [9]. A higher energy density, lower laser speeds, and adequate hatch spacing can reduce surface roughness.  
Denudation occurs when metallic particles surrounding the solidified melt track are depleted, causing porosities and a high level of surface roughness [10]. These particles may be incorporated into the melted pool or lost within the metal vapor plume [11]. Careful hatch spacing selection is advised to prevent denudation [12]. The vaporization of metallic powder can result in a shortage of alloying elements, causing defects like keyhole porosities, spatter, and cracks, significantly weakening the LPBF-printed part's mechanical properties. Due to their low boiling points, metals such as Mg, Zn, and Al can vaporize quickly [13]. Environmental effects include the loss of shielding inert gases during the LPBF process. Argon (Ar) is the most commonly used inert gas in the LPBF process. Other gases include nitrogen (N\({}_{2}\)) and helium (He). Inert gas conditions impact part density and surface quality [14].  
Figure 1: Types of surface quality defects in LPBF printing processes: (**a**) SEM micrograph imagery of top surface balling [5]; (**b**) simplified representation of oxide layer formation on metal powder substrate; (**c**) characteristic imagery of surface roughness post build [12]; (**d**) wide-field image of denuded zones for various scans with increasing laser power specifications (bottom to top) [11]; (**e**) simplified representation of metallic vaporization/evaporation occurring during the LPBF process; (**f**) comparison of inert gas and air atmospheric environments during the LPBF melting process [14].