## 2 Classification of LPBF Defects
### Surface-Quality Defects
Figure 3 displays the surface roughness profile of a 17-4 PH SS LPBF-printed part [13]. **Denotation** occurs when the powder particles around the solidified melt track are depleted. Denotation has been reported to cause porosities and high surface roughness from accumulating un-melted or partially melted particles between the layers [14]. Increased laser power, scan speeds, and environmental gas pressure can cause a decrease in pressure inside the vapor plume. The low pressure favors ambient gas flow towards the center of the melt track. The gas flow is powerful enough to sweep powder particles along its flow. These particles can be incorporated into the melt pool or ejected with the vapor plume [15]. Powder denotation can also occur when insufficient laser power completely melts the powder. The surface tension of the powder substrate enables partially melted particles to go back into the melt pool, depleting the powder around the melt track. Careful selection of the hatch spacing is recommended to avoid denotation [16]. Figure 4 highlights the effects of laser power on the denotation zone width for Ti6 Al4V powder [15]. Figure 3: Characterized surface roughness profile of LPBF 17-4 PH SS specimens [13]. Figure 2: Oxidation effects of SS316L surface quality, slight oxidation (**left**), and severe oxidation (**right**) [11]. **Vaporization** of metallic components of the powder substrate can lead to a loss of alloying elements, reduce part density, and cause microstructural defects. Defects include keyhole/gas porosities, spatter, and cracks. Consequently, the vaporization alters the metallic composition, severely weakening the mechanical properties of the LPBF-printed part, including the ultimate tensile strength, corrosion resistance, and microstructural creep. Vaporization occurs when the intensity of the vapor pressure at the molten melt pool surface is higher than the pressure in the surrounding environment. The surplus pressure difference drives the vapors containing the alloying elements to eject away from the surface [17]. Only certain volatile metals are very susceptible to vaporization at very high temperatures. Metals, including Mg, Zn, and Al, can quickly vaporize due to the elements' low boiling points [18]. Figure 5 shows how metal vaporization can disperse un-melted powder particles during scanning [17]. **Environmental effects** involve the protective shield of inert gases during the LPBF process. Inert gas conditions play a role in the outcome of surface quality. The three most common shield inert gases used in the LPBF process are argon (Ar), nitrogen (N\({}_{2}\)), and helium (He). The type of protective inert gas used during the printing process can affect the part density. One study reported that LPBF-printed SS316L parts under a He environment exhibited a 10% density loss compared to a fully dense part. Under Ar and N\({}_{2}\) environments, the parts had near total density values. The density decrease is due to the He environment restricting the laser interaction and causing some energy loss. An Ar gas supply can overcome the energy losses caused by metal vaporization and ionization processes [19]. Figure 6 compares the melt pool geometry during scanning with inert  
Figure 4: Denotation zone around the melt tracks for various laser power and a constant scan rate of 2 m/s [15]. Figure 5: Angular powder velocity tracing path caused by metal vaporization [17]. (N\({}_{2}\)) and air atmosphere, showing a much larger and more chaotic melt pool under an air environment for air compared to inert N\({}_{2}\) gas [19].