## 2 Classification of LPBF Defects
### Microstructure Defects  
Microstructure defects in LPBF, known as porosities, are voids/gaps that can negatively impact the microstructural integrity and part homogeneity. Heterogenic microstructures often occur through the evolution of the build with each subsequent scan. Porosities can be broken down into three main types, namely: (1) lack of fusion (LOF) porosities, (2) gas porosities, and (3) keyhole porosities. Porosities can form due to powder/elemental impurities. Each porosity type can be attributed to particular sets of process parameters and material properties. Porosities can lead to severe metallurgical defects in lower part density and adversely affect the texture and mechanical performance.  
**LOF porosities** are caused by insufficient laser power and high laser scanning speeds and usually occur at the melt track layer interface. Often associated with the balling effect, the high scan speed and low laser power provide insufficient time for the powder to melt [20]. Poor heat penetration causes the surface of the current layer to become rough and obstructs the molten melt pool flow, causing interlayer defects. Irregular powder particles, such as tiny satellites attached to powder particles, broken powder particles, and dendrite powder particles, have been shown to act as friction points that reduce the powder flowability, allowing for voids to form [21]. Figure 7 highlights LOF porosities in an SS316 LPBF-printed part [22].  
**Gas porosities** are typically caused by gas entrapment, the supersaturation of dissolved gases, or chemical reactions. It can also be attributed to process parameters, environmental conditions, and contaminants such as oxides, moisture, and foreign particles. Gas pores form when the equilibrium pressure of the gas overwhelms the combined atmospheric, hydrostatic, and capillary pressures during the LPBF process. Like LOF pores, increasing the laser scan speed while maintaining sufficient laser power allows less time for the powder to melt entirely, providing gas retention opportunities for the solidifying melt pool. Figure 8 highlights the location of gas pores in an SS316 LPBF-printed part [23].  
Figure 6: Comparison of melt pool during scanning with inert (N\({}_{2}\)) and air atmosphere [19].  
Figure 7: SEM imagery showing various LOF porosities locations (circled) [22].  
**Keyhole porosities** are also known as depression defects and typically occur when the laser power density is excessively high. The high laser power causes deeper melting pools to form, and the overheating causes the low melting point elements within the alloy to vaporize in the form of gas bubbles.  
The vaporization leads to the instability of the melt pool and creates a recoil pressure inside the melt pool that can push away the melted liquid [24]. As the depressed melt zone rapidly solidifies, the gases are trapped, causing the formation of spherical pores. Figure 9 highlights a keyhole pore in an SS316L LPBF-printed part [25].  
Impurities in the powder substrate can have a positive and/or negative influence on the mechanical behavior of the LPBF-printed part. Impurities have been known to enhance mechanical behavior, strength, and ductility but can also lead to lower corrosion resistance. The impurities most commonly associated with the LPBF process include carbon, oxygen, nitrogen, hydrogen, and chlorine [26]. Oxygen is the most common impurity and can cause oxides to form. Lower energy impurities lead to smaller grains, more grain boundaries, increased nucleation, and diminished corrosion resistance [27].