## 2 Classification of LPBF Defects
### Surface-Quality Defects  
Surface-quality defects pertain to defects that appear on the end surface of the part or the current scan's surface. Surface-quality defects are often an indication of more severe defects present within the part. The surface-quality defects include balling, surface roughness, surface oxidation, denudation, and vaporization. **Balling** results from insufficient wettability occurring during the deposition of the melt track and can lead to pores, higher surface roughness, and irregular melt tracks [5]. Balling occurs due to the combination of surface tension and capillary forces that drive the melt pool to shrink to its lower surface energy state. Poor contact with the underlying substrate causes spheroidic or ellipsoidal ball-shaped structures to form [6]. The balling phenomenon can be mainly attributed to the process parameters (i.e., laser power, scan velocity, hatch spacing, layer thickness) and the physical properties of the substrate material. The main properties contributing to balling initiation are the high surface tension and viscosity. Higher laser power induces more heat to form, decreasing the viscosity and increasing the wettability of the molten melt pool. Severe oxidation has also been observed to exacerbate wetting capabilities [7]. In extreme cases, balling can potentially obstruct the deposition process through the formation of humps and ripples on the surface. These humps and ripples are further accentuated as subsequent layers are stacked. They can potentially damage the recoater and lead to costly and time-consuming maintenance. Figure 1 displays the balling of the sintered AlSi10 Mg powder at various scan speeds [8]. **Surface oxidation** occurs when oxygen is present. Oxidation is a significant problem for alloys containing Si or Mg [9]. Even though most LPBF processes are performed under protective environments with a shielding inert gas flow, a small percentage of unwanted oxygen content (\(\sim\)0.1-0.2%) can be present [10]. Oxygen is often found in the fillings between the powder particles. Over time, passive oxide films can be formed on the powder's surface, immediately affecting the scan quality. The thickness of the oxide  
Figure 1: The severity of the balling phenomena increases as the scan speed increases, as shown by the LPBF printed surface profiles with scan speeds of (**a**) 250 mm/s, (**b**) 500 mm/s, (**c**) 750 mm/s and (**d**) 1000 mm/s (modified from [8]). film layer is vital in that nm thick oxide films can cause negligible damage, while \(\upmu\)m thick oxide films (10-100 \(\upmu\)m) can cause irregular geometries to form [9]. In the presence of oxide films, the film is broken up during laser re-melting, causing some oxide particles to solidify underneath the surface while others float to the surface and form a new oxide layer. As subsequently deposited oxide layers are stacked over each other, porosities can form, which can cause cracking and weak mechanical properties. Clean and dried powders are recommended to reduce surface oxidation. Figure 2 highlights the effects of slight and severe oxidation on the surface quality of processed SS316L. **Surface roughness** severity depends upon many factors, including process parameters (laser power, scan velocity, layer thickness, and hatch spacing). The surface quality is affected by two main behaviors, namely oxidation and powder adhesion on the surface [11]. Surface oxidation causes an oxide film, affecting the subsequent layers' adhesion to the surface. Process parameters directly affect the powder adhesion effectiveness and, in turn, the part surface roughness. For example, increasing the laser scanning velocity decreases the formation time of the melt pool length, which resultantly decreases the width of the melt pool. It encourages the melt pool to break down into an uneven surface during the current scan, leading to higher surface roughness for subsequent scans. The surface roughness can decrease by providing higher energy density, laser power at lower speeds, and adequate hatch spacing [12].