## 2 Overview of the L-PBF Process
### Defects
#### 2.3.1 Dimensional Accuracy and Surface Quality  
The factors affecting dimensional accuracy and surface quality are related to either the process parameters setting or the part's geometry. For example, the improper choice of slice thickness when preparing the design for printing will contribute to height errors and volume errors [40]. The staircase effect refers to the steplike feature present when the part's surface is approximated layer by layer for an angled surface [41]. The choice of laser process parameters [42] and scanning strategy [43] can contribute to any shrinkage, affecting dimensional accuracy. Curling is another defect, resulting from warping, and happens when a curl forms in the downward-facing area of a part that was supposed to be flat [44]. Curling is attributed to the uneven shrinkage between the top and bottom of the part being printed. Another defect that affects dimensional accuracy is the superelevated edge formation, as illustrated in Figure 3[45]. These errors are related to high energy density or the presence of critical features [46]. The recoater can impact parts with curled surfaces and elevated edges, leading to its wear and chipping, which in turn will affect the powder spreading, causing powder hopping, as discussed in Section 2.2. Using in situ sensors to monitor the scanned geometry might help identify dimensional defects resulting from improper layer thickness choice or errors slicing the stereolithography (STL) files, while monitoring the powder bed might help identify defects related to curling, superelevation, and recoater-related errors.  
One example of process parameter-related issues is the formation of balling droplets on the surface of printed parts. The balling phenomena are related to the formation of spheroidal drops due to the insufficient wetting of the previous layer and surface tension [29; 48]. The formation of these balling phenomena indicates inadequate selection of process parameters or oxide film formation on the previously melted layer [49]. Remelting the surface or the laser tracks with balling formed on them might help improve the surface quality and dimensional accuracy required. Another phenomenon witnessed in laser tracks is called humping formation, which is related to bulge formation on laser tracks. Humping formation is affected by the surface tension and Rayleigh instability, and Marangoni shear force [50]. If the geometry contains parts that are not supported, these parts are referred to as overhanging features. In this case, the meltpool is surrounded by powder instead of the previously melted layer; it is easier for heat to diffuse to the loose powder, thus causing overheating and cross formation, as illustrated in Figure 3b [47], which affects the geometric dimensioning and tolerancing (GD&T) [51]. Sometimes the proper choice of part's orientation can mitigate errors related to overhanging features [52]. However, it is not always feasible to apply it on complicated parts [53]. Cooling during overhang takes longer due to the unconsolidated powder beneath; thus, monitoring meltpool temperature and eventually controlling it can help reduce errors related to overhanging features in L-PBF parts [54].  
Figure 3: Dimensional accuracy and surface defects in metal L-PBF parts: (**a**) distortion of a part caused by detachment from build plate/support [45], (**b**) staircase effect in up-facing and cross formation in down-facing surfaces [47].