# Roles of Modeling and Artificial Intelligence in LPBF Metal Print Defect Detection: Critical Review
## 3 Advances in Numerical Modeling Techniques for LPBF Defect Detection
### Melt Pool Defect Modeling
#### 3.1.2 Thermo-Mechanical Simulations of LPBF Defects Using FVM
Small overhang angles (\(0^{\circ}\leq\theta\leq 15^{\circ}\)) do not affect the surface roughness since only a small area of the melt pool boundaries contact the powder, resulting in slight powder adhesion. Medium overhang angles (\(15^{\circ}<\theta\leq 50^{\circ}\)) caused enhanced powder adhesion by the increased contact area between the melt pool boundary and powder bed and the melt pool sinking. When \(\theta>50^{\circ}\), the surface roughness increases sharply due to the high level of contour waviness, powder adhesion, severe deformation, and cross formation. A DEM-CFD model in ANSYS was used to understand the formation mechanisms of various defects using Invar36, Cu10 Sn, and mixed Invar36/Cu10 Sn powders [71]. For the mixture powder, increasing the Cu10 Sn in the powder resulted in partially melted and unmelted Invar36 metallic particles being deposited on the lower layer. The condition is attributed to the relatively lower level of laser absorptivity, lower melting point, and higher thermal conductivity of the Cu10 Sn powder. The results indicate that inter-layer defects were more likely to occur when the powder layer thickness was increased. Intertrack defects can be avoided by optimizing the hatch spacing for each powder mixture with a different material composition. A CFD model of the LPBF process of AISI H13 steel using FLOW3D/FLOW Weld [72] was presented. As the laser power reached the value at which the molten pool entered a transition mode, a molten pool depression was created from the recoil pressure and Marangoni flow. It results in a convex shape once the melt pool solidifies, which can ultimately affect the surface quality of the printed layer and impact part quality. A unique approach to eliminate the chances of forming keyholes in LPBF by utilizing stable nano-size particles was developed and presented in the literature [73]. The powder material was Al6061, and the nanoparticles were TiC. The FLOW3D software was used to distinguish the induced keyhole pore mitigation mechanisms. Introducing nanoparticles into the mixture significantly increased the critical depth of the keyhole pores, preventing the keyhole's collapse by slowing the protrusion's development. Additionally, the inclusion of nanoparticles increased the viscosity, slowed the pore transportation, and resulted in the recapturing of pores by the keyhole. The decrease in the protrusion at the keyhole boundaries allowed for enough protrusion energy for the recoil pressure to push the protrusion, resulting in uniform liquid heating. A numerical study modeled the interaction between the heat source and Tantalum particles in the LPBF print using the TATM-MEX code [74]. Pore-induced microcracks, interconnected micro-pores, solidification-induced microcracks and micropores, microconcaves, microgrooves, and unsteady keyhole motion were observed. The molten pool behavior of GH4169 alloy powder was simulated with a low energy density and a triple-laser scanning strategy using OpenFOAM [75]. The balling defect phenomenon as a function of the parallel offset was observed, showing that the balling phenomenon becomes more severe as the parallel offset increases. Balling occurs because the surface tension in the area far away from the center of the lasers gravitates towards the minimum surface energy as the parallel offset increases, resulting in the molten pool's fracture to a small liquid droplet that instantaneously cools down and turns solid. A simple thermal scaling model using OpenFOAM was proposed to predict the threshold from balling to conduction mode in LPBF for Cu-7 Sn powder [76]. The porosity caused by balling defects in the samples is presented by the powder size distribution, the material's thermal conductivity, the substrate's pre-heating temperature, and the fraction of power reaching the solid substrate. The powder size was smaller than the laser spot size.