# 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
The material was a TiC/AlSi10 Mg composite, and the FVM code used was ANSYS Fluent. The high difference in thermal conductivity caused a sharp edge in the temperature distribution at the powder liquid boundary at the front of the melt pool. With a relatively high laser energy input per unit length, the center melted pool flows towards the rear part, resulting in a stack of molten material, which can contribute to poor surface quality. Also, as the liquid volume shrinks, the fluid surface tension property tends to break the melting vector into a series of individual droplets that splash from the molten track's surface. The splashing droplets cause interlayer pores, which, when combined with the shrinkage phenomenon, contribute to the formation of poor surface quality. A multiphase and multiphysics model using ANSYS was developed for the SLM of AlSi10 Mg to capture the free surface of the melts [65]. As the hatch spacing increased, more pore defects were observed, and the pores became progressively more irregularly shaped. The spherical shape of the tiny pores was attributed to air wrapping inside the melt. The irregularly shaped large poles are attributed to inadequate melting. A numerical investigation of how the particle size distribution significantly impacts the processability of AlSi10 Mg metallic particles was performed using an in-house DEM-CFD code [66]. The coarser powders were very sensitive to changes in the laser power regarding the melt pool characteristics, defect population, and defect size. The combination of a low level of laser power and coarse powder increases part pores. These pores originated from the insufficient and inhomogeneous absorption of laser energy, which contributed to the inadequate melting of powders at the sides of the melt pools. It was concluded that it is possible to use coarse powders to nearly build a fully dense geometry with sufficient laser beam energy. A computational model was developed for the LPBF process of Inconel 718 using FLOW3D [67]. Un-melted particles were located away from the pool, and particles that experienced incomplete melting were around the melt pool. The fluid convection in the molten pool significantly influences the pool shape and the liquid solidification conditions. The molten pool is elongated along the travel direction at fast scanning speeds, which can  
Figure 5: Temperature field, melt pool geometry, and flow patterns versus laser shape variation, (a–e) corresponding to laser incidence angles of \(60^{\circ},75^{\circ},90^{\circ},105^{\circ},\) and \(120^{\circ}\)[63]. influence the solidification rates and the printed layer's surface roughness. A mesoscale CFD model in ANSYS was developed for simulating the single-track LPBF melt pool behavior of mixed IN718/Cu10 Sn powder [68]. Gas porosities were mentioned to be potentially trapped inside the track if the powder particles were not melted, resulting in deteriorating properties. Additionally, to avoid insufficient intertrack fusion and pore surface quality of the printed layer, the hatch spacing was adjusted to be smaller in length than the liquid region width. A study presented numerical simulations to predict the melt pool temperature and estimate the melt track quality for Inconel 718 using FLOW3D [69]. Recoil pressure is known to drive keyhole and conduction modes in LPBF printing. A profound balling effect was observed at a low level of power and a low scanning speed. A recoil force-induced ejection tends to spout out at high scanning speeds. The surface tensions predominately depress and stretch neck formation. The Marangoni force speeds up the convection process faster than the heat transport by conduction, increases the occurrence of porosity between neighboring powders, and exacerbates the instability of the molten liquid pool. At lower scanning speeds, intense circulation in the melt pool enhances the wetting over the substrate, affecting the following layer's quality. A study investigated the mechanisms behind surface roughness evolution in overhang regions of 17-4 PH SS using FLOW3D [70].