# 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
Inappropriately specified hatch spacing during the LPBF process may cause inter-track printing issues in the melt pool, such as the formation of inter-track voids. Large hatch spacing during printing led to massive irregular LOF defects. A CFD model was created using FLOW3D to simulate the influence of defocusing on the single-track scanning process of LPBF-printed Ti6 Al4V [59]. Laser defocusing, laser beam energy, and speed are influential factors in coordinating the quality of LPBF-fabricated products. The defocusing position of the laser influences the melt pool shape and dimensions through the intensity distribution of the Gaussian laser. The negative-defocused laser creates deeper melt pools than the positive-defocused laser [59]. Deep depressions and keyholes were observed through developed modeling for LPBF printing using OpenFOAM to investigate laser beam absorption behavior [60]. Due to multiple reflections occurring inside keyholes, the redistribution of the beam intensity leads to recoil pressure variation over the keyhole wall, which results in vapor depression instabilities. A corrugated free surface is created from these instabilities, which can sometimes block the path of the laser beam and result in momentarily decreased laser absorptance. The absorption behavior is dependent on the scanning speed. At low scanning speeds, the beam falls inside the molten pool boundary. Still, for higher scanning speeds, a portion of the beam energy can irradiate into the powder bed ahead of the melt pool, which results in a slightly higher absorption due to multiple reflections in the powder bed. In a numerical investigation, authors performed melt pool simulations for single-track experiments conducted over various energy densities for Ti6 Al4V using FLOW3D Weld [61]. Melt pool instabilities caused by high energy densities were observed, which can lead to deep keyholes and a high surface roughness. As the laser energy increases, the fluctuation of the keyhole shape creates pinched-off voids that produce undesired porosity and negatively impact part quality. Microroughness occurs through surface ripples generated by vapor drag. A study modeled the temperature distribution, melt pool geometry, flow dynamics, and grain morphology of melted Ti6 Al4V powder during LPBF using OpenFOAM. The melt pool dimension and pore defect propagative rate can be partially due to the laser inclination angle. Increasing the incidence angle results in a giant vortex at the front and a smaller vortex at the rear end of the melt pool. It causes a thermal gradient increase and a solidification rate decrease, which can impact part quality. A low thermal gradient and a high solidification rate can be achieved by selecting a slight incidence angle [62]. A model was developed to predict pore defect formation mechanisms during the LPBF process of Ti6 Al4V using OpenFOAM [63]. Two LOF defects were produced: near-spherical pores and irregular pores. The incompletely melted powder particles and inter-powder gaps, caused by the meager beam power input, are direct contributors to the irregular pores. The formation of interlayer LOF pores is attributed to the thickness of the powder layer being greater than expected. Excessive hatch spacing caused inter-track LOF pores and created a rough surface on the overlapped melt tracks for multi-track simulations. The keyhole geometry and the liquid pool dynamics promoted the collapse of keyhole regionsand the formation of pore defects. Multi-track prints caused keyhole depth fluctuations due to the merging of the pores induced by the previously lower layers. Deep and middle pore defects were simultaneously generated during multi-track prints. The keyhole of the next track and the coalescence of the large pore from the previous track enlarge the pore at its spatial location [63]. An example of simulations of the LPBF melt pool is provided in Figure 5.  
A study proposed a coupled radiation transfer and thermal diffusion model for simulating the SLM process, including the melt pool [64].