**4.2 FEA model**

A recently developed and launched suite in ANSYS workbench called "ANSYS Additive" provides capabilities to simulate the complete metal additive printing process in L-PBF method. In addition to the common uses of creating and optimizing design solutions for AM purposes, it can also be used for understanding the metallurgical properties of the printed parts with porosity and microstructure prediction. Accessible data throughout the process ensures the traceability and hence enhances the feasibility of a parametric study using DOE (Design of experiments) technique.

In this method, we incorporate the layer-wise structure development and time discretization (exposure time of single layers is in milliseconds, but total build time

**Figure 5.** *Melt pool formation in L-PBF, (a) 3D view, (b) 2D view, (c) sectional view.*

can be hours) by using a detailed model for single layer and global model for whole structure (**Figure 6**). This is called lumped layer approach. The details of the theoretic development of this model can be found in ANSYS Additive training website [46].

Necessary material information to perform this simulation are density, thermal conductivity, heat capacity, structural properties such as Young's modulus, Poisson's ratios, thermal expansion co-efficient, and stress–strain data. Material properties of common AM metals can be obtained from pre-existing database of Workbench Additive and/or modified with specific case study or experimentally driven data.

Using this technique, it is possible to simulate the building of whole structure, without the complexity of looking into the details of each layer formation and related void generations. As we already know the parametric relationships between defect generation and process parameters from the aforesaid CFD model in Section 5.1, this FEA model complements it with the whole structural information of the printing process.

The main challenge for the FEA simulation is local discretization, by which we mean the dimensions of the laser spot are in μm, whereas the dimension of the whole structure is in cm. Moreover, exposure to a single layer occurs in ms (milli second), but the full built takes longer time, even couple of hours. This issue can be taken care of by applying a lumped layer approach in ANSYS additive. A quick simulation catches global stress/strain as well as the distortion that takes place during printing. The whole structure is simulated in a global simulation model, where several subsequent simulations are done with status and boundary conditions are updated consequentially. For such simulations, a few assumptions are made. These are:


*Multiscale Modeling Framework for Defect Generation in Metal Powder Bed Fusion Process… DOI: http://dx.doi.org/10.5772/intechopen.104493*

#### **Figure 6.** *Lumped layer approach; top: Single layer detailed model, bottom: Whole structure global model.*


This is acceptable as we are focusing on the melt pool in the CFD simulation, and the combined modeling scheme will provide a comprehensive identification of all the relevant phenomena in the printing process.

A sequentially weakly coupled transient thermal-structural analysis is performed in this FEA model. Similar to conventional structural or thermal analysis in ANSYS workbench, we can import any kind of CAD file to this additive suite. Next steps include body cartesian mesh generation, creating named selections for build, support, and base. The simulation wizard is capable of automatically generating the support and base, so the designer only needs to consider the design of build part. After that, the build settings should be defined. This includes machine setting, deposition thickness, hatch spacing, laser speed, time between layers etc. Thermal boundary conditions include preheat temperature, gas/powder temperature, convection coefficients, and cooldown temperature (usually room temperature). Common deposit thickness is 0.001 mm–0.1 mm. A very large model will need higher computation power and time, in such cases, High performance computing (HPC) will help.

Using result tracker for temperature and displacement, the user can control the progress of print process during the solution as shown in **Figure 7** [46]. Moreover, it is possible to switch between automatic and manual mode for result tracker.

#### **Figure 7.**

*Result tracking during solution in FEA model, top: Tracking global maximum temperature, bottom: Build progression shown from left to right.*

#### **Figure 8.** *Typical model tree of coupled AM FEA study.*

As mentioned earlier it is a weakly coupled FEA analysis, a transient thermal analysis is first conducted with appropriate properties and boundary conditions. Then the results are fed into a structural analysis model, and we would finally obtain the deformation, bucking etc. on the whole structure. A general model tree is shown in **Figure 8** for easy understanding of reader.
