# Design and Assessment of an Austenitic Stainless Alloy for Laser Powder Bed Additive Manufacturing
## 3 Results
### Experimental Validation
#### 3.2.2 Microstructure after AM Processing  
* Analysis of porosity  
To determine the porosity, the alloy produced through LPBF is characterized by SEM observations: a set of images is taken over a total surface of 4 mm\({}^{2}\). The images are then thresholded to capture the pore fraction and size distribution using ImageJ software 1.54f (Figure 5). An example is shown for the material built at P = 144 W and v = 1200 mm/s, displaying an average porosity of 0.05%, with a standard deviation of 0.02%. This rate is notably low compared with those observed in similar alloys, especially considering that standard processing parameters were used without any specific adjustment. Typically, the porosity rate reaches values of around 0.8-1% for common alloys like 316L [70,71]. Figure 6 illustrates the pore size distribution, which is roughly a lognormal distribution with an average size of 4.5 um and a median value of 3.4 um. The pore size remains less than or equal to 10 um. This is below the critical defect size that can initiate crack propagation, even under fatigue conditions. Indeed, assuming a very low crack propagation threshold of 2 MPa-m\({}^{1/2}\) to be conservative and a penny-shaped crack of radius 5 um associated with a conventional geometrical factor of \(2/\sqrt{\pi}\), cracking would be likely to occur only for a stress amplitude superior to 560 MPa, which is higher than the yield stress. Given that a pore is normally less critical than a crack and that the actual crack propagation threshold more likely lies between 5 and 10 MPa.m\({}^{1/2}\) for this category of alloys, it is almost certain that the observed pores cannot become critical defects and initiate any damage. * Microstructure analysis  
X-ray diffraction analysis was performed to identify the phases present in the powder and in the LPBF built material (Figure 7). The observed peaks indicate the presence of two phases in the powder. The primary FCC phase corresponds with the austenite phase (\(\gamma\)), with a theoretical lattice parameter a = 3.606 A, according to JCPDS card number 01-081-8775. Additionally, a minor BCC phase, estimated to constitute 3% of the material, corresponds with the ferrite phase, characterized by a theoretical lattice parameter a = 2.882 A (JCPDS card number 04-003-2920). The LPBF built material characterized in the same conditions shows the presence of only the austenite phase. The ferrite phase observed in the powder is therefore not detected in the LPBF built samples. It is possible that the  
Figure 5: (**a**) Observation of the pores by SEM (red arrows), (**b**) thresholding of the image to detect pores, and (**c**) detection using ImageJ software. ferrite phase was first formed during solidification, as predicted, and then transformed into austenite. The crystalline orientation in the melt pools is characterized using EBSD. The inverse pole figure (IPF) map is shown in Figure 8a, illustrating the crystalline orientation along the building direction (BD) with the laser scanning direction (SD) along the X and Y axes. Large grains with melt pool geometry are observed and high angle boundaries (>15\({}^{\circ}\)) are represented by black lines. Small variations in the IPF color code within grains suggest the presence of a post-solidification substructure, indicating that a significant portion of the FCC phase formed directly during the solidification step. This observation is consistent with predictions from the Scheil model (Figure 3a), suggesting a likely liquid \(\rightarrow\gamma\) solidification for much of the observed austenite. It is noteworthy that the microstructure does not consist of long columnar grains, as is commonly observed in the 316L alloy [72]. The crystalline orientation of each successive layer is different from the previous one, indicating that solidification does not occur by epitaxial growth between successive layers.