# Additive Manufacturing of PH 13-8 Mo Family: A Review
## 2 PBF-LB of PH 13-8 Mo Family  
### As-Built Microstructure  
In 2018, [PERSON] et al. [42] published the first paper about the microstructure of an almost entirely dense build of CX by PBF-LB. The microstructure consisted of martensite and austenite phases, which was confirmed later by different researchers [42; 43; 44; 45; 46; 47; 48; 49]. In addition to austenite and martensite, aluminium oxide inclusions were also found in the as-built microstructure of PBF-LB/CX by [PERSON] et al. [43]. [PERSON] et al. [50] also identified aluminium oxide inclusions in Uddeholm AM Corrax(r) produced through PBF-LB. Further research demonstrates that precipitation hardening can occur during the PBF-LB process [29; 46; 47; 51]. Transmission Electron Microscopy (TEM) revealed nano-precipitates ranging from 3-25 nm in the as-built state of PBF-LB/CX [29]. Subsequent analysis classified these nano-precipitates as NiAl, which were also observed in the as-built microstructures of both PBF-LB/CX (average diameter = 3.7 nm) and Corrax (average diameter = 4.3 nm) [46; 51]. The texture evaluation of the PBF-LB/CX was reported by [PERSON] et al. [45]. Electron backscattering diffraction (EBSD) confirmed the dominant texture in martensite (<111> || BD) and austenite (<011> || BD). On the other hand, as-built PBF-LB/Corrax exhibits a weak crystal texture, and an obvious preferred orientation was not found [46]. It is known that additively manufactured metals can exhibit anisotropic microstructures [38; 52]. Therefore, [PERSON] et al. [43] studied the anisotropy of as-built CX microstructure using PBF-LB. Dislocation density, lath size, grain boundary density, and residual stresses were found to be different when comparing the side and the top of the sample, resulting from different thermal histories. The influence of built direction on the microstructure of the PBF-LB/CX was unfolded by [PERSON] et al. [53; 54]. It was observed that the different built orientations caused different thermal histories and resulted in differences between the microstructures of the horizontal and vertical samples. The vertical specimen has lower retained austenite compared to the horizontal specimen due to the different cooling rates and different prior austenite grain (PAG) sizes. The vertical specimen had a bigger PAG size than the horizontal specimen. It is known that when the PAG size reduces, while austenite strengthens. Therefore, the transformation from austenite to martensite can be suppressed, which explains the higher retained austenite in horizontal material. Similar results were observed by [PERSON] et al. [55] and [PERSON] et al. [56] in terms of lower retained austenite and larger grain size in the vertical samples compared to the horizontal PBF-LB/CX and Corrax samples. The morphology of the grains changed due to the building orientation. Vertical material hasmostly cellular-columnar grains. On the other hand, equiaxed-columnar grains were the majority in the horizontal material [53]. The different morphologies can be seen in Figure 2. [PERSON] et al. [48] also observed fine equiaxed cells in the microstructure of as-built PBF-LB/CX due to high solidification rates. The microstructural evolution process was explained in three states by [PERSON] et al. [29] and [PERSON] et al. [51] during the PBF-LB/CX: the cell state (1), the cellular dendritic state (2), and the blocky state (3). The influence of linear energy density on the as-built CX by PBF-LB was studied by [PERSON] et al. [51]. CX powder was processed with a linear energy density in the range of 167-438 J /m, where 245 J /m was found to be the optimal value, as it resulted in the best relative density and surface roughness. [PERSON] et al. [46] investigated the influence of the laser energy density (volumetric) on the as-built PBF-LB/Corrax. It was also concluded that when the laser energy increased in a range of between 42.73 J \(\cdot\)mm\({}^{-3}\) and 78.19 J \(\cdot\)mm\({}^{-3}\), the relative density rose and then declined.