## 3 Microstructure  
### Microstructure in As-Built State  
Similar to the grain growth behavior in most LPBF alloys, as-built IN625 consists of some cellular dendrites and columnar grains with elongated grains in the building direction (see Figure 4) [49]. To be more specific, LPBF IN625 alloy has a typical gamma face-centered cubic (\(\gamma\)-FCC) matrix and a strong texture with \(<\)100\(>\) orientations parallel to the Z-axis direction [55]. Due to factors such as heterogeneous nucleation of the impurity elements and heat dissipation disorder at the boundary of the molten pool, some non \(<\)100\(>\) oriented grains also can undergo restricted growth, resulting in many fine grains. In the center of the molten pool, the size of the grains is relatively large and the orientation is more uniform. Moreover, precipitations like \(\gamma^{\prime}\), \(\gamma^{\prime\prime}\) phases, Laves phase, and some MC carbide are often observed in the as-built state. The presence of intergranular brittle Laves lead to the mechanical properties and creep properties of the direct LPBF parts much lower than the standards of aerospace forgings. [PERSON] et al. [63] also observed an intermetallic strengthening precipitate \(\gamma^{\prime}\) (Ni\({}_{3}\)Nb) positioned along the boundary of the molten pool. [PERSON] et al. [59] observed single austenite with large lattice distortion but no carbides and precipitates in the as-built IN625 alloy. In addition, element segregation and extremely high dislocation density exist in individual grains as well as in the whole component [50]. The process parameters of the LPBF, as well as the powder factor, will have a significant impact on the microstructure of the formed part. [PERSON] et al. [49] conducted a single track LPBF of IN625 experiment and found that the effect of laser power on the spacing of primary cells is greater than the effect from scanning speed. With the increase of laser power and the decrease of scanning speed, the track width, and melt depth increased, and the contact angle of the melt pool decreased with the increase of power, while the height and surface roughness of the melt pool had nothing to do with the scanning speed, which was consistent with the research results of [PERSON] et al. [30]. According to [PERSON]'s research [64], the laser scanning direction is also related to the texture characteristics of the LPBF of the IN625 alloy. When the bi-directional scanning is carried out, the grains all extend along the direction of \(<\)100\(>\), while the grain growth direction forms an angle of 60\({}^{\circ}\) with the forming direction  
Figure 4: Inverse pole figure of the cross-section of the as-built IN625 track. A represents the track, B represents the heat-affected zone, and C represents the substrate [49]. Figure 3: Schematic diagram of typical chevron features on the surface of the as-built IN625 sample [58]. when adopting unidirectional scanning, which is mainly caused by the asymmetry between heat flow direction and printing direction. As discussed in the work conducted by [PERSON] et al. [59], the microstructure of the as-built IN625 alloy sample is uniform and fine, showing obvious rapid solidification characteristics. The sample consists of a single austenite phase, and some carbon-oxygen inclusions are present inside. Using EBSD, [PERSON] et al. [55] reported that the texture was stronger under the condition of higher power when comparing 1000 W to 400 W. In summary, the main phase in as-built LPBF IN625 samples is Ni-matrix. Sometimes carbides and some second phases precipitated [63], sometimes they were not observed [59], which is mainly related to the process parameters. High laser power and slow laser scanning speed lead to more laser heat input, which increases the penetration and the width of the weld bead [30; 49] and also enhances the texture [55].