## 3 Microstructure
### Microstructure After Heat Treatments  
To eliminate or ameliorate the non-equilibrium phase, the Laves phase, and the micro-cracks existing inside the alloy after LPBF manufacturing, and the residual internal stress of the macro part, it is often necessary to perform a suitable heat treatment process on the as-built part. In general, there are mainly four types of post-treatments recommended for the LPBF of IN625: stress relief annealing, recrystallization annealing, solution treatment, and hot isostatic pressing [65; 66]. [PERSON] et al. [55] carried out heat treatment on as-built IN625 samples by heating them to 700 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, and 1150 \({}^{\circ}\)C and holding for one hour before air cooling. It was found that the microstructure of IN625 parts at the condition of annealing at 700 \({}^{\circ}\)C was the same as that of an as-built state. When annealed at 1000 \({}^{\circ}\)C, solute elements were homogenized and some fine grains were formed. The original molten pool boundary gradually disappeared and a lot of MC carbides were precipitated at the grain boundary. At 1150 \({}^{\circ}\)C, the grains grew significantly, and the carbides at the grain boundaries still existed and coarsened. Because of heat conduction in the LPBF process, most grains had obvious preferential orientation after heat treatment. Comparison of different annealing temperatures [50; 55], one can conclude that high-temperature annealing enables recrystallization (Figure 5a) of as-built IN625 sample, high Z-contrast precipitates are dissolved and MC carbides precipitate, and the grain boundaries become serration due to the pinning effect from MC carbides (Figure 5b).  
[PERSON] et al. [65] focused on the effect of annealing temperature on texture and grain boundary character distribution of as-built samples. After recovery annealing at 870 \({}^{\circ}\)C and 980 \({}^{\circ}\)C, the weak texture similar to the cube-oriented type can be observed. While conducting recrystallization annealing at 1500 \({}^{\circ}\)C, a strong [110] <01-1> (X-Y plane) texture appears. The fraction of low angle grain boundary  
Figure 5: X–Y plane microstructure of the LPBF of IN625 samples after annealing at 1150 \({}^{\circ}\)C. (**a**) Optical metallographic image, (**b**) grain boundary structure, and the carbides distribution [55].  
in the as-built sample was significantly reduced, concomitantly many \(\sum 3^{\text{n}}\) grain boundaries and some \(\sum 3^{\text{n}}\) grain clusters were generated. The \(\{110\}\) type texture instead of cube-oriented texture is beneficial for multiple twining, which can be used for grain boundary character distribution optimization. The study of [PERSON] et al. [60] showed that the dendrite structure of as-built IN625 was maintained when it was annealed below 1000 \({}^{\circ}\)C, and some new grains appeared at the interface between the deposition layer and the substrate, while some of them were recrystallized when it was annealed at 1100 \({}^{\circ}\)C. At 1200 \({}^{\circ}\)C, complete recrystallization occurred, and the grains became equiaxed and a large number of annealing twins appeared to release residual stress.  
In summary, the desired microstructure evolutions occurred when the annealing temperature is above 1000 \({}^{\circ}\)C [55,60]. Below 1000 \({}^{\circ}\)C, no obvious microstructure evolution was observed. High-temperature annealing heat treatments above 1000 \({}^{\circ}\)C are accompanied by the obvious occurrence of recrystallization and the release of residual stress [60]. In the range of medium temperature (several hundred \({}^{\circ}\)C to 1000 \({}^{\circ}\)C), the microstructure of IN625 almost did not change, which shows that the IN625 parts formed by LPBF have a high thermal stability [60]. More studies on the influence of various heat treatments on the microstructure evolution and residual stress release are necessary to better control the properties of LPBF IN625 parts through low-cost and highly efficient heat treatments.