## 4 Residual Stresses and Distortion  
During LPBF, a steep thermal gradient certainly exists between the melt pool and the relatively cold substrate (or previously consolidated layer), resulting in high residual stress inside the LPBF parts [65,67]. The accumulated residual stress will lead to distortion, geometric failure and even crack. Therefore, it is very important to study the residual stress level in IN625 alloy parts produced by LPBF. When directly printing the IN625 alloy tensile samples by LPBF [59], the residual stresses on the surface of the samples before and after annealing were all in tension. Before annealing, the residual stresses on the sample surface fluctuate considerably due to the different cooling conditions at different locations during laser forming, with an average value of 398 MPa [59]. After annealing, the average residual stress was 242 MPa, which is still at a considerably high level. To relieve the residual stress, [PERSON] et al. [68] preheated the LPBF substrate at 20 \({}^{\circ}\)C (room temperature), 150 \({}^{\circ}\)C, and 300 \({}^{\circ}\)C, and found that the residual stress decreased with the increase of preheating temperature. When the preheating temperature reached 300 \({}^{\circ}\)C, the residual stress (160.7 MPa) of the LPBF of IN625 alloy decreased by more than 50% compared with that without preheating. The residual stress increases with the increase of the temperature gradient. When the substrate is preheated, the heat input of laser can be greatly reduced, thereby reducing the temperature gradient; besides, preheating reduces the yield strength of the material, so the maximum residual stress can be reduced [69]. [PERSON] et al. [65] show that for the LPBF IN625 shaped specimens, the residual stress inside the specimen is still high even after annealing at 980 \({}^{\circ}\)C and subsequent two-step stress relief annealing of 16 h. However, when the annealing temperature reaches 1150 \({}^{\circ}\)C, the low angle grain boundaries and internal residual stress (local deformation), which is the driving force for recrystallization can be clearly eliminated. Similar results were obtained by [PERSON] et al. [50] and [PERSON] et al. [66]. Based on the above studies, we can reach a conclusion that residual stress can only be significantly eliminated by very-high-temperature heat treatments.  
In addition, distortion often occurs in LPBF formed IN625 alloy parts, as described in Figure 6. [PERSON] et al. [70] prepared IN625 lattice structures using LPBF, and some geometric distortion (Figure 6a) and visible oxidation would occur to the strut with a larger diameter. Distortion and roughness were easily observed on the downward-facing surfaces. An even higher heat input was used by [PERSON] et al. [49] under a laser power of 60 W and a scanning speed of 1500 mm/min. In this instance, the single track IN625 alloy fabricated by LPBF showed irregularity and distortion (Figure 6b) due to the decrease in viscosity of the molten metal and increased vapor recoil force. Whereas according to the discussion of [PERSON] et al. [67], there was no clear trend of influence of process parameters on surface residual stresses in LPBF IN625 parts.