## 3 Results and Discussions  
### Residual Stress in As-Built State  
Figure 2 shows the deformation, \(\delta\), measured when cutting the supporter legs of the maraging steel cantilevers with different thicknesses, \(T\), in the as-built state. The evolutions of the deformations while cutting each supporter leg are shown in Figure 2a. When \(T=0.5\) mm, cutting the legs caused significant and gradually increasing downward distortions. This behavior is very interesting because, typically, the deformation of the cantilever occurs in an upward direction due to the tensile stress on the top and the compressive stress near the bottom, which are a general consequence of the thermal shrinkage of each layer [24; 27; 28]. In the case of \(T=1.0\) mm, the cantilever showed smaller but still significant downward deformations compared with \(T=0.5\) mm. When the thicknesses of the cantilevers were 2.5 and 5 mm, the cantilever showed small amounts of upward deformations instead of downward deformations. The final deformations measured for the cantilevers with various \(t\) after cutting all the supporter legs are shown in Figure 2b. It can be seen clearly that the downward deformation of the cantilever was highest when the cantilever was the thinnest (i.e., \(T=0.5\) mm). With increases in the cantilever thickness, the downward deformation decreased sharply until \(T=1.25\) mm, where almost no deformation was occurred. With further increases in the thickness, the deformation became upward, and the amount of the upward deformation increased moderately with increasing thickness. The downward deformation behavior of the thin cantilevers could be attributed to the compressive stress state at the top of the cantilever. The occurrence of the compressive stress at the top was also observed by the X-ray diffraction measurements of the maraging steel produced by LPBF by [PERSON] et al. [22]. A possible cause of such a compressive stress state is the phase transformation from the body-centered tetragonal martensite to the face-centered cubic austenite that occurs during the LPBF process due to the intrinsic heat treatment (IHT) effect, which has been suggested and validated by phase transformation simulations by [PERSON] et al. [23]. According to this mechanism, when each maraging steel layer is deposited, the layer is nearly fully composed of martensite phase due to the rapid cooling right after the deposition. Then, each layer is heated again several times when the next layers are deposited. In this situation, second and third heating can cause some amount of the austenite phase to be retained after the cooling due to the IHT effect. Since the martensite to austenite phase transformation occurs with significant volumetric expansion and only the top layer remains in a near fully martensite state, such a phase transformation can cause a compressive stress state at the top LPBF layer. A high-resolution scanning transmission electron microscopy of the maraging steel produced by LPBF has shown that the layers in the middle consisted of very thin (\(\sim\)100 nm) austenite phases between \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Heat Treatment Condition** & **Hardness (HV)** & **Yield Strength (MPa)** & **Tensile Strength (MPa)** & **Elongation (\%)** \\ \hline As-built & 345 & 1110 & 1120 & 8.6 \\
850 \({}^{\circ}\)C solution treated & 595 & 1865 & 1940 & 2.9 \\ and 500 \({}^{\circ}\)C aged & 500 \({}^{\circ}\)C directly aged without & 595 & 2015 & 2050 & 2.3 \\ solution treatment & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical properties of LPBF processed 18 Ni-300 maraging steel in different heat treatment conditions used in this study [9]. martensite pockets [29], which supports the fact that some austenite can be retained after multiple heating and cooling due to the IHT effect. In the case of the maraging steel used in this study, the microstructural morphologies of the middle and the top layers were apparently the same, with solidification in the cell structures.