Microstructure and mechanical properties of selective laser melted 18Ni300 steel

Figure 3 shows the optical microstructure of the longitudinal section of the formed part at different laser powers. The laser beam scanned the metal powder at an angle of 90° to form molten pools, and the interaction between the molten pools formed a fish scale interface. When it was cooled to the martensite transformation temperature, part of austenite rapidly transformed into martensite. However, in the solidification process, solute elements easily microsegregated at the grain boundary, which hindered the martensitic transformation and caused residual austenite at the grain boundary. With the increase in laser power, the laser energy absorbed by the powder increased in short time, so the actual overlap ratio of adjacent welds increased. Also, the weld bead depth h and the width W increased.

Fig. 3

A hardness changing curve 4 were obtained. The law of the side surface hardness was the same as that of the upper surface hardness (Figure 4). Under the same laser power, the side hardness of the specimen was higher than that of the upper surface hardness. This was because in the layer-by-layer stacking, the formed martensite structure extended to the epitaxial boundary of the weld channel along the formation direction.

Fig. 4

The tensile parts were prepared according to GB/T 228.1-2010 (Figure 5). The engineering stress–strain curves of materials in different states are shown in Figure 6. From the curves, it can be seen that the slope of the elastic deformation stage of specimens under different laser powers is very small but with similar Young's modulus values. The tensile properties of 3D-printed pieces prepared with different laser powers are shown in Table 3. The tensile strength of 330 W and 300 W samples is the highest, but 300 W samples have higher elongation after fracture, showing good plasticity as well as micromorphology. Hence, the comprehensive mechanical properties of the samples are the best when the laser power is 300 W.

Fig. 5

Fig. 6

The fractures of tensile specimens were observed using SEM to further study the mechanism of tensile fractures. The fractured surface of the tensile specimens is composed of dimples in different sizes (Figure 7) due to the influence of energy density. When the laser power is 270 W, although the internal grain size is gradually refined and the grain boundary is increased, the crack propagation is hindered to some extent. However, due to the low power density, many defects appear in the formation process. In addition to deep holes in the tensile fractures, large particle inclusions appear in the holes. When the laser power is 300 W, a large number of equiaxed crystals are distributed on the fracture surface (Figure 7A). These depressions are evenly distributed and deep, showing high plasticity (Figure 7B). Therefore, the tensile strength and elongation of the samples are the highest. When the laser power is 330 W, the dimple size and depth of the fracture surface increase gradually. The fracture surface also has obvious tear marks, which reduce the tensile property (Figure 7C). When the laser power is increased to 360 W (Figure 7D), the surface dimples increase, and granular inclusions appear on the fracture surface, so the fracture mode changes to a plastic fracture.

Fig. 7

Density is an important influence factor on the mechanical properties of samples formed using SLM. The optimization of laser process parameters during SLM is also the first step in the research of SLM additive manufacturing. Figure 8 shows the density curve of the formed samples with laser power when the laser scanning speed is constant. The curves show that the density is significantly positively correlated with the laser power, meaning the energy density increases with the increasing density of samples.

Fig. 8

Based on the relative density measurements of the formed samples in the process optimization experiment, the optimized process of SLM and additive to martensitic stainless steel was determined. The laser power of 300 W was selected as the optimal process parameter. The surface of the sample was observed by SEM. Obvious grain boundaries appeared in the molten pool, and the microstructure of the central area was mainly cellular crystal (Figure 9). The formation of such crystals is mainly related to the cooling rate. A faster cooling rate can more promote the transformation of the initially generated planar crystal to the cell crystal. During SLM, the powder is heated and melted into molten metal. When the metal melt is continuously deposited to the crystal core, grains continue to grow, so the molten metal is solidified [21]. According to the hall patch relationship, the grain size is inversely proportional to the material strength. The fine grain structure of martensitic stainless steel indicates that the material has high strength, which is consistent with the previous tensile property results.

Fig. 9

Figure 10 shows a microstructure diagram of EBSD on the upper surface of a sample with a laser power of 300 W and 360 W. The upper surface EBSD inverse pole diagram (IPF) of 300 W (Figure 10A) shows that the columnar crystal grains exhibit weak texture due to the high thermal gradient. The formation of columnar crystals is caused by the high temperature gradient along the growth direction during SLM formation [22]. Moreover, the upper surface EBSD grain boundary diagram of 300 W (Figure 10A1) shows many small-angle grain boundaries in the upper surface, reflecting the thermal stress caused by rapid heating and cooling, and the formed sample has a high dislocation density [23]. The grain size of the sample is <10 μm (Figures 10A and 10A1), which imply that the sample of 300 W has better tensile properties than the sample of 360 W (Figures 10B and 10B1).

Fig. 10