Compared with traditional manufacturing methods, selective laser melting (SLM) can form complex components. As 316 L stainless steel has excellent mechanical properties, several studies on additive manufacturing of 316L stainless steel have been conducted. However, current research on the impact toughness of 316L prepared by SLM based on the heat treatment temperature is insufficient, and the effect on anisotropy has not been reported. Therefore, in this study, we use the SLM method to prepare 316L impact components with different forming orientations and compare the impact toughness and anisotropy of the SLM-formed 316L stainless steel before and after heat treatment at different temperatures. The study provides technical ideas for regulating the microstructure and properties of 316 L stainless steel parts prepared by SLM.

We use 316 L stainless steel spherical powder. The selected process parameters are as follows: laser power of 280 W, scanning speed of 1150 mm/s, layer thickness of 30 μm, and scanning spacing of 0.1 mm. Impact specimens and micro-characterization specimens are prepared in an SLM equipment. Three impact specimens, namely XZ-X, XY-Z, and XY-X, are prepared according to different printing and notch orientations (Fig.2). They are heated to 1050 ℃ and 1100 ℃ in the vacuum furnace and held for 1 h after air cooling. Three groups of comparison samples with different states are obtained, namely the SLM state, heat treatment state at 1050 ℃, and heat treatment state at 1100 ℃. Finally, the prepared impact samples are subjected to the Charpy impact test at room temperature and characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD),and transmission electron microscope (TEM).

The impact toughness of 316 L with different states and orientations shows an obvious trend. For different states, the impact toughness of SLM samples is the highest, followed by that of the heat treated sample at 1100 ℃, and that of the heat treated sample at 1050 ℃ is the lowest. For different orientations, the impact toughness of the XY-Z sample in the SLM state is the best, but it has the worst impact toughness after heat treatment (Fig.4). The phase composition characterized using XRD is found to be the single austenite phase (Fig.3). Using SEM and TEM to characterize the microstructure, it is found that 316L in the SLM state is composed of many fine crystals and cellular subgrains. After the heat treatment, recrystallization occurs, fine crystals transform into coarse grains, and subgrain boundaries gradually disappear. This phenomenon becomes more evident as the heat treatment temperature increases (Fig.7). Simultaneously, oxides rich in Si and Mn are observed in the SLM state (Fig.8). The size of the oxides increases significantly after heat treatment but does not increase after the heat treatment temperature reaches 1100 °C (Fig.9). EBSD characterization reveals that the grain size and proportion of large-angle grain boundaries increase after heat treatment, which becomes more evident as the heat treatment temperature increases (Fig.6 and Fig.10). Therefore, the effects of heat treatment on impact toughness and anisotropy are analyzed. Impact toughness is affected by oxide content, grain size, and large angle grain boundaries. Among these effects, the size of the oxide inclusions is dominant. The coarsening of the oxide after heat treatment significantly worsens the impact toughness. Although the increase in grain size and large-angle grain boundaries can improve the impact toughness, the effect on impact toughness is less than that of the oxide coarsening. It is believed that anisotropy is affected by the multi-layer structure and grain texture. The anisotropy of the SLM state is dominated by the multilayer structure, and the notch cracks of the XZ-X and XY-X samples expand between layers and are not hindered by the print layer. The notch cracks of the XY-Z samples expand perpendicular to the layer and are hindered layer by layer, releasing more energy when the impact is exerted (Fig.11). After heat treatment, the multilayer structure is destroyed, and the grain orientation of the XY plane is <110> (Fig.12), which is not conductive for increasing the impact toughness; therefore, the impact energy released by the XY-Z sample is lower.

The SLM sample exhibits the highest impact toughness, which decreases after heat treatment owing to an increase in inclusions. As the heat treatment temperature increases, the grain size and large-angle grain boundaries also increase, along with the impact toughness. The XY-Z sample in the SLM state has many impact obstacles and exhibits good toughness. After heat treatment, the obstacles are weakened, and the texture dominates. The <110> crystal orientation is not conducive to impact performance, and the toughness of the XY-Z sample decreases.