Microstructural Characterization and Mechanical Properties of 316L Stainless Steel Parts Prepared by Laser Powder Bed Fusion (L-PBF) Additive Manufacturing

Objective Laser powder bed fusion (L-PBF) is a representative technology in metal/alloy additive manufacturing. It utilizes a laser as the heat source with a small beam size, enabling the production of fine and intricate parts. During L-PBF additive manufacturing of complex structure parts, the forming direction is variable which increases the complexity of heat transfer and solidification. As a result, the thermal efficiency differs among different forming directions, which can impact the performance of the printed parts. However, the current research on the influence of forming direction on the microstructure and properties of L-PBF 316L stainless steel is not sufficiently systematic. Therefore, it is further necessary to deeply investigate the influence of formation direction. We investigated the correlation between the forming direction and the microstructure and properties of L-PBF 316L stainless steel and explored the variations in mechanical properties to provide valuable insights for the development and applications of L-PBF 316L stainless steel. Methods First, the 316L stainless steel parts were fabricated using L-PBF from two different forming directions of 0 degrees and 60 degrees. The microstructure and mechanical properties of the parts were investigated through metallographic and tensile tests. The phases of the 316L stainless steel powder and as-printed samples were determined using X-ray diffraction (XRD). The grain orientation distributions, grain sizes, and grain boundary angles of these samples at different deformation strains were characterized using electron backscattered diffraction (EBSD) and scanning electron microscope (SEM). Results and Discussions The results show that some porosity defects occur in the L-PBF 316L stainless steel (Fig.5). During the printing process, the rapid cooling rate leads to the retention of the alpha-Fe ferrite phase (Fig.6). Tensile testing shows that the samples printed from the forming direction of 60 degrees exhibit higher tensile strength than those from the forming direction of 0 degrees, while the samples printed from the forming direction of 0 degrees demonstrate better elongation than those from the forming direction of 60 degrees (Fig.7 and Table 3).In-situ tensile testing results indicate that there exist significant differences in grain boundary angles, phase contents,surface morphologies, grain orientations, and grain sizes among the L-PBF 316L stainless steel samples printed from different forming directions during the tensile deformation process. In both 0 degrees and 60 degrees, the samples exhibit predominantly high-angle grain boundaries before tensile deformation. However, as the deformation strains increase, the proportion (volume fraction) of low-angle grain boundaries gradually increases and finally surpasses that of high-angle grain boundaries (Fig.9). In terms of phase composition,the gamma-Fe face-centered cubic (FCC) phase account for over 98% in the sample before deformation, but its proportion (volume fraction)decreases while the alpha-Fe body-centered cubic (BCC) phase increases with increasing deformation strains (Fig.10 and Table 5). In terms of surface morphology, the samples underwent dislocation slip and twinning during the tensile process. The sample built from the forming direction of 0 degrees exhibits much more slip bands as well as a large amount of deformation twinning compared with the part printed from the forming direction of 60 degrees, which improves the tensile properties of the parts (Fig.11). In terms of grain orientation,the samples manufactured using L-PBF exhibit anisotropy. For the sample built from the forming direction of 0 degrees, the initial <101>parallel to Z1grain orientation gradually transforms to <001>parallel to X1 and <111>parallel to X1 during the tensile process. In contrast, for the sample built from the forming direction of 60 degrees, the initial <111>parallel to Z1 grain orientation gradually transforms to <111>parallel to X1 during tensile deformation. This difference in grain orientation is related to the formation of deformation twinning within the grains during tensile deformation, which induces grain orientation rotation (Fig.12). In terms of grain size, the L-PBF 316L stainless steel undergoes grain refinement with increasing deformation strain. The coarse columnar grains in the as-printed state are progressively fractured under external forces, leading to a reduction in grain size with increasing strain. The sample built from the forming direction of 0 degrees exhibits a higher degree of grain refinement and smaller grain size than the sample printed from the forming direction of 60 degrees (Fig.13). Conclusions The influence of two forming directions of 0 degrees and 60 degrees on the microstructure and mechanical properties of L-PBF 316L stainless steel was studied. The evolution of microstructure and grain orientation during tensile deformation of L-PBF 316L stainless steel was studied using in-situ tensile testing. There are some defects in the L-PBF 316L stainless steel, and a fish-scale-like melt pool occurred in the part printed from the forming direction of 60 degrees. The sample built from the forming direction of 60 degrees exhibits a high tensile strength, while the part printed from the forming direction of 0 degrees shows good elongation and plasticity. During in-situ tensile deformation, the proportion of low-angle grain boundaries and the alpha-Fe-BCC phase content increase, the grain size decreases, and the slip bands appear within the grains. Compared with the sample built from the forming direction of 60 degrees, these changes are much more significant in the part printed from the forming direction of 0 degrees. In the forming direction of 0 degrees, the initial <101>parallel to Z1 grain orientation gradually transforms to <001>parallel to X1 and <111>parallel to X1 during tensile deformation, while in the forming direction of 60 degrees, the initial<111>parallel to Z1 grain orientation gradually transforms to <111>parallel to X1