Metal additive manufacturing can be used to manufacture complex structural components that are difficult or even impossible to be produced using conventional methods. Recent development in constituent technologies has improved the understanding of process parameter-structure-property relationships for as-printed parts; 316L stainless steel (SS) is a face-centered cubic material, and the structure is not transformed when cooled to room temperature. Therefore, it is a good candidate material for analyzing the influence of heterogeneous microstructures on the mechanical performance of additive manufacturing (AM)-processed materials. Several studies have revealed that the strength and ductility of selective-laser-melted (SLM) 316L SS are higher than those of forged SS. This is because SLM parts have unique heterogeneous microstructures. Here, we review the recent SLM 316L SS, considering the process parameters, trans-scale structures, and mechanical properties. We provide a detailed review of SLM 316L SS with high strength and ductility and give insight into the future of this material.

First, defect formation mechanisms in SLM 316L SS are discussed. To summarize and compare the process-parameter-dependent relative density of as-printed samples, different energy density indices are adopted to calculate the resultant energy density under different processing conditions (i.e., different selective laser melting machines, spot diameters, and materials). Then, the melt pool evolutions with different process parameters are reported. We summarize the relationship between the melt pool geometry and crystallographic texture and present the melt pool morphology predicted through dimensionless analysis. Thereafter, the grain size and morphology, cellular structure, dislocation density, and nanoparticles of SLM 316L samples are discussed, focusing on the formation mechanism of cellular structures, followed by the presentation of the mechanical performance, including hardness, tensile properties, and corrosion behavior, of SLM 316L parts. Additionally, the effects of postdeposition heat treatment on the microstructures and tensile properties are also reviewed.

With an increase in various energy density indices, the relative density of the part increases first, remains constant, and then decreases (Fig. 2). Several dimensionless quantities, including R_HD, η_m, η_v, and KemLd*, are used to determine the minimum threshold for as-printed samples with high relative densities, and their values are 1.2, 2.6, 0.45, and 2.0, respectively (Fig. 3). The energy density imported into a powder bed has a significant impact on melt pool morphology (Figs. 5-8). However, contradictory results are observed when different energy density indices are used (Fig. 5), suggesting that there are limitations in using these indices as design parameters for selective laser melting. Comparing reported data for SLM and forged materials, it is observed that the grain size of SLM 316L SS is relatively large (Table 1). An anisotropic grain structure, namely, a checkerboard-like structure, is formed on the top surface, whereas a columnar grain structure is formed on the side plane of the as-built 316L SS, with the grain-size aspect ratio ranging from 1.4 to 15. Although the formation mechanism of the cellular structure is still not clarified, the structure plays a vital role in determining the mechanical performance of SLM parts. Nevertheless, as-printed materials have massive dislocation networks at the cell boundaries. Such cell structures with dislocations formed in SLM material are similar to the microstructure processed under severe plastic deformation processes. Many studies have reported that high dislocation densities, ranging from 10¹⁴ to 10¹⁵ m^-2, are obtained in as-built 316L SS, which contributes significantly to the enhanced tensile yield strength of SLM samples according to the Taylor hardening law. Recently, it has been reported that SLM 316L samples break through the strength-ductility tradeoff due to their hierarchical microstructure (Table 2 and Fig. 11). By tailoring laser process parameters, the melt pool shape, cell structure size, and size/content of nanoinclusions may change; hence, different strengthening mechanisms will be dominated at a certain printing process (Fig. 11). We infer that the hierarchically heterogeneous microstructure acts as a whole to resist tensile deformation under loading. Our experimental results collectively suggest that melt pool, grain, and cell structure boundaries are relatively weak regions in SLM parts, and the original grains of an SLM part are usually subdivided after tensile deformation (Figs. 12-13). Though the thermal stability of various microstructural features may differ (Fig. 14), independent of the process parameters and printing machines, 873 K is the temperature threshold above which SLM 316L SS exhibits classical strength-ductility tradeoff (Fig. 15).

In this study, we present a comprehensive overview of the evolution of the microstructures of SLM 316L SS from heterogeneous aspects. Unique microstructures, including the presence of crystalline grains, defects, melt pools, cellular structures, very high dislocation density similar to that of a severely plastically deformed material, and nanoinclusions, are formed in SLM 316L SS. Many studies have shown that the mechanical properties of SLM 316L SS are comparable with those of the wrought counterparts, though the mechanical performances may vary with process parameters and change locally within a part. Progress has been made in understanding SLM 316L SS, and the underlying strengthening mechanisms have been sufficiently revealed. Therefore, tailoring the structure and properties of SLM 316L based on scientific principles paves the way to AM metal parts with excellent mechanical properties. This review can serve as a valuable reference for understanding the current state of SLM 316L SS, the scientific gaps, and future research needed to advance this technology.