## 1 Introduction
In recent years, stainless steels have been intensively processed by AM technologies, mainly owing to the high mechanical properties that make them suitable for a wide range of applications in various industries such as the automotive, aerospace and petrochemical sectors [4; 29; 30]. AISI 316L steel is by far the most processed and studied, and its success is mainly related to its weldability, corrosion resistance, and tensile properties. Among the AM technologies, the DED process, which can provide a high grade of flexibility in the design and production of large AISI 316L components, could attract significant attention. In fact, large complex parts can be produced via the DED process with a reduction in the weight, the waste of expensive starting material, and the number of costly post-machining steps. In recent years, a growing body of literature has emerged in which the correlation among process parameters, microstructure, and mechanical properties of DED AISI 316L stainless steel (Figure 1) has been studied [31; 32; 33; 34; 35]. For instance, [PERSON] et al. studied the influence of the time interval between the deposition of layers and the mechanical properties of DED AISI 316L [36]. [PERSON] et al. investigated \begin{table}
\begin{tabular}{c c c} \hline \hline
**Acronym** & **Technology** & **Ref.** \\ \hline LENS & Laser Energy Net Shaping & [12; 13] \\ LMD & Laser Metal Deposition & [14; 15] \\ LC & Laser Cladding & [16; 17] \\ DMD & Direct Metal Deposition & [18; 19] \\ LAMP & Laser aided manufacturing process & [3; 20] \\ DLF & Direct Laser Fabrication & [21; 22] \\ LPF & Laser Powder Fusion & [23] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Different commercialized names of the directed energy deposition (DED) process. the effect of powder recycling and deposition pattern on the microstructure and mechanical properties of AISI 316L samples produced by the DED process [37; 38]. [PERSON] et al. evaluated the effect of DED process parameters on the evolution of the dimensional and surface quality, microstructure, internal surface, and mechanical performance of AISI 316L samples [39]. [PERSON] et al. studied the role of hatch rotation angle on the built quality of DED AISI 316L samples [40]. [PERSON] et al. examined the correlation between the porosity, density, and microstructure of AISI 316L samples produced via DED technology [41]. However, it should be highlighted that according to the existing body of literature on the processing of AISI 316L stainless steel via the DED process, microstructure and mechanical properties analyses are the dominant features that have been considered in the previous research (Figure 2). Hence, this paper provides an overview of the microstructure and properties of DED AISI 316L, and summarizes the main effects of the building parameters on the quality of the final products. In fact, the aim of this article is to review the additive manufacturing of AISI 316L alloy by DED in terms of microstructural development and mechanical properties of the samples produced with the optimal process parameters. First, the role of various factors, such as thermal history and process parameters, on the microstructure of DED AISI 316L is reviewed, and thereafter, the influence of different parameters, such as building direction, building parameters, and powder quality, on the mechanical properties of manufactured components is discussed. In general, the target here is not to assemble all of the existing literature about DED of AISI 316L alloy, but to clarify the importance and opportunities of this innovative process in this field. Figure 1: Publication trend in DED AISI 316L chronology between 2010 and 2020 in ScienceDirect (This graph is plotted according to a simple search of “316L” + “DED” at www.sciencedirect.com). Figure 2: Pie chart showing the percentage of publications at www.sciencedirect.com on different features of the processing of AISI 316L via the DED process.