## 1 Introduction
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Technique** & **Acronym** & **Reference** \\ \hline Laser cladding & LC & [9; 10] \\ Direct metal deposition & DMD & [11] \\ Direct light fabrication & DLF & [12; 13] \\ Laser direct casting & LDC & [14] \\ Laser forming & Lasform & [15] \\ Shape deposition manufacturing & SDM & [16] \\ Laser engineer net shaping & LENS & [17] \\ Laser powder fusion & LPF & [18] \\ Laser-aided direct-metal deposition & LADMD & [19] \\ Laser-based multi-directional metal deposition & LBMDMD & [20; 21] \\ Laser aided manufacturing process & LAMP & [2; 22] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Various commercialized directed energy deposition (DED) processes. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Alloy** & **Examples** & **Reference** \\ \hline Titanium alloy & Ti-22 Al-23 Nb, Ti-6 Al-4V, Ti-6.5 Al-3.5 Mo-1.5 Zr-0.3 Si, & [13; 15; 23; 24; 25; 26; 27; 28] \\  & Ti-5 Al-5 Mo-4V-1 Cr-1 Fe & & [11; 29; 30] \\ Steel & 10V, 15-5 PH, 410, AISI 309 Aement 10, A2, MM10 & [11; 29; 30] \\ Nickel based superalloy & CMSX-3, Haynes188, Haynes280, IN600, IN690, IN713, & [31; 32; 33; 34; 35] \\  & Rene 142, Rene N5 & & [36; 37; 38; 39; 40; 41; 42; 43; 44; 45] \\ Aluminum alloy & CP Al, 6061, 2024 & [36] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Common alloys processed by the DLD process. deposition [43; 44]. Turbine blades, engine cylinder heads, and blocks are examples of the application of DED as a repairing technique [45]. Despite all the merits, low powder efficiency and final rough surface are the main disadvantages of this process [42]. In addition, a large number of studies have shown that the thermal history of a component within the DED has a significant influence on the homogeneity of the microstructure and final mechanical properties of the components [46; 47; 48]. Ti-6 Al-4V is the most commonly used Ti alloy in the aerospace, aircraft, automotive and biomedical industries because of its excellent strength, fracture toughness, low specific gravity, and corrosion resistance [49; 50]. However, its further exploitation in other applications is challenging owing to its low thermal conductivity and high reactivity features which result in its poor machinability characteristics [51]. Moreover, due to the heat hardening phenomena during cutting, specific heat treatment operations are generally required. Finally, the high cost of titanium alloys with respect to other competitive materials, highlighted by the comparison of the cost of titanium, aluminum, and steel in the different forms of Table 3, remarkably affects their widespread use. For these reasons, it is necessary to reduce the cost of fabrication by using economic or near net shape processes: for example, AM technologies have been used to produce net shape components by reducing the cost of fabrication and broadening the applications of this alloy [52]. The aim of this work is to provide an overview of the additive manufacturing of Ti-6 Al-4V alloy by direct laser deposition in terms of microstructural development, mechanical performances, and process optimization. At the beginning, the influence of different factors such as thermal history and process parameters on microstructural evolution within the DED is explained and afterwards, the mechanical characteristics of manufactured components are described. Indeed, the target here is not to assemble all the existing literature about DED of Ti-6 Al-4V alloy, but to clarify the importance and opportunities of this innovative process in this field.