## 2 Metals Used in WDED and Types of Microstructure and Mechanical Properties
### Mg and Ni Alloys  
Magnesium (Mg) and its alloys are widely used in different applications because they have significant properties such as high balance performance, excellent specific strength and low density [96; 97; 98]. Studies on the WDED method for Mg alloys are still examining the effect of heating and other parameters on the mechanical properties, microstructure and defects [99]. Since working on Mg alloys has not been fully investigated, there are not many studies related to this field, but a number of studies are detailed in Table 6.  
The microstructure and mechanical properties of magnesium-based alloys fabricated by ultrasound-assisted wire-arc DED are related to the ultrasonic power in the process. Stable microstructures are produced by increasing the ultrasonic power for fabrication by the ultrasound-assisted wire-arc DED method, and the grain size is reduced by increasing the ultrasonic power. [PERSON] et al. [99] investigated the microstructure from the bottom to top of AZ 31 alloy. Both conditions in this research show the fully equiaxed grains. As can be seen from Figure 12, the grain size of the microstructure fabricated by the higher ultrasonic power is finer than that achieved with the lower power. The process parameter and mechanical properties of the nickel-based alloys and methods are given in Table 7.  
Figure 11: The microstructure of CuAl\({}_{9}\)Mn\({}_{2}\)/Udimet 500 composites: (**a**–**c**) 5 vol. % of Ud 500 and (**d**–**f**) 15 vol. % of Ud 500 [90]. Figure 12: The microstructure of the AZ 31 Mg alloy, from bottom to top, fabricated by ultrasound-assisted wire-arc DED method: (**a**) ultrasonic power 60 W and (**b**) ultrasonic power 90 W [99]. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Technique** & **Material** & **Process Parameters** & **Microstructure** & **Condition** & **YS (MPa)** & **UTS (MPa)** & **El (\%)** & **Hardness (GPa)** & **Reported by** \\ \hline \hline \multirow{8}{*}{**Ultrasound-assisted wire-arc DED**} & & Wire diameter: & & & & & & \\  & & 1.2 mm, & & & & & & \\  & & Ultrasonic frequency: & & & & & & \\  & & 15 kHz, AC voltage, & & & & & & \\  & & Ultrasonic power: 60 W, & Fully equiaxed grains. & Build & Build direction: & Build direction: & & \\  & & & & 76 MPa & 205 MPa & 11.6\% & & \\  & & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Mechanical properties of Mg alloys fabricated by WDED processes. Figure 13 shows the effect of the ultrasonic power on the fabrication of the porosity in the sample fabricated by the ultrasound-assisted wire-arc DED method [99]. This schematic clearly shows that with increasing ultrasonic power in the process, the percentage of the porosities is reduced. The nickel-based alloys are used in different applications, such as nuclear reactors, aircraft engines, and turbine blades, because of their high mechanical properties at high temperatures and high thermal stability [100; 101; 102]. The surfaces of these alloys have serious damage and significant loss of weight from the surface because of corrosion, chemical reactions during the process, wear, thermal fatigue, and erosion. Therefore, the cost of repairing damaged equipment can be significantly reduced in terms of energy and materials by replacing it with new equipment [8]. Recently, the DED methods have been used for the repair of damaged parts [103]. The microstructure and mechanical properties obtained with the DED method for nickel-based alloys are related to three zones [103], as shown in Figure 14:  
* Deposition (D)
* Diffusion line (FL)
* Substrate (S)  
The D zone fabricated by laser power-based DED has lower deformation suitability than that of the S zone and has a non-uniform structure.