# Applications of Magnesium and Its Alloys: A Review
## 3 Magnesium and Its Alloys as an Engineering Material
### Engineering Applications  
#### 3.2.1 Aerospace Applications  
As we briefly discussed, early applications for magnesium include fabricating incenti diary bombs, pyrotechnics, and flash for photography [6]. These military applications led magnesium to play a considerable role in World War I and World War II. In World War II, magnesium was also used to construct military aircraft and its components [49]. That paved magnesium's entry into the civil aerospace industry as there are considerable environmental and economic benefits of using magnesium in constructing aircraft. Firstly, magnesium's excellent castability, machinability, and ductility make it advantageous over other conventional metallic materials. More importantly, using a lighter metal like magnesium in place of heavier metals can improve the aircraft's fuel efficiency and reduce emissions. This translates to savings on fuel and a lower operational cost. To illustrate the savings potential, we can use a standard Boeing 747 passenger model as an example. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Materials** & **Density** & **Compressive** & **Tensile Strength** & **Elastic Modulus** & **Reference** \\  & **(g cm\({}^{3}\))** & **Strength (MPa)** & **(MPa)** & **(GPa)** & **(GPa)** & **(43)** \\ \hline _Magneisum_ & & & & & \\ Pure Magnesium & 1.74 & 20–115 & 90–190 & 45 & [43] \\ AZ31 Alloy (Extruded) & 1.78 & 83–97 & 241–260 & 45 & [43] \\ AZ91D Alloy (Die & 1.81 & 160 & 230 & 45 & [43] \\ Cast) & & & & & \\ \hline _Alternative Metals_ & & & & & \\ Aluminum Alloys & 2.7 & - & 170–560 & 70 & [44,45] \\ Stainless Steel & 7.9–8.1 & - & 480–620 & 189–205 & [43,46] \\ Cobalt-Chrome Alloys & 7.8–9.2 & - & 450–960 & 195–230 & [43,46] \\ Titanium Alloys & 4.4–4.5 & - & 550–985 & 100–125 & [43,46] \\ \hline _Biological Tissues_ & & & & & \\ Arterial Wall & - & - & 0.5–1.72 & 0.001 & [43,47] \\ Skin & - & - & 2.5–16 & 0.006–0.04 & [47] \\ Cancellous Bone & 1–1.4 & 1.5–9.3 & 1.5–38 & 0.01–1.57 & [43,47] \\ Cortical Bone & 1.8–2 & 160 Transverse & 35 Transverse & 5–23 & [43,47,48] \\  & 240 Longitudinal & 283 Longitudinal & 5–23 & [43,47,48] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Selected Mechanical Properties. By substituting all aluminum alloys with magnesium alloys, the aircraft could attain an overall weight reduction of \(\sim\)60.4 tonnes (t) or 28% of its \"operating empty weight\" [50]. The potential benefits of using magnesium in the aerospace industry led to several experiments throughout history where aircraft were constructed entirely with magnesium or magnesium as the base material. Notable examples include the military aircrafts Northrop XP-56, Lockheed F-80, Convair B-36, and Convair XC 99 [51]. Through these experiments, the inherent limitations and critical aspects exhibited from the use of magnesium alloys emerged. It predominantly includes implications arising from its flammability, surface durability, and corrosion resistance. This led to restrictions imposed by various authorities, and consequently, these enactments significantly dented the prospects of magnesium in modern aviation. Presently, with advancements in magnesium research, it is possible to manage these prevailing concerns with maintenance and repairs. However, performing these servicing activities will inevitably drive up its total cost of ownership [51]. Thus, despite significant improvements since the restrictions were first imposed, magnesium is still found mainly in engine- and transmission-related castings and landing gears only and not in external or structural applications [52]. The same is true for helicopters. Today, aluminum and its alloys are still highly favored over magnesium and its alloys for aerospace structures. However, that is set to change with more significant advancements in applied magnesium research, such as its material development and novel manufacturing technologies. After all, if its drawbacks can be mitigated, magnesium's intrinsic material properties will make it ideal for the aerospace industry.