# Advanced Metal Matrix Nanocomposites
## 2 Fabrication Methods
### Stir-Casting  
Stir-casting is a common, economical, and fairly simple method of producing metal matrix nanocomposites wherein the reinforcement particles are incorporated into the molten matrix metal (i.e., aluminum). A stirrer (impeller) is inserted into the molten metal and applies mechanical stirring with some rpm to get a uniform distribution of the particles with the molten metal. This process has been extensively used to incorporate ceramic, carbon nanotubes, graphene, and metal oxide particles to magnesium and aluminum matrices [22; 23; 24; 25]. The main challenges of this technique are (i) the tendency of nanoparticles to cluster due to high surface area and resulting high van der Waals forces between them; (ii) poor wettability of the solid nanoparticles with the molten metal; and (iii) high density of porosity due to possible entrapment of the air induced by the rotating stirrer [26; 27; 28]. Figure 1 shows the schematic of the stir casting process. [PERSON] and [PERSON] [23] studied the stir casting fabrication of A356--1 wt% SiC (aluminum powder with average grain size of 45 \(\upmu\)m and SiC nanoparticles with the average size of 20-50 nm) nanocomposite wherein a specific stirring rate and temperature were identified to secure a great combination of mechanical properties; the ideal combination of stirring temperature and stirring rate were 750 \({}^{\circ}\)C and 700 rpm, respectively.  
[PERSON] et al. [30] assessed the microstructure and mechanical properties of Al6061-nanocomposite, reinforced with nano-Al\({}_{2}\)O\({}_{3}\) particles (0.5, 1, and 1.5 wt%), produced by stir-casting. Upon casting, they extruded the nanocomposite materials at 550 \({}^{\circ}\)C and observed enhanced strength and ductility in the extruded samples compared with un-reinforced samples; increase in yield strength and tensile strength resulted with addition of nano-Al\({}_{2}\)O\({}_{3}\) while reduction in ductility. [PERSON] et al. [31] studied the mechanical properties and the microstructure of A356 aluminum alloys reinforced with 1 to 5% nano-SiC particle produced by the stir casting process with stirring at 550 \(\pm\) 50 rpm, stirring duration of 10 min, and pouring of the molten slurry at 680 \(\pm\) 20 \({}^{\circ}\)C increase of 41% in yield strength and 45% in ultimate tensile strength (UTS) was described.  
[PERSON] et al. [32] assessed the mechanical behavior of AZ31/Al\({}_{2}\)O\({}_{3}\) magnesium alloy nanocomposites prepared using ultrasound-assisted stir casting (nano-sized alumina powder with the average size of 30 to 70 nm) with different wt% of alumina nanoparticles (0.5, 1.0, and 2.0 wt%). The melt was subjected to ultrasonic treatment (UST) under two different conditions, i.e., UST performed outside the furnace during air-cooling and nearly isothermal (Iso) UST within the furnace (see Figure 2). Grain refinement was attributed to the improved heterogeneous nucleation due to the addition of reinforcing particles. The reinforcing agent dispersed by UST greatly improved the mechanical response of the fabricated composite material (Figure 3). The coefficient of thermal expansion (CTE) as well as Orowan strengthening mechanism were the two factors contributing to the strength of the produced composites. The extensive fracture analyses revealed quasi-static cleavage fracture (brittle nature) in the iso-UST composite materials.  
Figure 1: Typical stir-casting experimental setup [29].