## 2 Experimental Approach
AMC samples was measured using the principle of Archimedes. The sintered density was calculated in accordance with the relationship:  
\[\rho_{AMCs}=\frac{\rho_{l}\times w_{AMCs}}{\Delta w_{AMCs}}. \tag{1}\]  
In this expression, \(\rho_{l}\) is the density of liquid, \(w_{AMCs}\) is the weight of the sintered aluminum matrix composite (AMC) in the laboratory air and \(\Delta w_{AMCs}\) is the difference in the weight of the sintered AMC between the laboratory air and liquid. The variations in the sintered density of the Al hybrid composite for the different sintering conditions is shown in Figure 3. As the density of the reinforcing SiCp and Al\({}_{2}\)O\({}_{3}\)p is inherently higher than that of aluminum, its presence in the material led to a higher density of the aluminum matrix composite (AMC), which could be analyzed using the rule of mixtures theory [4]:  
\[(\rho_{c})=v_{r}\times\rho_{r}+(1-v_{r})\times\rho_{m}. \tag{2}\]  
In Equation (2), \(\rho_{c}\), \(\rho_{r}\) and \(\rho_{m}\) denote the density of the composite, the density of the particulate reinforcement and the density of the matrix. \(v_{r}\) is the volume percent of the particulate reinforcement. We concluded that the density of the engineered aluminum composite varied between the density of the matrix and the density of the particulate reinforcement. Theoretically, this was true, but experimentally the \"green\" and sintered density depended upon the conjoint and mutually interactive influences of the following: (i) the compaction pressure; (ii) the sintering condition; and (iii) the compressibility of the reinforcement particles. Due to this, the \"green\" and sintered density did not follow the trend shown or observed by the theoretical density of the composites. An increase in the addition of the alumina (Al\({}_{2}\)O\({}_{3}\)) particles to the aluminum tended to decrease the density. An excess in the addition of alumina particle reinforcements tended to cause a matrix swelling of the test samples subsequent to sintering. The \"green\" density did not follow the theoretical density for the addition of SiCp and Al\({}_{2}\)O\({}_{3}\)p to the aluminum metal matrix. It was evident that the \"green\" density of the aluminum matrix composite (AMC) compact actually decreased with an increase in the content of both the SiCp (>7 vol.%) and Al\({}_{2}\)O\({}_{3}\)p (>3 vol.%) reinforcing particulates. This is shown in Figure 3. This observation could essentially be attributed to the hard and non-deforming nature of the two chosen particulate reinforcements--i.e., (i) SiCp and (ii) Al\({}_{2}\)O\({}_{3}\)p--which tended to construct the deformation of the reinforcing particles as well as sliding and possible rearrangement during compaction [5; 6; 7]. The sintered density could be improved by increasing both the sintering temperature and time. Figure 3: Effect of addition of SiC\({}_{\text{p}}\)/Al\({}_{2}\)O\({}_{3}\)p on density for the different sintering conditions. The hybrid effect of the reinforcement was studied in the experiments. This effect is shown in Figure 4. The aluminum metal matrix composites (AMCs) containing 2 vol.% SiCp/3 vol.% Al\({}_{2}\)O\({}_{3}\)P and 7 vol.% SiCp/3 vol.% Al\({}_{2}\)O\({}_{3}\)p revealed an observable improvement in hardness. However, vice versa was found to be not true. An increase in the volume fraction of the reinforcing SiCp for a constant volume fraction of Al\({}_{2}\)O\({}_{3}\) particles contributed to increasing the hardness. An increase in the volume fraction of the alumina (Al\({}_{2}\)O\({}_{3}\)) particles in the composite microstructure reduced the sintered density (Figure 3) with a concomitant decrease in hardness, as shown in Figure 4.  
Furthermore, Rockwell hardness tests were carried out on a flat surface of the cylindrical sintered Al/7 vol.% SiCp specimens with a different modulus (Figure 5a). A similar trend in the variation (as observed for densification) of the hardness is shown in Figure 5b with respect to the modulus of the part.