## 3 Results and Discussion  
### Relative Density (RD)  
As tabulated in Table A1 (Appendix A), the RD of the cold-compacted Al-4 Cu/(Al\({}_{2}\)O\({}_{3}\))\({}_{\rm x}\) at different processing parameters after sintering was experimentally measured. It was revealed that increasing the compaction pressure from 500 MPa to 900 MPa and additionally decreasing the H/D value from 2 to 0.75 resulted in a significant increase in the RD. However, increasing the wt.% of the Al\({}_{2}\)O\({}_{3}\) showed a different performance.  
For Al/4 Cu at an H/D = 1.5, increasing the compaction pressure up to 500 MPa enhanced the RD to 85.27%. Increasing the pressure up to 600 MPa resulted in an RD increase of 0.23%. A further compaction pressure increases up to 900 MPa resulted in a 0.2%  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Response**} & \multirow{2}{*}{**F-Value**} & **Model** & **Adeq Precision** & \multirow{2}{*}{**R\({}^{2}\)**} & \multirow{2}{*}{**Adjusted R\({}^{2}\)**} & \multirow{2}{*}{**Predicted R\({}^{2}\)**} \\  & & **Significant** & **(Ratio \textgreater{} 4)** & & & \\ \hline Relative density (RD\%) & 78.25 & \textless{}0.0001 & 28.8251 & 0.9031 & 0.8915 & 0.8752 \\ Hardness (Hv) & 276.02 & \textless{}0.0001 & 56.9531 & 0.9849 & 0.9814 & 0.9755 \\ Yield strength (\(\sigma_{y}\)) & 184.74 & \textless{}0.0001 & 47.7030 & 0.9700 & 0.9647 & 0.9559 \\ Compression strength (\(\sigma_{\rm c}\)) & 122.52 & \textless{}0.0001 & 39.3438 & 0.9554 & 0.9476 & 0.9352 \\ Fracture strain (\(\varepsilon_{\rm f}\)) & 190.89 & \textless{}0.0001 & 44.7085 & 0.9784 & 0.9732 & 0.9639 \\ \hline \hline \end{tabular}
\end{table}
Table 2: ANOVA results of AMC’s responses.  
increase in the RD compared to 600 MPa. However, for an H/D = 1.25, the RD was 88.96% at a compaction pressure of 500 MPa, and further increases in the compaction pressure up to 600 and 900 MPa resulted in increases in the RD of 0.04% and 0.067%, respectively. Additionally, a decrease in the H/D to 1 compacted under 500 MPa caused the RD to increase to 89.85%, and increases in the compaction pressure to 600 and 900 MPa led the RD to increase by 0.05% and 0.17%, respectively. A further decrease in the H/D to 0.75 significantly increased the RD to 90.3% as the compaction pressure increased to 600 MPa. Moreover, cold compaction under a pressure of 900 MPa resulted in a further increase of 0.23% in the RD compared to that at 600 MPa.  
However, strengthening the Al-4 Cu with a 2.5 wt.% of Al\({}_{2}\)O\({}_{3}\) resulted in an increase in the RD to 90.45% at 600 MPa and an additional increase of 0.266% when compacted under 900 MPa compared to 600 MPa. In addition, the RD reached 91.44% and -92% with a further increase in the Al\({}_{2}\)O\({}_{3}\) to 5% compacted under 600 MPa and 900 MPa, respectively. Furthermore, increasing the wt.% of Al\({}_{2}\)O\({}_{3}\) to 7.5 and 10% under a compaction pressure of 600 MPa resulted in the RD decreasing to 90.27 and 89.67%, respectively.  
The RD of the studied Al-Cu/(Al\({}_{2}\)O\({}_{3}\))\({}_{\rm x}\) was increased (up to 5 Al\({}_{2}\)O\({}_{3}\) wt.%), then decreased. Thus, a reduction in the porosity occurred at Al\({}_{2}\)O\({}_{3}\) up to 5 wt.%, accompanied by a further rise with the Al\({}_{2}\)O\({}_{3}\) increase, which can be attributed to the uneven dispersion of the Al\({}_{2}\)O\({}_{3}\) powder in the Al-4 Cu matrix, in good agreement with [59]. Therefore, additional interparticle friction and agglomeration impeded the particles' rearrangement. The Al\({}_{2}\)O\({}_{3}\) uniform embedding and dispersal were limited to the 5 wt.% addition of Al\({}_{2}\)O\({}_{3}\) because of Al\({}_{2}\)O\({}_{3}\) particle agglomeration, which delimited the contact area amongst the matrix particles, restricted the rearrangement of particles, and prevented the Al\({}_{2}\)O\({}_{3}\) from being sufficiently absorbed into the Al-4 Cu powders.