s achieved in the composites with some agglomerations. Figure 4 and Figure 5 present the SEM micrographs of the 5.0 wt% and 7.5 wt% Al2O3 red mud particle-reinforced composites in which the particle clustering and agglomeration are clearly shown. Also, Figure 6 shows the SEM micrograph of the 10 wt% Al2O3 red mud particle-reinforced composite which shows a uniform distribution of reinforcement particles.
As a result, SEM observations of the microstructures revealed that the dispersion of the coarser sizes was more uniform while the finer particles led to agglomeration and segregation of the particles, and porosity. The
Figure 1. The variation of theoretical and experimental density with Al2O3 red mud particle content.
Figure 2. The variation of porosity with Al2O3 red mud particle content.
Figure 3. SEM micrograph of 2.5 wt% Al2O3 red mud particle-reinforced composite.
Figure 4. SEM micrograph of 5.0 wt% Al2O3 red mud particle-reinforced composite.
Figure 5. SEM micrograph of 7.5 wt% Al2O3 red mud particle- reinforced composite.
reason for the particle segregation is proposed as follows: the Al dendrites solidify first during solidification of the composite, and the particles are rejected by the solid-liquid interface, and hence, are segregated to the inter- dendritic region. This event occurred more easily with the finer particles  .
3.4. Hardness and Tensile Strength
Hardness tests were performed on a rock well hardness tester and the results of the tests are shown in Figure 7.
Figure 6. SEM micrograph of 10 wt% Al2O3 red mud particle-reinforced composite.
Figure 7. The variation of hardness (HRB) with Al2O3 red mud particle content.
Figure 8. The variation of tensile strength with Al2O3 particle content.
As shown, hardness increases with the amount of reinforcement particles present.
The results of the tensile strength tests are given in Figure 8. Which shows the variation of tensile strength with the weight fraction of reinforcing particles? As shown here, the tensile strength of the MMCs increased with increasing amount of particles like the hardness. Among all of the MMCs, the composites reinforced with 10% Al2O3 particles have the maximum hardness and tensile strength. Compared with the 6061 Al matrix alloy, the tensile strength and hardness of the MMCs are greater, and the addition of (red mud + Al2O3) particles has increased the tensile strength and hardness of the Al alloy. The increase in filler content contributes to the increase in tensile strength. The reason for the increase in tensile strength in spite of increase in porosity is the thermal mismatch between metal matrix and the reinforcement. This is a major mechanism for increasing the dislocation density of the matrix and therefore increasing the composite strength.
The optimum process conditions for production of Al2O3 red mud particle-reinforced aluminum alloy composites by the vortex method and subsequently extruded are found in the present work. SEM microstructures, density and porosity, and the tensile strength and hardness of MMCs were investigated. The following conclusions have been drawn.
1) AA6061 MMCs reinforced with different weight percentages of (Al2O3 + red mud) particles (up to 10 wt%) have been successfully fabricated by compocasting. The optimum conditions of the production process were that the pouring temperature was 710˚C; stirring speed was 450 RPM, and the stirring time after the completion of particle feeding was 5 min.
2) SEM observations of the microstructures showed that the coarser particles were dispersed more uniformly, while the finer particles led to agglomeration and segregation of particles and porosity.
3) The density and porosity of the composites increased with increasing weight percentage of reinforcement.
4) The tensile strength and hardness of MMCs increased, with increasing weight percentage of the reinforcement.
 Lloyd, D.J. (1994) Particle Reinforced Aluminum and Magnesium Matrix Composites. International Materials Reviews, 39, 1-23. http://dx.doi.org/10.1179/imr.19188.8.131.52
 Bayoumi, M.A. and Suery, M. (1988) Partial Remelting and Forming of Al-Si/Sic Composites in Their Mushy Zone. In: Fishman, S.G. and Dhingra, A.K., Eds., Proceeding of International Symposium on Advances in Cast Reinforced Metal Composites, ASM International Publications, Materials Park, 167-172.
 Kattamis, T.Z. and Cornie, J.A. (1988) Solidification Processing of Particulate Ceramic-Aluminum Alloy Composites. In: Fishman, S.G. and Dhingra, A.K., Eds., Cast Reinforced Metal Composites, ASM International Publications, Materials Park, 47.
 Lloyd, D.J. (1989) The Solidification Microstructure of Particulate Reinforced Aluminum/SiC Composites. Composites Science and Technology, 35, 159-179. http://dx.doi.org/10.1016/0266-3538(89)90093-6
 Gibson, P.R., Clegg, A.J. and Das, A.A. (1985) Production and Evaluation of Squeeze Cast Graphitic Al-Si Alloy. Journal of Materials Science and Technology, 1, 558-567.
 Rohatgi, P.K. (1991) Cast Aluminum Matrix Composites for Automotive Applications. Journal of Metals, 43, 10-15. http://dx.doi.org/10.1007/bf03220538
 Dellis, M.A., Keastenrmans, J.P. and Delannay, F. (1991) The Wear Properties of Aluminum Alloy Composite. Materials Science and Engineering, 135A, 253-257. http://dx.doi.org/10.1016/0921-5093(91)90572-5
 Joshi, S.S., Ramakrishnan, N., Sarathy, D. and Ramakrishnan, P. (1995) Development of the Technology for Discontinuously Reinforced Aluminum Composites. The First World Conference on Integrated Design and Process Technology, 1, 492-497.
 Das, A.A., Clegg, A.J., Zantont, B. and Yakouh, M.M. (1988) Solidification under Pressure: Aluminium and Zinc Alloys Containing Discontinuous Sic Fibre. In: Fishman, C.G. and Dhingra, A.K., Eds., Proceedings of the Cast Reinforced MMCs, ASM International Publications, Materials Park, 139-147.
 Prasad, B.K., Jha, A.K., Modi, O.P., Das, S. and Yegneswaran, A.H. (1995) Abrasive Wear Characteristics of Zn— 37.2, Al—2.5, Cu—0.2Mg Alloy Dispersed with Silicon Carbide Particles. Materials Transactions, JIM, 36, 1048- 1057. http://dx.doi.org/10.2320/matertrans1989.36.1048
 Pramanik, A., Zhang, L.C. and Arsenault, J.A. (2007) An FEM Investigation into the Behavior of Metal Matrix Composites: Tool-Particle Interaction during Orthogonal Cutting. International Journal of Machine Tools and Manufacture, 47, 1497-1506. http://dx.doi.org/10.1016/j.ijmachtools.2006.12.004
 McCoy, O.W. and Franklin, E.W. (1988) Dendiritic Segregation in Particle-Reinforced Cast Aluminum Composites. In: Fishman, S.G. and Dhingra, A.K., Eds., Proceedings of the International Symposium on Advances in Cast Reinforced Metal Composites, ASM International Publications, Materials Park, 237-242.