Preparation of composite: The preparation of samples was carried out using planetary ball milling set up for the purpose of uniform mixing of Al6061 and nano-silica powders. The set speed and time only regulated the proper mixing by avoiding formation residual stresses. The coincidence of powder rotation speeds and less amount time led to form a different compound as shown in Figure 3. A possible cause is the uneven rotation speed of the ball mill. The reasons behind the compound formation were analyzed with XRD and morphological studies. (i) The mesoporous structure formed with oxygen content in nano-silica. (ii) The pre-sintering process was carried out during the ball milling process to form a strong structure. (iii) Uncrystallized compounds were formed with porous structure leading to mesoporous structure [10,13,17].
Elastic behavior:
Yield strength of Al-nano silica composite was increasing up to 10wt% of nano silica content as represented graphically in Figure 6, this was due to the presence of lesser parts of silica content leads to enhance the strength and obstructing crack propagation by powder particles of Al6061 and nano silica. This obstructing phenomenon is due to Orowan strengthening mechanism. On the other hand, at higher silica content of beyond 10wt% nano silica content, the same silica particles make the ductile Al6061 matrix to brittle matrix by weakening bonds of Al6061 and nano silica powders. This trend was also strengthened by XRD and SEM graphs, which were represented in Figure 1 & Figure 7. Compressive strength of Al-Si composites was increasing till 12wt% of sample, as powders have highest compressibility at axial loads under UTM as shown in figure 4. At the mid portion of sample in height, the Al-Si composite was in stable metal form but in bulged shape, beyond that the samples started breaking. The incremental percentage of compressive strength of AlSi-12 is 182% compared to AlSi-0 composite as shown in figure 12. The similar trend of yield strength with AlSi-10 is 92% compared to AlSi-0 composite as shown in figure 6. The incremental values of compressive and yield strength were correlating to the behavior of brittle. It was also evident that the basic nature of Al6061 ductile was not lost by retaining the high compressive strength. This high compressive strength is suitable for bearing applications of Al6061-nano silica composite. However, the addition of nano silica ceramic particles is limited to 10wt% for optimum utilization.
Densification and Porosity:
Powder’s densities were calculated before and after the sintering process. Relative and sintered densities were represented in figure 8 and figure 9, respectively. The values showed the incremental percentage at AlSi-10 for relative density of 1.5%, whereas for sintered density at AlSi-12 of 11% growth compared with AlSi-0 composite. The incremental values were also due to following reasons: (i) The Al6061 particles mesh size and nano silica mesh size forms the solid solution strengthening due to interstitial movement of atoms during the sintering process. (ii) Sintering temperature of 5500C for all samples at a period of 3hrs was created enough period to relieve the stresses generated during the ball milling process and those were filled by nano silica particles. (iii) The ratio of nano silica particles also has an effect in filling porousness created during the pre-sintering process of powders. (iv) The pre-sintering process was carried out to match thermal equilibrium of nano silica particles and Al6061 particles, however the process of pre sintering also increased the reaction time for the oxidation process of Al6061 [15,16,21]. The Porosity in the Al6061 and nano silica powders were allowed as it is intended for applications with impregnated bearings.
Hardness:
The hardness of AlSi-12 was incremental at a percentage of 21 compared with AlSi-0 composite. The sacrificial yield strength of 8% for AlSi-10 was not remarkable with gain of hardness value to 21% when compared with AlSi-0 composite were represented in figure 11. This was also due to the presence of hard and brittle particles of nano silica content [9,22–24]. The presence of nano silica particles in the soft matrix of Al6061 created the hard and strong bonds. The movement of grains were obstructed by the nano silica particles at pressed locations. The average of 5 locations were taken for measurement, where the dislocation of grains was found to be hard. The reasons behind the better hardness values are (i) Dislocations of grains on the top surface is obstructing by the nano silica particles, (ii) The hard and brittle grains were formed on the surface during sintering process, (iii) The sintering process was also created heat treatment environment for Al6061 nano silica samples [25–27].
Morphological studies and X-Ray Diffraction:
The images of samples also reveal the clusters and aggregation of nano silica particles, which were mentioned in Figure 1. In Figure 1(a) The white particles represent the presence of nano silica particles [28]. The images were captured on broken specimens of flexural test, the behavior crack was quite evident from the boundaries of white regions of nano silica particles. Sintering time was also found to be a major influencing factor in achieving the crystallinity of the composite [9,29]. The reasons for agglomeration of particles is due to (i) with incremental values of nano silica particles, the similar behavior materials were cohesively attracted in turn formed the clusters of nano silica particles, (ii) The wettability of Al6061 and at AlSi-12wt% nano silica particles during ball milling process was created the agglomerations, though movement of tungsten carbide balls in milling process, (iii) More the wt% of nano silica particles more the volume of nano silica particles, which is dominating the less volume content of Al6061 though at highest wt% of Al6061, as reinforcements taken were in nano size [8,14].
Flexural rigidity: Fracture toughness value of composite was following the trend of incremental with percentage change of 28% for AlSi-12wt% compared with Pure Al6061 samples were represented in figure 13. These incremental values were due to the fracture load bearing behavior of nano silica particles in the matrix of Al6061. Three point bending tests were performed based on a bending moment equation for rectangular specimens applying point loads at mid portion by creating reaction loads at opposite sides of Al6061-Silica composite. As the sample is a mixture of Al6061 and nano silica powders, less indentation was noticed on the loading side at mid portion whereas other surfaces were facing the tensile load. Powders are much elongated during the sintering process, the ductility behavior was quite low compared with the conventional casted piece [30,31].
Strengthening Mechanism: Increasing the values of mechanical properties for Al6061 and nano silica composite was observed in experimental, and those values were theoretically analyzed with Orowan strengthening mechanism. The enhancement of properties was observed due to the obstruction of crack propagation under loading conditions [32–35]. The obstruction of crack was due to the grain refinement and dislocation movement during compaction and sintering process, it implies that Orowan mechanism has effect in this aspect. The improved values of sintered density at ALSi-12 of 11% growth compared with the AlSi-0 composite was noted. The hardness of AlSi-12 was incremental at a percentage of 21 compared with AlSi-0 composite. The sacrificial yield strength of 8% for AlSi-10 was not remarkable with gain of hardness value to 21% when compared with AlSi-0 composite.