Structural and Mechanical Characterization of Aluminum Matrix Nanocomposites Reinforced with SiC and Al2O3 Nanoparticles

Nowadays, aluminum matrix composites (AMCs), as part of metal matrix composites (MMCs), are highly demanded materials in many engineering applications. AMCs have superior characteristics [1-2]. With the improvement of nanoscience and nanotechnology, MMCs reinforced by nanoparticles (MMnCs) are recently being considered worldwide because of their talented properties that fit a great number of functional and structural applications [3]. Choosing the processing method to produce this material is affected by many factors, including the type of reinforcement and matrix, the required mechanical and thermal properties, and the extent of microstructural integrity desired [1, 3, 4]. In this study, stir casting was chosen for performing the task. Annigeri and Kumar [5] gave a detailed description of stir casting and showed that this method is simple, flexible and low cost with mass production. The main drawback of this method is the difficulty to achieve a homogenous composite, which can be improved by the use of modified stir casting, nanosized particles or both. The modified methods include ultrasonic stir casting [6-8], vacuum stir casting [6], hot water treatment of the particles, squeeze casting [9], and powder metallurgy processes [10, 11]. On the other hand, the improvement can be gained by decreasing the size of particles because the use of nanosized particles has the tendency of agglomeration and clustering, due to high surface energy, electrostatic, moisture adhesiveness and attractive Vander Waal’s bonding, which affect its uniform distribution during processing [11]. The characteristics of final composites are affected by the mixing ratio, the processing parameters, the heat treatment schedule and other factors [12-18]. Enormous studies have treated the effect of different parameters on the performance and characteristics of AMCs and AMNCs [1-27]. Most of these studies have investigated the mechanical properties of composite material, with rare research concerning damping property conducted [19-27]. From those studies, it can be concluded that the research on determining the damping of nano reinforcement particulates in AMNCs is limited. This study investigates the effect of nano Al₂O₃ and nano SiC reinforcement on the damping and some other mechanical properties by focusing on the effect of the weight ratio of the particulate. This study aims to:

a) fabricate many specimens of the aluminum matrix with different types of nanoparticles.

b) compare nanoparticles in terms of their effect on mechanical proprieties.

c) find the damping ratio for AMNCs.

In summary, this study aims to determine the effect of nanoparticles on mechanical and structural properties (fracture toughness, hardness and damping) of AMNCs.

3.1 Vibration tests

The damping coefficient of the composite cantilever was calculated according to the amplitude attenuation ratio of the free vibration attenuation curve. The logarithmic decrement (δ) for underdamped free vibrations on n cycles was found from the following equation:

$\delta=\frac{1}{n} \ln \left(\frac{x(t)}{x\left(t+n T_d\right)}\right)$                      （1）

The equivalent damping ratio was then determined from the following equation:

$\zeta=\frac{\delta}{\sqrt{\left.1-\delta^2\right.}}$                  (2)

The logarithmic decrement was measured graphically by experiment for many cycles in order to get more accurate results (Figure 3). The results of the tests are shown in Table 5 and Figures 3 and 4.

In such an experiment, results can be affected by many factors like the length and other dimensions of the beam, the distance between the spring and the support point, the stiffness of the spring and the material of the beam. All of these factors were kept constant except the material composition.

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Figure 3. One example of time-response used for damping calculation with the logarithmic decrement method

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Figure 4. Plot of the average damping ratio (Zeta) for different weight ratios of Al₂O₃

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Figure 5. Plot of the average damping ratio (Zeta) for different materials

The experimental damping ratio ($\zeta$) values ranged between 0.01 and 0.07, which are the same as those in the literature. Figure 4 shows the effect of the Al₂O₃ weight ratio on the beam damping. At a low ratio (less than 1%), the damping was lower than that of Al 6061 and this negative effect can be referred to poor manufacturing. After that ratio, the damping of the AMNCs became higher than the origin and reached the best value (about twice of the Al 6061) at the ratio of around 2%. As indicated in several studies [10, 25], the reinforcement weight fraction increased beyond a limit, and the composite showed a decline in characteristics due to the clustering and agglomeration of nanoparticles. The method must be improved in order to reduce the porosity and increase the homogenous distribution of nanoparticles.

This study compares between Al₂O₃ and SiC as nanoparticle reinforcement of Al 6061 with the same ratio (1.5 weight %), as shown in Figure 5. The Al₂O₃ obtained a higher damping ratio than SiC.

3.2 Hardness tests

Five readings were used for each sample composition at different positions in order to give more realistic values. Their average values were found and recorded. The coefficient of variation (CV), sometimes called relative standard deviation, of the five readings was calculated and found to be acceptable. As a general guideline, a CV less than 100% reflects low variability, while values greater than 100% reflect high variability. The results of these tests were shown in Table 6.

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Figure 6. Plot of the average surface hardness (HRB) for different weight ratios of Al₂O₃ particulates

Table 6. Hardness test results of different samples

Figure 6 shows the results of hardness tests conducted on the Al 6061 alloy and the AMNCs containing different weight percentages of Al₂O₃ particles. The hardness increased when more particles were added, clearly indicating that the presence of particulates in the matrix can improve the hardness of the composites. This agrees with the theory because the ceramic particles are harder than aluminum. When the AMNCs were compared with the pure material, the hardness values were less than those of the pure material until the weight ratio reached around 2%. After this ratio, the hardness of the AMNCs became higher than that of the Al 6061. According to the theoretical expectations, the hardness of the AMNCs must be higher than the original material in all ranges, but some practical errors reduced these values. Most of these errors belonged to the different parameters and techniques of the stir casting method.

Figure 7 shows a comparison between SiC and Al₂O₃ as nanoparticle reinforcement of Al 6061 with the same weight ratio (1.5%). As observed, the SiC composite exhibits higher hardness than the Al₂O₃ composite. However, both composites demonstrate lower hardness values compared to pure Al 6061, as previously discussed.

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Figure 7. Plot of the average surface hardness (HRB) for different materials

3.3 Impact strength tests

The Charpy impact test was used to assess the toughness and notch sensitivity of the samples. The energy absorbed by the material during fracture was measured, with brittle materials absorbing minimal energy and ductile materials absorbing a significantly higher amount before fracture.

Five different samples were used for each sample composition and their averages were found. The CV of the five readings around the averages was computed and found to be satisfactory. The results of the tests were shown in Table 7.

Table 7. Charpy impact test results of different samples

Figure 8 shows the toughness values for different ratios of Al₂O₃. The toughness increased slightly with the increase of the weight ratio. The toughness value of AMNCs was less than that of the pure material, which comes to an agreement with the role of mixture and the theoretical expectations. The degree of reduction is more than theory, which proves that the samples with reinforced particles become more brittle due to porosity formation. Well-controlled production can improve this drawback. The properties of AMNCs are also dependent on both the sintering temperature and time. Several studies have suggested that ultrasonic-assisted stir casting followed by squeeze casting can effectively reduce the porosity and increase the homogenous distribution of nanoparticles [6-8, 25]. One more problem is the relatively poor wettability of nanoparticles. This has been partially solved by heating the nanoparticles to high temperatures before mixing. Further improvements happened when the wetting agent was used.

Figure 9 shows a comparison between SiC and Al₂O₃ as nanoparticles reinforcement of Al 6061 with the same weight ratio (1.5%). As indicated in the figure, the toughness of the SiC composite is higher than that of the Al₂O₃ composite. However, it should be noted that the toughness values of both composites are significantly lower than that of the pure Al 6061.

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Figure 8. Plot of the average surface roughness for different weight ratios of Al₂O₃

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Figure 9. Plot of the average surface roughness for different materials

The test results showed that the impact energy of Al/Al₂O₃ and Al/SiC composites were mainly depended on the distribution of the particles in the matrix.

The presence of the ceramic particles in the alloy negatively affected the toughness value in which the composite became brittle. However, with the increasing percentage of the ceramic particles, the behavior of the composite changed in which the toughness value increased along with the particles, but it was still less than the desirable value and was still brittle.