In the present study, the damping behavior of hybrid composites has been investigated using dynamic mechanical analyzer (DMA). The composites were fabricated with 2, 4, 6, and 8% by weight of rice husk ash (RHA) and SiC in equal proportions using two stage stir casting process. Damping measurements of all the specimens were obtained by dynamic mechanical analyzer (DMA) at different frequencies in air atmosphere. Scanning electron microscope (model JSM-6610LV) was used to study the microstructural characterization of the hybrid composites. It was observed that the dislocation density, which results from the thermal mismatch between the reinforcement and the matrix and the porosity of composites, has a great influence on the damping capacity of hybrid composites. The dislocation damping mechanisms were discussed with regards to the Granato–Lucke theory.

The incorporation of different reinforcements into a matrix has led to the development of hybrid composites. Hybrid composites are becoming better substitutes for the conventional alloys because of characteristics like high stiffness, high strength and low density. Aluminum matrix composites with multiple reinforcements (hybrid composites) are finding increased applications because of improved mechanical and wear resistance, and hence are better substitutes for single reinforced composites. Hybrid composites have unique features that can be used to meet various design requirements in a more economical way than conventional composites. Also, hybrid composites provide a combination of properties such as tensile modulus, compressive strength and impact strength, which cannot be realized in composite materials. In recent times, hybrid composites have been established as highly efficient, high performance structural materials and their use is increasing rapidly. Hybrid composites are finding wide applications where high wear resistance is of importance [1]. However, at present, hybrid composites are using as bearing materials and turbine blades.

The effect of hybrid reinforcements on the microstructure and mechanical properties of pure magnesium has been investigated by Sankaranarayanan et al. [2]. Nanoscale alumina and titanium particulates were used as reinforcements. From their study, it can be concluded that the addition of reinforcement leads to significant grain refinement and exhibit higher microhardness, tensile and compressive properties when compared to monolithic magnesium. According to Akbarpour et al. [3] the addition of nanosized silicon carbide reinforcement lowers the grain growth rate and enhances the homogenization. The aging behavior and the mechanical properties of (SiCp+Ti)/7075Al hybrid composites has been investigated by Liu et al. [4]. Results confirmed that the precipitation hardening of the hybrid composites was delayed during the aging process. Also, an increase in the tensile strength of the hybrid composites was observed because of the precipitation hardening of the matrix alloy. A356 alloy modified with 0.03 mass% of strontium was reinforced with silicon carbide micro particles and graphite macro particles (Grp) to fabricate hybrid A356/SiCp/Grp composites via compo-casting technique and studied the aging behavior by Ilija Bobić et al. [5]. The results demonstrated that the composites reached maximum hardness, faster than the thixocast A356 alloy and time required to attain peak hardness decreases with the increase in the percentage of reinforcements. The strength of (SiCp+Ti)/7075Al hybrid composites with and without addition of Ti particles has been investigated by Weiping Chen et al. [6]. Results demonstrated that the strength was improved significantly with Ti addition, whilst their ductility was decreased. The thermal expansion behavior of micro-/nano-sized Al2O3 particles reinforced hybrid composite has been investigated by Zhibo Lei el al. [7]. The results revealed that the nanoparticle concentration had significant effect on the thermal expansion behavior of the composites. Surface integrity studies while drilling metal matrix and hybrid metal matrix composites has been performed by Rajmohan et al. [8]. Drilling tests were carried to investigate the effect of the various cutting parameters on the surface quality and the deformation of drilled surface due to drilling.

The fabrication of hybrid composites with low cost reinforcement would minimize the cost of the product with enhanced properties. Rice husk ash is one of such reinforcement, an agricultural waste byproduct, which is gaining more importance in recent days as a secondary reinforcement in the fabrication of composites. The advantages of using RHA is to produce low cost by-product thereby, reducing the cost of aluminum products [9,10], readily available with less cost, and often lower densities in comparison with most technical ceramics (such as boron carbide, alumina). In recent years, many researches have been reported the potentials and limitations of the use of RHA as reinforcement [11,12]. Prasad and Krishna [13] reported that the damping capacity increases with the addition of RHA particulates and increases further with heat treatment. Srikanth and Gupta [14] reported that the damping capacity of the pure magnesium matrix was enhanced with the addition of SiC particulates, and increases with the increase of the proportion of SiC particulates. Sudarshan and Surappa [15] showed that the addition of fly ash, an industrial waste byproduct into A356 exhibited improved damping capacity compared to base alloy. Zhang et al. [16] studied the damping behavior of SiC and graphite reinforced metal matrix composites. They reported that the damping capacity of aluminum could be significantly improved by the addition of either SiC or graphite particulates. The influence of CaO on damping capacity has been reported by Jang et al. [17]. They reported that Mg–CaO alloy can be regarded as a cost-effective damping material with enhanced mechanical properties. According to Schaller [18], an elegant way to reduce mechanical vibrations is to use high damping materials and this can be achieved by incorporating reinforcement in the matrix. A detailed study on the damping behavior of metal matrix composites has been studied and presented by Prasad and Shoba [19].

From the above literature, it is clear that several researchers carried investigations on hybrid composites; however damping behavior of hybrid composites is hardly seen. Hence, the present study aims at finding the damping characterization of Al/SiC/RHA hybrid composites at different frequencies using dynamic mechanical analyzer with an objective to develop high damping materials.

2ExperimentationIn the present work, SiC and RHA particulates were used as reinforcements and A356.2 was used as a matrix material. The chemical compositions of RHA and base alloy A356.2 are given in Tables 1 and 2 respectively. Pre-treatment was carried to RHA particulates before incorporating into the molten metal, to remove inorganic matter and carbonaceous material [13]. The reinforcement particulates were preheated to 700–800°C for 1h before incorporation into the molten melt to remove moisture. 1% by weight magnesium was added in the molten metal to improve the wettability between the matrix and the reinforcements. The detailed fabrication process of the hybrid composites was presented in earlier works [9]. Using this process, 2, 4, 6 and 8% by weight in equal proportions of RHA/SiC particle-reinforced hybrid composites were fabricated. Microstructural characterization of the hybrid composites was examined using scanning electron microscope (Model: JSM-6610LV) and optical microscope (OLUMPUS). The damping measurements were performed using a GABO Eplexor dynamic mechanical analyzer at frequencies ranging from 1Hz to 15Hz at room temperature using three point bending mode. All the damping experiments are performed at a static load of 50N, a dynamic load of 40N, and at constant strain amplitude (??) of 1×10−5. The schematic diagram of the experimental set up with three point bending arrangement is shown in Fig. 1. The damping capacity, i.e., tan δ as a function of frequency, was recorded. The samples of dimensions 30×12×1·5mm3 for damping measurements were cut from Ultra cut 843/Ultra cut f2 CNC wire electric discharge machine.

Fig. 2 shows the scanning electron micrograph of the hybrid composite. Micrograph of hybrid composites showing clearly the uniform distribution of RHA and SiC in the matrix. Fig. 3 shows the variation of damping capacity with the frequency for the unreinforced alloy. It was observed that the damping capacity of the base material was found to be 0.00549 at 1Hz, which indicates low damping for A356.2 alloy. Also, it could be observed that the damping capacity increases with the increase in the frequency. Fig. 4 shows the variation of damping capacity with frequency for different weight percentage of the reinforcement. From the plot it could be observed that the damping capacity increases with the increase in the % reinforcement. An increasing trend for damping capacity has been observed with frequency for 2, 4, 6 and 8% reinforced hybrid composites. Also, it was observed that the damping capacity for the 2% reinforced composites show similar trends as unreinforced alloy, however with no significant increase in the damping capacity.

It was noticed from the literature that the damping capacity of hybrid composites (in the present study) is more than single reinforced composites. To explain this behavior a comparison was made between the damping capacities of Al/RHA composites [13] and hybrid composites. Fig. 5a–c shows the variation of damping capacity with frequency for Al/RHA composites and Al/SiC/RHA hybrid composites. It was observed that hybrid composites exhibit higher damping capacity than Al/RHA composites for all % of reinforcement studied herein. Similarly, a comparison was made between the damping capacity of Al/SiC composites [20] and Al/SiC/RHA hybrid composites at 1Hz. Results of Ranjit Bauri [20] showed that the damping capacity of unreinforced aluminum alloy is 0.034 and an increase of maximum 50% was reported for 18% reinforced composites. The corresponding values are tabulated in Table 3. However, from the present study, the damping capacity of hybrid composites with 8% reinforcement increases by 3 times than the unreinforced alloy. Hence, it can be concluded that the hybrid composites exhibit higher damping capacity than monolithic alloy and single reinforced composites. The increase in damping capacity can be attributed to the following reasons:

Damping Capacity at 1Hz [20].

Unreinforced alloy | 8% SiC reinforced composite | 12% SiC reinforced composite | 18% SiC reinforced composite |
---|---|---|---|

0.034 | 0.037 | 0.046 | 0.051 |

Metal matrix composites are characterized by a large difference in the thermal expansion coefficient (CTE) of the matrix and the reinforcement (CTE of A356.2 is 21.4×10−6/°C, the CTE of RHA is 10.1×10−6/°C and the CTE of SiC is 4.3×10−6/°C). Even small temperature changes, generate thermal stresses in the aluminum matrix. These stresses can be partially released by dislocation generation in the vicinity of the interface. Thus, the dislocation density generated can be quite significant at the interface and can be predicted using the model of Taya and Arsenault [21] based on prismatic punching of dislocations at a ceramic particulate. The dislocation density ρ at the interface is given by Eq. (1)

For hybrid composites Eq. (1) can be modified as

where B is a geometric constant that depends on the aspect ratio (it varies between 12 for equiaxed particulate and 4 for whisker-like particulate), ?? is the thermal mismatch strain (the product of temperature change ΔT, during solidification of MMCs and CTE difference, Δα, between the reinforcement and matrix), Vr is the volume fraction of the reinforcement, b is the burgers vector, d is the average grain diameter of reinforcements.The CTE of the composites is relatively difficult to predict because it is influenced by several factors, which includes the internal structure of the composite, plasticity, etc. However, there are several analytical methods to predict CTE of the composites, which includes simple rule of mixtures and thermo-elastic energy principles like Kerner, and Turner models. Based on the Kerner model the CTE of the composites was predicted and presented in Table 4. The detailed calculations were presented in earlier works [9].

The dislocation density for the hybrid composites were then calculated based on Eq. (1) with an assumption for the burgers vector of 0.32nm for Al [13] and are tabulated in Table 4. From Table 4 it was observed that the dislocation density increases with the increase in the percentage of the reinforcement. Granato–Lucke mechanism [22] is a well-accepted theory that explains the damping mechanism by dislocations. When a dislocation is pinned between two particulates, it behaves like an elastic vibrating string. Thus, under applied cyclic loading the string vibrates and dissipate energy to the surroundings. The vibration string model is schematically illustrated in Fig. 6.

As the applied load is increased, the pinned dislocation may bow out at some weak pinning points and subsequently the further motion of the dislocation line undergo higher vibration amplitude. As the damping is the materials ability to dissipate energy, the increase in dislocation density results in the increase in the damping capacity. The dislocation based damping is expressed as follows.

where ao is a numerical factor of order 1, B is the damping constant, ω is the operating frequency, L is the effective dislocation loop length, which depends on the pinning distance, C is the dislocation line tension (≈0.5 Gb2), G is the shear modulus, b is burgers vector and ρ is the total dislocation density. From Eq. (3) it is clear that the damping depends on the dislocation density, frequency of cyclic stress and dislocation loop length. As the % of reinforcement increases the dislocation density increases, which results in the increase in damping capacity. Also, the damping capacity is directly proportional to the square of the frequency; the increase in frequency also results in enhancing the damping capacity of the hybrid composites.3.2Intrinsic damping of hybrid compositesThe improved damping capacity of the hybrid composites was due to the addition of RHA and SiC particulates in the matrix. The damping capacity of MMCs is directly related to the intrinsic damping of each of the individual constituents. From Fig. 3 it was observed that the damping capacity increases with increasing volume fraction of reinforcing particulates. Applying the rule of mixtures, the overall damping capacity of the hybrid composites is given by Eq. (4).

where ηRHA, ηSiC, ηA356 are the damping capacity of RHA, SiC and A356.2, respectively, and, VRHA, VSiC and VA356 are the volume fraction of RHA, SiC and A356.2, respectively.Using Eq. (4) the overall damping capacity was found at 1Hz for the hybrid composites and these results are found to be in good agreement with the experimental results. However, Eq. (4) is independent of frequency, the temperature and the percentage of reinforcement, the damping behavior cannot be predicted at different frequencies and at different percentages of reinforcement. However, the equation can be used to validate the damping capacity with experimental values.

3.3PorosityDuring the fabrication of composites, some porosity level is normal, because of the long particle feeding and the increase in surface area in contact with air. The porosity of the hybrid composites was measured using Eq. (5). The corresponding values are tabulated and presented in Table 5.

where ρth and ρm are the theoretical and measured densities respectively.The detailed measurement of theoretical and measured densities was presented in the earlier works [9]. It was observed that the porosity increases with the increase in the percentage of reinforcement. This feature is evident from the optical micrographs shown in Fig. 7. This is because of the higher reinforcement particles in the composite, leading more particle concentration regions, which increases the probability to introduce voids or pores. Based on the results reported by Zhang et al. [23], the sliding along bulk defects are responsible for high damping capacity. Hence, in the present study, the damping capacity increases with the increase in the percentage of reinforcement due to the relative motion of the reinforcement particulates in regions that voids exist.

4ConclusionsDamping characteristics of the unreinforced A356.2 alloy and its composites containing 2, 4, 6 and 8 weight percentage SiC and RHA in equal proportions were studied. From the study, the following conclusions are drawn:

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The damping capacity of the unreinforced alloy increases with the increase in the frequency. A damping capacity of 0.005 at 1Hz has been observed, which indicates that A356.2 alloy is a low damping material.

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The addition of micro sized particulates increases the damping capacity of the A356.2 alloy.

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It was observed that the damping capacity increases with the increase in the percentage of the reinforcement.

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The increase in the damping capacity can be attributed to the increase in dislocation density, which results from the thermal mismatch between the reinforcement and the matrix.

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Porosity also plays a crucial role in enhancing the damping capacity of the hybrid composites. The sliding of the reinforcement particulates along the voids during cyclic loading increase the energy dissipation, which is a direct measure of its damping capacity.

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Also, it can be concluded that the damping capacity of the hybrid composites is more than the composites with single reinforcement.

The authors declare no conflicts of interest.