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j m a t e r r e s t e c h n o l . 2 0 1 6;5(2):123–130

www.jmrt .com.br

Available online at www.sciencedirect.com

riginal Article

xperimental evaluation onto the dampingehavior of Al/SiC/RHA hybrid composites

ora Siva Prasada,∗, Chintada Shobab

Department of Mechanical Engineering, GITAM University, Visakhapatnam, IndiaDepartment of Industrial Engineering, GITAM University, Visakhapatnam, India

r t i c l e i n f o

rticle history:

eceived 31 January 2015

ccepted 14 August 2015

vailable online 23 November 2015

eywords:

orosity

islocation density

icrostructures

amping

a b s t r a c t

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 pro-

cess. 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.

ybrid composites © 2015 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier

Editora Ltda.

ture and mechanical properties of pure magnesium has been

. Introduction

he incorporation of different reinforcements into a matrixas led to the development of hybrid composites. Hybridomposites are becoming better substitutes for the conven-ional alloys because of characteristics like high stiffness,igh strength and low density. Aluminum matrix compositesith multiple reinforcements (hybrid composites) are find-

ng increased applications because of improved mechanicalnd wear resistance, and hence are better substitutes for sin-

le reinforced composites. Hybrid composites have uniqueeatures that can be used to meet various design requirementsn a more economical way than conventional composites.

∗ Corresponding author.E-mail: dorasivaprasad@gmail.com (D. Siva Prasad).

ttp://dx.doi.org/10.1016/j.jmrt.2015.08.001238-7854/© 2015 Brazilian Metallurgical, Materials and Mining Associa

Also, hybrid composites provide a combination of propertiessuch as tensile modulus, compressive strength and impactstrength, which cannot be realized in composite materials.In recent times, hybrid composites have been established ashighly efficient, high performance structural materials andtheir use is increasing rapidly. Hybrid composites are findingwide applications where high wear resistance is of importance[1]. However, at present, hybrid composites are using as bear-ing materials and turbine blades.

The effect of hybrid reinforcements on the microstruc-

investigated by Sankaranarayanan et al. [2]. Nanoscale alu-mina and titanium particulates were used as reinforcements.From their study, it can be concluded that the addition

tion. Published by Elsevier Editora Ltda.

124 j m a t e r r e s t e c h n o l . 2 0 1 6;5(2):123–130

Table 1 – Chemical composition of A356.2 Al Alloy matrix.

Si Fe Cu Mn Mg Zn Ni Ti

6.5–7.5 0.15 0.03 0.10

of reinforcement leads to significant grain refinement andexhibit higher microhardness, tensile and compressive prop-erties when compared to monolithic magnesium. Accordingto Akbarpour et al. [3] the addition of nanosized silicon car-bide reinforcement lowers the grain growth rate and enhancesthe homogenization. The aging behavior and the mechani-cal properties of (SiCp + Ti)/7075Al hybrid composites has beeninvestigated by Liu et al. [4]. Results confirmed that the pre-cipitation hardening of the hybrid composites was delayedduring the aging process. Also, an increase in the tensilestrength of the hybrid composites was observed because ofthe precipitation hardening of the matrix alloy. A356 alloymodified with 0.03 mass% of strontium was reinforced withsilicon 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 IlijaBobic et al. [5]. The results demonstrated that the compositesreached maximum hardness, faster than the thixocast A356alloy and time required to attain peak hardness decreaseswith the increase in the percentage of reinforcements. Thestrength of (SiCp + Ti)/7075Al hybrid composites with andwithout addition of Ti particles has been investigated by Weip-ing Chen et al. [6]. Results demonstrated that the strength wasimproved significantly with Ti addition, whilst their ductilitywas decreased. The thermal expansion behavior of micro-/nano-sized Al2O3 particles reinforced hybrid composite hasbeen investigated by Zhibo Lei el al. [7]. The results revealedthat the nanoparticle concentration had significant effect onthe thermal expansion behavior of the composites. Surfaceintegrity studies while drilling metal matrix and hybrid metalmatrix composites has been performed by Rajmohan et al.[8]. Drilling tests were carried to investigate the effect of thevarious cutting parameters on the surface quality and thedeformation of drilled surface due to drilling.

The fabrication of hybrid composites with low cost rein-forcement would minimize the cost of the product withenhanced properties. Rice husk ash is one of such reinforce-ment, an agricultural waste byproduct, which is gaining moreimportance in recent days as a secondary reinforcement inthe fabrication of composites. The advantages of using RHAis to produce low cost by-product thereby, reducing the costof aluminum products [9,10], readily available with less cost,and often lower densities in comparison with most techni-

cal ceramics (such as boron carbide, alumina). In recent years,many researches have been reported the potentials and lim-itations of the use of RHA as reinforcement [11,12]. Prasadand Krishna [13] reported that the damping capacity increases

Table 2 – Chemical composition of RHA.

Constituent Silica Graphite Calcium oxide

% 90.23 4.77 1.58

0.4 0.07 0.05 0.1

with the addition of RHA particulates and increases furtherwith heat treatment. Srikanth and Gupta [14] reported thatthe damping capacity of the pure magnesium matrix wasenhanced with the addition of SiC particulates, and increaseswith the increase of the proportion of SiC particulates. Sudar-shan and Surappa [15] showed that the addition of fly ash,an industrial waste byproduct into A356 exhibited improveddamping capacity compared to base alloy. Zhang et al. [16]studied the damping behavior of SiC and graphite reinforcedmetal matrix composites. They reported that the dampingcapacity of aluminum could be significantly improved by theaddition of either SiC or graphite particulates. The influenceof CaO on damping capacity has been reported by Jang et al.[17]. They reported that Mg–CaO alloy can be regarded as acost-effective damping material with enhanced mechanicalproperties. According to Schaller [18], an elegant way to reducemechanical vibrations is to use high damping materials andthis can be achieved by incorporating reinforcement in thematrix. A detailed study on the damping behavior of metalmatrix composites has been studied and presented by Prasadand Shoba [19].

From the above literature, it is clear that several researcherscarried investigations on hybrid composites; however damp-ing behavior of hybrid composites is hardly seen. Hence, thepresent study aims at finding the damping characterization ofAl/SiC/RHA hybrid composites at different frequencies usingdynamic mechanical analyzer with an objective to develophigh damping materials.

2. Experimentation

In the present work, SiC and RHA particulates were used asreinforcements and A356.2 was used as a matrix material.The chemical compositions of RHA and base alloy A356.2are given in Tables 1 and 2 respectively. Pre-treatment wascarried to RHA particulates before incorporating into themolten metal, to remove inorganic matter and carbona-ceous material [13]. The reinforcement particulates werepreheated to 700–800 ◦C for 1 h before incorporation intothe molten melt to remove moisture. 1% by weight mag-nesium was added in the molten metal to improve thewettability between the matrix and the reinforcements. The

detailed fabrication process of the hybrid composites waspresented in earlier works [9]. Using this process, 2, 4, 6and 8% by weight in equal proportions of RHA/SiC particle-reinforced hybrid composites were fabricated. Microstructural

Magnesium oxide Potassium oxide Ferric oxide

0.53 0.39 0.21

j m a t e r r e s t e c h n o l . 2 0 1 6;5(2):123–130 125

Servo motor

Spindle stroke gear

Transverse

Force transducer

Leaf spring

Dyn. Elongtransducer

Manualcontrol

Shaker ampliferPower supplyBlower forshaker

Liquidnitrogen

Vibro box

a

b

ple

cuowiatoaeifsfm

Fig. 1 – (a) GABO Eplexor DMA. (b) Sam

haracterization of the hybrid composites was examinedsing scanning electron microscope (Model: JSM-6610LV) andptical microscope (OLUMPUS). The damping measurementsere performed using a GABO Eplexor dynamic mechan-

cal analyzer at frequencies ranging from 1 Hz to 15 Hzt room temperature using three point bending mode. Allhe damping experiments are performed at a static loadf 50 N, a dynamic load of 40 N, and at constant strainmplitude (ε) of 1 × 10−5. The schematic diagram of thexperimental set up with three point bending arrangements shown in Fig. 1. The damping capacity, i.e., tan ı as a

unction of frequency, was recorded. The samples of dimen-ions 30 × 12 × 1·5 mm3 for damping measurements were cutrom Ultra cut 843/Ultra cut f2 CNC wire electric discharge

achine.

holder for three point bending mode.

3. Results and discussions

Fig. 2 shows the scanning electron micrograph of the hybridcomposite. Micrograph of hybrid composites showing clearlythe uniform distribution of RHA and SiC in the matrix. Fig. 3shows the variation of damping capacity with the frequencyfor the unreinforced alloy. It was observed that the dampingcapacity of the base material was found to be 0.00549 at 1 Hz,which indicates low damping for A356.2 alloy. Also, it couldbe observed that the damping capacity increases with theincrease in the frequency. Fig. 4 shows the variation of damp-

ing capacity with frequency for different weight percentageof the reinforcement. From the plot it could be observed thatthe damping capacity increases with the increase in the %

126 j m a t e r r e s t e c h n o l

Fig. 2 – Scanning electron micrograph of hybrid composite.

16141210864200.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

Dam

ping

cap

acity

Frequency (Hz)

Fig. 3 – Variation of damping capacity with frequency forunreinforced alloy.

1614121086420

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055

0.060

Dam

ping

cap

acity

Frequency (Hz)

Unreinforced alloy 2% reinforced hybrid composite 4% reinforced hybrid composite 6% reinforced hybrid composite 8% reinforced hybrid composite

Fig. 4 – Variation of damping capacity with frequency forunreinforced alloy and hybrid composites.

. 2 0 1 6;5(2):123–130

reinforcement. An increasing trend for damping capacity hasbeen observed with frequency for 2, 4, 6 and 8% reinforcedhybrid composites. Also, it was observed that the dampingcapacity for the 2% reinforced composites show similar trendsas unreinforced alloy, however with no significant increase inthe damping capacity.

It was noticed from the literature that the dampingcapacity of hybrid composites (in the present study) is morethan single reinforced composites. To explain this behavior

16141210864200.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

Dam

ping

cap

acity

Frequency (Hz)

Hybrid composite Al/RHA composite

16141210864200.01

0.02

0.03

0.04

0.05

0.06

Dam

ping

cap

acity

Frequency (Hz)

Hybrid composite Al/RHA composite

16141210864200.010

0.015

0.020

0.025

0.030

0.035

0.040

Dam

ping

cap

acity

Frequency (Hz)

Hybrid composite Al/RHA composite

a

b

c

Fig. 5 – Comparison of damping capacity for Al/RHA/SiCcomposite and Al/RHA composite for (a) 4% (b) 6% and (d)8%.

j m a t e r r e s t e c h n o l . 2 0 1 6;5(2):123–130 127

Table 3 – Damping Capacity at 1 Hz [20].

Unreinforced alloy 8% SiC reinforced composite 12% SiC reinforced composite 18% SiC reinforcedcomposite

0.034 0.037 0.046 0.051

Table 4 – Theoretical results of unreinforced and hybrid composites.

S. No. Weight (%) of reinforcement Estimated dislocationdensity, � (m−2)

CTE, (/◦C)

1 0.0 – 21.4 × 10−6

2 2.0 17.31 × 1011 17.44 × 10−6

3 4.0 21.32 × 1011 16.64 × 10−6

aosAwcmbA[mrvsraiac

3

MeaoEtbttati

�

�

particulates, it behaves like an elastic vibrating string. Thus,under applied cyclic loading the string vibrates and dissipate

Strong pinning point

L

Weak pinning point

4 6.0

5 8.0

comparison was made between the damping capacitiesf Al/RHA composites [13] and hybrid composites. Fig. 5a–chows the variation of damping capacity with frequency forl/RHA composites and Al/SiC/RHA hybrid composites. Itas observed that hybrid composites exhibit higher damping

apacity than Al/RHA composites for all % of reinforce-ent studied herein. Similarly, a comparison was made

etween the damping capacity of Al/SiC composites [20] andl/SiC/RHA hybrid composites at 1 Hz. Results of Ranjit Bauri

20] showed that the damping capacity of unreinforced alu-inum alloy is 0.034 and an increase of maximum 50% was

eported for 18% reinforced composites. The correspondingalues are tabulated in Table 3. However, from the presenttudy, the damping capacity of hybrid composites with 8%einforcement increases by 3 times than the unreinforcedlloy. Hence, it can be concluded that the hybrid compos-tes exhibit higher damping capacity than monolithic alloynd single reinforced composites. The increase in dampingapacity can be attributed to the following reasons:

.1. Dislocation damping

etal matrix composites are characterized by a large differ-nce in the thermal expansion coefficient (CTE) of the matrixnd the reinforcement (CTE of A356.2 is 21.4 × 10−6/◦C, the CTEf RHA is 10.1 × 10−6/◦C and the CTE of SiC is 4.3 × 10−6/◦C).ven small temperature changes, generate thermal stresses inhe aluminum matrix. These stresses can be partially releasedy dislocation generation in the vicinity of the interface. Thus,he dislocation density generated can be quite significant athe interface and can be predicted using the model of Tayand Arsenault [21] based on prismatic punching of disloca-ions at a ceramic particulate. The dislocation density � at thenterface is given by Eq. (1)

= BεVr

bd(1 − Vr)(1)

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

= Bε(VRHA + VSiC)

bd{

1 − (VRHA + VSiC)} (2)

23.99 × 1011 16.09 × 10−6

30.82 × 1011 15.06 × 10−6

where B is a geometric constant that depends on the aspectratio (it varies between 12 for equiaxed particulate and 4 forwhisker-like particulate), ε is the thermal mismatch strain (theproduct of temperature change �T, during solidification ofMMCs and CTE difference, �˛, between the reinforcement andmatrix), Vr is the volume fraction of the reinforcement, b is theburgers vector, d is the average grain diameter of reinforce-ments.

The CTE of the composites is relatively difficult to pre-dict because it is influenced by several factors, which includesthe internal structure of the composite, plasticity, etc. How-ever, there are several analytical methods to predict CTE ofthe composites, which includes simple rule of mixtures andthermo-elastic energy principles like Kerner, and Turner mod-els. Based on the Kerner model the CTE of the composites waspredicted and presented in Table 4. The detailed calculationswere presented in earlier works [9].

The dislocation density for the hybrid composites werethen calculated based on Eq. (1) with an assumption forthe burgers vector of 0.32 nm for Al [13] and are tabulatedin Table 4. From Table 4 it was observed that the disloca-tion density increases with the increase in the percentageof the reinforcement. Granato–Lucke mechanism [22] is awell-accepted theory that explains the damping mechanismby dislocations. When a dislocation is pinned between two

a b c d e f

Fig. 6 – Granato and Lucke vibration string model.

n o l

128 j m a t e r r e s t e c h

energy to the surroundings. The vibration string model isschematically illustrated in Fig. 6.

As the applied load is increased, the pinned dislocationmay bow out at some weak pinning points and subsequentlythe further motion of the dislocation line undergo highervibration amplitude. As the damping is the materials abilityto dissipate energy, the increase in dislocation density resultsin the increase in the damping capacity. The dislocation baseddamping is expressed as follows.

Q−1 = a0B�L4ω2

�2Cb2(3)

where ao is a numerical factor of order 1, B is the damping con-stant, ω is the operating frequency, L is the effective dislocationloop length, which depends on the pinning distance, C is thedislocation line tension (≈0.5 Gb2), G is the shear modulus, bis burgers vector and � is the total dislocation density. FromEq. (3) it is clear that the damping depends on the dislocationdensity, frequency of cyclic stress and dislocation loop length.As the % of reinforcement increases the dislocation densityincreases, which results in the increase in damping capac-ity. Also, the damping capacity is directly proportional to the

square of the frequency; the increase in frequency also resultsin enhancing the damping capacity of the hybrid composites.

Fig. 7 – Optical micrograph showing porosity of hybrid

. 2 0 1 6;5(2):123–130

3.2. Intrinsic damping of hybrid composites

The improved damping capacity of the hybrid composites wasdue to the addition of RHA and SiC particulates in the matrix.The damping capacity of MMCs is directly related to the intrin-sic damping of each of the individual constituents. From Fig. 3it was observed that the damping capacity increases withincreasing volume fraction of reinforcing particulates. Apply-ing the rule of mixtures, the overall damping capacity of thehybrid composites is given by Eq. (4).

�c = �RHAVRHA + �SiCVSiC + �A356{1 − (VRHA + VSiC)} (4)

where �RHA, �SiC, �A356 are the damping capacity of RHA, SiCand A356.2, respectively, and, VRHA, VSiC and VA356 are thevolume fraction of RHA, SiC and A356.2, respectively.

Using Eq. (4) the overall damping capacity was found at1 Hz for the hybrid composites and these results are found tobe in good agreement with the experimental results. However,Eq. (4) is independent of frequency, the temperature and thepercentage of reinforcement, the damping behavior cannotbe predicted at different frequencies and at different percent-

ages of reinforcement. However, the equation can be used tovalidate the damping capacity with experimental values.

composites (a) 2%, (b) 4%, (c) 6%, and (d) 8% at 20×.

j m a t e r r e s t e c h n o l . 2

Table 5 – Variation of porosity with % reinforcement.

S. No. Weight (%) of reinforcement Porosity

1 0.0 1.012 2.0 2.113 4.0 2.53

3

Dnicv

P

wr

dopohpibstcr

4

Diat

•

•

•

•

•

r

4 6.0 2.965 8.0 3.34

.3. Porosity

uring the fabrication of composites, some porosity level isormal, because of the long particle feeding and the increase

n surface area in contact with air. The porosity of the hybridomposites was measured using Eq. (5). The correspondingalues are tabulated and presented in Table 5.

orosity = �th − �m

�th(5)

here �th and �m are the theoretical and measured densitiesespectively.

The detailed measurement of theoretical and measuredensities was presented in the earlier works [9]. It wasbserved that the porosity increases with the increase in theercentage of reinforcement. This feature is evident from theptical micrographs shown in Fig. 7. This is because of theigher reinforcement particles in the composite, leading morearticle concentration regions, which increases the probabil-

ty to introduce voids or pores. Based on the results reportedy Zhang et al. [23], the sliding along bulk defects are respon-ible for high damping capacity. Hence, in the present study,he damping capacity increases with the increase in the per-entage of reinforcement due to the relative motion of theeinforcement particulates in regions that voids exist.

. Conclusions

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

The damping capacity of the unreinforced alloy increaseswith the increase in the frequency. A damping capacity of0.005 at 1 Hz has been observed, which indicates that A356.2alloy is a low damping material.

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

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

The increase in the damping capacity can be attributedto the increase in dislocation density, which results fromthe thermal mismatch between the reinforcement and thematrix.

Porosity also plays a crucial role in enhancing the damping

capacity of the hybrid composites. The sliding of the rein-forcement particulates along the voids during cyclic loadingincrease the energy dissipation, which is a direct measureof its damping capacity.

0 1 6;5(2):123–130 129

• Also, it can be concluded that the damping capacity of thehybrid composites is more than the composites with singlereinforcement.

Conflicts of interest

The authors declare no conflicts of interest.

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