An Investigation of Al–SiCp Composites

http://jcm.sagepub.comJournal of Composite Materials

DOI: 10.1177/002199803036245 2003; 37; 1839 Journal of Composite Materials

I. Ozdemir and M. Toparli An Investigation of Al-SiCp Composites Under Thermal Cycling

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An Investigation of AlSiCp CompositesUnder Thermal Cycling

I. OZDEMIR AND M. TOPARLI*Department of Metallurgical and Materials Engineering

Faculty of Engineering, Dokuz Eylul University

35100, Bornova, Izmir, Turkey

(Received January 7, 2003)(Revised May 1, 2003)

ABSTRACT: The effect of thermal cycling on the behavior of the aluminumsiliconmatrix alloy Al-7%Si-0.7%Mg (AlSi7) reinforced with 10% volume SiC particles hasbeen investigated experimentally and theoretically. Cast ingots of the matrix alloyand composite samples were extruded at 773K at an extrusion ratio of 10:1. Theextruded microstructures exhibit a more uniform distribution of the SiC particles. Inthis study, for determining the thermal stress and deformation on the compositematerials ABAQUS finite element software package was used. Thermal residualstresses developed during and after thermal cycling were also investigated. Thermalcycling tests were performed between 373 and 703K under a constant tensile load(150N) and without external load. The stress distributions in the composite duringheating and cooling were revealed. The axial displacement under constant externalload after one thermal cycling was 0.01672mm and kept increasing considerably.The maximum residual stresses were generated at the interfacial region duringthermal cycling. SEM micrographs showed that cracks were present in the compositestructure under repeated action of thermal cycling process (100 cycles).

KEY WORDS: particle-reinforced composites, thermal properties, finite elementanalysis, extrusion.

INTRODUCTION

METAL MATRIX COMPOSITES (MMCs) especially having light metal matrices such asAl, Mg, Ti etc., are rapidly becoming the strongest candidates as a structuralmaterial for many high-temperature [1], automotive [2], and aerospace applications [3,4].In MMCs, aluminum based metal matrix composites have attracted considerableacademic and industrial attention [5]. Relatively low ductility of most metal matrixcomposites has rendered conventional metal forming processes impractical and lowformability at high temperatures has restricted the use of their commercial applications.

*Author to whom correspondence should be addressed. E-mail: [email protected]

Journal of COMPOSITE MATERIALS, Vol. 37, No. 20/2003 1839

0021-9983/03/20 183912 $10.00/0 DOI: 10.1177/002199803036245 2003 Sage Publications


So, it is important that their thermal behaviors are well understood and the properties atelevated temperature need to be investigated. Considerable work has been carried out toinvestigate the ductility and behavior of metal matrix composites at different temperaturesunder isothermal and thermal cycling conditions [69].Among various thermal environments, thermal cycling can be considered as one of the

most severe environments. The mismatch of thermal expansion between matrix andreinforcement due to temperature change results in large internal stresses and mismatchstrain that affect the microstructure and mechanical properties of the composites. It is alsopossible that excessive plastic strains are developed [10]. In AlSiCp system, because thedifference of their coefficients of thermal expansion (CTE) values is the factor of about 6,this difference is sufficient to generate high local stresses during heating and cooling [3].The matrix deforms plastically to accommodate the smaller volume expansion of the SiCparticles.The numerical analyses of SiC particles-reinforced aluminum alloys have been

conducted by several researchers [1114]. Geni and Kikuchi [11] studied about the non-uniform distribution of SiC particle volume fraction and the aspect ratio in the matrix.Meijer et al. [12] investigated on thermal residual stress using unit cell models. Theyfound that the mechanical properties of the composite strongly depended on thereinforcement geometry. Tham et al. [13] investigated the influence of processing-induced voids on the deformation behavior of silicon carbide particulate-reinforcedaluminum metal-matrix composites synthesized by the disintegrated melt depositiontechnique using axisymmetric finite element model. Also they compared theoreticalwith experimental results. Shen et al. [14] discussed about particle shape and thermalresidual stress. These theoretical results show that the stress-strain relationship agreewith the experimental analysis.The aims of this investigation are to study the stress-strain relationship and thermal

behavior of aluminum composites containing SiC particles, under thermal cyclingconditions by using FEM calculations, and to use this information to produce MMCs withexcellent thermal performance.

MATERIALS AND EXPERIMENTAL DETAILS

The materials used in this work were produced by die casting technique, as describedearlier [15]. The nominal composition of alloy elements in the aluminum alloy matrixAl-7%Si-0.7Mg (AlSi7) is listed in Table 1. The machined ingots with an average diameterof 32mm and a height of 58mm were extruded at 773K with an extrusion ratio of 10 : 1and cooled in air. The porosity content of the samples was evaluated from the differencebetween the calculated and experimentally measured density of each sample by using the

Table 1. The materials and their chemical compositions.

Materials

Elements

Si Fe Cu Mn Mg Ni Zn Ti Cr Pb Sn V Al

AlSi7 Matrix 6.62 0.29 0.01 0.02 0.67 0.01 0.08 0.07 0.01 0.04 0.05 0.01 BalanceAl-10%SiCp 16.84 0.29 0.06 0.028 0.56 0.03 0.05 0.09 0.04 0.01 0.01 0.00 Balance

1840 I. OZDEMIR AND M. TOPARLI


Archimedean principle. The calculated densities of the samples were determined from thechemical analysis. The microstructure of the composite in the extruded state was examinedby optical microscopy (Figure 1).Firstly, mechanical properties of aluminum matrix material were obtained experimen-

tally. For the determination of the material properties at different temperatures, threesamples of each were prepared for tensile tests. All tests were performed on a computerizedAG-50kNG Shimadzu universal testing machine at ambient and high temperatures, usingcylindrical specimens with a diameter of 6mm and a gauge length of 20mm (Figure 2). Inaddition, the modulus of elasticity was measured by using extensometer attached to thetensile test machine. The applied strain rate was 4 104 s1 and the ASTM StandardE21-92 procedures were used to evaluate the results. Data of mechanical properties aregiven in Table 2.Thermal cycling tests were performed in a special machine designed and constructed for

this purpose (Figure 3). The specimens were heated in an electrical resistance movablefurnace and cooled by a forced air current. The heating and cooling rates were used 0.5and 0.35K/s respectively. The temperature in the central portion of the specimen wascontinuously measured by thermocouple. The total elongation was measured with aresolution of 0.01mm. Temperature cycling was between 373 and 703K and period foreach cycle was 270 s. The applied external load was 150N. Figure 4 shows the temperature

Figure 1. The microstructure of the composite AlSi7SiC in the as-extruded condition.

Figure 2. The configuration of tensile test specimen.

An Investigation of AlSiCp Composites Under Thermal Cycling 1841


profile of the heating and cooling during the thermal cycling test. As a result, thedeformation of the materials was characterized by measuring the specimens length at afixed point. The measured elongation after five cycles was compared with FEM results forthe same test conditions.

LOAD LOAD

Cooling Fan

ThermocoupThermocouple le

Movable Movable FurnaceFurnace

T2T2 ( (oC)C)

T1 (T1 (oC)C) SampleSample

Figure 3. Schematic illustration of the thermal cycling test unit used.

300

400

500

600

700

800

0 50 100 150 200 250 300Time (s)

Tem

pera

ture

(K

)

Figure 4. The temperature profile of the sample during the thermal cycling test.

Table 2. Mechanical properties of AlSi7 matrix.

Temperature (K) 273 373 473 573 703Modulus of elasticity (GPa) 71.2 68.1 62.2 42.3 21.1Poissons ratio 0.33 0.33 0.33 0.33 0.33Yield stress (MPa) 76 82 82 52 19Ultimate stress (MPa) 156 124 110 54 22



FINITE ELEMENT ANALYSIS

Due to complexity and the limit of the computer memory, it was not possible todetermine an exact solution of the SiC reinforcement in the matrix. In this study, a verysimple model is considered and a convenient approximation is conducted for the modelingof the uniform distribution of SiC particles. We have simulated the AlSiC compositematerials using axisymmetric finite element models and volume percentage of SiC wasfixed to 10%. It is assumed that SiC particles and Al matrix are isotropic and SiChomogenously distributed. The thermal cycling applied to the composite model, having aninitial temperature of 373K, was varied from 373 to 703K and from 703 to 373K (i.e. onecycle) and an external load (P ) of 150N was applied on the upper surface alonglongitudinal direction. The elastic-plastic capability of the ABAQUS [16] code wasadopted for the finite element simulation. The specimen was modeled with 2187 four-noded axisymmetric thermally coupled elements (CAX4T element type) as shown inFigure 5. It was assumed that the composites consist of a regular array of identicalparticles distributed in a homogeneous matrix. The SiC particle size was 100 mm and themaximum mesh size was selected as 10 mm. As a matter of fact that an analysis by Takaoand Taya [17] has shown that if the differences in size and distribution of thereinforcement are not extreme, then a relatively dilute composite can be modeled usingan average size and spacing without significant loss of accuracy.

Figure 5. The mesh pattern of axisymmetric finite element model.



It is assumed that if the specimen was cylindrical, it was possible to take the advantageof the axisymmetric geometry. The Al matrixSiC interface region was small-modeledusing edge-biased type which gives good results owing to the fine mesh application.The FEM analyses were done using the mechanical properties obtained at different

temperatures for the AlSi7 matrix as given in Table 2. The modulus of elasticity andPoisson ratio for SiC particles were taken as 450GPa and 0.17, respectively. These valuesand data of the thermal properties of AlSi7 matrix and SiC particles were taken from theliterature [18] and are given in Table 3.During the deformation of AlSiC metal matrix composite, aluminium alloy is able to

deform plastically while the SiC particles remain elastically. So, it was assumed that SiCparticles were elastic during this analysis. The Von Mises yield criterion is applied fordetermining the occurrence of plastic deformation. The equivalent Mises stress is given bythe expression:

m 1 2 2 2 3 2 3 1 2

2

s1

where, 1, 2, and 3 are the three principal stresses. Whenever m reaches the yieldstrength, the material begins to plastic deform. The behavior of the aluminum matrixmaterial was assumed to be due to multilinear kinematic hardening which means that thereal stressstrain curves can be approximated by a series of straight lines. This type ofhardening law allows the Baushinger effect to be represented [19].

RESULTS AND DISCUSSION

Figure 1 shows the micrograph of the composite Al-10%SiCp in the extruded condition.Under the optical microscope, generally a uniform SiCp distribution was observed andsome pores resolvable. Extrusion process increases the mechanical properties by reducingporosity [20,21] and improving the SiC particle dispersion. The porosity values of thecomposite in the as-cast state and after the extrusion are presented in Table 4.The finite element program was organized to calculate thermal stress and deformation

for one cycle. In order to minimize error, time interval was selected to use the smallestvalue. The displacement of the AlSiCp system at the 270 s of the thermal cycling is givenin Figure 6. Maximum axial displacement occurs at the AlSiC particle interface andcontinues throughout the AlSi7 matrix alloy which shows a plastic behavior. The value ofdisplacement was calculated as 0.01672mm. On the other hand, after one cycle,displacement of the AlSiCp system without loading was calculated as only 0.00152mm(Figure 7). It is clear that in the case of thermal cycling test under load (150N), the axial

Table 3. Thermal properties of the AlSi7 matrix and SiC particles.

AlSi7 SiC

Coefficient of thermal expansion, (1/K) 23.6106 4106Thermal conductivity, k (W/mK) 200 100Heat capacity, cp (J/kgK) 900 675Density, (kg/m3) 2670 3200



displacement value increased considerably in comparison to that of the thermal cyclingtest without loading. Experimental results confirm this analysis such that the compositesample exhibited a fairly high elongation to failure (over 80%) after 100 thermal cycles(Figure 8).

0,0E+00

4,0E-06

8,0E-06

1,2E-05

1,6E-05

2,0E-05

0 0,05 0,1 0,15Distance from the center of SiC particle, (mm)

Dis

plac

emen

t, (m

) SiC AlSi7

InterfaceInterface

Figure 6. Displacement (m) of the AlSiCp system at the 270 s of the thermal cycling.

1,0E-07

4,0E-07

7,0E-07

1,0E-06

1,3E-06

1,6E-06

1,9E-06


Dis

plac

emen

t , (m

)

SiC AlSi7

IntInterface

rface

Figure 7. Displacement (m) of the AlSiCp system without loading after one cycle.

Table 4. The densities and porosity of the matrix alloy and the composites in theas-cast and extruded states.

Material

CalculatedDensity

Dc (g/cm3)

Experimental DensityDe (g/cm

3)Porosity(%)

As-Cast Extruded As-Cast Extruded

Matrix alloy 2.6917 2.6558 2.6670 1.33 0.92AlSi7/SiC/10p 2.7427 2.6162 2.7135 4.61 1.06



Figures 9 and 10 show the distribution of Von Mises stress and maximum principalstress on the composite material, respectively. As seen in Figure 9, maximum Von Misesstress value occurs on the interface. On the other side, maximum tensile principal stressesoccur on the aluminum matrix and maximum compressive principal stresses occur on the

Figure 8. SEM micrograph of thermally cycled AlSi7/SiC/10p composite (a) and microprobe line profile(b) of aluminum showing crack in the structure.

0

20

40

60

80

100

0 0,05 0,1 0,15

Stre

ss, (M

Pa)

SiC AlSi7

Interface Interface

Distance from the center of SiC particle, (mm)Figure 9. Von Mises stress (MPa) distribution of the AlSiCp system at the 270 s of the thermal cycling.



SiC particle as seen in Figure 10. Maximum tensile and compressive principal stressesvalue are 46.5 and 123.0MPa, respectively. Moreover, the residual Von Mises stressdistributions in the composite were calculated by FEM, and the results are illustrated inFigure 11. It is noted that the maximum stresses were generated at the interfacial regionduring cycling because of the temperature gradients inside the specimen and differentthermal expansion coefficients of SiC particle and the AlSi7 matrix. Ho and Saigal [22]found that thermal residual stresses strongly affect the mechanical behavior of thecomposites. Penn et al. [23] revealed that tensile residual stresses, present within MMCsbefore the application of external load, have often been regarded as defects or inherentflaws, and may initiate and amplify some of the early damage, such as microcracking, thatoccurs when MMCs are mechanically loaded. For example, tensile matrix residual stressesproduced by thermal cycling can degrade the composite integrity [24]. Therefore, under

-130

-110-90

-70

-50-30

-10

1030

50

00,05 0,1 0,15

Distance from the center of SiC particle, (mm)

Stre

ss, (M

Pa)

SiC

IntInterface

rface

AlSi7

Figure 10. Maximum principal stress (MPa) distribution of the AlSiCp system at the 270 s of the thermalcycling.

0

30

60

90

120

150


Stre

ss, (M

Pa)

AlSi7

IntInterface

rface

SiC

Figure 11. Residual Von Mises stress (MPa) distribution of the AlSiCp system at the 270 s of the thermalcycling.



continued regeneration of these stresses through thermal cycling, composite deformedplastically and microscopic damage accumulation such as cavitation and cracking can bedeveloped in the matrix as observed experimentally in Figure 8.Figure 12 shows equivalent plastic strain distribution of the AlSiCp system. Plastic

deformation is observed at Al matrix and SiC particle interface on the aluminum. It can befound from Figure 12 that the predicted maximum plastic strain amplitude during onethermal cycling was about 8.17%.

CONCLUSION

SiC particulate reinforced AlSi7 matrix base composites are produced under ordinaryfoundry conditions, and cast ingots are hot extruded to improve mechanical properties.The thermal cycling analysis is studied in the extrusion process experimentally andtheoretically. Temperature cycling was between 373 and 703K and period for each cyclewas 270 s. Theoretical study was confirmed to only one cycle. After the experimental andtheoretical evaluation, the following points are concluded.

1. After thermal cycling tests, maximum displacement was calculated in the direction ofthe applied load at the interface of the AlSi7 matrixSiC particle in the compositesystem. The values calculated were 0.01672 and 0.00152mm with and without loading,respectively. This result indicated that composite samples are easily deformed to havelarge elongations as a result of repeated thermal cycling under load.

2. Maximum tensile principal stress values and maximum compressive principal stressvalues were found for the aluminum matrix and SiC particles, respectively. The resultscalculated by the finite element method indicated that the stresses in the composite werenot uniform.

3. Maximum residual stresses (99MPa) were calculated at the interfacial region, whichexceeded the yield point and caused plastic flow in the matrix.

4. Experimentally, the composite sample exhibited a fairly high elongation to failure (over80%) after repeated thermal cycling (100 cycles). In addition, microscopic damage suchas cracking was observed in the matrix.

0

0,02

0,04

0,06


Stra

inSiC

AlSi7

IntInterface

rface

Figure 12. Equivalent plastic strain distribution of the AlSiCp system at the 270 s of the thermal cycling.



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An Investigation of Al–SiCp Composites

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