Materials Science and Engineering A 532 (2012) 275– 281
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Materials Science and Engineering A
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icrostructure and mechanical properties of Al/SiO2 composite produced by CARrocess
ajid Hashemia, Roohollah Jamaatib,∗, Mohammad Reza Toroghinejadb
Department of Mechanical Engineering, Isfahan University of Technology, Isfahan 84156-83111, IranDepartment of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
r t i c l e i n f o
rticle history:eceived 5 October 2011eceived in revised form 24 October 2011ccepted 25 October 2011
a b s t r a c t
Continual annealing and roll-bonding (CAR) process was used in this study as a very effective method forfabrication of the Al/15 vol.% SiO2 metal matrix composite. The microstructure of the produced compos-ites after 6 cycles of the CAR process was showed an excellent distribution of SiO2 particles in the AA1100matrix without any porosity. Particle breaking was one of the most important phenomena that can occur
vailable online 3 November 2011
eywords:etal matrix composite
AR processicrostructure
in CAR process. The results also were indicated that when the number of CAR cycle was increased, thetensile strength of the manufactured composites was improved, but their elongation was decreased atfirst step and then was increased. The tensile strength of the composite was 1.77 and 1.63 times higherthan the same values was obtained for annealed and monolithic aluminum strips, respectively, while theelongation value was decreased slightly.
echanical properties
. Introduction
Composites are materials made up of two or more materials thatre combined in a way that allows the materials to stay distinct anddentifiable. The purpose of composites is to allow the new materi-ls to have strengths from both materials, often times covering theriginal materials weaknesses. Composites are usually classified byhe type of matrix and reinforcement they use. The reinforcement ismbedded into a matrix that holds it together. The reinforcementsre used to strengthen the composites. Common composite typesnclude metal matrix composite (MMC), polymer matrix compositePMC), ceramic matrix composite (CMC) [1].
There has been a wide interest in developing metal matrixomposites due to their unique mechanical properties such asightweight and high elastic modulus. Aluminum is a lightweightnd relatively weak metal. Its applications are limited whenigh-modulus and strength are required. Although high-strengthluminum alloys have been developed, addition of alloying ele-ents and microstructural control can play but only a small role
n enhancing their stiffness. The demands for lightweight, high-odulus, and high-strength materials have led to the development
f MMCs [2–5].
Aluminum matrix composites (AMCs) are prepared using both
he solid state route (blending, compacting, and sintering ofowders) and the liquid state route such as infiltration, stir casting,
squeeze casting, and spray forming [6,7]. Each of these processeshas its own limitations such as non-uniform distribution of the rein-forcement, large content of porosity, undesirable chemical reaction,and poor adhesion between the reinforcement and matrix. Also,since expensive equipment is required and the processing routesare usually complex, the cost to produce AMCs by these methodsis high [7].
To overcome the aforementioned problems, the present authorswere invented a process using roll bonding process and named itCAR (continual annealing and roll-bonding) process [7–10]. Up tonow, only Al/Al2O3 and Al/SiC composites have been manufacturedby CAR process and no reports are available on any other com-posite. Many potential applications of AMC strips prompted thepresent study to investigate the feasibility of the CAR process forfabrication of Al/SiO2 MMC strips. Thus, the objective of this studyis to manufacture Al/SiO2 particulate composites by the CAR pro-cess. The influence of CAR on the microstructure and mechanicalproperties of Al/SiO2 MMC strips will also evaluate.
2. Experimental procedure
As-received strips of commercial purity aluminum alloy(AA1100) of 150 mm length, 50 mm width, and 0.5 mm thickness,were annealed at 643 K for 2 h, and the SiO2 powder with aver-
age particle size of less than <50 �m were used as raw materials.The chemical composition of aluminum strip used in this study isshown in Table 1.
First step: Six aluminum strips were degreased in acetone bathnd scratch brushed with a stainless steel wire brush 0.26 mm iniameter. After surface preparation, the SiO2 particles were uni-ormly dispersed between the strips which were then stacked overach other and fastened at both ends by steel wires. Attention waslso paid to proper alignment of the six strips prior to rolling. Theoll bonding process was carried out with no lubrication, using aaboratory rolling mill with a loading capacity of 20 tons. The rollonding process was carried out with an amount of reduction equalo 50% (corresponding to a von Mises equivalent strain εvM of 0.8er cycle).
Second step: After the first step, the roll bonded strip wasut in half and was annealed at the same time and tempera-ure. Then, two strips without adding reinforcement particles weretacked together and finally, the roll bonding process was car-ied out with an amount of reduction equal to 50%. To achieve aniform distribution of reinforcement particles in the matrix andlso to remove porosity in the interfaces of aluminum/aluminumnd aluminum/SiO2, the above procedure was repeated again upo six cycles without adding reinforcement particles. Finally, after
he last cycle, the annealing treatment was performed on thetrips.
The schematic illustration of CAR process for fabrication of thel/SiO2 MMC is shown in Fig. 1.
Fig. 1. Schematic illustration showin
142 8 4639 37 19
Samples for SEM observations were cut from the CAR-processedstrips and these were mounted in bakelite. Then, these sampleswere polished using 80–4000 grit water-proof SiC paper. Finally,the polishing was finished on a cloth using diamond paste of 3 �m.
Scanning electron microscopy (SEM) PHILIPS XL30 was usedfor microstructural observation to investigate how well the SiO2particles were distributed in the produced MMCs at different CARcycles.
The tensile test samples were machined from the CAR-processedstrips, according to the ASTM E8M tensile sample, oriented alongthe rolling direction. The gauge width and length of the tensile testsamples were 6 and 25 mm, respectively. The tensile tests wereconducted at room temperature on a Hounsfield H50KS testingmachine at an initial strain rate of 1.67 × 10−4 s−1. Total elonga-tion of the samples was measured from the difference between thegauge lengths before and after testing.
3. Results and discussion
3.1. Microstructural observation
Fig. 2 shows the microstructures of RD–TD (rollingdirection–transverse direction) plane of the Al/15 vol.% SiO2composite produced by CAR process for various number of cycles
g the steps of the CAR process.
M. Hashemi et al. / Materials Science and
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ig. 2. SEM micrographs of the Al/15 vol.% SiO2 composite at RD–TD plane for: (a)rst, (b) fourth, and (c) sixth CAR cycle.
1, 4, and 6). After first cycle (first step), there were very largearticle free zones and big agglomerated and clustered particlesFig. 2(a)). The length of particle free zones reached ∼200 �m inhe RD and TD. In fact, after first step, the produced compositehowed a non-uniform distribution of the SiO2 reinforcement inluminum matrix. Also, Fig. 2(a) illustrates that there was largeontent of porosity due to adding SiO2 particles to the aluminumatrix and thereby formation of clusters. In the first step where
articles were added between aluminum strips, due to largeurface to volume ratio of SiO2 particles and their attractive vaner Waals interactions, the reinforcement particles easily formedig clusters. Fig. 2(b) illustrated that after the fourth cycle, theig agglomerated particles were separated and fragmented, andany clusters were formed. Separation and fragmentation of SiO2
articles was occurred in rolling direction. This is reasonable,ecause in the roll bonding process with 50% reduction, the lengthf strips become twice. In fact, the plastic flow of samples inhe rolling direction is very high. According to Fig. 2(b), the size
Engineering A 532 (2012) 275– 281 277
of particle free zones was still large (∼150 �m). After the sixthcycle (Fig. 2(c)), the clusters, porosity, and particle free zoneswere disappeared and thus, the composite strip was completelymodified and changed to a uniform Al/SiO2 composite.
The cluster characteristics consisted of:
1. Fraction of clusters in the matrix.2. Particle volume fraction in the cluster.3. Cluster shape.
In fact, if the fraction of clusters in the matrix and particle vol-ume fraction in the clusters decrease and the sphericity of clustershape increases, the produced composite displays better proper-ties [11]. According to Fig. 1, when the number of CAR cycle wasincreased, the aforementioned cluster characteristics improvedand these characteristics for sixth cycle CAR-processed samplesbecame completely modified. In other words, when the number ofCAR cycle was increased, the uniformity of SiO2 particles in the alu-minum matrix improved. These phenomena are attributed to highdeformability of matrix after annealing treatment. Plastic flow ofaluminum matrix causes the particle clusters to be broken and theSiO2 particles to be separated from each other, and as a result to bemore uniformly distributed in all parts of the matrix.
To date, many attempts have been made to explain the mech-anism of cold roll bonding process. According to the techniquesused to establish atom/atom bonds between strip layers, fourtheories have been so far proposed to explain the mechanismsinvolved in the cold roll bonding process. These include film,energy barrier, diffusion bonding, and joint recrystallization theo-ries [4,12–15]. It was found that the film theory is major mechanismin the roll bonding process. During rolling, two opposing brittlesurface oxide layers produced after surface preparation break upcoherently to expose the underlying metals which are extrudedunder normal roll pressure through widening cracks in the sur-face oxide layers from both sides of the interfaces. Also, duringrolling, the metal plastically deforms and extends. In the presenceof SiO2 particles between strips, the clusters of particles openedup and, consequently, got uniformly distributed in the aluminummatrix. Furthermore, regarding the film theory, aluminum flowedthrough the widening cracks in the surface oxide layers. The inter-face was, therefore, a combination of SiO2 particles and bondedareas of extruded aluminum. Consequently, the opening of thesurface oxide layer produced after surface preparation allows formetal/metal contact and roll bonding to take place. In this way,the annealed matrix can easily open the oxide layer due to its highplasticity, subsequently allowing for a strong bonding and a greateruniformity.
Particle breaking is one of the most important phenomena thatcan occur in CAR process. Since the hardness of ceramic particles isvery high, the reinforcement particles can break the thin oxide layerof annealed matrix. In other words, when the particles distributein the matrix, individually, and surround by the matrix, only, theparticle breaking will not occur (in high CAR cycle). But, in initialCAR cycles that there are many clusters in the matrix, the particlessurround by other particles. These particles have same hardness.In other hands, the rolling process involves two types of stressesat the point of contact between the roll and strip, the compres-sive radial pressure increases by decreasing the roll gap and thetangential shear stress due to the friction between the roll andthe strip. Both stresses have horizontal and vertical components.Therefore, the matrix is allowed to flow in various directions. Thisled to increasing the pressure on particles in the clusters. Thus, the
particle breaking phenomenon occurred. Fig. 3 shows the particlebreaking phenomenon in the Al/15 vol.% SiO2 composite producedby one CAR cycle. It was observed that one big particle broke andsix smaller particles created.
278 M. Hashemi et al. / Materials Science and Engineering A 532 (2012) 275– 281
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ig. 3. SEM micrograph of the Al/15 vol.% SiO2 composite at RD–TD plane for firstAR cycle.
Figs. 4 and 5 illustrate the microstructures of RD–TD plane of thel/15 vol.% SiO2 composites at various magnifications for first and
ourth CAR cycle, respectively. As mentioned before, the porosityalue increased after the first step due to adding SiO2 particles tohe aluminum matrix and thereby formation of the clusters of SiO2.owever, by increasing the number of CAR cycle, SiO2 particles
eparate from each other and rolling pressure increases, and as aesult the porosity decreases. This result is completely consistentith previous researches [7–10].
ig. 4. SEM micrograph of the Al/15 vol.% SiO2 composite at RD–TD plane for firstAR cycle.
Fig. 5. SEM micrograph of the Al/15 vol.% SiO2 composite at RD–TD plane for fourth
CAR cycle.
There are two types of the porosity in observed microstructuresof CAR-processed composites:
1. Porosity due to adding reinforcement particles to matrix and for-mation of the particle clusters. In this case, since the plastic flowof matrix into the clusters is insufficient, the porosity creates.This type of porosity takes place in initial CAR cycles, especiallyin the first step (Fig. 4).
2. Porosity due to polishing the sample and weak bonding of thereinforcement/matrix interfaces. During polishing treatment,
since the bond strength of particle/matrix is weak, the particlesseparate from matrix and therefore, the porosity creates. Thistype of porosity occurs in initial and middle CAR cycles (Fig. 5).
M. Hashemi et al. / Materials Science and Engineering A 532 (2012) 275– 281 279
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Fig. 7. The tensile strength of composite and monolithic samples versus the numberof CAR cycle. (For interpretation of the references to color in the artwork, the reader
ig. 6. SEM micrograph of the Al/15 vol.% SiO2 composite at RD–ND plane for sixthAR cycle.
In order to decreasing both two type of porosity, it is essen-ial to increase the number of CAR cycle. In fact, increasing theumber of CAR cycle lead to improving the bond strength of par-icle/matrix and also increasing the plastic flow of matrix into thearticle clusters. Therefore, the porosity decreases.
SEM micrograph of the Al/15 vol.% SiO2 composite at RD–NDrolling direction–normal direction) plane for sixth CAR cycle ishown in Fig. 6. After six cycles, there are 161 bonded interfacescross the thickness of the sample. According to Fig. 6, since thenterfaces, even the last one, were invisible, it can be concludedhat a good bonding with no delamination between layers waschieved under the CAR conditions selected. If the rolling reductionad been insufficient for bonding, the bonded interfaces betweenhe layers would have been clearly seen. It is very important thathe subsequent rolling and annealing treatment after each CARycle helps produce a strong bonding for aluminum/aluminum andluminum/SiO2 interfaces.
Indeed, the difficulty of achieving a uniform distribution of par-icles in the matrix is one of the problems associated with theroduction of cast metal matrix composites [2,11,16]. In cast MMCs,he distribution of the reinforcement particles in the matrix alloy isnfluenced by several factors including the rheological behavior ofhe matrix melt; the particle incorporation method; interactions ofhe particles and the matrix before, during, and after mixing; andhe changing particle distribution during solidification [2,11,17].s was shown in previous researches [2,11,18], the post solidifi-ation processing of cast metal matrix composites by extrusion orolling can modify the particle distribution, but complete decluster-ng cannot be achieved. According to microstructural observations,t is useful to employ the CAR process in the manufacture of highlyniform metal matrix composites.
.2. Mechanical properties
The variation of tensile strength of the Al/SiO2 composites andhe monolithic aluminum produced by the CAR process versus the
umber of cycle is shown in Fig. 7. For the monolithic aluminumtrip (produced by the same process and without SiO2 particles), theesults indicated that strength is far less sensitive to the number ofycle. Due to the annealing process at the end of each CAR cycle,
is referred to the web version of the article.)
a sequence of recovery, recrystallization, and grain growth occursand the work hardening effect is almost eliminated [19]. However,it can be seen that the tensile strength of composite increased byincreasing the number of CAR cycle. After first step, the tensilestrength reached 119 MPa, which was 1.45 time higher than whatwas obtained for the initial material (82 MPa). In the first step, SiO2particles were added to aluminum matrix, and therefore particlevolume fraction in the matrix was increased. It is well known thatincreasing the amount of reinforcement particles in the matrix sig-nificantly increases the tensile strength [20–24]. This is attributedto the role of ceramic particles in strengthening of matrix. Strength-ening mechanisms of the first step are related to the following threemajor effects of the SiO2 particles in the aluminum matrix:
1. During the tensile test, the SiO2 particles act as a barrier todislocation movement and create dislocation pile-up in theirneighborhood and add strength.
2. After the roll bonding and subsequent annealing, the SiO2 par-ticles hinder grain growth during annealing of the compositesthus also improve strength.
3. After annealing and during cooling treatment, the SiO2 par-ticles generate dislocations in the composites as a result ofheavily built multidirectional thermal stress at the reinforce-ment/matrix interfaces induced by the large difference of thecoefficient of thermal expansion (CTE) between the matrix andreinforcement.
As mentioned before, in the second step, CAR process was donewithout adding SiO2 particles. In this step, by increasing the numberof cycle, the uniformity of particles in the matrix was improved,better bonding was created between the layers, and the porositywas removed, which resulted in higher strengths. In second step,the tensile strength values up to the fifth CAR cycle had a steepslope but dwindle at cycles above 5. In other words, after a specificnumber of CAR cycle, the tensile strength of composite reached itsmaximum value and then saturated by increasing the number of
cycle. The tensile strength of the composite after six CAR cycleswas 1.77 and 1.63 times higher than the same values obtained forannealed and monolithic aluminum strips, respectively. All these
280 M. Hashemi et al. / Materials Science and
Fig. 8. The elongation of composite and monolithic samples versus the number ofCr
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AR cycle. (For interpretation of the references to color in the artwork, the reader iseferred to the web version of the article.)
esults are in agreement with those obtained from microstructuralbservations.
The variation of elongation of the Al/15 vol.% SiO2 compositend the monolithic aluminum versus the number of CAR cycle isllustrated in Fig. 8. For the monolithic aluminum strip, the resultsndicated that elongation is far less sensitive to the number of CARycle. But, the variation of elongation for composite sample is sig-ificant. For MMC, the elongation was decreased dramatically from7% (for the annealed raw material) to 14% in the first step. Thisignificant decreasing is related to adding SiO2 particles to the alu-inum matrix in this step. When hard SiO2 particles added into theatrix, the dislocations movement is hindered as a result. There-
ore, the elongation value to fracture is reduced.Elongation values at the end of first step were 14% and 35%
or aluminum/15 vol.% SiO2 composite and monolithic aluminum,espectively. But those at the end of second step were 27% and2%. It should be noted that the second step greatly increased thelongation of composite (92%) due to following factors:
. Uniformity: This factor has an important effect on the elonga-tion value. The agglomeration and clustering of particles lead todecreasing the distance between interfaces and thus, the crackswhich are initiated in the interfaces will propagate and linkup with other cracks promptly, and the ductility of compositedecreases. When the distribution of SiO2 particles in the alu-minum matrix changes to be more uniform and homogeneous,the distance between the Al/SiO2 interfaces increases. Therefore,during plastic deformation, the cracks which are initiated in theinterfaces will propagate and link up with other cracks later, andthe ductility of composite improves. Since the uniformity of par-ticles was improved due to increasing the number of CAR cycle(Section 3.1), it can be concluded that by increasing the CAR cycle,elongation of composite improve.
. Porosity: Porosity in metal matrix composites can act as prefer-ential crack initiation sites and propagation paths for the cracks[21–25]. As explained in Section 3.1, when the number of CARcycle was increased, the porosity content in the compositesdecreased, thus enhancing the elongation.
. Bonding quality: In case of the metal matrix composites, fracturemechanism dominated by the crack initiation at the inter-face propagates through the interface and links up with othercracks and reinforcement/matrix interfaces cause the failure.
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Engineering A 532 (2012) 275– 281
As expressed in Section 3.1, by increasing the number of CARcycle, the bonding quality of particle/matrix and matrix/matrixbecomes stronger due to the greater rolling pressure and anneal-ing treatment which imparts a higher elongation to the product.Thus, the bonding quality plays a vital role for improving theductility.
As can be seen from the results of this work, the CAR processcan be a useful procedure for producing high-performance com-posites and for obtaining a highly uniform particles distribution inthe matrix. Although no investigations have been reported on theinfluence of the CAR process on the uniformity of particle distribu-tion and mechanical properties during fabrication of the Al/SiO2,our results suggest that the CAR process might be very useful inthis regard as well.
4. Conclusions
In the present work, the CAR process was used for fabrica-tion of the Al/SiO2 composite. The microstructure and mechanicalproperties of the manufactured MMC were investigated. When thenumber of CAR cycle was increased, the uniformity of SiO2 parti-cles in the AA1100 matrix and bonding quality between SiO2 andaluminum improved. The produced composite by six CAR cyclesshowed a homogeneous distribution and strong bonding betweenparticles and matrix without any porosity. In the present work, theparticle breaking was one of the most important phenomena thatcan be occurred in CAR process. Also, there were two types of theporosity in observed microstructures of CAR-processed compos-ites: (a) porosity due to adding reinforcement particles to matrixand formation of the particle clusters, and (b) porosity due to pol-ishing the sample and weak bonding of the reinforcement/matrixinterfaces. The Al/SiO2 composite was exhibited a higher tensilestrength than the annealed and monolithic samples so that the ten-sile strength of the Al/15 vol.% SiO2 MMC was 1.77 and 1.63 timeshigher than that of the annealed and monolithic samples, respec-tively. In addition, the tensile strength of CAR-processed MMC wasincreased in both steps by increasing the number of cycle. In otherhands, the elongation of CAR-processed MMC was decreased in firststep but it was increased in second step as a result of increasing thenumber of CAR cycle.
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